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Comparison of the emission of Eu2+ in MFCl (M = Sr, Ba) and Gd3+ in YOCl
JOURNAL OF
Journal of Luminescence 51(1992) 283—292 North-Holland
Comparison of the emission of Eu2~in MFC1 (M and Gd3~in YOC1
=
Sr, Ba)
J. Sytsma and G. Blasse Debye Research Institute, University of Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands Received 7 August 1991 Revised 13 November 1991 Accepted 13 November 1991
Transition probabilities of the 6P 8S 2~in MFCI (M = Sr, Ba) are reported, and compared with values found for Gd3~ in YOCI. The 7/2 transition —‘ 7/2 transition probabilities of Eu are larger for Eu2~.This is ascribed to the lower position of the opposite-parity states of Eu2~. Experimentally we show that the intensity of the vibronic lines relative to the zero-phonon line is about equal for both ions, as is to be expected from theory.
1. Introduction In their ground states the Eu2~and Gd3~ions have a 4f7 configuration. The ground state is 8S states originate from the 77/2. andThe thelower 4f65dexcited configurations. The position of 4f 4f65d configuration is strongly dependent of the the host lattice. For Gd3~this position is at much higher energy than for Eu2~for Gd3~the lowest excited state originates always from the 4f7 configuration, for Eu2~this is not always the case. If the 4f65d configuration of Eu2~ is at low energy, only the band emission of the parity al65d —~8S lowed 4f 7/2transition will be 2~ observed, [1]. In This is the case in, e.g., BaFBr:Eu compounds which center gravity the 4f65d levels in is at highthe energy andofthe crystaloffield splitting is small, the lowest excited state of the Eu2~ ion is the 6P 7/2 term level [21. At low temperatures emission will 6P 8S then occur by the parity-forbidden 7/2 —‘ 7/2 several transition. This emission has been studied in Eu2~systems [3—6]. The Gd3~emission consists of sharp zero-phonon lines and weak vibronic lines. Weak, in this sense, means that the ratio r of the integrated intensity of the vibronic lines to the integrated 0022-2313/92/$05.OO © 1992
—
intensity of the zero-phonon lines is of the order of 0.1—0.01 [7]. An analysis of the ratio r was given in ref. [81for Gd3~ in several compounds. It should be realized, however, that the value of r is notprobability absolute, since thefrom purely electronictotransition differs compound cornpound. To obtain absolute (purely) electronic and vibronic) transition probabilities, decay measurements are necessary in addition to the relative intensity measurements. This has been done for Gd3~in different host lattices for the transitions 6P 8S 7/2, 617/2 — 7/2 [9]. The extreme values of the vibronic transition probability A~Ib 6P differ two orders of magnitude, AV~bofLaF the 7/2 ~ transition is 2 s_I forviz. Gd3~in 3, and 114 3~in YOC1 [101. This is ascribed to s~ the for Gd host lattice dependence of the polarizability a of the ligands and the different positions of the optical absorption of the host lattices involved [8,9]. 2~in From literature it isline known that at EuLHeT MFC1 (Mthe Sr, Ba) shows emission [111.The compounds MFC1 (M Sr, Ba) have the PbFCI structure [121just as YOC1 has. This makes MFC1 : Eu2~(M Sr, Ba) not only isoelectronic, but also isostructural with YOC1 : Gd3t It is our aim to compare the zero-phonon and vibronic =
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=
=
284
J. Sytsma, G. Blasse
/ Comparison
transition probabilities of Eu2~in MFC1 (M Sr, Ba) with those found for Gd3~in YOC1 [10]. In order to do so, the 4f7 luminescence of Eu2~in BaFCI and SrFC1 is measured. Absolute transition probabilities of the purely electronic and the vibronic 6P 8S 2~ in —* are 7/2 derived transitions BaFC1 and 7/2 SrFC1 from of theEu spectral data and decay measurements. The transition probabilities found for the Eu2~ compositions are much larger than for Gd3~in YOC1. This is ascribed to the lower position of the 4f65d states of Eu2t =
2. Experimental 2.1. Sample preparation Several authors have published a procedure for the preparation of MFX : Eu2~(M Sr, Ba and X Cl, Br) [13,141. Most of these have in common that the formation of the host lattice is achieved at a low temperature, and the incorporation/reduction of the Eu ion at higher temperatures. We prepared MFX: Eu2~(M Sr, Ba) by mixing stoichiometrically MF 2, MC12 and EuF3. In a reducing atmosphere the composition is heated 4 h to 400°C,fired at that temperature for 4 h and thereafter for 4 h at 900°C. The crystal structure is checked powder The absence of Eu3by + X-ray is checked by diffraction. measuring the diffuse reflection spectrum. The final meas3~ urernents are performed with Eureconcentration too low toonbesamples observed in athe flection spectrum. The Eu2~ concentration was about 0.3 at% in order to avoid energy migration over the Eu2~ions. The samples were stored and measured in a dry atmosphere since the materials are known to be hygroscopic. =
=
2 + and Gd3 ±
of the emission of Eu
ter equipped with two model 1280 double monochromators. The high resolution emission spectra are obtamed after excitation with a frequency-doubled dye laser, pumped by a Nd-YAG laser. For details see ref. [15]. The monochromator is a Spex 1269. During the luminescence measurements the temperature of the sample can be varied between 1.5 and 300 K. FIR experiments have been performed with a Bomem DA3 FTIR instrument at the AKZO Research Laboratories in Arnhem. The set-up consists of a high pressure mercury lamp, 6 and 12 p~mMylar beam splitters and a DTGS detector with a polyethylene window. The powdered sample is measured in the diffuse reflectance mode at a resolution of 8 cm~’. A Rarnan spectrum was measured at the Inorganic Chemistry Laboratory of the University of Amsterdam. The sample is irradiated with a SP 171 Argon laser and the Raman spectrum is recorded with a Jobin-Yvon Ramanor HG2S spectrometer.
=
2.2. Optical instrumentation Diffuse reflection spectra are measured on a Perkin Elmer Lambda 7 spectrometer. Low resolution spectra are measured with a Perkin Elmer MPF 44-B spectrofluorometer, described in detail in ref. [6], and a Spex Fluorolog spectrofluorome-
3. Results 3.1. Low resolution measurements 2~ (M Sr, Ba) Theat compositions Eu at about 27500 show LHeT a lineMFC1: emission cm~’.This emission is ascribed to the 6P 7/2 ~8 S 2~ ion. The excitation 7/2 transition of the Eu spectra of this emission are given in fig. 1. There is a broad excitation region, which ranges from about 28000 to 41 000 cm~ and corresponds to transitions to the 4f65d configuration. The splitting of the 4f6 subconfiguration into seven 7F~ levels can be observed. This shows that the ex=
change interaction between the 4f6 electrons and the Sd electron is small [2]. The theoretical splitting of the 4f6 configuration [161 is indicated in fig. 1 by arrows. The agreement between the positions of the arrows and the maxima in the excitation spectra is satisfying. At room temperature the emission spectrum consists of a broad band. This is ascribed to the
J. Sytsma, G. Blasse
27500
32500
37500
Energy (cm’)
/
2 + and Gd3 +
285
Comparison of the emission of Eu
42500
3
4
6
7
9
10
1000/T (K-’)
--.>
Fig. 1. Excitation spectrum of the 6P 2~in SrFCI (dashed line) and BaFCI line) at liquid 712 (solid ~ emission of Eu helium ting temperature. of the 4f6 configuration The arrowsinindicate the 4f55d theconfiguration. theoretical split-
6P 8S intensity to the integrated 4f65d —~8S Fig. 2. The ratio R of the integrated 7/2 —~ 7/2 line 2~in SrFCI (+) and BaFCI (z~),as a7/2band function intensity of temperafor Eu ture. Fits are to eq. (1).
4f65d —8S 7/2 transition which occursThe dueratio to 65d states. thermal the 4f R of thepopulation integratedof line intensity to the integrated band intensity is given by
R
=
gfAf A exp(~EB_L/kBT).
(1)
ci
Here, A1 is the average radiative transition prob8S ability of the 1 states to the 7/2 ground state, g~ is the degeneracy of the 1 states and L~EB_L is the energy difference between the two emitting levels. By low resolution spectra as measuring a function the of temperature, R emission is deter-
of the dataEu2~in to eq. (1) yields 702 and 592 cm~’for SrFC1 andL~EB_L BaFC1, respectively. The low resolution spectra at LHeT of the 6P 8S 7/2 —* 7/2 emission transition are presented in fig. 3. They consist of a zero-phonon line (mdicated by z.p.) and several, much weaker, vibronic lines. For a discussion of these spectra, see see=
tion 4.
3.2. High resolution measurements 2~ 3.2.1. WithSrFCl:Eu the laser excitation tuned to 35715 cm~
~,
____ I
mined as a function of temperature (fig. 2). A fit
26500
27000
27500
Energy (cm’)
— —.>
28000
the emission spectrum of Eu2~ in SrFC1 is
.
26500
27000
27500
Energy (cm’)
28000
— —.>
Fig. 3. Low resolution emission spectra of the 6P 5S 2~in SrFCI (a) and BaFCI (b) at 4.2 K. The label ‘z.p.’ stands for zero-phonon line. The labels I and7/2 II —~ indicate 7/2 transition vibronic of regions Eu involving one and two phonons, respectively. The zero-phonon lines are off scale.
286
J. Sytsma, G. Blasse
/ Comparison
2 + and Gd3 +
of the emission of Eu
emission with increasing temperature. With increasing a third 5). zero-phonon is observed temperature at 27505 cm~(fig. According line to the
40
site symmetry of the Eu2~ion, viz. C 4~,four lines 6P are expected in the purely electronic 7/2 transition. The total 24 CFinsplitting BaFC1 the is small, CF splitting viz. 23 cm~’(fig. of the 6P 5). For8SEu 7/2 —~ 7/2 transition is 34 cm~ (see below and fig. 7). The third and fourth CF cornponent are very close in that case. Therefore, we assume 6P that the8Sthird and fourth CF component of 2~in the SrFC1 7/2 —are7/2 transition of toozero-phonon close to be resolved. EuThe relative intensity of the lines varies exponentially with temperature as is expected from —+
____________________________ -50
50
150
250
Energy
350
450
(cm’)
Fig.the 4. High emission spectrum vibronic of 6P resolution 8S~~~ transition of Eu2~of in the SrFCI at 4.2lines K. Two zero-phonon 7/2 —* lines are observed at this temperature (extreme left).
the relation ~ recorded at liquid helium temperature (LHeT). Two zero-phonon lines are observed (see fig. 4). The positions of these two lines are 27482 and 27491 cm~’.They are assigned to the 6p~) 2 6P~ and the 2zero-phonon transitions. Here stands nent of the for 6P the j.tth crystal field (CF) compo7/2 term 6P~) level. The small energy 6P~) difference between the 2and the 2CF component (9 cm~1) makes it possible that the 6P~) 2component is thermally populated at LHeT. The integrated intensity ratio of the two zerophonon lines at LHeT equals 0.25. 6p~2) The 2 8S assignment of the second line to the 7/2 transition is confirmed by measuring the ._~
—~
j(~)=
j(I).
E_exp( A~P~
—
z~EOL1)/knT).
(2)
Here, I~ denotes the integrated intensity, and A(~the transition probability of of the the 6P~~ zero-phonon transition. The contribution third and fourth CF component are taken together and will be labeled by p~ (3 + 6P~ 4). ~ and is the energy difference between the 6P~2 the 2CF component. Using eq. (2) the ratio ~ is determined to be 1.58 and the ratio ~ is found to be 1.71. The decay curve at LHeT is exponential and the decay time is T 236.6 ~xs. 8S At lower energy than the 7/2 transition, vibronic lines are observed (fig. 4). The ratio of the integrated vibronic intensity to the zerophonon intensity equals 0.20 ±0.01 at LHeT. We assume that the value of r~ is independent of =
=
—*
__________________________________
/
3&4
the CF components, jx, so that r~ r. We are now able to estimate the zero-phonon transition probabilities and the 6P~ 8S vibronic transition probability of the 7/2 ~ 1, 2, (3 + 4)) transitions from the set of equations =
1
/
/
27460
27480
Energy
27500
(cm’)
—~
=
27520
“>
Fig. 5. High resolution 2~in emission SrFCI atspectrum 80 K. The of the lines are —* labeled transition of Eu with the CF component index ~.c.
T[(A~+A~,)
+exp(~E(21)/kaT)(A~+A~)J r ‘2p xl1+expu~E /k i
B
Tii ,~
(3)
J. Sytsma, G. Blasse
r~ = r
=
/
2 + and Gd3 +
287
Comparison of the emission of Eu
(A~,+ exp(~E~2’~/kBT)A~,) (2)~’
x (A~ + exp(z~Et2l)/kBT)AZP) (4) and the known ratios of A~/A(~and A~3’4~ zp /A~ zp• Nonradiative processes to the ground state can be neglected. The results are presented in table 1. Averaging over the CF component index ~a yields the average values of the zero-phonon /
____________________________ —50
transition probability and the vibronic transition probability of the 6P 8S 2~ —~error 7/2transition of Eu in SrFC1 (table 1). 7/2 The in the transition probabilities is about 10%, and is mainly due to the uncertainty in the relative intensity measurements. We find Af=AZP+AVIb=444l s~.The ratio (g~A~/g~A~) is obtained from the fit to eq. (1). It is not exactly clear which value should be taken for the ratio (g~/g~)[1,17]. Experimental cvidence exists [181 for (gf/gd) 4. The radiative 65d —~8S transition 7/2transition is nowprobability estimated of as the Ad 4f 2.34 x 1O~ ~—1 =
=
3.2.2. BaFC1: Eu2 The results for BaFC1 : Eu2~are quite similar to those of SrFCI:Eu2t The °P~2 8S —* 2 7/2 and 8S the 7/2 transitions are observed at LHeT at 27571 and 27582 cm after excitation +
—*
~,
50
150
Energy
250
350
450
(cm’)
Fig. 6. High resolution emission spectrum of the vibronic lines of the _~8~ transition of Eu2~in BaFCI at 4.2 K. Two zero-phonon lines are observed at this temperature (extreme left).
at 35715 cm~1.The integrated intensity ratio of the two zero-phonon lines at LHeT is 0.01. The energy difference between the °P~,? 2 6).the 1(Fig.and CFTo components is only 11 C~components cm~ observe the other of the op 7/2 term level, the emission spectra are measured as a function of temperature. Figure 7 shows the ~ 8S 7/2 emission lines at 70 K for cxcitation at 35715 cm’. Four CF components are observed. The positions of the CF components are given in table 1. The total CF splitting is only
Table 1 2~6P 6P Data on the Eu 7,,2 emission in SrFCI and BaFCI: positions of the crystal field components of the 7/2 term level, the ratio r of the integrated vibronic intensity to the integrated zero-phonon line intensity, and the radiative transition probabilities, separated into ~ and AVIb, at 4.2 K. The error in the transition probabilities is about 10% and is mainly due to errors in the relative intensity measurements. See also text. 2~ BaFCI: Eu2~ YOCI: Gd3~ Sample: SrFCl: Eu (in cm~1) 702 592 ~35000 27482 27571 6P~,~~(incm1) ~=2 27491 27582 (31934)~ ~=(3+4) 27505 27605 r~ = r 0.20 0.16 0.23 i- (in ~is) 236 240 1200 = 1 3449 3592 A~(ins~) ~.c=2 5449 3919 ~=(3+4) 5907 6499 690 575 A~j~(ins~) is=2 1090 627 1181 1240 A,~(ins~) 3701 3503 494 AVIb(ins~) 740 561 114
288
J. Sytsma, G. Blasse
/
2 ± and Gd3 ±
Comparison of the emission of Eu
component index, we can calculate A~. These are tabulated in table 1 together with the values of the zero-phonon and vibronic transition probabilities averaged over the crystal field components, A, 0 and AVIb. The error in the transition probabilities is again about 10%. With AfAZ0 +AVjb 4064 s~for 6P the radiative transition probability of the 7/2 transition, we find a~ (gf/gd)~4.08 X 10~s~, using the results of the fit to eq. (1) and (gf/gd) 47s~. (see above). With this we find A~ 1.63 x 10
1
4-
=
—
=
27560
27580
27600
27620
=
Energy Fig. 7. High resolution emission(ciii’) spectrum --.> of the 6P~ —~
=
transition of Eu2~ in BaFCI at 70 K. The lines are labeled with the CF component index j.c.
4. Discussion 34 cm~ the CF components 3 and 4 are very close. The contribution of these CF components are taken together, since these lines could not be resolved totally. The decay curve of the emission are exponential. The decay time at LHeT is r 240 p~s. At energies lower than the 6P~) 2~~*8S7/2 zero-phonon transition, many vibronic lines are observed (fig. 6). Assignment of the parity of the vibrations is possible with the Raman IR 2~ given in fig. and 8. The spectrum BaFC10.16 : Eu±0.01. This yields A~ value of r of amounts 3592 s_i and A~ 575 s_i for the first CF component. Using eq. (2) we obtain A~ 3919 and A~4~ 6499 s1. With the assumption that the ratio r~ is independent of the CF
In this section we consider the results presented table 1 for andYOC1 make : aGd3~[10]. comparison with the results in obtained 4.1. Theory
=
=
=
=
=
To interpret the results, 6P we first 8S develop an expression for r for the 7/2 7/2 transition, using existing theories describing vibronic and zero-phonon transition of rare-earth ions. The ratio of the probabilities total integrated vibronic intensity to total integrated zero-phonon intensity can be expressed as ‘~b AV~b r ~. (5) A, 0 —‘
=
—
=
‘ZP
a
4-
_____
~ —50
50
150
250
350
450
—50
50
Energy (ciii’)
150
2Energy ~.
Fig. 8. Raman (a) and infrared (b) spectra of BaFCI : Eu
250
(cxxi’)
350
450
J. Sytsma, G. Blasse
/ Comparison of the
The vibronic process can be separated into two processes [9,19], the M process and the ~ process. In the M process an infrared (IR)-active vibrational mode mixes opposite-parity wave functions into the wave functions of the 4ffl term levels. The ~ process is equal to the classical one-phonon replica (Frank—Condon approximalion). Theoretical calculations on the M process have been made by Stavola et al. [20], Richardson [21] and Judd [22]. The transition probability for the transition from the initial vibrational state <0 I to the final vibrational state p) due to the M process can be written as [22,9] 4’rr 1 AVIbM Process,p
0
=
289
the integrated zero-phonon intensity is given by [23] .~
‘~b.i
=
e
I
S
I,~
(9)
i!
where S is the Huang—Rhys factor. The purely electronic transition probability can be written as A =AED +AMD (10 ZP
ZP
‘
ZP
where A,~ (A~) stands for the electric (magnetic)-dipole transition probability. The electric-dipole transition probability is cxpressed in1 terms of the Judd—Ofelt theory [26,27]. ED 64’rr4e2p3 A, 3 0 4~e~ 3hc
2 32 3 (2J + 1) 64~eii nX 3hc
(U(2))2
2 + and Gd3 +
emission of Eu
—
—
2
R6
X
~ (2J+1)
u1~(U~),
(11)
A=2,4,6
P, x P~-.E~(1, 2)
+
,
(6)
where
with the Judd—Ofelt parameter ~ ~
A)[B,+Bfl.
5_(2,k+1)E(2t+1)~(t,
4 g+— Na R3
P..=20~e
2
(7)
a2~.
(8)
and 10976 =
defined by
15
2
(1
—
02)
Here, is the frequency of the zero-phonon transition, n~ is the local field correction term, N is the number of ligands surrounding the metal ion, a the polarizability of the ligands, R is the metal ion to ligand distance, (1 72)2 is a screening factor and
. (12) Here [B, + Bfl is defined by eqs. (18) and (19) in ref. [26] and is a measure of the strength of the crystal field. For our purpose it is sufficient to take in the
summation of eq. (12) only t 1, describing the parity mixing. The term E(1, A) is defined by =
ii
—
=
E( 1, A)
=
2 ~
{
(n’l’)
A l
1 3
I l’Xl’ I ~ <4f I r I n’l’Xn’l’ r X ~ x
1 ~
I f) 14f) .
(13)
The summation in eq. (13) runs over all values of n’ and I’ where 4f6(n’l’) is an excited configuration. ~E(4O_~~’t’)is the energy difference between the 4f6 configuration and the (n’l’) configuration. The reduced matrix elements (U’~) are tabulated for many transitions in the rate earths ions by Carnall et al. [28].
290
1 Sytsma, G. Blasse
2 ± and Gd3 ±
/
Comparison of the emission of Eu
The ratio of the total integrated vibronic intensity to the total integrated zero-phonon intensity can now be expressed as ‘vib
AvibMprocessp
r=—=E zp
+
eS ~,
.
(14)
.
r on the energy difference ~ occurring in eq. (15) via the term E2(1 2), vanishes. The other terms in eq. (15) will not differ much for the compositions MFC1 (M Sr, Ba) and YOC1. The value of S is almost the same for =
A,
~
0
all these compositions (see below). This implies
t!
Before proceeding we note the following. From the tables by Detrio [25] and Carnall 8S [28] it becomes clear that for the °P7/2— 7/2 transition of Gd3~only the term with A 2 gives a significant contribution to the electric-dipole transition probability. This is also true for the °P 7/2 8~7/2 2t The value for the magnetic-ditransition of Eu pole transition probability of the 6P 7/2 3~is A~D 122 s [10,28]. The transition of Gd value of A~,Dfor the 6P 8S 7/2 order7,~2transition of 2~ will be of the same of magnitude. Eu Experimental from the casewith of 2~in LiBaF evidence follows 2~occupies a site Eu 3, where inversion symmetry. TheEu tip 8S 7~transition is due toprocess. the magnetic-dipole vibronic The decay timeinteraction measured and for this transition is 2.5 ms [29].The relative intensity of the vibronic lines to the zero-phonon lines is ‘large’ [29]. Thus A~,D(°P 8S 2~)< 7/2 7,,2, Eu 250 s—’. The total zero-phonon transition probability of the 6P 8S 3~in YOC1 is 7/2 7~2transition of Gd A, 6P 8S 0 2~in 494MFCI s~. (M For the 7/2 the —* values 7/2 transition of Sr, Ba) of A,~are Eu about a factor6P7—8 larger. 8S From this we conclude that for the 7/2 —~ 7/2transition in approximation A,0 in eq. (14) can be replaced by A~. Doing so and taking only terms with A 2, we find 2 p~ r= ~ 15R6[B 4E2(1, (T”)) 1 +Bf] R =
._~
that r in first approximation will have the same numerical value for the 2~ in MFC1 (M corresponding Sr, Ba) andtransitions Gd3~ in of Eu YOCI. This is in good agreement with our experiments (see table 1). =
4.2. Discussion of the results
—‘
=
—~
picture of the experiments is clear. TheThe linegeneral emission is ascribed to the 6P 8S 7/2 —* 7/2 transition the broad band emission, occurring at higher and temperatures, is ascribed to the 4f65d
—*
—~
~ 7/2 transition. This with [4— the 2~isincomparable other systems results 6,11,17].found for Eu The energy differences ~EB_L are not in agreement with the values found by Sommerdijk and Bril [30]. They have, however, only measured the value of R at 77 and 300 K. This makes their data less reliable. It was not possible to measure ~EB_L directly from an emission and an excita-
—~
=
=
=
[~~+
+
~
.
.
2)1
(15)
This equation shows that r is independent of so that r is independent of the position of the corresponding zero-phonon transition. The term with P~ is only of importance if a large amount of covalency is present [22]. If this term is neglected in first approximation, the dependence of v,
tion spectrum, because line it was not4f65d possible observe a zero-phonon in the —~8Sto 7/2 transition. 6P 8S The 7/2 — 7/2 transition consists of a zero-phonon line and several vibronic lines (figs. 3, 4 and 6). Assignment of the vibronic lines is possible with the Raman and JR spectra of BaFCI vibronic as given in spectrum, fig. 8. The except JR spectrum for the resembles fact that the vibrational peaks are shifted towards higher energy, the more so the higher their energy. The same holds for a comparison using the SrFC1 IR spectrum. The vibronic lines in figs. 3, 4 and 6 thus originate from local modes. These spectra show that the maximum energy is at about 300 cm The vibronic lines in region II in fig. 3 are hence ascribed to vibronic transitions involving two phonons. The largest energy difference between the zero-phonon line and the vibronic lines
I Sytsma, G. Blasse
/
2 + and Gd3 +
291
Comparison of the emission of Eu
in this region amounts to 679 and 575 cm1 for SrFCI : Eu2~ and BaFCI : Eu2~,respectively. For region I these values are 320 and 291 cm_i, respectively. A comparison with the vibrational spectra learns that the vibronics in region I are due to coupling with the highest IR-active vibrations. These in region II are due to coupling with two of these vibrations. This excludes the M process for 2~ the lines 679 and 575 cm Therefore, in SrFC1 : Eu and BaFC1at : Eu2~,respectively. these lines are due to the ~ process. The relative intensity of these vibronic lines must obey eq. (9). From this equation the Huang—Rhys factor is derived: S 0.11 for Eu2~in SrFC1 as well as in BaFC1. This value is very close to the value observed for Gd3 in YOC1. There the value of S was obtained in a completely different way, viz, from the vibronic intensity in the 617/2 _~*8S7/2 21)2 for that transition transition. The value of (Ut is negligibly small [251,so that according to eq. (6) no vibronic lines are induced by the M process in this transition. The value the same for Eu2~in MFC1of(MS isSr,thus Ba) about and Gd3~in YOCI. From the discussion above it follows that r may =
+
=
be somewhat larger than r(MFC1 : Eu2 ~) in agreement with our observations. The transition probabilities for Eu2~in MFCI (M Sr, Ba) are all much larger than for Gd3~in YOC1 (table 1). The zero-phonon transition probabilities differ by a factor 7—8, the vibronic tradition probabilities by a factor 5. The differences in ~ are due to differences in A~ (see above). For the 6P 8S 7/2 only7,,.2 is given by eq. (11), with thetransition, term A A~ 2 being significant. Unfortunately, the value of (U~2~) is not known for the 6P 2~, 7/2much ~ 7/2 but it will not differ fromtransition the value of of Eu (Ut2~) for the 6P 8S 3t There7/2 —* 7/2 transition of Gd that the fore, we conclude from the experiments difference in A,p(6P 8S 2~ and 7/S) for Eu of the Gd3 originates from7/2—* the different values energy difference to the first opposite-parity state ~iE4f_5d, which appears in the expression for A~ 0 via the Judd-Ofelt parameter Q 2. The linear relation between AV~band A,0 expressed in eq. (5) and the particular values of r now6Pprovide8Sthe same argument to explain 2~)>AVIb(6P 8S why AVLb( 3~). 7/2 —~ 7/2, Eu In conclusion 7/2-_* 7/2,Gd we have measured the transition probabilities of the 6P 8S —* They 7/2 transition of 2~in MFC1 (M Sr,7/2 Ba). are all much Eu larger than the corresponding values found for Gd3~in YOC1. This is explicitly shown to be due to the lower position of the opposite-parity states of Eu2t Experimentally we have shown that the relative intensity of vibronic lines to zero-phonon line does not differ much, as is to be expected from existing theories. =
—*
=
+
=
not much for This corresponding in thesediffer compositions. agrees withtransitions the experimental results: the value of r 0.16, 0.20 and 0.23 for the 6P 2~ in 7/2 —~~~7/2 of Eu SrFCI, BaFC1 and Gd3~in transition YOC1, respectively, The other vibronic lines in region II are hard to assign with reasonable accuracy. However, within the experimental error region II is obtamed from region I by subtracting the highest vibrational frequency (320 and 291 cm~ for SrFCI and BaFC1, respectively). This suggests that the excited state of Eu2~is distorted. As noted above, the contribution of P 1~, will increase as the amount of covalency increases, The presence of oxygen in YOC1 makes this composition more covalent3~) than MFC1 (M Sr, > P~(MFC1 : Eu2~). Ba). term Thuscontaining P~(YOClP: Gd The 2(1, in eq. contains E to 2) in the denominator; 1~, E2(1 2) 15 is proportional the inverse of ~E41~”. The larger energy difference L~1E4~5dfor Gd3~enhances the contribution of P~,so that r(YOCI : Gd3~)is expected to =
=
Acknowledgements The investigations were partly supported by the Netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Foundation Pure Research (NWO) andfortheAdvancement Netherlands ofFoundation for Technical Research (STW). We wish to thank Dr. D.J. Stufkens (University of Amsterdam) for measuring the Raman spectrum, and Drs. M.G.J. van Leur (AKZO Research Laboratories, Amhem) for measuring the JR spectrum. The assist-
292
1 Sytsma, G. Blasse
/ Comparison
ance of E.Th.G. Lutz to obtain these spectra is greatfully acknowledged.
Notes added in proof
2 ± and Gd3 +
of the emission of Eu
[9] J. Sytsma, W. van Schaik and G. Blasse, J. Phys. Chem. Solids 52 (1991) 419. [10] J. Sytsma, G.F. Imbusch and G. Blasse, J. Chem. Phys. 91 (1989) 1456. An erratum to this paper: J. Chem. Phys. 92 (1990) 3249. [11] J.L. Sommerdijk, iMP.). Verstegen and A. Bril, J. Lumm. 8 (1974) 502. [12] M. Sauvage, Acta Cryst. B 30 (1974) 2789.
Recent experiments on Eu2~in LiBaF AMDI6D ~~zp ~ ‘7/2
__
8c~
‘~
~7/2’
r~ 2+\
~u
,
._. —
inn
S
(A
-~ .
.
3 give iA eye-
rink et al., to be published.) The assumption 6P made in section 7.4.2 that (Ut21) for the 7/22~and ~ ~ Gd3~is transition will not confirmed by differ much for Eu the explicit calculations by Downer of the (U ) values between the ground state and the excited states for Eu2~ and for Gd3~ (M.C. Downer, Ph.D. Thesis, Harvard (1983)). (A)
References [1] J.P. Spoonhower and M.S. Burberry, J. Lumin. 43 (1989) 221. [2] F.M. Ryan, W. Lehmann, D.W. Feldman and J. Murphy, J. Electrochem. Soc. 121 (1974) 1475. [3] M.V. Hoffmann, J. Electrochem. Soc. 118 (1971) 933. [4] J.L. Sommerdijk and A. Bril, J. Lumin. 11(1976) 363. [5] J.L. Sommerdijk, P. Vries and A. Bril, Philips. J. Res. 33 (1978) 117. [6] A. Meyerink, J. Nuyten and G. Blasse, J. Lumin. 44 (1989) 19. [7] G. Blasse, J. Sytsma and L.H. Brixner, Chem. Phys. Lett. 155 (1989) 64. [8] G. Blasse and L.H. Brixner, Inorg. Chim. Acta 169 (1990) 25.
[13] A.L.N. Stevels and F. Pingault, Philips. Res. Reports 30 (1975) 227. [14] H.P. Beck, J. Solid State Chem. 17 (1976) 275. [15] A.J. de Vries, M.F. Hazenkamp and G. Blasse, J. Lumin. 42 (1988) 275. [16] N.C. 3227. Chang and J.B. Gruber, J. Chem. Phys. 41(1964) [17] R.A. Hewes and M.V. Hoffmann, J. Lumin. 3 (1971) 261. [18] L.H. Brixner J.D. Bierlein and V. Johnson, Current Topics Mat. Sci. 4 (1980) 47. [19] J. Dexpert-Ghys and F. Auzel, J. Chem. Phys. 80 (1984) 4003. [20] M. Stavola and D.L. Dexter, Phys. Rev. B 20 (1979) 1867. [21] T.R. Faulkner and F.S. Richardson, Mol. Phys. 35 (1978) 1141. [22] BR. Judd, Physica Scripta 21(1980) 543. [23] B. Henderson and G.F. Imbusch Optical spectroscopy of inorganic solids (Oxford University Press, Oxford, 1989). [24] W.T. Carnall, P.R. Fields and K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [25] J.A. Detrio, Phys. Rev. B 4 (1971) 1422. [26] B.R. Judd, Phys. Rev. 127 (1962) 750. [27] G.S. Ofelt, J. Chem. Phys. 37 (1962) 339. [28] W.T. Carnall, H. Crosswhite and H.M. Crosswhite, in: Energy level structure and transition probabilities of the trivalent Lanthanides in LaF 3, Argonne National Laboratory Report (1977). [29] N.S. Al’tshuler, S.L. Korableva, L.D. Livanova and AL. Stolov, Soy. Phys. Solid State 15 (1974) 2155. [30] Some values of the transition probabilities in their tables ‘Results’ are not reproducible with their very own data mentioned in their text.
Journal of Luminescence 51(1992) 283—292 North-Holland
Comparison of the emission of Eu2~in MFC1 (M and Gd3~in YOC1
=
Sr, Ba)
J. Sytsma and G. Blasse Debye Research Institute, University of Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands Received 7 August 1991 Revised 13 November 1991 Accepted 13 November 1991
Transition probabilities of the 6P 8S 2~in MFCI (M = Sr, Ba) are reported, and compared with values found for Gd3~ in YOCI. The 7/2 transition —‘ 7/2 transition probabilities of Eu are larger for Eu2~.This is ascribed to the lower position of the opposite-parity states of Eu2~. Experimentally we show that the intensity of the vibronic lines relative to the zero-phonon line is about equal for both ions, as is to be expected from theory.
1. Introduction In their ground states the Eu2~and Gd3~ions have a 4f7 configuration. The ground state is 8S states originate from the 77/2. andThe thelower 4f65dexcited configurations. The position of 4f 4f65d configuration is strongly dependent of the the host lattice. For Gd3~this position is at much higher energy than for Eu2~for Gd3~the lowest excited state originates always from the 4f7 configuration, for Eu2~this is not always the case. If the 4f65d configuration of Eu2~ is at low energy, only the band emission of the parity al65d —~8S lowed 4f 7/2transition will be 2~ observed, [1]. In This is the case in, e.g., BaFBr:Eu compounds which center gravity the 4f65d levels in is at highthe energy andofthe crystaloffield splitting is small, the lowest excited state of the Eu2~ ion is the 6P 7/2 term level [21. At low temperatures emission will 6P 8S then occur by the parity-forbidden 7/2 —‘ 7/2 several transition. This emission has been studied in Eu2~systems [3—6]. The Gd3~emission consists of sharp zero-phonon lines and weak vibronic lines. Weak, in this sense, means that the ratio r of the integrated intensity of the vibronic lines to the integrated 0022-2313/92/$05.OO © 1992
—
intensity of the zero-phonon lines is of the order of 0.1—0.01 [7]. An analysis of the ratio r was given in ref. [81for Gd3~ in several compounds. It should be realized, however, that the value of r is notprobability absolute, since thefrom purely electronictotransition differs compound cornpound. To obtain absolute (purely) electronic and vibronic) transition probabilities, decay measurements are necessary in addition to the relative intensity measurements. This has been done for Gd3~in different host lattices for the transitions 6P 8S 7/2, 617/2 — 7/2 [9]. The extreme values of the vibronic transition probability A~Ib 6P differ two orders of magnitude, AV~bofLaF the 7/2 ~ transition is 2 s_I forviz. Gd3~in 3, and 114 3~in YOC1 [101. This is ascribed to s~ the for Gd host lattice dependence of the polarizability a of the ligands and the different positions of the optical absorption of the host lattices involved [8,9]. 2~in From literature it isline known that at EuLHeT MFC1 (Mthe Sr, Ba) shows emission [111.The compounds MFC1 (M Sr, Ba) have the PbFCI structure [121just as YOC1 has. This makes MFC1 : Eu2~(M Sr, Ba) not only isoelectronic, but also isostructural with YOC1 : Gd3t It is our aim to compare the zero-phonon and vibronic =
Elsevier Science Publishers B.V. All rights reserved
=
=
284
J. Sytsma, G. Blasse
/ Comparison
transition probabilities of Eu2~in MFC1 (M Sr, Ba) with those found for Gd3~in YOC1 [10]. In order to do so, the 4f7 luminescence of Eu2~in BaFCI and SrFC1 is measured. Absolute transition probabilities of the purely electronic and the vibronic 6P 8S 2~ in —* are 7/2 derived transitions BaFC1 and 7/2 SrFC1 from of theEu spectral data and decay measurements. The transition probabilities found for the Eu2~ compositions are much larger than for Gd3~in YOC1. This is ascribed to the lower position of the 4f65d states of Eu2t =
2. Experimental 2.1. Sample preparation Several authors have published a procedure for the preparation of MFX : Eu2~(M Sr, Ba and X Cl, Br) [13,141. Most of these have in common that the formation of the host lattice is achieved at a low temperature, and the incorporation/reduction of the Eu ion at higher temperatures. We prepared MFX: Eu2~(M Sr, Ba) by mixing stoichiometrically MF 2, MC12 and EuF3. In a reducing atmosphere the composition is heated 4 h to 400°C,fired at that temperature for 4 h and thereafter for 4 h at 900°C. The crystal structure is checked powder The absence of Eu3by + X-ray is checked by diffraction. measuring the diffuse reflection spectrum. The final meas3~ urernents are performed with Eureconcentration too low toonbesamples observed in athe flection spectrum. The Eu2~ concentration was about 0.3 at% in order to avoid energy migration over the Eu2~ions. The samples were stored and measured in a dry atmosphere since the materials are known to be hygroscopic. =
=
2 + and Gd3 ±
of the emission of Eu
ter equipped with two model 1280 double monochromators. The high resolution emission spectra are obtamed after excitation with a frequency-doubled dye laser, pumped by a Nd-YAG laser. For details see ref. [15]. The monochromator is a Spex 1269. During the luminescence measurements the temperature of the sample can be varied between 1.5 and 300 K. FIR experiments have been performed with a Bomem DA3 FTIR instrument at the AKZO Research Laboratories in Arnhem. The set-up consists of a high pressure mercury lamp, 6 and 12 p~mMylar beam splitters and a DTGS detector with a polyethylene window. The powdered sample is measured in the diffuse reflectance mode at a resolution of 8 cm~’. A Rarnan spectrum was measured at the Inorganic Chemistry Laboratory of the University of Amsterdam. The sample is irradiated with a SP 171 Argon laser and the Raman spectrum is recorded with a Jobin-Yvon Ramanor HG2S spectrometer.
=
2.2. Optical instrumentation Diffuse reflection spectra are measured on a Perkin Elmer Lambda 7 spectrometer. Low resolution spectra are measured with a Perkin Elmer MPF 44-B spectrofluorometer, described in detail in ref. [6], and a Spex Fluorolog spectrofluorome-
3. Results 3.1. Low resolution measurements 2~ (M Sr, Ba) Theat compositions Eu at about 27500 show LHeT a lineMFC1: emission cm~’.This emission is ascribed to the 6P 7/2 ~8 S 2~ ion. The excitation 7/2 transition of the Eu spectra of this emission are given in fig. 1. There is a broad excitation region, which ranges from about 28000 to 41 000 cm~ and corresponds to transitions to the 4f65d configuration. The splitting of the 4f6 subconfiguration into seven 7F~ levels can be observed. This shows that the ex=
change interaction between the 4f6 electrons and the Sd electron is small [2]. The theoretical splitting of the 4f6 configuration [161 is indicated in fig. 1 by arrows. The agreement between the positions of the arrows and the maxima in the excitation spectra is satisfying. At room temperature the emission spectrum consists of a broad band. This is ascribed to the
J. Sytsma, G. Blasse
27500
32500
37500
Energy (cm’)
/
2 + and Gd3 +
285
Comparison of the emission of Eu
42500
3
4
6
7
9
10
1000/T (K-’)
--.>
Fig. 1. Excitation spectrum of the 6P 2~in SrFCI (dashed line) and BaFCI line) at liquid 712 (solid ~ emission of Eu helium ting temperature. of the 4f6 configuration The arrowsinindicate the 4f55d theconfiguration. theoretical split-
6P 8S intensity to the integrated 4f65d —~8S Fig. 2. The ratio R of the integrated 7/2 —~ 7/2 line 2~in SrFCI (+) and BaFCI (z~),as a7/2band function intensity of temperafor Eu ture. Fits are to eq. (1).
4f65d —8S 7/2 transition which occursThe dueratio to 65d states. thermal the 4f R of thepopulation integratedof line intensity to the integrated band intensity is given by
R
=
gfAf A exp(~EB_L/kBT).
(1)
ci
Here, A1 is the average radiative transition prob8S ability of the 1 states to the 7/2 ground state, g~ is the degeneracy of the 1 states and L~EB_L is the energy difference between the two emitting levels. By low resolution spectra as measuring a function the of temperature, R emission is deter-
of the dataEu2~in to eq. (1) yields 702 and 592 cm~’for SrFC1 andL~EB_L BaFC1, respectively. The low resolution spectra at LHeT of the 6P 8S 7/2 —* 7/2 emission transition are presented in fig. 3. They consist of a zero-phonon line (mdicated by z.p.) and several, much weaker, vibronic lines. For a discussion of these spectra, see see=
tion 4.
3.2. High resolution measurements 2~ 3.2.1. WithSrFCl:Eu the laser excitation tuned to 35715 cm~
~,
____ I
mined as a function of temperature (fig. 2). A fit
26500
27000
27500
Energy (cm’)
— —.>
28000
the emission spectrum of Eu2~ in SrFC1 is
.
26500
27000
27500
Energy (cm’)
28000
— —.>
Fig. 3. Low resolution emission spectra of the 6P 5S 2~in SrFCI (a) and BaFCI (b) at 4.2 K. The label ‘z.p.’ stands for zero-phonon line. The labels I and7/2 II —~ indicate 7/2 transition vibronic of regions Eu involving one and two phonons, respectively. The zero-phonon lines are off scale.
286
J. Sytsma, G. Blasse
/ Comparison
2 + and Gd3 +
of the emission of Eu
emission with increasing temperature. With increasing a third 5). zero-phonon is observed temperature at 27505 cm~(fig. According line to the
40
site symmetry of the Eu2~ion, viz. C 4~,four lines 6P are expected in the purely electronic 7/2 transition. The total 24 CFinsplitting BaFC1 the is small, CF splitting viz. 23 cm~’(fig. of the 6P 5). For8SEu 7/2 —~ 7/2 transition is 34 cm~ (see below and fig. 7). The third and fourth CF cornponent are very close in that case. Therefore, we assume 6P that the8Sthird and fourth CF component of 2~in the SrFC1 7/2 —are7/2 transition of toozero-phonon close to be resolved. EuThe relative intensity of the lines varies exponentially with temperature as is expected from —+
____________________________ -50
50
150
250
Energy
350
450
(cm’)
Fig.the 4. High emission spectrum vibronic of 6P resolution 8S~~~ transition of Eu2~of in the SrFCI at 4.2lines K. Two zero-phonon 7/2 —* lines are observed at this temperature (extreme left).
the relation ~ recorded at liquid helium temperature (LHeT). Two zero-phonon lines are observed (see fig. 4). The positions of these two lines are 27482 and 27491 cm~’.They are assigned to the 6p~) 2 6P~ and the 2zero-phonon transitions. Here stands nent of the for 6P the j.tth crystal field (CF) compo7/2 term 6P~) level. The small energy 6P~) difference between the 2and the 2CF component (9 cm~1) makes it possible that the 6P~) 2component is thermally populated at LHeT. The integrated intensity ratio of the two zerophonon lines at LHeT equals 0.25. 6p~2) The 2 8S assignment of the second line to the 7/2 transition is confirmed by measuring the ._~
—~
j(~)=
j(I).
E_exp( A~P~
—
z~EOL1)/knT).
(2)
Here, I~ denotes the integrated intensity, and A(~the transition probability of of the the 6P~~ zero-phonon transition. The contribution third and fourth CF component are taken together and will be labeled by p~ (3 + 6P~ 4). ~ and is the energy difference between the 6P~2 the 2CF component. Using eq. (2) the ratio ~ is determined to be 1.58 and the ratio ~ is found to be 1.71. The decay curve at LHeT is exponential and the decay time is T 236.6 ~xs. 8S At lower energy than the 7/2 transition, vibronic lines are observed (fig. 4). The ratio of the integrated vibronic intensity to the zerophonon intensity equals 0.20 ±0.01 at LHeT. We assume that the value of r~ is independent of =
=
—*
__________________________________
/
3&4
the CF components, jx, so that r~ r. We are now able to estimate the zero-phonon transition probabilities and the 6P~ 8S vibronic transition probability of the 7/2 ~ 1, 2, (3 + 4)) transitions from the set of equations =
1
/
/
27460
27480
Energy
27500
(cm’)
—~
=
27520
“>
Fig. 5. High resolution 2~in emission SrFCI atspectrum 80 K. The of the lines are —* labeled transition of Eu with the CF component index ~.c.
T[(A~+A~,)
+exp(~E(21)/kaT)(A~+A~)J r ‘2p xl1+expu~E /k i
B
Tii ,~
(3)
J. Sytsma, G. Blasse
r~ = r
=
/
2 + and Gd3 +
287
Comparison of the emission of Eu
(A~,+ exp(~E~2’~/kBT)A~,) (2)~’
x (A~ + exp(z~Et2l)/kBT)AZP) (4) and the known ratios of A~/A(~and A~3’4~ zp /A~ zp• Nonradiative processes to the ground state can be neglected. The results are presented in table 1. Averaging over the CF component index ~a yields the average values of the zero-phonon /
____________________________ —50
transition probability and the vibronic transition probability of the 6P 8S 2~ —~error 7/2transition of Eu in SrFC1 (table 1). 7/2 The in the transition probabilities is about 10%, and is mainly due to the uncertainty in the relative intensity measurements. We find Af=AZP+AVIb=444l s~.The ratio (g~A~/g~A~) is obtained from the fit to eq. (1). It is not exactly clear which value should be taken for the ratio (g~/g~)[1,17]. Experimental cvidence exists [181 for (gf/gd) 4. The radiative 65d —~8S transition 7/2transition is nowprobability estimated of as the Ad 4f 2.34 x 1O~ ~—1 =
=
3.2.2. BaFC1: Eu2 The results for BaFC1 : Eu2~are quite similar to those of SrFCI:Eu2t The °P~2 8S —* 2 7/2 and 8S the 7/2 transitions are observed at LHeT at 27571 and 27582 cm after excitation +
—*
~,
50
150
Energy
250
350
450
(cm’)
Fig. 6. High resolution emission spectrum of the vibronic lines of the _~8~ transition of Eu2~in BaFCI at 4.2 K. Two zero-phonon lines are observed at this temperature (extreme left).
at 35715 cm~1.The integrated intensity ratio of the two zero-phonon lines at LHeT is 0.01. The energy difference between the °P~,? 2 6).the 1(Fig.and CFTo components is only 11 C~components cm~ observe the other of the op 7/2 term level, the emission spectra are measured as a function of temperature. Figure 7 shows the ~ 8S 7/2 emission lines at 70 K for cxcitation at 35715 cm’. Four CF components are observed. The positions of the CF components are given in table 1. The total CF splitting is only
Table 1 2~6P 6P Data on the Eu 7,,2 emission in SrFCI and BaFCI: positions of the crystal field components of the 7/2 term level, the ratio r of the integrated vibronic intensity to the integrated zero-phonon line intensity, and the radiative transition probabilities, separated into ~ and AVIb, at 4.2 K. The error in the transition probabilities is about 10% and is mainly due to errors in the relative intensity measurements. See also text. 2~ BaFCI: Eu2~ YOCI: Gd3~ Sample: SrFCl: Eu (in cm~1) 702 592 ~35000 27482 27571 6P~,~~(incm1) ~=2 27491 27582 (31934)~ ~=(3+4) 27505 27605 r~ = r 0.20 0.16 0.23 i- (in ~is) 236 240 1200 = 1 3449 3592 A~(ins~) ~.c=2 5449 3919 ~=(3+4) 5907 6499 690 575 A~j~(ins~) is=2 1090 627 1181 1240 A,~(ins~) 3701 3503 494 AVIb(ins~) 740 561 114
288
J. Sytsma, G. Blasse
/
2 ± and Gd3 ±
Comparison of the emission of Eu
component index, we can calculate A~. These are tabulated in table 1 together with the values of the zero-phonon and vibronic transition probabilities averaged over the crystal field components, A, 0 and AVIb. The error in the transition probabilities is again about 10%. With AfAZ0 +AVjb 4064 s~for 6P the radiative transition probability of the 7/2 transition, we find a~ (gf/gd)~4.08 X 10~s~, using the results of the fit to eq. (1) and (gf/gd) 47s~. (see above). With this we find A~ 1.63 x 10
1
4-
=
—
=
27560
27580
27600
27620
=
Energy Fig. 7. High resolution emission(ciii’) spectrum --.> of the 6P~ —~
=
transition of Eu2~ in BaFCI at 70 K. The lines are labeled with the CF component index j.c.
4. Discussion 34 cm~ the CF components 3 and 4 are very close. The contribution of these CF components are taken together, since these lines could not be resolved totally. The decay curve of the emission are exponential. The decay time at LHeT is r 240 p~s. At energies lower than the 6P~) 2~~*8S7/2 zero-phonon transition, many vibronic lines are observed (fig. 6). Assignment of the parity of the vibrations is possible with the Raman IR 2~ given in fig. and 8. The spectrum BaFC10.16 : Eu±0.01. This yields A~ value of r of amounts 3592 s_i and A~ 575 s_i for the first CF component. Using eq. (2) we obtain A~ 3919 and A~4~ 6499 s1. With the assumption that the ratio r~ is independent of the CF
In this section we consider the results presented table 1 for andYOC1 make : aGd3~[10]. comparison with the results in obtained 4.1. Theory
=
=
=
=
=
To interpret the results, 6P we first 8S develop an expression for r for the 7/2 7/2 transition, using existing theories describing vibronic and zero-phonon transition of rare-earth ions. The ratio of the probabilities total integrated vibronic intensity to total integrated zero-phonon intensity can be expressed as ‘~b AV~b r ~. (5) A, 0 —‘
=
—
=
‘ZP
a
4-
_____
~ —50
50
150
250
350
450
—50
50
Energy (ciii’)
150
2Energy ~.
Fig. 8. Raman (a) and infrared (b) spectra of BaFCI : Eu
250
(cxxi’)
350
450
J. Sytsma, G. Blasse
/ Comparison of the
The vibronic process can be separated into two processes [9,19], the M process and the ~ process. In the M process an infrared (IR)-active vibrational mode mixes opposite-parity wave functions into the wave functions of the 4ffl term levels. The ~ process is equal to the classical one-phonon replica (Frank—Condon approximalion). Theoretical calculations on the M process have been made by Stavola et al. [20], Richardson [21] and Judd [22]. The transition probability for the transition from the initial vibrational state <0 I to the final vibrational state p) due to the M process can be written as [22,9] 4’rr 1 AVIbM Process,p
0
=
289
the integrated zero-phonon intensity is given by [23] .~
‘~b.i
=
e
I
S
I,~
(9)
i!
where S is the Huang—Rhys factor. The purely electronic transition probability can be written as A =AED +AMD (10 ZP
ZP
‘
ZP
where A,~ (A~) stands for the electric (magnetic)-dipole transition probability. The electric-dipole transition probability is cxpressed in1 terms of the Judd—Ofelt theory [26,27]. ED 64’rr4e2p3 A, 3 0 4~e~ 3hc
2 32 3 (2J + 1) 64~eii nX 3hc
(U(2))2
2 + and Gd3 +
emission of Eu
—
—
2
R6
X
~ (2J+1)
u1~(U~),
(11)
A=2,4,6
P, x P~-.E~(1, 2)
+
,
(6)
where
with the Judd—Ofelt parameter ~ ~
A)[B,+Bfl.
5_(2,k+1)E(2t+1)~(t,
4 g+— Na R3
P..=20~e
2
(7)
a2
(8)
and 10976 =
defined by
15
2
(1
—
02)
Here, is the frequency of the zero-phonon transition, n~ is the local field correction term, N is the number of ligands surrounding the metal ion, a the polarizability of the ligands, R is the metal ion to ligand distance, (1 72)2 is a screening factor and
. (12) Here [B, + Bfl is defined by eqs. (18) and (19) in ref. [26] and is a measure of the strength of the crystal field. For our purpose it is sufficient to take in the
summation of eq. (12) only t 1, describing the parity mixing. The term E(1, A) is defined by =
ii
—
=
E( 1, A)
=
2 ~
{
(n’l’)
A l
1 3
I l’Xl’ I ~ <4f I r I n’l’Xn’l’ r X ~ x
1 ~
I f) 14f) .
(13)
The summation in eq. (13) runs over all values of n’ and I’ where 4f6(n’l’) is an excited configuration. ~E(4O_~~’t’)is the energy difference between the 4f6 configuration and the (n’l’) configuration. The reduced matrix elements (U’~) are tabulated for many transitions in the rate earths ions by Carnall et al. [28].
290
1 Sytsma, G. Blasse
2 ± and Gd3 ±
/
Comparison of the emission of Eu
The ratio of the total integrated vibronic intensity to the total integrated zero-phonon intensity can now be expressed as ‘vib
AvibMprocessp
r=—=E zp
+
eS ~,
.
(14)
.
r on the energy difference ~ occurring in eq. (15) via the term E2(1 2), vanishes. The other terms in eq. (15) will not differ much for the compositions MFC1 (M Sr, Ba) and YOC1. The value of S is almost the same for =
A,
~
0
all these compositions (see below). This implies
t!
Before proceeding we note the following. From the tables by Detrio [25] and Carnall 8S [28] it becomes clear that for the °P7/2— 7/2 transition of Gd3~only the term with A 2 gives a significant contribution to the electric-dipole transition probability. This is also true for the °P 7/2 8~7/2 2t The value for the magnetic-ditransition of Eu pole transition probability of the 6P 7/2 3~is A~D 122 s [10,28]. The transition of Gd value of A~,Dfor the 6P 8S 7/2 order7,~2transition of 2~ will be of the same of magnitude. Eu Experimental from the casewith of 2~in LiBaF evidence follows 2~occupies a site Eu 3, where inversion symmetry. TheEu tip 8S 7~transition is due toprocess. the magnetic-dipole vibronic The decay timeinteraction measured and for this transition is 2.5 ms [29].The relative intensity of the vibronic lines to the zero-phonon lines is ‘large’ [29]. Thus A~,D(°P 8S 2~)< 7/2 7,,2, Eu 250 s—’. The total zero-phonon transition probability of the 6P 8S 3~in YOC1 is 7/2 7~2transition of Gd A, 6P 8S 0 2~in 494MFCI s~. (M For the 7/2 the —* values 7/2 transition of Sr, Ba) of A,~are Eu about a factor6P7—8 larger. 8S From this we conclude that for the 7/2 —~ 7/2transition in approximation A,0 in eq. (14) can be replaced by A~. Doing so and taking only terms with A 2, we find 2 p~ r= ~ 15R6[B 4E2(1, (T”)) 1 +Bf] R =
._~
that r in first approximation will have the same numerical value for the 2~ in MFC1 (M corresponding Sr, Ba) andtransitions Gd3~ in of Eu YOCI. This is in good agreement with our experiments (see table 1). =
4.2. Discussion of the results
—‘
=
—~
picture of the experiments is clear. TheThe linegeneral emission is ascribed to the 6P 8S 7/2 —* 7/2 transition the broad band emission, occurring at higher and temperatures, is ascribed to the 4f65d
—*
—~
~ 7/2 transition. This with [4— the 2~isincomparable other systems results 6,11,17].found for Eu The energy differences ~EB_L are not in agreement with the values found by Sommerdijk and Bril [30]. They have, however, only measured the value of R at 77 and 300 K. This makes their data less reliable. It was not possible to measure ~EB_L directly from an emission and an excita-
—~
=
=
=
[~~+
+
~
.
.
2)1
(15)
This equation shows that r is independent of so that r is independent of the position of the corresponding zero-phonon transition. The term with P~ is only of importance if a large amount of covalency is present [22]. If this term is neglected in first approximation, the dependence of v,
tion spectrum, because line it was not4f65d possible observe a zero-phonon in the —~8Sto 7/2 transition. 6P 8S The 7/2 — 7/2 transition consists of a zero-phonon line and several vibronic lines (figs. 3, 4 and 6). Assignment of the vibronic lines is possible with the Raman and JR spectra of BaFCI vibronic as given in spectrum, fig. 8. The except JR spectrum for the resembles fact that the vibrational peaks are shifted towards higher energy, the more so the higher their energy. The same holds for a comparison using the SrFC1 IR spectrum. The vibronic lines in figs. 3, 4 and 6 thus originate from local modes. These spectra show that the maximum energy is at about 300 cm The vibronic lines in region II in fig. 3 are hence ascribed to vibronic transitions involving two phonons. The largest energy difference between the zero-phonon line and the vibronic lines
I Sytsma, G. Blasse
/
2 + and Gd3 +
291
Comparison of the emission of Eu
in this region amounts to 679 and 575 cm1 for SrFCI : Eu2~ and BaFCI : Eu2~,respectively. For region I these values are 320 and 291 cm_i, respectively. A comparison with the vibrational spectra learns that the vibronics in region I are due to coupling with the highest IR-active vibrations. These in region II are due to coupling with two of these vibrations. This excludes the M process for 2~ the lines 679 and 575 cm Therefore, in SrFC1 : Eu and BaFC1at : Eu2~,respectively. these lines are due to the ~ process. The relative intensity of these vibronic lines must obey eq. (9). From this equation the Huang—Rhys factor is derived: S 0.11 for Eu2~in SrFC1 as well as in BaFC1. This value is very close to the value observed for Gd3 in YOC1. There the value of S was obtained in a completely different way, viz, from the vibronic intensity in the 617/2 _~*8S7/2 21)2 for that transition transition. The value of (Ut is negligibly small [251,so that according to eq. (6) no vibronic lines are induced by the M process in this transition. The value the same for Eu2~in MFC1of(MS isSr,thus Ba) about and Gd3~in YOCI. From the discussion above it follows that r may =
+
=
be somewhat larger than r(MFC1 : Eu2 ~) in agreement with our observations. The transition probabilities for Eu2~in MFCI (M Sr, Ba) are all much larger than for Gd3~in YOC1 (table 1). The zero-phonon transition probabilities differ by a factor 7—8, the vibronic tradition probabilities by a factor 5. The differences in ~ are due to differences in A~ (see above). For the 6P 8S 7/2 only7,,.2 is given by eq. (11), with thetransition, term A A~ 2 being significant. Unfortunately, the value of (U~2~) is not known for the 6P 2~, 7/2much ~ 7/2 but it will not differ fromtransition the value of of Eu (Ut2~) for the 6P 8S 3t There7/2 —* 7/2 transition of Gd that the fore, we conclude from the experiments difference in A,p(6P 8S 2~ and 7/S) for Eu of the Gd3 originates from7/2—* the different values energy difference to the first opposite-parity state ~iE4f_5d, which appears in the expression for A~ 0 via the Judd-Ofelt parameter Q 2. The linear relation between AV~band A,0 expressed in eq. (5) and the particular values of r now6Pprovide8Sthe same argument to explain 2~)>AVIb(6P 8S why AVLb( 3~). 7/2 —~ 7/2, Eu In conclusion 7/2-_* 7/2,Gd we have measured the transition probabilities of the 6P 8S —* They 7/2 transition of 2~in MFC1 (M Sr,7/2 Ba). are all much Eu larger than the corresponding values found for Gd3~in YOC1. This is explicitly shown to be due to the lower position of the opposite-parity states of Eu2t Experimentally we have shown that the relative intensity of vibronic lines to zero-phonon line does not differ much, as is to be expected from existing theories. =
—*
=
+
=
not much for This corresponding in thesediffer compositions. agrees withtransitions the experimental results: the value of r 0.16, 0.20 and 0.23 for the 6P 2~ in 7/2 —~~~7/2 of Eu SrFCI, BaFC1 and Gd3~in transition YOC1, respectively, The other vibronic lines in region II are hard to assign with reasonable accuracy. However, within the experimental error region II is obtamed from region I by subtracting the highest vibrational frequency (320 and 291 cm~ for SrFCI and BaFC1, respectively). This suggests that the excited state of Eu2~is distorted. As noted above, the contribution of P 1~, will increase as the amount of covalency increases, The presence of oxygen in YOC1 makes this composition more covalent3~) than MFC1 (M Sr, > P~(MFC1 : Eu2~). Ba). term Thuscontaining P~(YOClP: Gd The 2(1, in eq. contains E to 2) in the denominator; 1~, E2(1 2) 15 is proportional the inverse of ~E41~”. The larger energy difference L~1E4~5dfor Gd3~enhances the contribution of P~,so that r(YOCI : Gd3~)is expected to =
=
Acknowledgements The investigations were partly supported by the Netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Foundation Pure Research (NWO) andfortheAdvancement Netherlands ofFoundation for Technical Research (STW). We wish to thank Dr. D.J. Stufkens (University of Amsterdam) for measuring the Raman spectrum, and Drs. M.G.J. van Leur (AKZO Research Laboratories, Amhem) for measuring the JR spectrum. The assist-
292
1 Sytsma, G. Blasse
/ Comparison
ance of E.Th.G. Lutz to obtain these spectra is greatfully acknowledged.
Notes added in proof
2 ± and Gd3 +
of the emission of Eu
[9] J. Sytsma, W. van Schaik and G. Blasse, J. Phys. Chem. Solids 52 (1991) 419. [10] J. Sytsma, G.F. Imbusch and G. Blasse, J. Chem. Phys. 91 (1989) 1456. An erratum to this paper: J. Chem. Phys. 92 (1990) 3249. [11] J.L. Sommerdijk, iMP.). Verstegen and A. Bril, J. Lumm. 8 (1974) 502. [12] M. Sauvage, Acta Cryst. B 30 (1974) 2789.
Recent experiments on Eu2~in LiBaF AMDI6D ~~zp ~ ‘7/2
__
8c~
‘~
~7/2’
r~ 2+\
~u
,
._. —
inn
S
(A
-~ .
.
3 give iA eye-
rink et al., to be published.) The assumption 6P made in section 7.4.2 that (Ut21) for the 7/22~and ~ ~ Gd3~is transition will not confirmed by differ much for Eu the explicit calculations by Downer of the (U ) values between the ground state and the excited states for Eu2~ and for Gd3~ (M.C. Downer, Ph.D. Thesis, Harvard (1983)). (A)
References [1] J.P. Spoonhower and M.S. Burberry, J. Lumin. 43 (1989) 221. [2] F.M. Ryan, W. Lehmann, D.W. Feldman and J. Murphy, J. Electrochem. Soc. 121 (1974) 1475. [3] M.V. Hoffmann, J. Electrochem. Soc. 118 (1971) 933. [4] J.L. Sommerdijk and A. Bril, J. Lumin. 11(1976) 363. [5] J.L. Sommerdijk, P. Vries and A. Bril, Philips. J. Res. 33 (1978) 117. [6] A. Meyerink, J. Nuyten and G. Blasse, J. Lumin. 44 (1989) 19. [7] G. Blasse, J. Sytsma and L.H. Brixner, Chem. Phys. Lett. 155 (1989) 64. [8] G. Blasse and L.H. Brixner, Inorg. Chim. Acta 169 (1990) 25.
[13] A.L.N. Stevels and F. Pingault, Philips. Res. Reports 30 (1975) 227. [14] H.P. Beck, J. Solid State Chem. 17 (1976) 275. [15] A.J. de Vries, M.F. Hazenkamp and G. Blasse, J. Lumin. 42 (1988) 275. [16] N.C. 3227. Chang and J.B. Gruber, J. Chem. Phys. 41(1964) [17] R.A. Hewes and M.V. Hoffmann, J. Lumin. 3 (1971) 261. [18] L.H. Brixner J.D. Bierlein and V. Johnson, Current Topics Mat. Sci. 4 (1980) 47. [19] J. Dexpert-Ghys and F. Auzel, J. Chem. Phys. 80 (1984) 4003. [20] M. Stavola and D.L. Dexter, Phys. Rev. B 20 (1979) 1867. [21] T.R. Faulkner and F.S. Richardson, Mol. Phys. 35 (1978) 1141. [22] BR. Judd, Physica Scripta 21(1980) 543. [23] B. Henderson and G.F. Imbusch Optical spectroscopy of inorganic solids (Oxford University Press, Oxford, 1989). [24] W.T. Carnall, P.R. Fields and K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [25] J.A. Detrio, Phys. Rev. B 4 (1971) 1422. [26] B.R. Judd, Phys. Rev. 127 (1962) 750. [27] G.S. Ofelt, J. Chem. Phys. 37 (1962) 339. [28] W.T. Carnall, H. Crosswhite and H.M. Crosswhite, in: Energy level structure and transition probabilities of the trivalent Lanthanides in LaF 3, Argonne National Laboratory Report (1977). [29] N.S. Al’tshuler, S.L. Korableva, L.D. Livanova and AL. Stolov, Soy. Phys. Solid State 15 (1974) 2155. [30] Some values of the transition probabilities in their tables ‘Results’ are not reproducible with their very own data mentioned in their text.