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Some optical properties of zinc silicate phosphors
Physica
VI,
no
8
Augustus
SOME OPTICAL OF ZINC SILICATE
1939
PROPERTIES PHOSPHORS
by I?. A. I
Zusammenfassung 1). Zinksilikat Phosphorc uncl Zink-Bcrylliumsilikat Phosphore, aktiviert mit Mangan, sincl Miscl~l~ristalle van Zinltsililtat ocler Zink-Bcrylliumsililcat mit Mangansililiat. 2) Mischltristalle cler Reihe zwischen 0 uncl 50 molqa Mn zeigcn bei 25” und -180°C Emission in zwei Bantlen mit Maxima bci 5200 uncl 6100 1%. 3) Die Banclen, welche von Zinksilikat--Mn Phosphoren cmittiert wcrden ltijnnen, sincl Elelrtronenspriingen im Mn++ Ion zuzuschreiben. 4) Mischltristalle Zinlcsilikat-Mangansilikat 0--SOY& zeigen Absorption in clrei verschieclenen Gebieten. Das crste Absorptionsgebiet wirtl such an reinem Zinksililcat gefunclcn uncl ist als tine Iiristallabsorption aufzufassen. Das zweite Absorptionsgebiet tritt nur bei Einfiihrung von Mangan auf, hat aber such die Eigenschaften einer Kristallabsorption. Das dritte ist ein System von Banclen; jeclcr liegt ein Elcktroneniibergang im nrn++ Ion zugruncle. 5) Einstrahlung in alle obengenannten Absorptionsgebiete ruft Lumineszenz in den fiir Nln++ charakteristischcn Emissionsbanden hervor. Einstrahlung in die zwei Kristallabsorptionsgebiete gibt Fluoreszenz nebst Phosphoreszenz, Einstrahlung in die charakteristischcn Rlangan.4bsorptionsbanclcn erzcugt nur Fluorcszenz.
1. Introduction. Zinc silicate phosphors are of great importance in industry, as well with regard to excitation with electrons and alphaparticles, as for irradiation with ultra violet light and X-rays. The behaviour of these phosphors during the electronic bombardment has been invcstigatcd by N o t t i n g h a m 1) and M a r t i n and Headrickz), while Nelson and Johnson3) and Bcese4) stuclied the afterglow. I< a r 1 i k and M i c h a i 1 o v a “) found some laws, governing the scintillation process. Results obtained up till now about the optical excitation by several investigators are not very satisfactory. -
764 -
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
765
F o u n d “) and M a r d e n and B e e s e 7) state that excitation of willemite exclusively occurs in a. closed spectral range between2200and30008; Rtittenauerzo), Jenkins and B o w t e 11”) and S c h ij n “). however, communicate that excitation can be caused by the neon resonance lines 736 and 740 A as well, while R ii t t e n a u e r’s work lo) proves that also irradiation with . Hg 3650 A produces luminescence. This controverse led us to the investigation of the excitation spectrum and its interpretation. Besides the problem of changes in the spectral distribution of the emission upon introducing Be,SiO, or SiO, was touched. 2. Formation of mixed crystals. A combined study of the spectra of excitation, absorption and emission seemed to be most promising. Since characteristic manganese absorption is only very faint in the phosphors, containing but a few per cent of manganese, we tried to make products of a higher manganese content. Preliminary experiments showed that in the system Zn,SiOcMn,SiO, at 1200°C a homogeneous phase occurs with the structure of the mineral phenakite, Be,SiO,, covering a composition range between 0 and about fifty mol per cent of manganese *). Herefrom it follows immediately that Zn,SiO, phosfihors, containing always less tlzan 50~201 y0 Mn,SiO,, are really solid solutions of Mn,Si04 in Zn,SiO,. By chemical methods no ions of a valency higher than two could be detected. It is still an open question, however, whether the Mn++ ions substituting the Zn+ + ions at normal lattice positions are responsible for the emission, or those at interstitial positions. Also in the system Zn,SiOd-Be,Si04 mixed crystal formation could be proved. Thus the Zn-Be silicates activated by manganese presumably are perfectly comparable with the Zn,SiO,-Mn phosphors. 3. Exfierimental technipe. In absorption experiments a quartz plate covered with a layer of the powder under investigation was interposed between a continuous light source and the spectrograph; *) The products were made by heating together ZnO (from NH,),ZnO, from Mn(NO,), solution) and silica (from aethylsilicate), in a CO-CO, appropriate composition.
solution), MnO atmosphere of
766
F. A.
KRijGER
for measurements at -180°C this plate was plunged into liquid nitrogen in a Dewar vessel with two non silvered slits. Photographs in the visible part of the spectrum were made by means of a Fuess glass spectrograph; in the ultra violet a Leitz quartz spectrograph was used. An incandescent lamp with quartz bulb, filled with krypton under high pressure, and a hydrogen discharge tube were used as continuous light sources. For monochromatic excitation we employed mercury discharge tubes (a low pressure tube producing mainly 2537 A, a high pressure tube in connection with a Wood filter yielding practically only light of 3650 A). The emission bands were either photographed or measured by means of a double monochromator in combination with a potassium photocell. In the visible range we used Ilford Special Rapid Panchromatic Plates and Agfa spektral total hart Platten; Agfa Isochrom Platten were employed in the ultra violet. Since the sensibility of the latter plates falls off at about 2200 A, they were sensibilised for the far ultra violet by covering them with a thin layer of blue fluorescent Senca pumping oil. Thus all ultra violet light transmitted by quartz blackens the plate. For photographing the excitation spectrum spectrally decomposed light was projected upon a mica foil, covered on the front side with a thin layer of the phosphor, behind which was placed a thin filter and a photographic plate. The filter must be exclusively transparent to light of the wavelengths of the emission band of the phosphor under investigation *). As a green filter, Wratten Gelatine 58A was satisfactory. For the red a combination of the green blue filter Nr. 5 N 5 and the red filter Nr. 27 was necessary t). *) H e y n e and P i r a n i rr), using the same principle, covered the glass side of their photographic plate with the fluorescent powder and exposed this side to the spectrum, thus using the glass of the plate itself as a filter. M a r d e n C.S. 7) took a thin layer of shellack as a filter. Both filters, however, are not satisfactory for the whole spectral range; besides they show luminescence themselves. t) A single red filter, showing no luminescence itself, was not available. Since luminescence of organic dyestuffs always lies immediately on the long wavelength side of the absorption band, a combination of filters could help; the ultra violet was cut off with a green blue filter (5 N 5) whose absorption edge lies at 4840 A, showing fluorescence in a band from 4500-5600 A. The red flter 27 absorbed the remaining visible light below 5800 A as well as the fluorescence of the first filter; the absorption of the visible light did not cause any appreciable fluorescence in the red filter.
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
767 ’
4. Emission *). Unactivated Zn,SiO, showed a faint green luminescence, in a band obviously identical with the one emitted by ZnSiO,-1 y0 Mn. Probably our samples contained still some minute traces of manganese (cf. R a n d a 11”‘)). In the system Zn,Si04 . Mn,Si04, products containing a few percents of manganese are luminescent at room temperature emitting a green band with maximum at 5300 A. With increasing manganese content the colour of emission light becomes more yellowish : this is due to a slight rise of the red side of the emission band. At the same time the, intensity decreases.
EA t
450
500
550
6
-A
Fig. 1. Emission spectra of Zn,SiO,-Mn a. 25°C. b. -180°C. c. -180°C (ordinate of the maximum equal to that of the curve a).
5%.
made
Upon irradiation with light of 3650 A, the concentration giving the optimum of light emission lies at 4% Mn 10); above this concentration the luminescence is falling off rapidly. This is not the case at liquid nitrogen temperature; all products between 0 and 50% Mn show a bright luminescence, changing from green to yellowish green with the increase of manganese concentration. This enables us to follow the colour shifting through the whole phenakite phase. Fig. 1 and Fig. 2 (a and b) show very clearly that the broadening of tke *)
I am indebted
to Mr. J.
R i e m e n s for measuring
some of the emission
bands.
F.A.KRijGER
768
I
EA T
500
550
600
Fig. 2. Emission spectra a. Zn,SiO,-Mn,SiO, 5%. b. Zn,SiO,-Mn,SiO, 20%. c. MnSiO..
a. b. c. d.
at -180°C
of:
Fig. 3. Emission spectra at room temperature Zn,SiO,-Mn 1%. Zn,SiO,-Mn 1% (= Zn,SiO, . SiO,-Mn 1%). Zn,SiO,-Be,SiO, 10% - Mn 5%. Zn,SiO,-Be,SiO, 5% - Mn 2%.
of:
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
769 ,
nayrow green emission band with increasing maagalzese content is due to the occurrence of another emission band, with a maximzlm at a$$roximately 6 100 A. In patent literature we find two methods for making the emission of zinc silicate phosphors shift to the red. According to the first method the phosphors are made with an excess of SiOz, after the second one Be,SiO, is added, combined with an increase of the manganese content. The effects obtained are shown in fig. 3. It is seen that SiOz excess produces a broadening to the red side (fig. 3a and b) comparable with the effect due to increasing manganese content in stoechiometrical ZnzSiO,. In the Zn-Be silicates a red band occurs, with a maximum at 6100 A, obviously identical with that visible at -180°C in the phenakite phase of the system Zn,Si04-Mn,Si04. The r&e of the berylh’tim seems to be to make possible the emission at room temperature of a red emission band, which in the system Zn,SiOb-Mn,SiOc is only emitted with a considerable intensity at very low temperatures. 5. Absorption. Pure zinc silicate shows a faint absorption over the whole ultra violet ; at about 2200 A the absorption increases rapidly.: even very thin layers do not transmit any light of a shorter wave length (fig. 4a). The introduction of manganese into the lattice has no detectable effect upon this absorption edge *), but causes a) an increased absorption in the tail on the long wave length side of the edge, b) the occurrence of a system of narrow bands between 3000 and 6000 it. Both absorptions are increased when more manganese is added to the silicate. The rise of the tail proceeds so rapidly that, while at the composition ZnaSiO,. 1y0 Mn, it is still to be recognised as such (fig. 4b), products with 5-50 mol% Mn all show a new, fairly sharp edge at 3000 A (fig. 4~). The absorption in the bands in the ultra violet are to be seen in fig. 4c, for the visible in fig. 5a and b t). At liquid nitrogen tempera*). Also in mixed crystals ZnlSiOI . BezSi04 the limit of the crystal absorption could be found to shift appreciably. t) Since we were working with powders, no well defined absorption coefficients can be given. We obtained the ordinates of our figures as the differences in height on photometer 49 Physica VI not
. ..
770
B. A. KROGER
ture the bands become sharper, without any appreciable shifting *). The absorption of &--Be silicates containing manganese is given in fig. 5c; they show the same bands as Zn,Si04 . 10% MnzSi04, also in the ultra violet. ABSORPTION t
\/-\A
225
a. b. c.
Fig. Zn,SiOd. Zn,SiO,-Mn Zn,SiO,-Mn
4.
250 Absorption
300 spectra
at room
350 temperature
I
400
450
of:
l”/b. 5-20%.
For comparison the absorption spectrum of synthetical MnSiO, is given ; the unsharp limit of the crystal absorption lies at about 2500 a (fig. 6a--b). The absorption bands occur at the same wavelength, but the intensities are slightly changed, besides some quite new bands appear (fig. 5d, e, fig. 6, b, c). Some further substances prove to give the same absorption bands as the products studied above, MnSiO, (fig. 5/z), MnO (fig. 5g), K u t z e 1 n i g g’s layer lattice phosphor CdJ,-MnCl, 50% (fig. 5f) and some manganese salts 13). The bands are tabulated in table I. The green phase between 95 and 100% Mn,Si04 had no very characteristic features; it was much like MnO, showing a very broad absorption band at about 6000 A and a continuous absorption from 5000 A to the violet. curves of absorption photographs and of those showing the background only (the latter were made by making photographs of the light source directly, choosing a time of exposure suitable to give comparable blackening.) *) There may be a-shift of about 5 cm- 1, as was reported by G i e I e s s e n IJ) for AlnClz . 4aq.
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
A BSORPTION
‘ tfxl
a. b. c. d. e. f. g. It.
Fig. 5. Absorption Zn,SiO,-Mn,SiO, Zn,SiO,-Mn,SiO,. Zn,SiO,-Be,SiO,. MnSiO, at --180°C. MnSiO, at 25°C. Cd J,--MnCI, 50% MnO -18OT. MnSiO, (the mineral
425
450
spectra in the visible part of the spectrum. 10% at 25°C. 10% at -180°C. 10% - Mn 5% at -180°C.
-180°C. rhodonite)
-180°C.
771
772
F. A. KRGGER
TABLE
I Wavelengths
Absorption Zn2Si04. MnlSiOJ
ZnzSi04. BezSi04
MnSiO,
0%;
20% 493 406 469 443 433 424 422 419.5
433
444 434
422.5
424 422
416.4 413.3 409 406.7 302 373 359
Mo
MnO
433 425.2 424 421.5
.
416.4
MnC124ac
421.5 416.4
409 406
373.5 361 343 326 318.5
318
5%
MnSiOJ Mineral Rhodonite)
in rnp
374 361 343
369 363 342 318
6. Excitation. The method described in section 3 gave beautiful photographs of the excitation spectrum; the photometer curves of these are given in fig. 7, 8, 9 and 10. ABSORPTION t
22.5
250
300
350 -
a. thin b. thick c. band
Fig. 6. Absorption spectra layer, 25°C. layer, 25°C. system at -180°C. enlarged.
of MnSiO,.
400 XINmp
SOME OPTICAL
PROPERTIES
OF ZINC SILICATE
773
PHOSPHORS
Excitation takes place in three different parts : A) in bands in the ultra violet as well as in the visible ; these
200
225
2.50
300
350 -A
Fig. a. b. c. d.
71 Excitation Zn,SiOl-Mn Zn,SiO,-Mn Zn,SiO,-Mn Zn,SiO ,--Mn
Fig. 8. Excitation n. Zn,SiO,-Be,SiO, Zn,SiO,-Be,SioO, c. Zn,SiO,-Be,SiO, b.
4.50
at room temperature.
spectra of zinc silicate phosphors lo/,,. 5%. 2% (band system enlarged). 5% ( ,, ,, ,, 1.
225
400 INmp
2.50
spectra
of zinc-beryllium temperature. . 10% - Mn 5%. . 10% - Mn 5% 5% -Mn 2%.
silicate
(bapd ( ,,
phosphors
system ,,
at room
enlarged). ,, )
774
F. A.
KRGGER
bands, tabulated in table II, are identical with those found absorption. B) in a broad band between 2200 and 3000 A with a maximum 2500 f-i.
in at
If1
a t
Y--N \ . 0’ \ . N-A /’
‘\ b ‘\
/q
Fig. a. b. c.
C
\
I
I
am
\\
250
225
300
350
400 -XlNmp
450
9. Excitation spectra of zinc silicate phosphors with an excess of SiO, Zn,SiO, . SiO, Mn - 4%. Zn,SiO, . SiO, Mn - 1%. enlarged). Zn,SiO, . SiO, Mn - 4% ‘(band q&tern
l
1
400
I
425
I
450
Fig. 10. Visible part of the excitation a. Zn,SiO, . SiO, . Mn 4p/,. b. Zn,SiO,-Be,SiO, . 10% - Mn 5%.
-
I
500 -AIN
550
mp
spectrum
of
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
775 .
PHOSPHORS
C) in an as far as we know unlimited range below 2200 A. Irradiation in the ranges B) and C) causes fluorescence and phosphorescence simultaneously : heating after irradiation at low temperature causes luminescence. In A), however, only fluorescence occurs ; not the slightest amount of energy can be frozen in here *). TABLE Excitation
II Wavelengths
Bands
Zn2Si04 . MnzSiO, O.l-20%
ZnzSiOd-SiOz-Mn
44.5 421 385 377 361 341 320
Zn*SiOl---;nO+-
10 y0 0
472.6 445 433 423
508 472.6 445 433 423
378 362 340
377 361 335 320
in rnp
.
CdSiOs-Mn
410 350
The spectral distribution of the emission is independent of the way of excitation, even if the emission is built up of several bands (e.g. Zn-Be silicate, Mn). I t
Fig.
i
-.-
EXCITATION Cd GO3 - 1 %&in ABSORPTION * u ABSORPTION CdSi03
11. Excitation and absorption spectra silicate at room temperature.
of cadmium
Some results with cadmium silicate, although this substance was not investigated thoroughly, may be quoted here (fig. 11) ; they *) Attention must be drawn of ZnS-Mn phosphors 19).
to the great
resemblance
with
the properties
of excitation
776
F. A.
KRijGER
prove that the type of excitation spectrum found at the Zn and &-r-Be silicates is not limited to these substances: Cd silicate too shows excitation in a spectral range corresponding to the crystal absorption, further in a band on the long wave length side of this crystal absorption and in certain bands, which are undoubtedly due to the manganese. 7. Discussion of results. G i s o 1 f 14) found recently that in the system ZnS-CdS the edge of the lattice absorption is in nearly linear relationship with the composition of the mixed crystals. The results on the absorption of ZnzSiO, and Zn,SiOq . Mn,SiO, mixed crystals, asgiven in fig. 4, prove that in the system Zn,SiO,-Mn$SiO, the absorption spectrum behaves quite differently. The edge of the crystal absorption found on pure willemite shows no marked shift on introducing manganese; instead of this a new edge erects itself. Both edges seem to be due to true lattice absorption. Probably for the first edge we have to deal with an electronic transition involving a zinc energy band, while for the second absorption this role is played by a manganese energy band. That the absorption edge remains practically at the same place is most probably due to the fact that the M a d e 1 u n g energy changes but little, the cell dimensions from Zn,SiO, to Zn,SiO.+ . MnzSiO, . 20% only increasing with 0.5%. The energy absorbed in one of these lattice absorption bands must be transported through the lattice to the centra of emission. H o fs t a d t e r 15) recently found photoconductivity in a band beginning at 3000 A (no doubt identical with our band “B”). Obviously the transport of energy through the lattice, at least to a certain amount, takes place by means of electrons in the conduction band. Trapping of these electrons on their way, followed by their slow liberation by heat, produces the phosphorescence. For the part of the absorbed energy causing fluorescence other mechanisms remain possible. (exponential decay !) The nature of the centra’of emission can best be approached by summarizing the following points : a) No manganese of higher valency than 2 can be found. b) A group of bands occur in absorption in Zn,SiO., . MnzSiO, mixed crystals; these same bands are found in other substances containing divalent manganese.
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
777 .
c) Irradiation into the absorption bands of the Zn,SiO, . Mn,SiO, mixed crystals yields fluorescence in red, yellow or green bands. d) All manganese phosphors emit bands in the red, yellow or green 16) lz) 21) *). e) If the emission spectrum consists of several superposed bands, the ratio of the intensities of these bands remains the same, whatever happens to be the way of excitation. The last fioint e) illustrating that all emission bands mast be eyitted by one and the same kind of emitting centra, the points a), b), c), d) undozlbtedly prove that this centre m%st be correlated with the divalent
Mn++ ion. Hence the emission bands with maxima between 5300 and 6600 A result from electronic transitions in Mn+ + ions. Herewith an answer is given to the question already put forward by Lecoq de Boisbaudran in 1887,astothevalencyof manganese in phosphors 17). B r ii n i n g h a u s’ la) statement, attributing the red and the green emission of glasses to manganese ions with different valency, must be incorrect ; we just proved that these different bands are due to the same center, the Mn++ ion. As the manganese absorption bands are found at about the same wavelengths in quite different substances, the energy levels in the Mn++ ion seem to be rather unaffected by the surrounding field. Hence the absorption bands, due to the Mn++ ions at interstitial positions, may practically coincide with those caused by manganese at ordinary lattice positions. A conclusion about the point whether the Mn++ ions acting as a center belong to the first or second kind, is thus impossible. The problem why the relative intensity of the different emission bands is dependent on the composition of the matrix lattice in which the emitting Mn+ + ions are embodied is still open to discussion too. From our experiments it can be concluded that with the change in emission the absorption spectrum is also markedly altered, some new bands coming up, others fading away. Irradiation into the new absorption bands does not produce, however, the new emission band exclusively, but actually causes exactly the same luminescence as is yielded by all other kinds of excitation (point e). Hence we only may conclude that both the emission and the *) The activated
green emission by manganese.
band
is found
also in A1203, CaS04,
CdSO.+;
SrS, Zn borate,
all
778
SOME
OPTICAL
PROPERTIES
OF, ZINC
SILICATE
PHOSPHORS
absorption spectra are varied under the same influence, probably a change in the field of the surrounding crystal lattice. We are also unable to give an explanation of the fact observed by R ii t t e n a u e r 10) that the optimum Mn concentration is dependent on the irradiating wavelength; we only can stipulate the point that the wavelengths, used by R ii t t e n a u e r *), are situated in the three different parts of the excitation spectrum. I am indebted to Dr. J. H. G i s o 1 f for some technical advises and for the many very helpful discussions on the subject we have been dealing with. Eindhoven; 11th April 1939. Received June 7th 1939.
REFERENCES 1) 2) 3) 4) 5)
W. B. S.T. R. B. N. C. B. K (1938).
Nottingham, J. Appl. Phys. 8, 762, 1937; 10, 73, 1939. Martin andL.B. Headrick, J.Appl. Phys.lO, llb(1939). Bull. Am. Phys. Sot. 13, 7, (1938). Nelson and R. P. Johnson, B e e s e, J. Opt. Sot. Am. 20, 26 (1939). a r 1 i k and E. K a r a M i c h a i 1 o w a, Sitz. Ber. Ak. Wien IIa, 137, 363
C. G. F o u n d, Trans. Ill. Eng. Sot. 33, 186 (1938). 7) J. W. Mar den, N. C. Be ese and G. Me is t er, Trans. Ill. Eng. Sot. 34,bS (1939). 8) H. G. J e n k i n s and J. N. B o w t e 11 , Trans. Far. Sot. 35, 155 ( 1939). 9) M. S c h 6 n, Trans. Far. Sot. 35, lb2 ( 1939). 10) A. R ii t ten au e r, 2. techn. Phys. 19, 148 (1938). 11) G. H e y n e and M. P i r a n i, 2. techn. Phys. 14, 31 (1933). 12) J.T. Randall, Trans. Far. Sot. 35, 11 (1939). 13) J. G i e 1 e s s e n, Ann. Phys. (5) 22, 541 (1935). 14) J. H. G is o 1 f, Physica 6, 84 (1939). 15) R. Hofstadter, Phys.Rev.54,864(1938). lb) J. T. R a n d a 11, Nature 142, 113 (1938). 17) L e c o 9 d e B o i s b a u d r a n, C. R. AC. Sci. Paris 105, 1228 (1887). 18) L. B r ii ni n g h au s, J. Phys. Rad. 12,398 (1931). 19) F. A. I< r 6 g e r, Physica (I, 369 (1939). 20) A. R ii t t e n a u e r, Phys. 2. 37, 810 (1936). 21) J. T. R a n d a 11, Proc. roy. Sot. London 170, 272 (1939). b)
*) For irradiation with ively 0.5, 2.5 and 4%.
736, 2537 and
3650
A this optimum
concentration
was respect-
VI,
no
8
Augustus
SOME OPTICAL OF ZINC SILICATE
1939
PROPERTIES PHOSPHORS
by I?. A. I
Zusammenfassung 1). Zinksilikat Phosphorc uncl Zink-Bcrylliumsilikat Phosphore, aktiviert mit Mangan, sincl Miscl~l~ristalle van Zinltsililtat ocler Zink-Bcrylliumsililcat mit Mangansililiat. 2) Mischltristalle cler Reihe zwischen 0 uncl 50 molqa Mn zeigcn bei 25” und -180°C Emission in zwei Bantlen mit Maxima bci 5200 uncl 6100 1%. 3) Die Banclen, welche von Zinksilikat--Mn Phosphoren cmittiert wcrden ltijnnen, sincl Elelrtronenspriingen im Mn++ Ion zuzuschreiben. 4) Mischltristalle Zinlcsilikat-Mangansilikat 0--SOY& zeigen Absorption in clrei verschieclenen Gebieten. Das crste Absorptionsgebiet wirtl such an reinem Zinksililcat gefunclcn uncl ist als tine Iiristallabsorption aufzufassen. Das zweite Absorptionsgebiet tritt nur bei Einfiihrung von Mangan auf, hat aber such die Eigenschaften einer Kristallabsorption. Das dritte ist ein System von Banclen; jeclcr liegt ein Elcktroneniibergang im nrn++ Ion zugruncle. 5) Einstrahlung in alle obengenannten Absorptionsgebiete ruft Lumineszenz in den fiir Nln++ charakteristischcn Emissionsbanden hervor. Einstrahlung in die zwei Kristallabsorptionsgebiete gibt Fluoreszenz nebst Phosphoreszenz, Einstrahlung in die charakteristischcn Rlangan.4bsorptionsbanclcn erzcugt nur Fluorcszenz.
1. Introduction. Zinc silicate phosphors are of great importance in industry, as well with regard to excitation with electrons and alphaparticles, as for irradiation with ultra violet light and X-rays. The behaviour of these phosphors during the electronic bombardment has been invcstigatcd by N o t t i n g h a m 1) and M a r t i n and Headrickz), while Nelson and Johnson3) and Bcese4) stuclied the afterglow. I< a r 1 i k and M i c h a i 1 o v a “) found some laws, governing the scintillation process. Results obtained up till now about the optical excitation by several investigators are not very satisfactory. -
764 -
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
765
F o u n d “) and M a r d e n and B e e s e 7) state that excitation of willemite exclusively occurs in a. closed spectral range between2200and30008; Rtittenauerzo), Jenkins and B o w t e 11”) and S c h ij n “). however, communicate that excitation can be caused by the neon resonance lines 736 and 740 A as well, while R ii t t e n a u e r’s work lo) proves that also irradiation with . Hg 3650 A produces luminescence. This controverse led us to the investigation of the excitation spectrum and its interpretation. Besides the problem of changes in the spectral distribution of the emission upon introducing Be,SiO, or SiO, was touched. 2. Formation of mixed crystals. A combined study of the spectra of excitation, absorption and emission seemed to be most promising. Since characteristic manganese absorption is only very faint in the phosphors, containing but a few per cent of manganese, we tried to make products of a higher manganese content. Preliminary experiments showed that in the system Zn,SiOcMn,SiO, at 1200°C a homogeneous phase occurs with the structure of the mineral phenakite, Be,SiO,, covering a composition range between 0 and about fifty mol per cent of manganese *). Herefrom it follows immediately that Zn,SiO, phosfihors, containing always less tlzan 50~201 y0 Mn,SiO,, are really solid solutions of Mn,Si04 in Zn,SiO,. By chemical methods no ions of a valency higher than two could be detected. It is still an open question, however, whether the Mn++ ions substituting the Zn+ + ions at normal lattice positions are responsible for the emission, or those at interstitial positions. Also in the system Zn,SiOd-Be,Si04 mixed crystal formation could be proved. Thus the Zn-Be silicates activated by manganese presumably are perfectly comparable with the Zn,SiO,-Mn phosphors. 3. Exfierimental technipe. In absorption experiments a quartz plate covered with a layer of the powder under investigation was interposed between a continuous light source and the spectrograph; *) The products were made by heating together ZnO (from NH,),ZnO, from Mn(NO,), solution) and silica (from aethylsilicate), in a CO-CO, appropriate composition.
solution), MnO atmosphere of
766
F. A.
KRijGER
for measurements at -180°C this plate was plunged into liquid nitrogen in a Dewar vessel with two non silvered slits. Photographs in the visible part of the spectrum were made by means of a Fuess glass spectrograph; in the ultra violet a Leitz quartz spectrograph was used. An incandescent lamp with quartz bulb, filled with krypton under high pressure, and a hydrogen discharge tube were used as continuous light sources. For monochromatic excitation we employed mercury discharge tubes (a low pressure tube producing mainly 2537 A, a high pressure tube in connection with a Wood filter yielding practically only light of 3650 A). The emission bands were either photographed or measured by means of a double monochromator in combination with a potassium photocell. In the visible range we used Ilford Special Rapid Panchromatic Plates and Agfa spektral total hart Platten; Agfa Isochrom Platten were employed in the ultra violet. Since the sensibility of the latter plates falls off at about 2200 A, they were sensibilised for the far ultra violet by covering them with a thin layer of blue fluorescent Senca pumping oil. Thus all ultra violet light transmitted by quartz blackens the plate. For photographing the excitation spectrum spectrally decomposed light was projected upon a mica foil, covered on the front side with a thin layer of the phosphor, behind which was placed a thin filter and a photographic plate. The filter must be exclusively transparent to light of the wavelengths of the emission band of the phosphor under investigation *). As a green filter, Wratten Gelatine 58A was satisfactory. For the red a combination of the green blue filter Nr. 5 N 5 and the red filter Nr. 27 was necessary t). *) H e y n e and P i r a n i rr), using the same principle, covered the glass side of their photographic plate with the fluorescent powder and exposed this side to the spectrum, thus using the glass of the plate itself as a filter. M a r d e n C.S. 7) took a thin layer of shellack as a filter. Both filters, however, are not satisfactory for the whole spectral range; besides they show luminescence themselves. t) A single red filter, showing no luminescence itself, was not available. Since luminescence of organic dyestuffs always lies immediately on the long wavelength side of the absorption band, a combination of filters could help; the ultra violet was cut off with a green blue filter (5 N 5) whose absorption edge lies at 4840 A, showing fluorescence in a band from 4500-5600 A. The red flter 27 absorbed the remaining visible light below 5800 A as well as the fluorescence of the first filter; the absorption of the visible light did not cause any appreciable fluorescence in the red filter.
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
767 ’
4. Emission *). Unactivated Zn,SiO, showed a faint green luminescence, in a band obviously identical with the one emitted by ZnSiO,-1 y0 Mn. Probably our samples contained still some minute traces of manganese (cf. R a n d a 11”‘)). In the system Zn,Si04 . Mn,Si04, products containing a few percents of manganese are luminescent at room temperature emitting a green band with maximum at 5300 A. With increasing manganese content the colour of emission light becomes more yellowish : this is due to a slight rise of the red side of the emission band. At the same time the, intensity decreases.
EA t
450
500
550
6
-A
Fig. 1. Emission spectra of Zn,SiO,-Mn a. 25°C. b. -180°C. c. -180°C (ordinate of the maximum equal to that of the curve a).
5%.
made
Upon irradiation with light of 3650 A, the concentration giving the optimum of light emission lies at 4% Mn 10); above this concentration the luminescence is falling off rapidly. This is not the case at liquid nitrogen temperature; all products between 0 and 50% Mn show a bright luminescence, changing from green to yellowish green with the increase of manganese concentration. This enables us to follow the colour shifting through the whole phenakite phase. Fig. 1 and Fig. 2 (a and b) show very clearly that the broadening of tke *)
I am indebted
to Mr. J.
R i e m e n s for measuring
some of the emission
bands.
F.A.KRijGER
768
I
EA T
500
550
600
Fig. 2. Emission spectra a. Zn,SiO,-Mn,SiO, 5%. b. Zn,SiO,-Mn,SiO, 20%. c. MnSiO..
a. b. c. d.
at -180°C
of:
Fig. 3. Emission spectra at room temperature Zn,SiO,-Mn 1%. Zn,SiO,-Mn 1% (= Zn,SiO, . SiO,-Mn 1%). Zn,SiO,-Be,SiO, 10% - Mn 5%. Zn,SiO,-Be,SiO, 5% - Mn 2%.
of:
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
769 ,
nayrow green emission band with increasing maagalzese content is due to the occurrence of another emission band, with a maximzlm at a$$roximately 6 100 A. In patent literature we find two methods for making the emission of zinc silicate phosphors shift to the red. According to the first method the phosphors are made with an excess of SiOz, after the second one Be,SiO, is added, combined with an increase of the manganese content. The effects obtained are shown in fig. 3. It is seen that SiOz excess produces a broadening to the red side (fig. 3a and b) comparable with the effect due to increasing manganese content in stoechiometrical ZnzSiO,. In the Zn-Be silicates a red band occurs, with a maximum at 6100 A, obviously identical with that visible at -180°C in the phenakite phase of the system Zn,Si04-Mn,Si04. The r&e of the berylh’tim seems to be to make possible the emission at room temperature of a red emission band, which in the system Zn,SiOb-Mn,SiOc is only emitted with a considerable intensity at very low temperatures. 5. Absorption. Pure zinc silicate shows a faint absorption over the whole ultra violet ; at about 2200 A the absorption increases rapidly.: even very thin layers do not transmit any light of a shorter wave length (fig. 4a). The introduction of manganese into the lattice has no detectable effect upon this absorption edge *), but causes a) an increased absorption in the tail on the long wave length side of the edge, b) the occurrence of a system of narrow bands between 3000 and 6000 it. Both absorptions are increased when more manganese is added to the silicate. The rise of the tail proceeds so rapidly that, while at the composition ZnaSiO,. 1y0 Mn, it is still to be recognised as such (fig. 4b), products with 5-50 mol% Mn all show a new, fairly sharp edge at 3000 A (fig. 4~). The absorption in the bands in the ultra violet are to be seen in fig. 4c, for the visible in fig. 5a and b t). At liquid nitrogen tempera*). Also in mixed crystals ZnlSiOI . BezSi04 the limit of the crystal absorption could be found to shift appreciably. t) Since we were working with powders, no well defined absorption coefficients can be given. We obtained the ordinates of our figures as the differences in height on photometer 49 Physica VI not
. ..
770
B. A. KROGER
ture the bands become sharper, without any appreciable shifting *). The absorption of &--Be silicates containing manganese is given in fig. 5c; they show the same bands as Zn,Si04 . 10% MnzSi04, also in the ultra violet. ABSORPTION t
\/-\A
225
a. b. c.
Fig. Zn,SiOd. Zn,SiO,-Mn Zn,SiO,-Mn
4.
250 Absorption
300 spectra
at room
350 temperature
I
400
450
of:
l”/b. 5-20%.
For comparison the absorption spectrum of synthetical MnSiO, is given ; the unsharp limit of the crystal absorption lies at about 2500 a (fig. 6a--b). The absorption bands occur at the same wavelength, but the intensities are slightly changed, besides some quite new bands appear (fig. 5d, e, fig. 6, b, c). Some further substances prove to give the same absorption bands as the products studied above, MnSiO, (fig. 5/z), MnO (fig. 5g), K u t z e 1 n i g g’s layer lattice phosphor CdJ,-MnCl, 50% (fig. 5f) and some manganese salts 13). The bands are tabulated in table I. The green phase between 95 and 100% Mn,Si04 had no very characteristic features; it was much like MnO, showing a very broad absorption band at about 6000 A and a continuous absorption from 5000 A to the violet. curves of absorption photographs and of those showing the background only (the latter were made by making photographs of the light source directly, choosing a time of exposure suitable to give comparable blackening.) *) There may be a-shift of about 5 cm- 1, as was reported by G i e I e s s e n IJ) for AlnClz . 4aq.
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
A BSORPTION
‘ tfxl
a. b. c. d. e. f. g. It.
Fig. 5. Absorption Zn,SiO,-Mn,SiO, Zn,SiO,-Mn,SiO,. Zn,SiO,-Be,SiO,. MnSiO, at --180°C. MnSiO, at 25°C. Cd J,--MnCI, 50% MnO -18OT. MnSiO, (the mineral
425
450
spectra in the visible part of the spectrum. 10% at 25°C. 10% at -180°C. 10% - Mn 5% at -180°C.
-180°C. rhodonite)
-180°C.
771
772
F. A. KRGGER
TABLE
I Wavelengths
Absorption Zn2Si04. MnlSiOJ
ZnzSi04. BezSi04
MnSiO,
0%;
20% 493 406 469 443 433 424 422 419.5
433
444 434
422.5
424 422
416.4 413.3 409 406.7 302 373 359
Mo
MnO
433 425.2 424 421.5
.
416.4
MnC124ac
421.5 416.4
409 406
373.5 361 343 326 318.5
318
5%
MnSiOJ Mineral Rhodonite)
in rnp
374 361 343
369 363 342 318
6. Excitation. The method described in section 3 gave beautiful photographs of the excitation spectrum; the photometer curves of these are given in fig. 7, 8, 9 and 10. ABSORPTION t
22.5
250
300
350 -
a. thin b. thick c. band
Fig. 6. Absorption spectra layer, 25°C. layer, 25°C. system at -180°C. enlarged.
of MnSiO,.
400 XINmp
SOME OPTICAL
PROPERTIES
OF ZINC SILICATE
773
PHOSPHORS
Excitation takes place in three different parts : A) in bands in the ultra violet as well as in the visible ; these
200
225
2.50
300
350 -A
Fig. a. b. c. d.
71 Excitation Zn,SiOl-Mn Zn,SiO,-Mn Zn,SiO,-Mn Zn,SiO ,--Mn
Fig. 8. Excitation n. Zn,SiO,-Be,SiO, Zn,SiO,-Be,SioO, c. Zn,SiO,-Be,SiO, b.
4.50
at room temperature.
spectra of zinc silicate phosphors lo/,,. 5%. 2% (band system enlarged). 5% ( ,, ,, ,, 1.
225
400 INmp
2.50
spectra
of zinc-beryllium temperature. . 10% - Mn 5%. . 10% - Mn 5% 5% -Mn 2%.
silicate
(bapd ( ,,
phosphors
system ,,
at room
enlarged). ,, )
774
F. A.
KRGGER
bands, tabulated in table II, are identical with those found absorption. B) in a broad band between 2200 and 3000 A with a maximum 2500 f-i.
in at
If1
a t
Y--N \ . 0’ \ . N-A /’
‘\ b ‘\
/q
Fig. a. b. c.
C
\
I
I
am
\\
250
225
300
350
400 -XlNmp
450
9. Excitation spectra of zinc silicate phosphors with an excess of SiO, Zn,SiO, . SiO, Mn - 4%. Zn,SiO, . SiO, Mn - 1%. enlarged). Zn,SiO, . SiO, Mn - 4% ‘(band q&tern
l
1
400
I
425
I
450
Fig. 10. Visible part of the excitation a. Zn,SiO, . SiO, . Mn 4p/,. b. Zn,SiO,-Be,SiO, . 10% - Mn 5%.
-
I
500 -AIN
550
mp
spectrum
of
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
775 .
PHOSPHORS
C) in an as far as we know unlimited range below 2200 A. Irradiation in the ranges B) and C) causes fluorescence and phosphorescence simultaneously : heating after irradiation at low temperature causes luminescence. In A), however, only fluorescence occurs ; not the slightest amount of energy can be frozen in here *). TABLE Excitation
II Wavelengths
Bands
Zn2Si04 . MnzSiO, O.l-20%
ZnzSiOd-SiOz-Mn
44.5 421 385 377 361 341 320
Zn*SiOl---;nO+-
10 y0 0
472.6 445 433 423
508 472.6 445 433 423
378 362 340
377 361 335 320
in rnp
.
CdSiOs-Mn
410 350
The spectral distribution of the emission is independent of the way of excitation, even if the emission is built up of several bands (e.g. Zn-Be silicate, Mn). I t
Fig.
i
-.-
EXCITATION Cd GO3 - 1 %&in ABSORPTION * u ABSORPTION CdSi03
11. Excitation and absorption spectra silicate at room temperature.
of cadmium
Some results with cadmium silicate, although this substance was not investigated thoroughly, may be quoted here (fig. 11) ; they *) Attention must be drawn of ZnS-Mn phosphors 19).
to the great
resemblance
with
the properties
of excitation
776
F. A.
KRijGER
prove that the type of excitation spectrum found at the Zn and &-r-Be silicates is not limited to these substances: Cd silicate too shows excitation in a spectral range corresponding to the crystal absorption, further in a band on the long wave length side of this crystal absorption and in certain bands, which are undoubtedly due to the manganese. 7. Discussion of results. G i s o 1 f 14) found recently that in the system ZnS-CdS the edge of the lattice absorption is in nearly linear relationship with the composition of the mixed crystals. The results on the absorption of ZnzSiO, and Zn,SiOq . Mn,SiO, mixed crystals, asgiven in fig. 4, prove that in the system Zn,SiO,-Mn$SiO, the absorption spectrum behaves quite differently. The edge of the crystal absorption found on pure willemite shows no marked shift on introducing manganese; instead of this a new edge erects itself. Both edges seem to be due to true lattice absorption. Probably for the first edge we have to deal with an electronic transition involving a zinc energy band, while for the second absorption this role is played by a manganese energy band. That the absorption edge remains practically at the same place is most probably due to the fact that the M a d e 1 u n g energy changes but little, the cell dimensions from Zn,SiO, to Zn,SiO.+ . MnzSiO, . 20% only increasing with 0.5%. The energy absorbed in one of these lattice absorption bands must be transported through the lattice to the centra of emission. H o fs t a d t e r 15) recently found photoconductivity in a band beginning at 3000 A (no doubt identical with our band “B”). Obviously the transport of energy through the lattice, at least to a certain amount, takes place by means of electrons in the conduction band. Trapping of these electrons on their way, followed by their slow liberation by heat, produces the phosphorescence. For the part of the absorbed energy causing fluorescence other mechanisms remain possible. (exponential decay !) The nature of the centra’of emission can best be approached by summarizing the following points : a) No manganese of higher valency than 2 can be found. b) A group of bands occur in absorption in Zn,SiO., . MnzSiO, mixed crystals; these same bands are found in other substances containing divalent manganese.
SOME
OPTICAL
PROPERTIES
OF ZINC
SILICATE
PHOSPHORS
777 .
c) Irradiation into the absorption bands of the Zn,SiO, . Mn,SiO, mixed crystals yields fluorescence in red, yellow or green bands. d) All manganese phosphors emit bands in the red, yellow or green 16) lz) 21) *). e) If the emission spectrum consists of several superposed bands, the ratio of the intensities of these bands remains the same, whatever happens to be the way of excitation. The last fioint e) illustrating that all emission bands mast be eyitted by one and the same kind of emitting centra, the points a), b), c), d) undozlbtedly prove that this centre m%st be correlated with the divalent
Mn++ ion. Hence the emission bands with maxima between 5300 and 6600 A result from electronic transitions in Mn+ + ions. Herewith an answer is given to the question already put forward by Lecoq de Boisbaudran in 1887,astothevalencyof manganese in phosphors 17). B r ii n i n g h a u s’ la) statement, attributing the red and the green emission of glasses to manganese ions with different valency, must be incorrect ; we just proved that these different bands are due to the same center, the Mn++ ion. As the manganese absorption bands are found at about the same wavelengths in quite different substances, the energy levels in the Mn++ ion seem to be rather unaffected by the surrounding field. Hence the absorption bands, due to the Mn++ ions at interstitial positions, may practically coincide with those caused by manganese at ordinary lattice positions. A conclusion about the point whether the Mn++ ions acting as a center belong to the first or second kind, is thus impossible. The problem why the relative intensity of the different emission bands is dependent on the composition of the matrix lattice in which the emitting Mn+ + ions are embodied is still open to discussion too. From our experiments it can be concluded that with the change in emission the absorption spectrum is also markedly altered, some new bands coming up, others fading away. Irradiation into the new absorption bands does not produce, however, the new emission band exclusively, but actually causes exactly the same luminescence as is yielded by all other kinds of excitation (point e). Hence we only may conclude that both the emission and the *) The activated
green emission by manganese.
band
is found
also in A1203, CaS04,
CdSO.+;
SrS, Zn borate,
all
778
SOME
OPTICAL
PROPERTIES
OF, ZINC
SILICATE
PHOSPHORS
absorption spectra are varied under the same influence, probably a change in the field of the surrounding crystal lattice. We are also unable to give an explanation of the fact observed by R ii t t e n a u e r 10) that the optimum Mn concentration is dependent on the irradiating wavelength; we only can stipulate the point that the wavelengths, used by R ii t t e n a u e r *), are situated in the three different parts of the excitation spectrum. I am indebted to Dr. J. H. G i s o 1 f for some technical advises and for the many very helpful discussions on the subject we have been dealing with. Eindhoven; 11th April 1939. Received June 7th 1939.
REFERENCES 1) 2) 3) 4) 5)
W. B. S.T. R. B. N. C. B. K (1938).
Nottingham, J. Appl. Phys. 8, 762, 1937; 10, 73, 1939. Martin andL.B. Headrick, J.Appl. Phys.lO, llb(1939). Bull. Am. Phys. Sot. 13, 7, (1938). Nelson and R. P. Johnson, B e e s e, J. Opt. Sot. Am. 20, 26 (1939). a r 1 i k and E. K a r a M i c h a i 1 o w a, Sitz. Ber. Ak. Wien IIa, 137, 363
C. G. F o u n d, Trans. Ill. Eng. Sot. 33, 186 (1938). 7) J. W. Mar den, N. C. Be ese and G. Me is t er, Trans. Ill. Eng. Sot. 34,bS (1939). 8) H. G. J e n k i n s and J. N. B o w t e 11 , Trans. Far. Sot. 35, 155 ( 1939). 9) M. S c h 6 n, Trans. Far. Sot. 35, lb2 ( 1939). 10) A. R ii t ten au e r, 2. techn. Phys. 19, 148 (1938). 11) G. H e y n e and M. P i r a n i, 2. techn. Phys. 14, 31 (1933). 12) J.T. Randall, Trans. Far. Sot. 35, 11 (1939). 13) J. G i e 1 e s s e n, Ann. Phys. (5) 22, 541 (1935). 14) J. H. G is o 1 f, Physica 6, 84 (1939). 15) R. Hofstadter, Phys.Rev.54,864(1938). lb) J. T. R a n d a 11, Nature 142, 113 (1938). 17) L e c o 9 d e B o i s b a u d r a n, C. R. AC. Sci. Paris 105, 1228 (1887). 18) L. B r ii ni n g h au s, J. Phys. Rad. 12,398 (1931). 19) F. A. I< r 6 g e r, Physica (I, 369 (1939). 20) A. R ii t t e n a u e r, Phys. 2. 37, 810 (1936). 21) J. T. R a n d a 11, Proc. roy. Sot. London 170, 272 (1939). b)
*) For irradiation with ively 0.5, 2.5 and 4%.
736, 2537 and
3650
A this optimum
concentration
was respect-