- Email: [email protected]
Trivalent cations in fluorescent zinc sulphide
Physica XVI, no 3
TRIVALENT
M a a r t 1950
CATIONS IN FLUORESCENT ZINC SULPHIDE
b y F. A. K R O G E R a n d J. D I K H O F F Philips Research Laboratories, N.V. Philips' Gloeilampenfabrieken Eindhoven,Netherlands
Summary Incorporation of monovalent cations in a lattice consisting of divalent ions is only possible to an appreciable extent when the lack of positive charge resulting from the substitution of a monovalent cation for a divalent one is compensated for. This compensation can be effected by a simultaneous incorporation of monovalent anions, or of cations of a valency higher than two. On this basis it is explained that ZnS becomes fluorescent by the monovalent activators Ag +, Cu +, Au + and Zn + when halogens or trivalent cations are present. Some of the trivalent ions incorporated in this way are found to cause effects of their own (electron traps, fluorescence, killing of fluorescence due to the other centres). An atomic model of the centres of fluorescence is given.
Introduction. F l u o r e s c e n t s m p h i d e s of zinc a n d c a d m i u m are usually p r e p a r e d b y h e a t i n g t o g e t h e r the p u r e sulphides, m a d e b y precipitation f r o m aqueous solutions, a c o m p o u n d of an a c t i v a t o r like Ag, Cu, Au or Mn, a n d a certain a m o u n t of halides of the alkali or alkaline e a r t h g r o u p metals. T h e l a t t e r c o m p o u n d s h a v e a double function. I n the first place t h e y act as crystallizing a g e n t b y causing the a p p e a r a n c e of a m o l t e n p h a s e at the n o r m a l firing t e m p e r a t u r e s at which p u r e sulphides are still solid 1); in this connection t h e y are called fluxes. I n the second place t h e y h a v e a specific influence. Glassner2), RandallS), Guntz4), Strange 5) a n d R o t h s c h i 1 d 6) h a v e shown t h a t the specific influence is m a i n l y due to the anions CI-, B r - a n d I - , while F - is ineffective. According to S t r a n g e 5) a n d K r 6 g e r et al. 7), s) the specific effects c a n be e x p l a i n e d on the basis of the a s s u m p t i o n t h a t the a c t i v a t o r ions f o r m i n g the centres are i n c o r p o r a t e d in the m a t r i x lattice --
2.~7 - -
298
F.A.
KROGER
AND
J. DIKHOFF
at normal lattice sites, replacing zinc (or cadmium). If the activators are monovalent ions as is the case with copper, silver and gold, incorporation at such sites either involves the formation of one anion vacancy for each pair of monovalent cations, or the occupation of interstitial sites by half of the activator ions. If monovalent anions like C1- or B r - are incorporated at the same time, however, complications of this kind are avoided, the lack of charge on the cation sites being compensated by a corresponding lack of charge on the anion sites. K r U g e r , Hellingman and S m i t T ' ) , ~) have shown that chlorine actually enters the lattice in amounts equivalent to the concentration of fluorescence centres. Compensation of the lack of positive charge due to the incorporation of monovalent cations at lattice sites can in principle also be effected b y the simultaneous incorporation of cations of valency higher than two; with trivalent cations for instance one monovalent and one trivalent ion replacing two zinc ions *). Now several patents exist in which the simultaneous incorporation of monovalent cations and cations of a valency of three or higher is claimed to have a beneficial effect on the fluorescence 11), 1~), is) and the principle of compensation of charge has also been recognised (W e s c h 11) : saturation of valencies). Yet a favourable effect found especially with the use of gadolinium as a trivalent ion was not explained merely b y the compensation principle but was attributed to an additional mechanism. As gadolinium is known to cause a characteristic ultraviolet fluorescence in many systems, it was assumed that excitation of a zinc sulphide phosphor containing this ion primarily causes the emission of this fluorescence, the visible fluorescence of the true activator (Ag, Cu) being secundarily excited b y the ultra-violet radiation 11), 3,). If this explanation were correct, the effect would be restricted to ions causing an ultra-violet fluorescence. In this paper it will be shown that this is not the case, all trivalent ions that can be incorporated in the sulphide lattice having a similar effect. Whenever there are differences in intensity between the fluorescence of various systems this is not due to a particularly fa*) T h e p r i n c i p l e of c o m p e n s a t i o n of c h a r g e is b y n o m e a n s n e w . It is well k n o w n in m i n e r a l o g y , in w h i c h it g o v e r n s a l a r g e n u m b e r of s u b s t i t u t i o n s ; V e r w e y e t al. h a v e p r o p o s e d i t s use in t h e r e g u l a t i o n of t h e v a l e n c y of ions in s e m i - c o n d u c t o r s 9), w h i l e o n e of t h e a u t h o r s h a s g i v e n a n a p p l i c a t i o n in t h e field of f l u o r e s c e n c e ( C a F , - - U) lo). In a r e c e n t p u b l i c a t i o n W e 11 s 33) h a s u s e d it to e x p l a i n t h e f a v o u r a b l e effect of N a in C a S - - Bi p h o s p h o r s .
T R I V A L E N T C A T I O N S IN F L U O R E S C E N T ZINC S U L P H I D E
299
vourable mechanism in the systems with a strong fluorescence, but to unfavourable conditions (quenchers) in the weak ones. From a comparison of the spectra of the fluorescence promoted by the halogen ions on the one hand and the trivalent ones on the other, conclusions as to the nature of the centres of fluorescence are drawn.
Experimental § 1. Preparation. Pure, halide-free zinc sulphide was made by precipitation from an aqueous solution of zinc sulphate by hydrogen sulphide. The precipitate was filtered, washed with H2S-water and then dried at 140°C. After milling the powder was preheated at 1000°C in a current of H2S. The sulphide obtained was a fine purely white powder, showing the ultra-violet fundamental fluorescence of ZnS at liquid-air temperature. Samples of this sulphide were wetted with aqueous solutions of A g N Q , C u N Q or AuCI~ *) and of the nitrates of the trivalent and tetravalent ions A1, Ga, In, Sc, Y, La, Ce, Pr, Gd, Sm, Yb, Zr and Th in amounts to give a concentration of 10-4 gram atoms per mol ZnS both for the monovalent and for the trivalent or tetravalent ions. Borium was added as boric acid, silicium as SiO 2 obtained by hydrolysis of ethyl silicate. Blanks containing monovalent and trivalent ions alone were also made. The samples were dried, thoroughly mixed, and then heated for half an hour at 1200°C in a stream of H2S. In order to ascertain the transformation of the oxides of the trivalent and tetravalent ions into the corresponding sulphides, care was taken for a good circulation of the atmosphere through the powder.
§ 2. General observations. The fluorescence of the samples was studied with excitation by ~t = 3650 A at room temperature. The spectral distribution of the fluorescence was measured with the aid of a double monochromator and photo-multiplier. Peak positions of fluorescence bands are given in Table I. The samples containing Ag, Cu or Au alone are grey and show no fluorescence. *) A gold solution w i t h o u t chlorine would have been preferable; the only other possibility, a solution of Au in KCN solutions, however, gave products with a very poor fluorescence. As the amount of chlorine added in this way was very low, while heating in H2S w i t h o u t t r i v a l e n t ions gave non-fluorescent products, it is believed t h a t the chlorine ev aporates entirely in the firing process and does not in va l i da t e the results of the experiments.
300
A N D J. D I K H O F F
F.A. KROGER
TABLE
I
M a x i m a of fluorescence bands in ZnS containing Ag, Cu, A u or Zn together with trivalent or tetravalent cations or monovalent anions; excitation by ,~.= 3650 A at room temperature. A'dded ions
Zn wurtzite D
B A1 Sc Y Ga In La Ce Pr Sm Gd Yb Si Ge Zr Th CI, Br, I
Ag sphalerite
wurtzite
Cu w urt z i t e
B
Au wurtzite m
m
4600
?
4450; 5100 4310 5280 4360; 5250 5200 5290 4340 4500; 5100 5280 4490 4500;5160;5700 4700;5280;5800 4400; 5700 4490; 585C 680O 68O0 6500 ? 4500; 5150 5280 4330 ? 4950; 5500 4300;4900;5400 5300 ? 5100;lines 5280;lines 4330;lines 4450 ?
4600
lines
E
D
m
? ?
4320 4310 4310
4500; 5100 5150
5290 5300
4460
4350
4450; 5!60
4700; 5300
n
4520
Samples containing B, Sc, Y, La, Ce, Sm, Gd, Yb, Si, Zr and Th alone are also non-luminescent. Samples containing A1 or Ga, however, show a blue fluorescence in a band identical with that found with self-activated ZnS containing chlorine. The product containing Pr shows a weak yellow fluorescence, while that containing In has a weak orange fluorescence. Nearly all products containing the monovalent ions together with trivalent ions have a considerable fluorescence, with the exception of B and Sm, which have no effect at all. Among the tetravalent ions Si seems to have a positive effect, but Zr and Th are ineffective. The products containing Ag, Cu or Au, together with A1, Y, Yb, La or Gd, show a strong fluorescence in the blue, green and yellowish green, in bands corresponding exactly to those appearing with similar phosphors made with halogen instead of trivalent cations. The same bands occur with products containing Ag, Cu or Au together with Ce, Pr, Sc, Ga or In, b u t these products show other bands in addition. The latter bands are different for the various trivalent ions, but they are the same for one particular trivalent ion in combination with different monovalent cations; they must therefore be'due to the trivalent ions.
TRIVALENT
CATIONS
IN FLUORESCENT
ZINC SULPHIDE
301
The proportion between the various bands in the fluorescence spect r u m varies from case to case; with Z n S - A g - Pr for instance the fluorescence occurs mainly in the blue silver band, while with Z n S - A g - In the red indium band is preponderant. Apart from such differences the proportion is dependent upon the condition of excitation (temperature, exciting intensity) as is normal for doubleactivated ZnS luminophors. It is remarkable that, while several rare earth ions are effective in promoting the fluorescence of ZnS, Sm was found to be ineffedtive. This is the more surprising since fluorescence due to samarium in ZnS has been mentioned by various authors 15), le), although it must be emphasized that both R i e h l and S c h l e e d e have been unable to prepare such a phosphor 17). The explanation probably is that samarium, which is known in the divalent form, is incorporated as such without the simultaneous incorporation of monovalent ions and therefore shows neither the characteristic lines of the Sm 3 ~ ion nor the bands of Cu, Ag, Au or Zn. The various double-activated products show marked differences in their phosphorescence properties and have different glow curves. Phosphors containing A1, Gd, In or Yb, show a more or less normal phosphorescence: in combination with silver the after-glow is very short, but with copper or gold there is a moderate after-glow. Products containing La or Y have practically no after-glow, in agreement with a claim made in a patent by W e s c h and K a m m 13) ; apparently these ions act as killers. The properties of products containing Pr or Ce depend on the monovalent cations present ; in combination with Cu or Au there is a normal after-glow in the green copper or gold band, but in combination with Ag the after-glow is in the Pr or Ce bands. Ga and Sc finally cause an exceptionally strong afterglow in the bands of the activator (Ag, Cu, Au). Systems containing Ga moreover show the peculiar effect that the fluorescence is very weak while the phosphorescence is strong *). Apparently this ion has various functions, causing both deep traps and killer centres of the cobalt type **) (while it also causes a fluorescence of its own - - and therefore also forms fluorescence centres; see § 5). *) O w i n g to t h i s p r o p e r t y t h e s y s t e m s a r e s u i t a b l e for use in r a d a r t u b e s . **) W i t h a n o r m a l killer t h e p h o s p h o r e s c e n c e is s u p p r e s s e d w h i l e t h e f l u o r e s c e n c e r e m a i n s u n w e a k e n e d ; t h i s is t h e case for n i c k e l in ZnS p h o p h o r s . C o b a l t a c t s d i f f e r e n t l y in Z n S - Cu a n d Z n S - A u , in w h i c h it w e a k e n s t h e f l u o r e s c e n c e b u t l e a v e s t h e p h o s p h o r e s c e n c e u n a f f e c t e d . F o r a n e x p l a n a t i o n see ref. 18).
302
F. A. KROGER AND J. DIKHOFF
§ 3. The bands o[ Ag, Cu, A u or Zn. Zinc sulphide phosphors activated with Ag, Cu or Au, and made in the presence of chlorides, bromides, or iodides (or of HC1, HBr or HI) are known to show a blue band which is usualiy attributed to an excess of zinc, another blue band which is due to silver, a green and a blue. band due to copper e), 8) and a greenish blue and a yellowish green band due to gold 19), 20), 21). Products made with addition of trivalent cations instead of halogen show the zinc band in two cases (AI, Ga); the blue silver band, the two copper bands and the yellowish green gold band appear in all cases, but .g the greenish-blue gold band is not ,t~mma observed. Apparently this band is not promoted by trivalent cations. The position of the bands ns-~0"*A.e - t0"~X is exactly the same for products containing halogens or trivalent ions as is demonstrated for the silver band in Fig. 1.The intensity of the blue fluorescence of the various samples for various methods of excitation is given in table II; it is seen that the best results are obFig. 1. S p e c t r a l d i s t r i b u t i o n of tained with A1 as a trivalent ion. For t h e f l u o r e s c e n c e of Z n S a c t i v a t e d this reason A1 has been used for a with silver together with chlorine quantitative study of the influence or v a r i o u s t r i v a l e n t ions. of trivalent ions. In a series of products containing a constant amount of AI(10 - 4 gram atoms per mol. ZnS) but an increasing concentration of Ag ( 0 - - 2 . I0 --4 g. atoms per mol ZnS) the fluorescence varies gradually from white-blue, in a band with a maximum at 4600 A, to deep blue in a band with a maximum at 4350 A. The first band is the band of self-activated ZnS, the latter the band of ZnS - Ag proper. Therefore addition of silver causes a suppression of the zinc band, just as was observed with products containing chlorine 7). This effect can be explained as follows: in order to incorporate a certain amount of A13~ in pure ZnS an equivalent amount of Zn + is formed. When Ag ÷ is present, however, these ions enter the lattice instead of the zinc ions, and reduction of Zn z+ to Zn ÷ does not occur. This means that the variation of the fluorescence spectrum is ,
,
TRIVALENT
CATIONS IN FLUORESCENT
ZINC SULPHIDE
303
TABLE II Relative intensities of the blue band of Z n S - A g filter BG 12 ZnS - - 5.10 -5 A g with addition of
10-4 10-* 10-4 10- i 10-4 10 -4 10- I 10- i 10-~
Firing conditions 80H2S--20HCI ; 1 h 1200°C HzS; 1 h 1200°C H2S ; 1 h 1200°C H2S; i h 1200°C H2S ; 1 h 1200°C H~S; l 11 1200°C H2S; I h 1200°C HzS; l h 1200°C H2S; l h 1200°C H2S ; 1 11 1200°C HzS; 1 h 1200°C
A1 y Ga Sc Pr Gd La Yb Si
measured through the Schott
I a for e x c i t a t i o n at 25°C b y = 3650 £
cathode rays (20 kV)
~. = 2537 A
100
100
I00
92
105 49
105 66 4.2 45 47 83 47 31.5
38 7.7 29.5 31
3.6 40 32
59 22 12
53 25 33
2.5 0.2
15
20.5 2
0.6
not simply due to an increase of the concentration of silver centres but to a simultaneous decrease of the concentration of zinc centres. A series of products containing a constant concentration of Ag (10 -4) and an increasing concentration of A1 ( 0 - 3.10 -4) shows a gradual increase of the intensity of the fluorescence in the blue Ag band from zero upwards. Maximum intensity is reached when the concentrations of A1 and Ag are about equal (Fig. X), as is to 20kV]
lft
F i g . 2. I n t e n s i t y amounts cathode
of
of the blue fluorescence
aluminium;
excitation
by
r a y s o r u l t r a - v i o l e t ( 2 . = 2 5 3 7 •) at room temperature.
~-,~'~/,/~"-x=a~
__g
t
=_~_~[,/ i 0 - - ti v-s
510 s,49 xAI
,,s-s., i ~-,
-
i v-~
~CA/
be expected on the basis of the principle of compensation of charge. A decrease of the fluorescence at concentrations of aluminium greater than the silver concentration does not appear: the excess of aluminium gives rise to monovalent zinc and the zinc band appears next to the silver band. In the presence of a slight excess of aluminium, silver can dissolve in ZnS up to a concentration of at least 10-3 gram atoms per tool. With increasing silver content the maximum of the fluores-
304
F . A . KROGER AND J. DIKHOFF
cence band shifts towards a longer wavelength (from 4330 to 4450A) ; at the same time the long-wave limit of the centre absorption shifts over the same distance (fig. 3). This suggests that the shift of the
4 I
"0
?
/!.;,.:.,6 jD-3A.
~ • Jf
I ~
~ \
....
Fig. 3. S p e c t r a l d i s t r i b u t i o n of t h e f l u o r e s c e n c e a n d t h e d i f f u s e r e f l e c t i o n for ZnS c o n t a i n i n g i n c r e a s i n g conc e n t r a t i o n s of silver a n d aluminium.
refl¢,¢¢~
III
tI 3500
1000
4500
I~'00
fluorescence is due to self-absorption. Gold behaves similarly to silver, but owing to the difference in colour between the gold band the zinc band the replacement of the zinc fluorescence b y the 2n-~
i
A
X
of ZnS c o n t a i n i n g 10 - 4 A1 a n d v a r i o u s a m o u n t s of gold.
//
~
*000
-
,
*N]0
~
~
.
5~0
"°i=
.
ZnS-~-~AI-xAu
~
.
~kin~
.
60O0
gold fluorescence is much more evident. In a series of products containing 10-4 A1 and increasing amounts of Au, for instance, the product without gold shows the blue zinc band while the sample
/ ,' X~
I
O'~i' - ' ~ -
3.10-6
I
e~2~-7
I
10-S 3.10-S
fO-A
340-~ D CAu
Fig. 5. I n t e n s i t v of t h e blue Znb a n d a n d t h e y e l l o w i s h - g r e e n Aub a n d for ZnS - 10 - 4 A1, c o n t a i n i n g v a r i o u s a m o u n t s of gold.
containing 10 - 4 Au shows the yellowish green gold band (fig. 4). Fig. 5 shows the variation of the intensity of the two bands with the gold concentration; the measurements were carried out through
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
305
the S c h o t t filters BG 12 for the blue, and OG 2 for the yellow" band. It is seen that the zinc band is totally suppressed in all products for which the gold concentration exceeds the aluminium concentration. The increase of the gold band in a series with 5.10 -s Au and increasing A1 concentration is shown in Fig. 6. lfl
Fig. 6. I n t e n s i t y of t h e y e l l o w i s h g r e e n gold b a n d for ZnS - - 5 . 1 0 - s Au containing various amounts of aluminium,
/ ~/'/ /
~ nZnS-S.10-$ ~ o ~ t ~ aAu-x ~ At
/ / o '
j.jo-~
~-s
j,~-s
Jo--'
~o-, .r w
The copper system is more intricate due to the appearance of a blue and a green copper band. In order to avoid intensity dependence of the proportion between these bands the measurements were carried out at liquid air temperatures, at which this effect is suppressed. In a series of products ZnS - 10-4 A 1- xCu in which R=~
A / ~,t-
ZnS-xCu-~41 'R--19~[b ~,ue - - IO-'A| " . . . . ." Ig~en for
Ltn-sa, ~,:{II
3./O-e
/0"~"
3.10- s
~
3404
b Co, t ~
Fig. 7. I n t e n s i t y of t h e blue a n d g r e e n b a n d s of Z n S - - 1 0 - 4 AI c o n t a i n i n g v a r i o u s a m o u n t s of c o p p e r , a n d tile r a t i o b e t w e e n t h e i n t e n s i t i e s of t h e g r e e n a n d blue b a n d s for Z n S - 1 0 - s , 10 _4 a n d 3.10 - 4 AI w i t h v a r i o u s a m o u n t s of c o p p e r . E x c i t a t i o n b y ~t = 3650 2k a t --1B0°C,
the copper content is varied, the product without copper shows the blue zinc band with a maximum at 4600 A. Incorporation of copper first causes a gradual replacement of this band by the green copper band (maximum 5100 A). At higher copper concentrations the blue Physica XVI
20
306
F . A . KROGER AND J. DIKHOFF
copper band with a maximum at 4400 A appears; this band becomes predominant when the copper concentration exceeds the aluminium concentration. The variation in the intensity of the blue and green bands is shown in Fig. 7, clearly demonstrating the appearance of the blue bands at extreme copper concentrations and the green in between. In the same figure the ratio green : blue is given for this series and also for similar series with a higher (3.10 -4) and lower (10-5) aluminium content. In alle cases the zinc band is predominant at low copper concentrations, but the amount of copper necessary to suppress it is the larger the higher the aluminium concentration. The explanation is obvious when it is borne in mind that the concentration of zinc centres depends directly upon the aluminium concentration, the concentrations of Zn + and A13 ~ being equal; the mechanism b y which the copper band replaces the zinc band is exactly the same as discussed above for the case of silver. The transition from copper green to copper blue also shifts to higher copper concentrations with increasing aluminium concen-
OJ-blt~
~O"e
I"
Fig. 8. C o n t o u r s of e q u a l v a l u e s for t h e r a t i o b e t w e e n t h e i n t e n s i t i e s of green a n d blue b a n d s in t h e s y s t e m s Z n S - C u - A 1 ; e x c i t a t i o n b y ~t = 3 6 5 0 A a t - 180°C. ~CAI
tration, and in all cases it takes place close to the point at which the concentrations of copper and aluminium are equal. The values of the ratio green: blue over the entire system Z n S - x A l - y C u are shown in Fig. 8. When this contour diagram is compared with similar diagrams in our paper dealing with the properties of ZnS - Cu made in an atmosphere of H2S - HC1 (fig. 4 and 6 of ref. 8) the similarity is o b v i o u s : a p p a r e n t l y an increasing amount of HC! in the H2S atmosphere corresponds to an increase of the aluminium
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
307
content. This is exactly as is to be expected on the basis of our theory where it is assumed that an increase of the amount of HC1 in the atmosphere results in an increase of the amount of chlorine incorporated in the sulphide. Although there is a great similarity between the systems containing C1 and those containing AI, there are nevertheless some quantitative differences: the maximum of the green: blue ratio, for instance is about twice as high with A1 than with C1. Similar differences are found when comparing systems containing v a rious trivalent ions. Z n S - 10-4 A1- 5.10 - s Cu for instance is nearly pure green fluorescent and the same is true of systems of a corresponding composition but containing Ga, Yb, Sc, P r or Ce instead of AI. Products containing Y or Gd, however, show a much bluer fluorescence, while the fluorescence of a product with La is practically pure blue (fig. 9). La
Fig. 9. Spectral distribution of
AI
the
fluorescence
of
ZnS-5.10--SCu,
containing various trivalent ions. The hatched peak on the curve for ZnS- C u - Pr is due to Pr-fluorescence. Similarly the long-wave tail of the fluorescence of ZnS- C u - Sc is due +n the scandium. In the system ZnS-Cu(C1) it has been observed that refiring of a sample showing the blue copper band in H2S at a temperature at which the chlorine concentration cannot change, e.g. 500°C, causes the fluorescence to change from blue to green and this observation is one of the main reasons for the assumption that Cu2C1 is the blue centre in th; ~, system. When a blue fluorescent phosphor of the aluminium system is subjected to the same treatment no variation of fluorescence is found. Retiring in a sulphur atmosphere, however, does indeed produce such a change, while heating in hydrogen causes a change in the opposite direction (Fig. 10). Therefore the
308
F.A.
KR(SGER A N D J. D I K H O F F
difference is only quantitative, the nature of the centres in the two systems being essentially the same. In view of the evidence so far given it seems apparent that the activation of ZnS phosphors is governed by the principle of charge compensation. Yet there is one observation which might seem to contradict this. According to S m i t h 2~) silver can activate ZnS without halogens being present, provided the product is made in the presence of ZnO or is fired in an oxidizing atmosphere (SOv A
?
2ns-s.to-scu-to-~A! ex~ 36S0~ -IeO=C
/UgX\\ 'k~\
Joo
,
45oo
O= A,rea,*d t^ ~O'C ln S~
~ c=a,~tj~atns°ec~"v° Fig. 10. Variation of the spectral distribution of the fluorescence of ZnS--5.10 s C u - - 1 0 -5 A1 upon retiring at 500°C in various at, , --,mospheres.
5oo0
550o
CO2). By repeating these experiments we have been abie to reproduce these results, and similar results were obtained with copper and gold. It must be emphasised, however that the effects are extremely weak. A possible explanation will be discussed in a subsequent paper.
§ 4. The nature o/the Ag, Cu, Au and Zn centres. In the papers 7),s) dealing with ZnS containing Ag or Cu together with chlorine it has been shown that although the incorporation of the activator ions is governed by the principle of charge compensation, resulting in the formation of solid solutions like Z n S - - A g C 1 and ZnS--CuC1, the metal ion and the halogen need not necessarily occupy neighbouring sites in the lattice. Therefore there were still three possibilities for the centres of fluorescence, viz 1) the activator A + with its neighbours, 2) the halogen X - with its beighbours, and 3) A+X - with neighbours. Similar possibilities exist, of course, for the case where the activator ions are incorporated together with trivalent cations 133+, these possibilities being 1) A + with surroundings, 2) B 3+ with surroundings, and 3) A + and ]33+ at neighbouring cation sites, plus the surroundings. The second possibilities are ruled out by the fact that zinc sulphide activated with the various activators Ag, Cu, Au or Zn shows dif-
T R I V A L E N T CATIONS IN F L U O R E S C E N T ZINC S U L P H I D E
309
ferent bands, while on the other hand the peaks of systems CO,ltaining different halogens (CI-, B r - , I - ) or different trivalent cations (A13+, Gd 3+, a.o.) do not show the slightest difference. For the latter reason the third possibility can also be discarded; if ions as different as C1- and A13+ were to belong to the centre, marked differences should be expected. Therefore only the first possibility remains open, and it must be concluded that this represents the actual situation: the centres of fluorescence consist of monovalent activator ions surrounded by four sulphur ions. The blue copper centre is slightly different and consists of a monovalent copper ion and a copper atom close by 8), but also in this case the halogens or the trivalent cations are at distant sites and do not belong to the actual centre. This does not mean, however, that all the incorporated ions are in this position; we have to expect an equilibrium between associated and dissociated pairs A+X - and A+B 3+ just as is the case in liquid solutions. In the case of Ag, Cu and Au, the equilibrium will be that corresponding to the high firing temperature or at least to a relatively high temperature passed during cooling; a variation of the equilibrium during cooling requires the migration of cations and therefore will not occur to an appreciable extent at low temperatures. In the case of self-activated ZnS the situation is different because the monovalent zinc which is held responsible for the fluorescence can associate and dissociate from its stabiliser (CI-, A13+) without a migration of the ions themselves, the migration of an electron being sufficient: Zn 2+ + Zn + -+ Zn + + Zn 2+ Such a migration of electrons, however, has either no activation energy at all or only very little. Therefore in this case the association-dissociation equilibrium will be that corresponding to room temperature or an only slightly higher temperature. If the energy involved in the dissociation is large, dissociation will not occur at all; it will only occur to an appreciable extent when this energy is small (or even negative). This energy will depend on the ions involved and therefore will be different for C1-. B r - , and various trivalent ions. This m a y explain why the zinc band appears only with the halogens and with A1a+ and Ga a+, while the Cu, Ag and Au bands on the other hand appear much more generally. The blue copper centre consisting either of a Cu + ion and a
310
F . A . K R O G E R A N D J. D I K H O F F
copper atom, or of two Cu + ions and a sulphur vacancy containing an electron near by s), m a y also dissociate, forming single Cu + ions and therefore green centres. A dissociation of this type has been proposed to explain the dependence of the colour of the fluorescence of copper phosphors upon the rate of cooling after preparation s). The differences observed between products made with various trivalent cations as mentioned in § 3 must be explained either by a dissociation of this type or by a difference in the "normal" A÷X - or A+B 3+ dissociation for the blue and green copper centres. The concept of association-dissociation m a y also explain the absence of fluorescence in products containing fluorine as a halogen if it is assumed that in this case the incorporated A + F - is completely associated. The difference with the other halogens m a y be due to an almost purely electrostatic binding in the case of fluorine while the other halogens form largely covalent bonds. The fluorescence caused by Ag, Cu, Au and Zn, is known to be due to an electronic transition between the conduction band, or a level close to this band 23), 24), and a level above the upper filled band (centre level). When part of the zinc in ZnS is replaced by cadmium the edge of the fundamental absorption band and the maxima of the fluorescence bands shift to the long-wave side, the the separation between the activator level and the full band remaining unchanged. The incorporation of cadmium has two effects; in the first place the zinc and cadmium ions interact thoroughly,, forming one mixed conduction band; in the second place the lattice constant is increased, resulting in a decrease of the electrostatic forces and therefore in a lowering of the levels of electrons at positive ions and a rise of the levels of electrons at negative ions 25). As the full band is supposed to be due to S2- ions, and the e m p t y band to (Zn, Cd) + ions, these two effects together explain the shift of the fundamental absorption. On the other hand, from the behaviour of the fluorescence the nature of the centre level can be deduced. If this level were to be a level of the activator itself, present at a cation site, the expansion of the lattice would cause a decrease of the energetical separation between this level and the full band; but when the centre level is due to an electron at an anion site this separation remains unchanged. The latter is found to be the case. Therefore the centrelevel must be due to an electron at an anion site.
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
311
Levels of electrons at an anion site must be due either to vacant anion sites containing an electron or to sulphur ions, or to ions replacing sulphur, e.g. halogens. The first possibility seems very unlikely for normal phosphors like ZnS-AgC1, particularly since the formation of phosphors has been found to be governed by the principle of c o m p e n sation of charge, which is just based on the avoidance of vacancies ; on the other hand it has been shown that halogen ions are not present in the centres. Therefore the only possibility left is that the centre level is due to sulphur ions. On the other hand we know that the activators Ag, Cu, Au, Zn have an essential influence on the fluorescence. Therefore it seems logical to assume that the centre level is a level due to sulphur ions, perturbed by the presence of an activator ion at an adjacent site. According to Dr K 1 a s e n s of this laboratory, who suggested this possibility several years ago, the influence of a monovalent cation on the level of an adjacent sulphur ion is in the right direction and of the order of magnitude as required for the centre level. The direction of the effect is easily seen for a simple electrostatic model: the lack of positive charge resulting from the replacement of Zn 2+ by A + causes a decrease of the electrostatic potential at an adjacent S2- site, and accordingly a rise of the electron energy levels at that site. For an exact calculation of the effect polarization, repulsive forces and v a n d e r W a a 1 s attraction forces should be taken into account; it is actually these effects which must be responsible for the differences between the levels of centres due to Ag, Cu, Au, Na, Li 14). Centres of the same type are probably also present in alkaline earth sulphide and oxide phosphors *).
§ 5. The bands o/Ga, In, Sc, Pr and Ce. When trivalent cations are used to aid the incorporation of monovalent activator ions, these ions are incorporated at the same time and it must be expected that these also cause extra levels in the energy spectrum of the system. When the conditions are favourable these ions may therefore Mso form centres of fluorescence. This happens to be the case with Ga, In, Sc, Pr and Ce, as is indicated by the fact that new bands appear in the fluorescence spectrum of double-activated phosphors, in *) T h e m o d e l L e n a r d S e ) and Tomaschek~7) p r o p o s e d for t h e c e n t r e s in s u c h s y s t e m s s h o w s s o m e r e s e m b l a n e e w i t h t h e one we a r r i v e d a t n o w .
312
F.A. KROGER AND J. DIKHOFF
addition to the bands of the " n o r m a l " activators. The bands due to scandium, gallium, and indium, are broad bands with maxima respectively at 5250 A, 5700 A and 6800 A (fig. 11). Our experiments indicate that incorporation of cadmium does not shift the position of these bands *). Accordingly the bands are of the characteristic type, corresponding to electronic transitions between levels of these
I
~
"ZnS-5°fO-$Ag-~-4Gatlnr Sc
I r,,\ II
\\
°
.
Fig. 11. Spectral distributions fluorescence bands due to Ga, In and Sc. ions. The same is undoubtedly true of bands due to Ce and Pr. Cerium causes a double band with maxima at 4900 and 5400 A, very similar to the band found with SrS - Ce ~6). The band is a doublet of the type usually found in the ultra-violet fluorescence of systems containing trivalent cerium 27), 28); the separation between the two E1 A#ZnS)
Ce
.
" ~ t1~ "~\ ' ~ le x "~
doo F i g . 12. -Ag-Ce;
gsoo
~o
o:ZnS-Ag-Ce b#Zn-fO~=d ;S-Ag-Ce . . . . . c:SrS-Ce
~oo
r2oo
Spectral distribution f o r Z n S - A g - Ce a n d (0.9 Z n - 0.1 C d ) for comparison the fluorescence of SrS-Ce according to U r b a c h e t al. 26a,b) is a l s o g i v e n .
maxima is 1900 cm -I and corresponds closely to the doublet separation found in such systems. In agreement with this view the *) It m u s t be stated t h a t owing to the width of the bands the results are not entirely certain.
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
313
position of the bands is not changed by incorporation of CdS (Fig. 12) **). The system ZnS - Ag - Ce behaves similarly to ZnS - Ag - Cu (C1); the distribution of excitation energy is dependent upon the exciting intensity, a low exciting intensity favouring the green cerium fluorescence, a high one the blue silver fluorescence. Further phosphorescence occurs in the green band(s) while at low temperatures the blue and at high temperatures the green is predominant. Fig. 13 shows the temperature-dependence curves for the blue and green bands separately, measured with the S c h o t t filters BG 12 in combination with noviol A, and OG 2, for excitation by ,l = 3650 A of two intensities (I,,c = 1, I,xc = 23). The figure clearly shows both the transfer of energy from blue to green at the llt
the blue silver band and the green c e r i u m b a n d s of Z n S - A g - Ce f o r t w o e x c i t i n g i n t e n s i t i e s of Jl = 3 6 5 0 •.
-~o
s ~ - ~ . o
~.. -. ~ " ~" ~60
temperature at which the blue is quenched, and the intensity dependence of this phenomenon. Just as with Ag .... Cu, this phenomenon can be explained by the theory of S c h 6 n - K 1 a s e n s ~.9). It mu.st be remembered, however, that while the activator level involved in the fluorescence transition is situated a short distance above the filled band, this cannot be true of the ground level of trivalent cerium. It must therefore be assumed that cerium causes an additional level in this position, the fluorescence process involving a transformation of excitation energy in a sort of resonance process as also assumed for ZnS - Mn (ref. 18, p. 234) and Zn2SiO 4 - MnS°). The orbital energy scheme is shown in Fig.14. In this figure the resonance process is indicated by an arrow connecting the transitions empty band - "Ce" and Ce3 + - Ce3+*. The products containing Pr show fluorescence in a large number **) A variation in the position of the cerium bands with the activator concentration as observed for S r S - C e by B a n k s and W a r d 2 e d ) is not a r e a l s h i f t caused by variation of the lattice dimensions: it is probably due to self-absmption, as was also found with othei cerium luminophors 27).
F. A. KI~OGER AND J . DIKHOFF
31 4
of sharp lines in the green and red part of the spectrum, similar to what has been found with other phosphors containing Pr 3+ 31). This fluorescence appears under all conditions, but its intensity relative to the band of the main activator varies with the mech-
to_t_., I
NNNNN} c.~.
Fig. 14. Orbital energy scheme for Z n S - - A g - - Ce.
anism of excitation: thus the lines are relatively weak in the cathodo-fluorescence, but they are stronger upon excitation b y = 3650 A. Fig. 15 gives a schematic representation of the line spectra for various products. The remarkable fact is observed that the I III
I
I
~0"q~-/0-*A I
~s-lo-*pr-s,t~ I
II L III
III ~X/n
Fig. 15. Schematic representation of the Pr-lines in the fluorescence spect r u m of ZnS containing Ag, Cu and Au together with Pr.
spectra, though all consisting of the same lines, show marked differences in the relative intensities of the various lines, dependent upon the presence of different monovalent ions; in the systems containing copper there is also a marked difference between the Pr spectrum of the products containing an excess of Cu which shows a
T R I V A L E N T CATIONS IN F L U O R E S C E N T ZINC S U L P H I D E
315
blue, and those containing an excess of Pr which show a green copper fluorescence. This indicates that there is a strong interaction between the monovalent ions and the praeseodymium, contrary to what has been observed for the Ag, Cu, Au and Zn bands. We therefore believe that while the latter bands are due to the free activator ions, the Pr bands originate nlainly in associated Pr3+A + pairs, only a part of the fluorescence, in common to all systems, being due to free Pr 3+ ions. A similar explanation m a y hold for the two types of Pr spectra (a and fl) observed with CaO and SrO 31) and for the various samarium spectra observed with C a O - S m prepared with different fluxes 3~), while also the remarkable differences between the rare earth spectra of various samples of MgO, observed b y T 0 m a s c h e k 3,.,) m a y be explained in the same way; the differences in internal structure, to which T om a s c h e k has attributed this effect, then would consist in the presence of monovalent ions (including LJ and Na)14). Products containing silver and copper together with praseodymium, of the general composition Z n S - 10-4 P r - 1 0 - 4 ( A g + Cu), show the remarkable property that the praseodymium fluorescence of the copper type (Fig. 15c) predominates over that of the silver type (Fig. 15b) for silver to copper ratios smaller than ten; only when this ratio is greater than ten does the silver type become preponderant. This must be explained either b y differences in the degree of dissociation of Ag - - Pr and Cu - - Pr pairs, or b y differences in the probability of the transfer of excitation energy from lattice states to the praseodymium ions. Eindhoven, 4th October 1949. Received 7-1-50.
REFERENCES 1) W. P r i m a k , R. K e i t h Osterheld and R. W a r d , 69, 1283 (1947). 2) J. G l a s s n e r , Thesis, Berlin 1938. 3) J . T . R a n d a l l , Trans. F a r a d a y Soe. 35, 2 (1939).
J. Am. chem. Soc.
316
T R I V A L E N T CATIONS IN F L U O R E S C E N T ZINC S U L P H I D E
4) A. A. G u n t z , Ann. chim .(10) 5, 157 (1925). 5) ]'. W. S t r a n g e, Proc. phys. Soc. (London) 55, 364 (1943). 6) S. R o t h s c h i l d , Trans. Faraday Sot., 42, 635 (1946). 7a) F. A. K r / s g e r and J. E. H e l l i n g m a n , J. electroehem. Soc. 9:], 156 (1948). 7b) F.A. K r / s g e r and J . E . H e l l i n g m a n , J. electroehem. Soc. 95, 68 (1949). 8) F. A. K r / s g e r , J. E. H e l l i n g m a n and N. W. S m i t , Physica 15,990(1949). 9) E . J . ~r. V e r w e y, Bull. Soe. china, de France, nr 3-4, DI 46 (1949). E.J.W. Verwey, P.W. Haayman and F.C. R o m e i j n , Chem. Weekbl., 49, 705 (1948). 10) F . A . K r / 5 g e r , Physiea 14, 483 (t948). 11) French Patent Spec. 869 336. 12) L. W e s e h , D.R.P. 747 208. 13) L. W e s c h and K. K a m m, D.R.P. 682 660. 14) F . A . K r / 5 g e r , J. opt. Soc. Am. :19, 670 (1949). 15) British Pat. Spec. 484 696. 16) R. T o m a s e h e k , Ann. Physik 75, 124 (1924). 17) N. R ie h], Physik und techn. Anw. der Lumineszenz, Berlin 1941, p. 27. 18) F . A . K r/5 g e r, Some aspects of the Luminescence of Solids (1948) p. 232-233. 19) S . T . H e n d e r s o n , Proc. roy. Soc. (London) A I 7 3 , 323 (1939). 20) C . J . M i 1 n e r, "Irans. Faladay Soc. 35, 101 (1939). 21) H. W. L e v e r e n z , U.S. Pat. 2402757. 22) A . L . S m i t h, J. electrochem. Soc. 93, 324 (1948). 23) N. F. M o t t and R . W . G u r n e y, Electronic processes in ionic crystals, Oxford 1940, p. 213. 24) W. S c h o t t k y, Zusammenfassende Bearbeitung der bisherigen experimentellen Ergebnisse zu einer Theorie der Phosphore (1944); (unpublished). 25) F . A . K r/5 g e r, Some aspects of the Luminescence of Solids, New York-Amsterdam, 1948, chapter I. 26a) D. P e a r l m a n , N. R. N a i l and F. U r b a c h , Collectedpapels of the symposium on fluorescence held at Cornell University 1946, John Wiley and Sons, New York 1948, p. 359. b) F. U r b a c h , D. P e a r l m a n and H. H e m m e n d i n g e r , J. opt. Soc. Am. 36, 372 (1946). c) P. B r a u e r , Z. Naturf. !, 70(1948). d) E. B a n k s and R. W a r d , J. electrochem. Soc., 96, 297 (1949). 27) F . A . K r 6 g e r and J. B a k k e r , Physica 8, 628 (1941). 28) H . C . F r o e l i c h , J. electrochem. Soe. 91, 161 (1947); 95, 254 (1949). 29) M. S c h 6 n , Z. P h y s i k 1 1 9 , 4 6 3 ( 1 9 4 2 ) ; F o r s c h . u. Fortschritte 19, no. 23/24, 1943; Chem. Bcr. 1942, p. 544; Ann. Physik (3) 6, 333 (1948). H.A. Klasens, Nature 158, 306 (1946). 30) F . A . K r / s g e r and W. H o o g e n s t r a a t e n , Physica 14, 425 (1948). 31) H. G o b r e c h t Ann. Physik 28, 673 (1937). H. L a n g e , Ann. Physik 31, 609 (1938). R. T o m a s c h e k Ann. Physik 75, 124 (1925). 32) R. T o m a s e h e k , Trans. Faraday Soc.,:]5, 148 (1939). Chem. Ber. (I942) 517. Ergebn. Exakt Naturw. 20, 269 (1942). 33) A . F . W e 11 s, Ministry of Home-Security Report RC(C) 119 (Gr. Britain). 34) R. T o m a s c h e k , Ann. Physik75, 109(142)(1924). 35) R. T o m a s e h e k and O. D e u t s c h b e i n , Ann. Physik 16, 930 (1933). 36) p. L e n a r d , Ann. Physik 31, 661 (1910). 37) R. T o m a s c h e k , Ann. Physik 75, 561 (1924); Physik. Z. 25, 643 (1924).
TRIVALENT
M a a r t 1950
CATIONS IN FLUORESCENT ZINC SULPHIDE
b y F. A. K R O G E R a n d J. D I K H O F F Philips Research Laboratories, N.V. Philips' Gloeilampenfabrieken Eindhoven,Netherlands
Summary Incorporation of monovalent cations in a lattice consisting of divalent ions is only possible to an appreciable extent when the lack of positive charge resulting from the substitution of a monovalent cation for a divalent one is compensated for. This compensation can be effected by a simultaneous incorporation of monovalent anions, or of cations of a valency higher than two. On this basis it is explained that ZnS becomes fluorescent by the monovalent activators Ag +, Cu +, Au + and Zn + when halogens or trivalent cations are present. Some of the trivalent ions incorporated in this way are found to cause effects of their own (electron traps, fluorescence, killing of fluorescence due to the other centres). An atomic model of the centres of fluorescence is given.
Introduction. F l u o r e s c e n t s m p h i d e s of zinc a n d c a d m i u m are usually p r e p a r e d b y h e a t i n g t o g e t h e r the p u r e sulphides, m a d e b y precipitation f r o m aqueous solutions, a c o m p o u n d of an a c t i v a t o r like Ag, Cu, Au or Mn, a n d a certain a m o u n t of halides of the alkali or alkaline e a r t h g r o u p metals. T h e l a t t e r c o m p o u n d s h a v e a double function. I n the first place t h e y act as crystallizing a g e n t b y causing the a p p e a r a n c e of a m o l t e n p h a s e at the n o r m a l firing t e m p e r a t u r e s at which p u r e sulphides are still solid 1); in this connection t h e y are called fluxes. I n the second place t h e y h a v e a specific influence. Glassner2), RandallS), Guntz4), Strange 5) a n d R o t h s c h i 1 d 6) h a v e shown t h a t the specific influence is m a i n l y due to the anions CI-, B r - a n d I - , while F - is ineffective. According to S t r a n g e 5) a n d K r 6 g e r et al. 7), s) the specific effects c a n be e x p l a i n e d on the basis of the a s s u m p t i o n t h a t the a c t i v a t o r ions f o r m i n g the centres are i n c o r p o r a t e d in the m a t r i x lattice --
2.~7 - -
298
F.A.
KROGER
AND
J. DIKHOFF
at normal lattice sites, replacing zinc (or cadmium). If the activators are monovalent ions as is the case with copper, silver and gold, incorporation at such sites either involves the formation of one anion vacancy for each pair of monovalent cations, or the occupation of interstitial sites by half of the activator ions. If monovalent anions like C1- or B r - are incorporated at the same time, however, complications of this kind are avoided, the lack of charge on the cation sites being compensated by a corresponding lack of charge on the anion sites. K r U g e r , Hellingman and S m i t T ' ) , ~) have shown that chlorine actually enters the lattice in amounts equivalent to the concentration of fluorescence centres. Compensation of the lack of positive charge due to the incorporation of monovalent cations at lattice sites can in principle also be effected b y the simultaneous incorporation of cations of valency higher than two; with trivalent cations for instance one monovalent and one trivalent ion replacing two zinc ions *). Now several patents exist in which the simultaneous incorporation of monovalent cations and cations of a valency of three or higher is claimed to have a beneficial effect on the fluorescence 11), 1~), is) and the principle of compensation of charge has also been recognised (W e s c h 11) : saturation of valencies). Yet a favourable effect found especially with the use of gadolinium as a trivalent ion was not explained merely b y the compensation principle but was attributed to an additional mechanism. As gadolinium is known to cause a characteristic ultraviolet fluorescence in many systems, it was assumed that excitation of a zinc sulphide phosphor containing this ion primarily causes the emission of this fluorescence, the visible fluorescence of the true activator (Ag, Cu) being secundarily excited b y the ultra-violet radiation 11), 3,). If this explanation were correct, the effect would be restricted to ions causing an ultra-violet fluorescence. In this paper it will be shown that this is not the case, all trivalent ions that can be incorporated in the sulphide lattice having a similar effect. Whenever there are differences in intensity between the fluorescence of various systems this is not due to a particularly fa*) T h e p r i n c i p l e of c o m p e n s a t i o n of c h a r g e is b y n o m e a n s n e w . It is well k n o w n in m i n e r a l o g y , in w h i c h it g o v e r n s a l a r g e n u m b e r of s u b s t i t u t i o n s ; V e r w e y e t al. h a v e p r o p o s e d i t s use in t h e r e g u l a t i o n of t h e v a l e n c y of ions in s e m i - c o n d u c t o r s 9), w h i l e o n e of t h e a u t h o r s h a s g i v e n a n a p p l i c a t i o n in t h e field of f l u o r e s c e n c e ( C a F , - - U) lo). In a r e c e n t p u b l i c a t i o n W e 11 s 33) h a s u s e d it to e x p l a i n t h e f a v o u r a b l e effect of N a in C a S - - Bi p h o s p h o r s .
T R I V A L E N T C A T I O N S IN F L U O R E S C E N T ZINC S U L P H I D E
299
vourable mechanism in the systems with a strong fluorescence, but to unfavourable conditions (quenchers) in the weak ones. From a comparison of the spectra of the fluorescence promoted by the halogen ions on the one hand and the trivalent ones on the other, conclusions as to the nature of the centres of fluorescence are drawn.
Experimental § 1. Preparation. Pure, halide-free zinc sulphide was made by precipitation from an aqueous solution of zinc sulphate by hydrogen sulphide. The precipitate was filtered, washed with H2S-water and then dried at 140°C. After milling the powder was preheated at 1000°C in a current of H2S. The sulphide obtained was a fine purely white powder, showing the ultra-violet fundamental fluorescence of ZnS at liquid-air temperature. Samples of this sulphide were wetted with aqueous solutions of A g N Q , C u N Q or AuCI~ *) and of the nitrates of the trivalent and tetravalent ions A1, Ga, In, Sc, Y, La, Ce, Pr, Gd, Sm, Yb, Zr and Th in amounts to give a concentration of 10-4 gram atoms per mol ZnS both for the monovalent and for the trivalent or tetravalent ions. Borium was added as boric acid, silicium as SiO 2 obtained by hydrolysis of ethyl silicate. Blanks containing monovalent and trivalent ions alone were also made. The samples were dried, thoroughly mixed, and then heated for half an hour at 1200°C in a stream of H2S. In order to ascertain the transformation of the oxides of the trivalent and tetravalent ions into the corresponding sulphides, care was taken for a good circulation of the atmosphere through the powder.
§ 2. General observations. The fluorescence of the samples was studied with excitation by ~t = 3650 A at room temperature. The spectral distribution of the fluorescence was measured with the aid of a double monochromator and photo-multiplier. Peak positions of fluorescence bands are given in Table I. The samples containing Ag, Cu or Au alone are grey and show no fluorescence. *) A gold solution w i t h o u t chlorine would have been preferable; the only other possibility, a solution of Au in KCN solutions, however, gave products with a very poor fluorescence. As the amount of chlorine added in this way was very low, while heating in H2S w i t h o u t t r i v a l e n t ions gave non-fluorescent products, it is believed t h a t the chlorine ev aporates entirely in the firing process and does not in va l i da t e the results of the experiments.
300
A N D J. D I K H O F F
F.A. KROGER
TABLE
I
M a x i m a of fluorescence bands in ZnS containing Ag, Cu, A u or Zn together with trivalent or tetravalent cations or monovalent anions; excitation by ,~.= 3650 A at room temperature. A'dded ions
Zn wurtzite D
B A1 Sc Y Ga In La Ce Pr Sm Gd Yb Si Ge Zr Th CI, Br, I
Ag sphalerite
wurtzite
Cu w urt z i t e
B
Au wurtzite m
m
4600
?
4450; 5100 4310 5280 4360; 5250 5200 5290 4340 4500; 5100 5280 4490 4500;5160;5700 4700;5280;5800 4400; 5700 4490; 585C 680O 68O0 6500 ? 4500; 5150 5280 4330 ? 4950; 5500 4300;4900;5400 5300 ? 5100;lines 5280;lines 4330;lines 4450 ?
4600
lines
E
D
m
? ?
4320 4310 4310
4500; 5100 5150
5290 5300
4460
4350
4450; 5!60
4700; 5300
n
4520
Samples containing B, Sc, Y, La, Ce, Sm, Gd, Yb, Si, Zr and Th alone are also non-luminescent. Samples containing A1 or Ga, however, show a blue fluorescence in a band identical with that found with self-activated ZnS containing chlorine. The product containing Pr shows a weak yellow fluorescence, while that containing In has a weak orange fluorescence. Nearly all products containing the monovalent ions together with trivalent ions have a considerable fluorescence, with the exception of B and Sm, which have no effect at all. Among the tetravalent ions Si seems to have a positive effect, but Zr and Th are ineffective. The products containing Ag, Cu or Au, together with A1, Y, Yb, La or Gd, show a strong fluorescence in the blue, green and yellowish green, in bands corresponding exactly to those appearing with similar phosphors made with halogen instead of trivalent cations. The same bands occur with products containing Ag, Cu or Au together with Ce, Pr, Sc, Ga or In, b u t these products show other bands in addition. The latter bands are different for the various trivalent ions, but they are the same for one particular trivalent ion in combination with different monovalent cations; they must therefore be'due to the trivalent ions.
TRIVALENT
CATIONS
IN FLUORESCENT
ZINC SULPHIDE
301
The proportion between the various bands in the fluorescence spect r u m varies from case to case; with Z n S - A g - Pr for instance the fluorescence occurs mainly in the blue silver band, while with Z n S - A g - In the red indium band is preponderant. Apart from such differences the proportion is dependent upon the condition of excitation (temperature, exciting intensity) as is normal for doubleactivated ZnS luminophors. It is remarkable that, while several rare earth ions are effective in promoting the fluorescence of ZnS, Sm was found to be ineffedtive. This is the more surprising since fluorescence due to samarium in ZnS has been mentioned by various authors 15), le), although it must be emphasized that both R i e h l and S c h l e e d e have been unable to prepare such a phosphor 17). The explanation probably is that samarium, which is known in the divalent form, is incorporated as such without the simultaneous incorporation of monovalent ions and therefore shows neither the characteristic lines of the Sm 3 ~ ion nor the bands of Cu, Ag, Au or Zn. The various double-activated products show marked differences in their phosphorescence properties and have different glow curves. Phosphors containing A1, Gd, In or Yb, show a more or less normal phosphorescence: in combination with silver the after-glow is very short, but with copper or gold there is a moderate after-glow. Products containing La or Y have practically no after-glow, in agreement with a claim made in a patent by W e s c h and K a m m 13) ; apparently these ions act as killers. The properties of products containing Pr or Ce depend on the monovalent cations present ; in combination with Cu or Au there is a normal after-glow in the green copper or gold band, but in combination with Ag the after-glow is in the Pr or Ce bands. Ga and Sc finally cause an exceptionally strong afterglow in the bands of the activator (Ag, Cu, Au). Systems containing Ga moreover show the peculiar effect that the fluorescence is very weak while the phosphorescence is strong *). Apparently this ion has various functions, causing both deep traps and killer centres of the cobalt type **) (while it also causes a fluorescence of its own - - and therefore also forms fluorescence centres; see § 5). *) O w i n g to t h i s p r o p e r t y t h e s y s t e m s a r e s u i t a b l e for use in r a d a r t u b e s . **) W i t h a n o r m a l killer t h e p h o s p h o r e s c e n c e is s u p p r e s s e d w h i l e t h e f l u o r e s c e n c e r e m a i n s u n w e a k e n e d ; t h i s is t h e case for n i c k e l in ZnS p h o p h o r s . C o b a l t a c t s d i f f e r e n t l y in Z n S - Cu a n d Z n S - A u , in w h i c h it w e a k e n s t h e f l u o r e s c e n c e b u t l e a v e s t h e p h o s p h o r e s c e n c e u n a f f e c t e d . F o r a n e x p l a n a t i o n see ref. 18).
302
F. A. KROGER AND J. DIKHOFF
§ 3. The bands o[ Ag, Cu, A u or Zn. Zinc sulphide phosphors activated with Ag, Cu or Au, and made in the presence of chlorides, bromides, or iodides (or of HC1, HBr or HI) are known to show a blue band which is usualiy attributed to an excess of zinc, another blue band which is due to silver, a green and a blue. band due to copper e), 8) and a greenish blue and a yellowish green band due to gold 19), 20), 21). Products made with addition of trivalent cations instead of halogen show the zinc band in two cases (AI, Ga); the blue silver band, the two copper bands and the yellowish green gold band appear in all cases, but .g the greenish-blue gold band is not ,t~mma observed. Apparently this band is not promoted by trivalent cations. The position of the bands ns-~0"*A.e - t0"~X is exactly the same for products containing halogens or trivalent ions as is demonstrated for the silver band in Fig. 1.The intensity of the blue fluorescence of the various samples for various methods of excitation is given in table II; it is seen that the best results are obFig. 1. S p e c t r a l d i s t r i b u t i o n of tained with A1 as a trivalent ion. For t h e f l u o r e s c e n c e of Z n S a c t i v a t e d this reason A1 has been used for a with silver together with chlorine quantitative study of the influence or v a r i o u s t r i v a l e n t ions. of trivalent ions. In a series of products containing a constant amount of AI(10 - 4 gram atoms per mol. ZnS) but an increasing concentration of Ag ( 0 - - 2 . I0 --4 g. atoms per mol ZnS) the fluorescence varies gradually from white-blue, in a band with a maximum at 4600 A, to deep blue in a band with a maximum at 4350 A. The first band is the band of self-activated ZnS, the latter the band of ZnS - Ag proper. Therefore addition of silver causes a suppression of the zinc band, just as was observed with products containing chlorine 7). This effect can be explained as follows: in order to incorporate a certain amount of A13~ in pure ZnS an equivalent amount of Zn + is formed. When Ag ÷ is present, however, these ions enter the lattice instead of the zinc ions, and reduction of Zn z+ to Zn ÷ does not occur. This means that the variation of the fluorescence spectrum is ,
,
TRIVALENT
CATIONS IN FLUORESCENT
ZINC SULPHIDE
303
TABLE II Relative intensities of the blue band of Z n S - A g filter BG 12 ZnS - - 5.10 -5 A g with addition of
10-4 10-* 10-4 10- i 10-4 10 -4 10- I 10- i 10-~
Firing conditions 80H2S--20HCI ; 1 h 1200°C HzS; 1 h 1200°C H2S ; 1 h 1200°C H2S; i h 1200°C H2S ; 1 h 1200°C H~S; l 11 1200°C H2S; I h 1200°C HzS; l h 1200°C H2S; l h 1200°C H2S ; 1 11 1200°C HzS; 1 h 1200°C
A1 y Ga Sc Pr Gd La Yb Si
measured through the Schott
I a for e x c i t a t i o n at 25°C b y = 3650 £
cathode rays (20 kV)
~. = 2537 A
100
100
I00
92
105 49
105 66 4.2 45 47 83 47 31.5
38 7.7 29.5 31
3.6 40 32
59 22 12
53 25 33
2.5 0.2
15
20.5 2
0.6
not simply due to an increase of the concentration of silver centres but to a simultaneous decrease of the concentration of zinc centres. A series of products containing a constant concentration of Ag (10 -4) and an increasing concentration of A1 ( 0 - 3.10 -4) shows a gradual increase of the intensity of the fluorescence in the blue Ag band from zero upwards. Maximum intensity is reached when the concentrations of A1 and Ag are about equal (Fig. X), as is to 20kV]
lft
F i g . 2. I n t e n s i t y amounts cathode
of
of the blue fluorescence
aluminium;
excitation
by
r a y s o r u l t r a - v i o l e t ( 2 . = 2 5 3 7 •) at room temperature.
~-,~'~/,/~"-x=a~
__g
t
=_~_~[,/ i 0 - - ti v-s
510 s,49 xAI
,,s-s., i ~-,
-
i v-~
~CA/
be expected on the basis of the principle of compensation of charge. A decrease of the fluorescence at concentrations of aluminium greater than the silver concentration does not appear: the excess of aluminium gives rise to monovalent zinc and the zinc band appears next to the silver band. In the presence of a slight excess of aluminium, silver can dissolve in ZnS up to a concentration of at least 10-3 gram atoms per tool. With increasing silver content the maximum of the fluores-
304
F . A . KROGER AND J. DIKHOFF
cence band shifts towards a longer wavelength (from 4330 to 4450A) ; at the same time the long-wave limit of the centre absorption shifts over the same distance (fig. 3). This suggests that the shift of the
4 I
"0
?
/!.;,.:.,6 jD-3A.
~ • Jf
I ~
~ \
....
Fig. 3. S p e c t r a l d i s t r i b u t i o n of t h e f l u o r e s c e n c e a n d t h e d i f f u s e r e f l e c t i o n for ZnS c o n t a i n i n g i n c r e a s i n g conc e n t r a t i o n s of silver a n d aluminium.
refl¢,¢¢~
III
tI 3500
1000
4500
I~'00
fluorescence is due to self-absorption. Gold behaves similarly to silver, but owing to the difference in colour between the gold band the zinc band the replacement of the zinc fluorescence b y the 2n-~
i
A
X
of ZnS c o n t a i n i n g 10 - 4 A1 a n d v a r i o u s a m o u n t s of gold.
//
~
*000
-
,
*N]0
~
~
.
5~0
"°i=
.
ZnS-~-~AI-xAu
~
.
~kin~
.
60O0
gold fluorescence is much more evident. In a series of products containing 10-4 A1 and increasing amounts of Au, for instance, the product without gold shows the blue zinc band while the sample
/ ,' X~
I
O'~i' - ' ~ -
3.10-6
I
e~2~-7
I
10-S 3.10-S
fO-A
340-~ D CAu
Fig. 5. I n t e n s i t v of t h e blue Znb a n d a n d t h e y e l l o w i s h - g r e e n Aub a n d for ZnS - 10 - 4 A1, c o n t a i n i n g v a r i o u s a m o u n t s of gold.
containing 10 - 4 Au shows the yellowish green gold band (fig. 4). Fig. 5 shows the variation of the intensity of the two bands with the gold concentration; the measurements were carried out through
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
305
the S c h o t t filters BG 12 for the blue, and OG 2 for the yellow" band. It is seen that the zinc band is totally suppressed in all products for which the gold concentration exceeds the aluminium concentration. The increase of the gold band in a series with 5.10 -s Au and increasing A1 concentration is shown in Fig. 6. lfl
Fig. 6. I n t e n s i t y of t h e y e l l o w i s h g r e e n gold b a n d for ZnS - - 5 . 1 0 - s Au containing various amounts of aluminium,
/ ~/'/ /
~ nZnS-S.10-$ ~ o ~ t ~ aAu-x ~ At
/ / o '
j.jo-~
~-s
j,~-s
Jo--'
~o-, .r w
The copper system is more intricate due to the appearance of a blue and a green copper band. In order to avoid intensity dependence of the proportion between these bands the measurements were carried out at liquid air temperatures, at which this effect is suppressed. In a series of products ZnS - 10-4 A 1- xCu in which R=~
A / ~,t-
ZnS-xCu-~41 'R--19~[b ~,ue - - IO-'A| " . . . . ." Ig~en for
Ltn-sa, ~,:{II
3./O-e
/0"~"
3.10- s
~
3404
b Co, t ~
Fig. 7. I n t e n s i t y of t h e blue a n d g r e e n b a n d s of Z n S - - 1 0 - 4 AI c o n t a i n i n g v a r i o u s a m o u n t s of c o p p e r , a n d tile r a t i o b e t w e e n t h e i n t e n s i t i e s of t h e g r e e n a n d blue b a n d s for Z n S - 1 0 - s , 10 _4 a n d 3.10 - 4 AI w i t h v a r i o u s a m o u n t s of c o p p e r . E x c i t a t i o n b y ~t = 3650 2k a t --1B0°C,
the copper content is varied, the product without copper shows the blue zinc band with a maximum at 4600 A. Incorporation of copper first causes a gradual replacement of this band by the green copper band (maximum 5100 A). At higher copper concentrations the blue Physica XVI
20
306
F . A . KROGER AND J. DIKHOFF
copper band with a maximum at 4400 A appears; this band becomes predominant when the copper concentration exceeds the aluminium concentration. The variation in the intensity of the blue and green bands is shown in Fig. 7, clearly demonstrating the appearance of the blue bands at extreme copper concentrations and the green in between. In the same figure the ratio green : blue is given for this series and also for similar series with a higher (3.10 -4) and lower (10-5) aluminium content. In alle cases the zinc band is predominant at low copper concentrations, but the amount of copper necessary to suppress it is the larger the higher the aluminium concentration. The explanation is obvious when it is borne in mind that the concentration of zinc centres depends directly upon the aluminium concentration, the concentrations of Zn + and A13 ~ being equal; the mechanism b y which the copper band replaces the zinc band is exactly the same as discussed above for the case of silver. The transition from copper green to copper blue also shifts to higher copper concentrations with increasing aluminium concen-
OJ-blt~
~O"e
I"
Fig. 8. C o n t o u r s of e q u a l v a l u e s for t h e r a t i o b e t w e e n t h e i n t e n s i t i e s of green a n d blue b a n d s in t h e s y s t e m s Z n S - C u - A 1 ; e x c i t a t i o n b y ~t = 3 6 5 0 A a t - 180°C. ~CAI
tration, and in all cases it takes place close to the point at which the concentrations of copper and aluminium are equal. The values of the ratio green: blue over the entire system Z n S - x A l - y C u are shown in Fig. 8. When this contour diagram is compared with similar diagrams in our paper dealing with the properties of ZnS - Cu made in an atmosphere of H2S - HC1 (fig. 4 and 6 of ref. 8) the similarity is o b v i o u s : a p p a r e n t l y an increasing amount of HC! in the H2S atmosphere corresponds to an increase of the aluminium
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
307
content. This is exactly as is to be expected on the basis of our theory where it is assumed that an increase of the amount of HC1 in the atmosphere results in an increase of the amount of chlorine incorporated in the sulphide. Although there is a great similarity between the systems containing C1 and those containing AI, there are nevertheless some quantitative differences: the maximum of the green: blue ratio, for instance is about twice as high with A1 than with C1. Similar differences are found when comparing systems containing v a rious trivalent ions. Z n S - 10-4 A1- 5.10 - s Cu for instance is nearly pure green fluorescent and the same is true of systems of a corresponding composition but containing Ga, Yb, Sc, P r or Ce instead of AI. Products containing Y or Gd, however, show a much bluer fluorescence, while the fluorescence of a product with La is practically pure blue (fig. 9). La
Fig. 9. Spectral distribution of
AI
the
fluorescence
of
ZnS-5.10--SCu,
containing various trivalent ions. The hatched peak on the curve for ZnS- C u - Pr is due to Pr-fluorescence. Similarly the long-wave tail of the fluorescence of ZnS- C u - Sc is due +n the scandium. In the system ZnS-Cu(C1) it has been observed that refiring of a sample showing the blue copper band in H2S at a temperature at which the chlorine concentration cannot change, e.g. 500°C, causes the fluorescence to change from blue to green and this observation is one of the main reasons for the assumption that Cu2C1 is the blue centre in th; ~, system. When a blue fluorescent phosphor of the aluminium system is subjected to the same treatment no variation of fluorescence is found. Retiring in a sulphur atmosphere, however, does indeed produce such a change, while heating in hydrogen causes a change in the opposite direction (Fig. 10). Therefore the
308
F.A.
KR(SGER A N D J. D I K H O F F
difference is only quantitative, the nature of the centres in the two systems being essentially the same. In view of the evidence so far given it seems apparent that the activation of ZnS phosphors is governed by the principle of charge compensation. Yet there is one observation which might seem to contradict this. According to S m i t h 2~) silver can activate ZnS without halogens being present, provided the product is made in the presence of ZnO or is fired in an oxidizing atmosphere (SOv A
?
2ns-s.to-scu-to-~A! ex~ 36S0~ -IeO=C
/UgX\\ 'k~\
Joo
,
45oo
O= A,rea,*d t^ ~O'C ln S~
~ c=a,~tj~atns°ec~"v° Fig. 10. Variation of the spectral distribution of the fluorescence of ZnS--5.10 s C u - - 1 0 -5 A1 upon retiring at 500°C in various at, , --,mospheres.
5oo0
550o
CO2). By repeating these experiments we have been abie to reproduce these results, and similar results were obtained with copper and gold. It must be emphasised, however that the effects are extremely weak. A possible explanation will be discussed in a subsequent paper.
§ 4. The nature o/the Ag, Cu, Au and Zn centres. In the papers 7),s) dealing with ZnS containing Ag or Cu together with chlorine it has been shown that although the incorporation of the activator ions is governed by the principle of charge compensation, resulting in the formation of solid solutions like Z n S - - A g C 1 and ZnS--CuC1, the metal ion and the halogen need not necessarily occupy neighbouring sites in the lattice. Therefore there were still three possibilities for the centres of fluorescence, viz 1) the activator A + with its neighbours, 2) the halogen X - with its beighbours, and 3) A+X - with neighbours. Similar possibilities exist, of course, for the case where the activator ions are incorporated together with trivalent cations 133+, these possibilities being 1) A + with surroundings, 2) B 3+ with surroundings, and 3) A + and ]33+ at neighbouring cation sites, plus the surroundings. The second possibilities are ruled out by the fact that zinc sulphide activated with the various activators Ag, Cu, Au or Zn shows dif-
T R I V A L E N T CATIONS IN F L U O R E S C E N T ZINC S U L P H I D E
309
ferent bands, while on the other hand the peaks of systems CO,ltaining different halogens (CI-, B r - , I - ) or different trivalent cations (A13+, Gd 3+, a.o.) do not show the slightest difference. For the latter reason the third possibility can also be discarded; if ions as different as C1- and A13+ were to belong to the centre, marked differences should be expected. Therefore only the first possibility remains open, and it must be concluded that this represents the actual situation: the centres of fluorescence consist of monovalent activator ions surrounded by four sulphur ions. The blue copper centre is slightly different and consists of a monovalent copper ion and a copper atom close by 8), but also in this case the halogens or the trivalent cations are at distant sites and do not belong to the actual centre. This does not mean, however, that all the incorporated ions are in this position; we have to expect an equilibrium between associated and dissociated pairs A+X - and A+B 3+ just as is the case in liquid solutions. In the case of Ag, Cu and Au, the equilibrium will be that corresponding to the high firing temperature or at least to a relatively high temperature passed during cooling; a variation of the equilibrium during cooling requires the migration of cations and therefore will not occur to an appreciable extent at low temperatures. In the case of self-activated ZnS the situation is different because the monovalent zinc which is held responsible for the fluorescence can associate and dissociate from its stabiliser (CI-, A13+) without a migration of the ions themselves, the migration of an electron being sufficient: Zn 2+ + Zn + -+ Zn + + Zn 2+ Such a migration of electrons, however, has either no activation energy at all or only very little. Therefore in this case the association-dissociation equilibrium will be that corresponding to room temperature or an only slightly higher temperature. If the energy involved in the dissociation is large, dissociation will not occur at all; it will only occur to an appreciable extent when this energy is small (or even negative). This energy will depend on the ions involved and therefore will be different for C1-. B r - , and various trivalent ions. This m a y explain why the zinc band appears only with the halogens and with A1a+ and Ga a+, while the Cu, Ag and Au bands on the other hand appear much more generally. The blue copper centre consisting either of a Cu + ion and a
310
F . A . K R O G E R A N D J. D I K H O F F
copper atom, or of two Cu + ions and a sulphur vacancy containing an electron near by s), m a y also dissociate, forming single Cu + ions and therefore green centres. A dissociation of this type has been proposed to explain the dependence of the colour of the fluorescence of copper phosphors upon the rate of cooling after preparation s). The differences observed between products made with various trivalent cations as mentioned in § 3 must be explained either by a dissociation of this type or by a difference in the "normal" A÷X - or A+B 3+ dissociation for the blue and green copper centres. The concept of association-dissociation m a y also explain the absence of fluorescence in products containing fluorine as a halogen if it is assumed that in this case the incorporated A + F - is completely associated. The difference with the other halogens m a y be due to an almost purely electrostatic binding in the case of fluorine while the other halogens form largely covalent bonds. The fluorescence caused by Ag, Cu, Au and Zn, is known to be due to an electronic transition between the conduction band, or a level close to this band 23), 24), and a level above the upper filled band (centre level). When part of the zinc in ZnS is replaced by cadmium the edge of the fundamental absorption band and the maxima of the fluorescence bands shift to the long-wave side, the the separation between the activator level and the full band remaining unchanged. The incorporation of cadmium has two effects; in the first place the zinc and cadmium ions interact thoroughly,, forming one mixed conduction band; in the second place the lattice constant is increased, resulting in a decrease of the electrostatic forces and therefore in a lowering of the levels of electrons at positive ions and a rise of the levels of electrons at negative ions 25). As the full band is supposed to be due to S2- ions, and the e m p t y band to (Zn, Cd) + ions, these two effects together explain the shift of the fundamental absorption. On the other hand, from the behaviour of the fluorescence the nature of the centre level can be deduced. If this level were to be a level of the activator itself, present at a cation site, the expansion of the lattice would cause a decrease of the energetical separation between this level and the full band; but when the centre level is due to an electron at an anion site this separation remains unchanged. The latter is found to be the case. Therefore the centrelevel must be due to an electron at an anion site.
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
311
Levels of electrons at an anion site must be due either to vacant anion sites containing an electron or to sulphur ions, or to ions replacing sulphur, e.g. halogens. The first possibility seems very unlikely for normal phosphors like ZnS-AgC1, particularly since the formation of phosphors has been found to be governed by the principle of c o m p e n sation of charge, which is just based on the avoidance of vacancies ; on the other hand it has been shown that halogen ions are not present in the centres. Therefore the only possibility left is that the centre level is due to sulphur ions. On the other hand we know that the activators Ag, Cu, Au, Zn have an essential influence on the fluorescence. Therefore it seems logical to assume that the centre level is a level due to sulphur ions, perturbed by the presence of an activator ion at an adjacent site. According to Dr K 1 a s e n s of this laboratory, who suggested this possibility several years ago, the influence of a monovalent cation on the level of an adjacent sulphur ion is in the right direction and of the order of magnitude as required for the centre level. The direction of the effect is easily seen for a simple electrostatic model: the lack of positive charge resulting from the replacement of Zn 2+ by A + causes a decrease of the electrostatic potential at an adjacent S2- site, and accordingly a rise of the electron energy levels at that site. For an exact calculation of the effect polarization, repulsive forces and v a n d e r W a a 1 s attraction forces should be taken into account; it is actually these effects which must be responsible for the differences between the levels of centres due to Ag, Cu, Au, Na, Li 14). Centres of the same type are probably also present in alkaline earth sulphide and oxide phosphors *).
§ 5. The bands o/Ga, In, Sc, Pr and Ce. When trivalent cations are used to aid the incorporation of monovalent activator ions, these ions are incorporated at the same time and it must be expected that these also cause extra levels in the energy spectrum of the system. When the conditions are favourable these ions may therefore Mso form centres of fluorescence. This happens to be the case with Ga, In, Sc, Pr and Ce, as is indicated by the fact that new bands appear in the fluorescence spectrum of double-activated phosphors, in *) T h e m o d e l L e n a r d S e ) and Tomaschek~7) p r o p o s e d for t h e c e n t r e s in s u c h s y s t e m s s h o w s s o m e r e s e m b l a n e e w i t h t h e one we a r r i v e d a t n o w .
312
F.A. KROGER AND J. DIKHOFF
addition to the bands of the " n o r m a l " activators. The bands due to scandium, gallium, and indium, are broad bands with maxima respectively at 5250 A, 5700 A and 6800 A (fig. 11). Our experiments indicate that incorporation of cadmium does not shift the position of these bands *). Accordingly the bands are of the characteristic type, corresponding to electronic transitions between levels of these
I
~
"ZnS-5°fO-$Ag-~-4Gatlnr Sc
I r,,\ II
\\
°
.
Fig. 11. Spectral distributions fluorescence bands due to Ga, In and Sc. ions. The same is undoubtedly true of bands due to Ce and Pr. Cerium causes a double band with maxima at 4900 and 5400 A, very similar to the band found with SrS - Ce ~6). The band is a doublet of the type usually found in the ultra-violet fluorescence of systems containing trivalent cerium 27), 28); the separation between the two E1 A#ZnS)
Ce
.
" ~ t1~ "~\ ' ~ le x "~
doo F i g . 12. -Ag-Ce;
gsoo
~o
o:ZnS-Ag-Ce b#Zn-fO~=d ;S-Ag-Ce . . . . . c:SrS-Ce
~oo
r2oo
Spectral distribution f o r Z n S - A g - Ce a n d (0.9 Z n - 0.1 C d ) for comparison the fluorescence of SrS-Ce according to U r b a c h e t al. 26a,b) is a l s o g i v e n .
maxima is 1900 cm -I and corresponds closely to the doublet separation found in such systems. In agreement with this view the *) It m u s t be stated t h a t owing to the width of the bands the results are not entirely certain.
TRIVALENT CATIONS IN FLUORESCENT ZINC SULPHIDE
313
position of the bands is not changed by incorporation of CdS (Fig. 12) **). The system ZnS - Ag - Ce behaves similarly to ZnS - Ag - Cu (C1); the distribution of excitation energy is dependent upon the exciting intensity, a low exciting intensity favouring the green cerium fluorescence, a high one the blue silver fluorescence. Further phosphorescence occurs in the green band(s) while at low temperatures the blue and at high temperatures the green is predominant. Fig. 13 shows the temperature-dependence curves for the blue and green bands separately, measured with the S c h o t t filters BG 12 in combination with noviol A, and OG 2, for excitation by ,l = 3650 A of two intensities (I,,c = 1, I,xc = 23). The figure clearly shows both the transfer of energy from blue to green at the llt
the blue silver band and the green c e r i u m b a n d s of Z n S - A g - Ce f o r t w o e x c i t i n g i n t e n s i t i e s of Jl = 3 6 5 0 •.
-~o
s ~ - ~ . o
~.. -. ~ " ~" ~60
temperature at which the blue is quenched, and the intensity dependence of this phenomenon. Just as with Ag .... Cu, this phenomenon can be explained by the theory of S c h 6 n - K 1 a s e n s ~.9). It mu.st be remembered, however, that while the activator level involved in the fluorescence transition is situated a short distance above the filled band, this cannot be true of the ground level of trivalent cerium. It must therefore be assumed that cerium causes an additional level in this position, the fluorescence process involving a transformation of excitation energy in a sort of resonance process as also assumed for ZnS - Mn (ref. 18, p. 234) and Zn2SiO 4 - MnS°). The orbital energy scheme is shown in Fig.14. In this figure the resonance process is indicated by an arrow connecting the transitions empty band - "Ce" and Ce3 + - Ce3+*. The products containing Pr show fluorescence in a large number **) A variation in the position of the cerium bands with the activator concentration as observed for S r S - C e by B a n k s and W a r d 2 e d ) is not a r e a l s h i f t caused by variation of the lattice dimensions: it is probably due to self-absmption, as was also found with othei cerium luminophors 27).
F. A. KI~OGER AND J . DIKHOFF
31 4
of sharp lines in the green and red part of the spectrum, similar to what has been found with other phosphors containing Pr 3+ 31). This fluorescence appears under all conditions, but its intensity relative to the band of the main activator varies with the mech-
to_t_., I
NNNNN} c.~.
Fig. 14. Orbital energy scheme for Z n S - - A g - - Ce.
anism of excitation: thus the lines are relatively weak in the cathodo-fluorescence, but they are stronger upon excitation b y = 3650 A. Fig. 15 gives a schematic representation of the line spectra for various products. The remarkable fact is observed that the I III
I
I
~0"q~-/0-*A I
~s-lo-*pr-s,t~ I
II L III
III ~X/n
Fig. 15. Schematic representation of the Pr-lines in the fluorescence spect r u m of ZnS containing Ag, Cu and Au together with Pr.
spectra, though all consisting of the same lines, show marked differences in the relative intensities of the various lines, dependent upon the presence of different monovalent ions; in the systems containing copper there is also a marked difference between the Pr spectrum of the products containing an excess of Cu which shows a
T R I V A L E N T CATIONS IN F L U O R E S C E N T ZINC S U L P H I D E
315
blue, and those containing an excess of Pr which show a green copper fluorescence. This indicates that there is a strong interaction between the monovalent ions and the praeseodymium, contrary to what has been observed for the Ag, Cu, Au and Zn bands. We therefore believe that while the latter bands are due to the free activator ions, the Pr bands originate nlainly in associated Pr3+A + pairs, only a part of the fluorescence, in common to all systems, being due to free Pr 3+ ions. A similar explanation m a y hold for the two types of Pr spectra (a and fl) observed with CaO and SrO 31) and for the various samarium spectra observed with C a O - S m prepared with different fluxes 3~), while also the remarkable differences between the rare earth spectra of various samples of MgO, observed b y T 0 m a s c h e k 3,.,) m a y be explained in the same way; the differences in internal structure, to which T om a s c h e k has attributed this effect, then would consist in the presence of monovalent ions (including LJ and Na)14). Products containing silver and copper together with praseodymium, of the general composition Z n S - 10-4 P r - 1 0 - 4 ( A g + Cu), show the remarkable property that the praseodymium fluorescence of the copper type (Fig. 15c) predominates over that of the silver type (Fig. 15b) for silver to copper ratios smaller than ten; only when this ratio is greater than ten does the silver type become preponderant. This must be explained either b y differences in the degree of dissociation of Ag - - Pr and Cu - - Pr pairs, or b y differences in the probability of the transfer of excitation energy from lattice states to the praseodymium ions. Eindhoven, 4th October 1949. Received 7-1-50.
REFERENCES 1) W. P r i m a k , R. K e i t h Osterheld and R. W a r d , 69, 1283 (1947). 2) J. G l a s s n e r , Thesis, Berlin 1938. 3) J . T . R a n d a l l , Trans. F a r a d a y Soe. 35, 2 (1939).
J. Am. chem. Soc.
316
T R I V A L E N T CATIONS IN F L U O R E S C E N T ZINC S U L P H I D E
4) A. A. G u n t z , Ann. chim .(10) 5, 157 (1925). 5) ]'. W. S t r a n g e, Proc. phys. Soc. (London) 55, 364 (1943). 6) S. R o t h s c h i l d , Trans. Faraday Sot., 42, 635 (1946). 7a) F. A. K r / s g e r and J. E. H e l l i n g m a n , J. electroehem. Soc. 9:], 156 (1948). 7b) F.A. K r / s g e r and J . E . H e l l i n g m a n , J. electroehem. Soc. 95, 68 (1949). 8) F. A. K r / s g e r , J. E. H e l l i n g m a n and N. W. S m i t , Physica 15,990(1949). 9) E . J . ~r. V e r w e y, Bull. Soe. china, de France, nr 3-4, DI 46 (1949). E.J.W. Verwey, P.W. Haayman and F.C. R o m e i j n , Chem. Weekbl., 49, 705 (1948). 10) F . A . K r / 5 g e r , Physiea 14, 483 (t948). 11) French Patent Spec. 869 336. 12) L. W e s e h , D.R.P. 747 208. 13) L. W e s c h and K. K a m m, D.R.P. 682 660. 14) F . A . K r / 5 g e r , J. opt. Soc. Am. :19, 670 (1949). 15) British Pat. Spec. 484 696. 16) R. T o m a s e h e k , Ann. Physik 75, 124 (1924). 17) N. R ie h], Physik und techn. Anw. der Lumineszenz, Berlin 1941, p. 27. 18) F . A . K r/5 g e r, Some aspects of the Luminescence of Solids (1948) p. 232-233. 19) S . T . H e n d e r s o n , Proc. roy. Soc. (London) A I 7 3 , 323 (1939). 20) C . J . M i 1 n e r, "Irans. Faladay Soc. 35, 101 (1939). 21) H. W. L e v e r e n z , U.S. Pat. 2402757. 22) A . L . S m i t h, J. electrochem. Soc. 93, 324 (1948). 23) N. F. M o t t and R . W . G u r n e y, Electronic processes in ionic crystals, Oxford 1940, p. 213. 24) W. S c h o t t k y, Zusammenfassende Bearbeitung der bisherigen experimentellen Ergebnisse zu einer Theorie der Phosphore (1944); (unpublished). 25) F . A . K r/5 g e r, Some aspects of the Luminescence of Solids, New York-Amsterdam, 1948, chapter I. 26a) D. P e a r l m a n , N. R. N a i l and F. U r b a c h , Collectedpapels of the symposium on fluorescence held at Cornell University 1946, John Wiley and Sons, New York 1948, p. 359. b) F. U r b a c h , D. P e a r l m a n and H. H e m m e n d i n g e r , J. opt. Soc. Am. 36, 372 (1946). c) P. B r a u e r , Z. Naturf. !, 70(1948). d) E. B a n k s and R. W a r d , J. electrochem. Soc., 96, 297 (1949). 27) F . A . K r 6 g e r and J. B a k k e r , Physica 8, 628 (1941). 28) H . C . F r o e l i c h , J. electrochem. Soe. 91, 161 (1947); 95, 254 (1949). 29) M. S c h 6 n , Z. P h y s i k 1 1 9 , 4 6 3 ( 1 9 4 2 ) ; F o r s c h . u. Fortschritte 19, no. 23/24, 1943; Chem. Bcr. 1942, p. 544; Ann. Physik (3) 6, 333 (1948). H.A. Klasens, Nature 158, 306 (1946). 30) F . A . K r / s g e r and W. H o o g e n s t r a a t e n , Physica 14, 425 (1948). 31) H. G o b r e c h t Ann. Physik 28, 673 (1937). H. L a n g e , Ann. Physik 31, 609 (1938). R. T o m a s c h e k Ann. Physik 75, 124 (1925). 32) R. T o m a s e h e k , Trans. Faraday Soc.,:]5, 148 (1939). Chem. Ber. (I942) 517. Ergebn. Exakt Naturw. 20, 269 (1942). 33) A . F . W e 11 s, Ministry of Home-Security Report RC(C) 119 (Gr. Britain). 34) R. T o m a s c h e k , Ann. Physik75, 109(142)(1924). 35) R. T o m a s e h e k and O. D e u t s c h b e i n , Ann. Physik 16, 930 (1933). 36) p. L e n a r d , Ann. Physik 31, 661 (1910). 37) R. T o m a s c h e k , Ann. Physik 75, 561 (1924); Physik. Z. 25, 643 (1924).