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EPR and optical study of Ni2+ ions in CsCaF3 and CsCdF3
.I. Phys. Chew. Soli& Vol. 55, No. 3, pp. 263472, 1994 Copyright 0 1994 Ekier .Tcknce Ltd Printed in Gnat Britain. All rights reserved 0022s3697/94 $7.00 + 0.00
Pergamon
EPR AND OPTICAL
STUDY OF Ni’+ IONS IN CsCaF, AND CsCdF,
B. VILLACAMPA, R. CASES, V. M. ORERA and R. ALCAL;Z~ Instituto de Ciencia de Materiales de Aragcin, Universidad de Zaragoza-CSIC, Plaza San Francisco s/n, 50009 Zaragoza, Spain (Received 24 September 1993; accepted 15 December 1993) Ahatract-Ni2+ ions in CsCaFj and CsCdFS have been studied by optical and magnetic resonance techniques. The EPR spectrum of Ni*+ in an octahedral en~iro~ent with small distortions due to random strains in the crystal has been measured. g and superhyperfine (SHF) tensors have been obtained and discussed. From the analysis of the SHF constants the IQ‘*+-F- distances are estimated and compared with those in other fluoroperovskites. The optical absorption and photoluminescence spectra have been measured at different temperatures. A dynamical Jahn-Teller (J-T) coupling of the excited electronic levels of Niz+ to the lattice vibrations is tentatively proposed in order to account for some of the results. Luminescence decay measurements show that there is a strong change in the lifetime of the ‘I;a@ ) level going from Ca to Cd crystals as previously observed in similar fluoroperovskites having Rb Instead of Cs. Finally, a new excitation for the NI‘*+ luminescence has been observed in 300 K X-irradiated crystals and the possible mechanisms involved in this process are discussed. 1yeywords: Ni’+, fluoroperovskite, EPR, luminescence.
1. ~RODU~ON In some recent papers [l-3] we have reported the spectroscopic properties of Ni2+ ions in RbCaF, and RbCdF3. Nickel impu~ties enter these crystals in a divalent cation substitutional position and most of their optical properties agree with those given in the literature for Ni’+ ions in similar environments. There are, however, some results that cannot easily be understood. So, the structure observed in the low temperature (10 K) optical spectra of Ni2+ in octahedral symmetry has usually been associated with the transitions among the energy levels calculated in a static approximation including crystal field and spin-orbit (SO) interactions and with the phonon replicas corresponding to the vibrational modes of the matrix 14-71, but in our case we have not been able to account for our experimental results using these approximations. This can be due to the influence of the dynamic Jahn-Teller (J-T) effect that has not been taken into account in these calculations. However, since the two Rb compounds show some structural phase transitions at temperatures higher than 10 K [8,9] it also could happen that the disagreement among the observed lines positions and those calculated following the usual procedure were related with the lowering of the point symmetry of Ni2+ due
TAuthor to whom correspondence should be addressed.
to the phase transition. To check this point it seems interesting to perform similar measurements in other fluoroperovskites such as CsCdF, and CsCaF, that do not present these structural phase transitions. Resides, although the crystal field (10 Dq) and Racah parameters (B and C) of Ni*+ were very similar in both RbCaF, and RbCdF, crystals, as expected, due to the simiIarity of the two Ni2+ environments, the lifetime of the ‘Yf%level measured at 10 K was much smaller in the Cd (490 p(s) compound than in the Ca one (830~s) [2]. Also, it was found that room temperature (RT) X-irradiation was able to produce Ni+ and Ni3+ ions in the Ca compound but not in the Cd one [3]. At the same time a new radiation induced absorption band was observed at about 3OOnm in both matrices such that after excitation with light in this band the Ni2+ emissions were found [2]. To test if these results are general or only appear in the Rb crystals we should perform similar studies in other fluoro~rovs~tes. Finally it is known that there is an approximate dependence of the cubic crystal field parameter Dq for divalent 3d ions in similar octahedral environments, with the fifth power of their distances to the nearest neighbour anions. Taking the Dq values obtained for N?+ in RbCdF, and RbCaF, and those given in the literature corresponding to the same ion in KZnF, [lo] and KMgF, ill], as a function of the F-Ni2+ distance, no agreement with the fifth 263
B. VILLACAMPAetal.
264
power law can be found unless we assume a strong relaxation of the lattice around Ni*+ ions in the Rb compounds, This relaxation expected because of the difference among the Ni*+ ionic radius (0.69 A) and those of Ca2+ (0.99A;) and Cd*+ (0.97~~ can be estimated from the superhyperfine (SHF) interaction parameters obtained from magnetic resonance measurements but for the Rb crystals the SHF structure was not resolved. Again in this case a comparison with the Cs compounds seems to be interesting. We present in this paper a spectroscopic study of NiZC in CsCdF, and CsCaF, single crystals. Optical absorption, photoluminescence, lifetime and EPR measurements have been performed at different temperatures. The structure of the optical spectra as well as the lifetime of the ‘Tk level have been measured and the results are discussed and compared with those in other fluoroperovskites. The changes in the optical properties of these crystals induced by Xirradiation have also been studied. Finally we have analyzed the spin-Hamiltonian (SH) parameters derived from the EPR experiments, including the SHF interactions with F- ions, and approximate values for the Ni**-F- distances have been obtained. 2. EXPERIMENTAL Single crystals of Ni-doped CsCdF, and CsCaF,, were grown by the Bridgman technique using a RF heated furnace and vitreous carbon crucibles. The nominal NiFz content ranged from 0.1 to 1%. Optical absorption measurements were taken with a Hitachi U3400 spectrophotometer. Photoluminescence spectra corresponding to transitions from the *T, level were obtained after excitation with the 457.9 nm line of a 20 W CW Coherent Ar+ laser (Innova 200) and detecting through a 0.5 m Spex monochromator with either a Hamamatsu R-928 photomultiplier or a Si avalanche photodiode (C30955E) from RCA. The ‘T2-+ 3A, luminescence was excited with light from a 300 W Xe lamp and detected with a cooled PbS detector. Lifetime measurements of the ‘T2 level were performed exciting with a pulsed dye laser (pulse width 2 ns) and using a Tektronix 2430 digital oscilloscope. The lifetime of the 3T, level was not measured because the response time of the PbS detector was too slow. Sample temperatures between 10 and 300 K were achieved with a CTI-Cryogenics close cycle cryorefrigerator. EPR spectra were obtained in a Varian E-112 spectrometer working in the X-band. For measurements at low temperatures a continuous flow helium cryostat (ESR900} from Oxford Inst~ments was
used. Magnetic field values were determined with a NMR gaussmeter model ER035 from Bruker. The diphenyl-picryl-hydrazyl (DPPH) resonance line (g = 2.0037 f 0.0002) was used to calibrate the microwave frequency. 3. EXPERIMENTAL RESULTS AND DISCUSSION
EPR results The EPR spectra of “as grown” Ni doped CsCaF, and CsCdF, crystals have been measured at different tem~ratures between 300 and 10 K for different orientations of the static magnetic field. The results are almost identical for both compounds except for small displacements of the line positions, Thus we will refer mainly to the data corresponding to the Cd compound. At 300 K the lines are very broad and it is difficult to determine their positions. At liquid nitrogen temperature (LNT) an isotropic spectrum of three lines is observed (Fig, 1): a broad line at g x 2.36 (-280 mT) with a peak to peak width of about 75 mT (this is an average value because the linewidth is sample dependent), a narrower one (peak to peak width -2 mT), centered in the same g-value as the broad line but with the phase inverted and another line (width N 15 mT) at g x 4.7 f_ 140 mT). The two first lines, that will be called normal line (NL) and inverted line (IL), respectively, are symmetric. The other line, labeled half field line (HFL), is strongly asymmetric being more extended towards the low field side. There is also a structure of six lines centered at _ 325 mT due to a small concentration of unwanted Mn2+ impurities. This spectrum is very similar to the one previously reported for Ni2+ ions in the Rb fluoroperovskites [I, 31 and it has been explained as being due to Ni’+ ions in an octahedral environment. In this symmetry the ground state of Ni’+ is a non-Kramer S = 1 state (3.42,(3F)), The broad line corresponds to the I----NLI
*
L
175
275
RfmT)
3 5
Fig. 1. EPR spectrum of Ni2+ in CsCdF, measured at LNT with S/j{ 100). The structure at about 325 mT corresponds to traces of Mn*+ impurities.
EPR and optical study of Ni2+ ions
265
10) 0 ]_+1) magnetic dipole transitions. Residual strains in the lattice produce a distribution of zero field splitting and this accounts for the large width of the NL. The g-value obtained from the position of this line using the following SH: H=g/YSB
T=lOK
(11
cannot be accurately determined due to the width of the line. The approximate value is g x 2.36 for both CsCdF, and CsCaF, . These values can be compared with those reported in the literature for Ni*+ in other fluoroperovskites [12-141 (Table 1). It can be seen that the g-shift with respect to the free electron value (g, = 2.0023) is bigger when the lattice parameter increases. This is in qualitative agreement with the predictions of the approximate expression: g = g, - 8dlA,
265
275
285
295
B(mT)
T=IOK 1
(2)
where 1 is the SO coupling constant and A = 10 D, is the octahedral crystal field parameter that decreases when the NiZ+-F- distance increases. The approximate value of A obtained from the optical absorption spectra measured at LNT (see below) is A = 6250 cm-’ in CsCdF, . Using this value and that of the experimental g-factor, we obtain from eqn (2): 1 = -280 cm-’ for CsCdF, that corresponds to a reduction of about 15% with respect to the free ion value I = -324cm-‘. Similar reductions have been found for Ni2+ ions in other fluoroperovskites and can be associated with bonding effects [15]. The inverted line has been explained by Smith et al. [16] as due to a broadening (due to cross relaxation between the I- 1) o 10) and 10) 9 11) transitions) of the absorption lines corresponding to Ni2+ centers with a minimum strain, in such a way that the shift of the resonance lines due to zero field splitting is smaller than the homogeneous width of the EPR line. Finally, the HFL is associated with the spin forbidden transition 11) o I - 1) that becomes partly allowed because of the strain-spin coupling that induces a mixing of the 10) state with both 11) and I-- 1). The asymmetry of this line can be also explained taking into account these effects [17].
I
1
125
I
I
135
I
I
145 B(mT)
Fig. 2. EPR spectrum of Nir+ in CsCdF, measured at 10K. Superhyperbne structure appears superimposed to both the DQ transition (a) and the HFL (b) (see the text). g//(100).
When measured at 10 K the EPR spectrum shows some new features superimposed on the lines observed at LNT. In both the g x 2.36 and g z 4.7 regions, a sharp structure appears which depends on the orientation of the static magnetic field B with respect to the crystal axes. These two regions, measured with B parallel to a (100) direction, are shown in Fig. 2. The structure corresponding to the high field region (Fig. 2a) can be understood as follows [15]. In a S = 1 system, double quantum (DQ) transitions can be observed. This happens in our case for those Ni2+ ions in an undistorted octahedral environment and for them the corresponding linewidth is narrow so it can easily be observed superimposed to the broad line due to distorted centers. The
Table 1. Bonding parameters in several fluoroperovskites. A constants are given in x 1O-4cm-’ Sample
KM@, KZnF, CsCdF, CsCaF,
g 2.28 2.29 2.36 2.36
An 65.9t 56.7 55.38 54.65
tData from Hall et al. [12]. $Data from Tomono et al. [13]. pur measurements.
‘4, 25.8t 21.1 19.15 17.95
A, 39.2 37.0$ 31.2 30.1
1 155
&b-f,% A 3.6 10.1 2.5 l.OS$ 3.1 9.3 3.2 9.45
f,%
&A)
a(A)
0.55 0.53 0.45 0.43
1.99 2.01 2.05 2.06
3.973 4.055 4.525 4.562
266
~.VILLACAMPA
SHF interaction with the surrounding fluorine nuclei gives the observed structure. The lines correspond to the I- 1) C+ 10) and 10) e I+ 1) transitions and the positions can be obtained using the following SH: H=gBSB+?
,=I
[A,(S,I:.,+S,.i:,,)+A,,S,II]
(3)
with S = 1 and I’= l/2. A,, and A, are the SHF interaction parameters and the zj axes are along the Ni2+-F- bonding directions. The values of the SH parameters obtained by fitting the line positions calculated using eqn (3) to the observed ones are given in Table 1 together with other values reported in the literature for Ni2+ in similar environments. The values found for CsCdF, are very close to the ones previously obtained by Ziaei [14] using ENDOR. With respect to the structure that appears in the g z 4.7 region (Fig. 2b), it is due to the SHF splitting of the I - 1) o (+ 1) line because of the interaction with the octahedron of fluorines. The values of the SHF constants can be used in order to get some information on the unpaired spin densities in the 2s (f,) and 2p (,f, and jn) fluorine orbitals. We will follow the arguments given by Hall et al. [12]. The SHF interaction of the magnetic electrons of Ni2+ and the nuclei of the six surrounding fluorines can be described using orthogonalized molecular orbitals. The (T antiboding orbitals are given by: Qi.*~,.,=N[d,*_?*-(1/2)1’(Y,
f Yy,- VI?- Y,)],
CD 3:2~.2 = NV,:2 ,2 - (A ‘/(12)‘“)(2Yy, + 2Ye - Y, - y‘l and the 7~antibonding
FY,- y's>1
(41
orbitals are:
?r.,, = M L&J- ( 1PV, (P1 +
@yz = Mfd,.z- (WMP, cp:,=1M[d,-(li2)A,(P,+P,
P2 - P4 - Ps
fP3
-
117
Ps - P6)lr
(5)
-ps-P6)1.
The ligand functions Y, , YJ2. . . Y, of the er orbitals are assumed to be composed by the contributions of 2s and 2pcr fluorine orbitals (neglecting the 1s contribution) and may be written as: i ‘Y = i,,s + &,prr. In the case of rc orbitals the ligand functions are 2~7~. The ligands 1,2,3 and 456 are on the positive and negative x, y and z axes, respectively. The SHF interaction parameters A,, and A, can be written as: A, = A, + 2(A, + A, - A,)
(6)
A, = A, - (A, -t- A, - A,),
(7)
et al.
where A,$is the contact interaction through the fluorine 2s orbital, A, is the dipole-dipole interaction: Ad = ggnajIn/R 3 and A, and A,, give the interactions through fluorine 2pa and 2pn orbitals. On the other hand, the spin densities are approximately given by: j;= (l/3)N2L: = 2 A,J/AzI, f,=(1/3)N21:=2A,SIA,,
(8)
fK=(l~4fMZii::=2AxS/AZII,
with S the spin of the Ni2+ ground state and AZr= @/3)ngPg,P,II1/ OhI2 = 1.503 cm ‘, A, = (2/.5)g~gJ,,(r
-‘&
= 0.0429 cm-‘,
(91
where g,,, & are the nuclear g-factor and the nuclear magneton, ]+ (0)2sI is the value of the wavefunction of the 2s fluorine orbital at the fluorine nucleus and (r -3)2p is the average value of (r -“) for the 2p orbital. Using these equations together with some corrections discussed in [12] and [14], the spin densities can be obtained if we give a value to the Ni’+-F- distance in order to calculate A,. We have used the average value between the divalent cation-anion spacing in the pure lattice and the Ni*“-F- distance in CsNiF,, but the values of the spin densities are almost the same if the distance used to calculate A, is changed within reasonable limits. We also give in Table 1 the spin densities corresponding to Ni2+ ions in other cubic fluoroperovskites, obtained from the data given in the literature [li, 19. It can be seen that j, decreases when the lattice parameter of the host crystal increases. We have also tried to get a quantitative relationship among the spin densities and the Ni*+-Fdistances. It is known that in a purely ionic bonding approximation I, is related with the standard overlap integral (2.r Id,) by: ;C,= (3)“*(2s ]cf,). In some recent papers [IS, 191 it has been shown that covalency effects can be approximately taken into account assuming that 1, = c (3)“*(2s Id,), where c is a constant with a value close to 1 for different 3d ions. With this approximation we have obtained the value of the constant c for Ni2+ from the .f, value in KMgF, assuming that the Ni2+-F- distance is I .99 ,&, the mean value of the Mg2+-F- in KMgF, and the Ni’+-F- distance in KNiF,. In this way we have found c = 1.45. Using this value and the SHF interaction parameters given in Table 1 we have estimated the Ni*‘-F- distance for the different matrices. The results are also given in Table 1. We can see that the estimated Ni’+-F- distance increases with the lattice
EPR and optical study of Ni*+ ions
WAV~MBER
267
(cm“)
Fig. 3. RT absorption spectra of Ni2+ m CsCdF,. The transitions are from the ‘A2 ground state to the levels indicated in the figure.
parameter but more slowly than the distance between the fluorine ions and the divalent cation in the pure crystal. This indicates that in CsCdF, and CsCaF, there is an inward relaxation of the fluorine octahedron towards the Ni2+ ion, as expected because of the smaller ionic radius of Ni*+ as compared with Ca2+ and Cd*+. These results are in agreement with those derived from the optical measurements.
Optical rest&s The optical absorption spectrum of Ni doped CsCdF, single crystals has been measured at different temperatures. In the Ca crystal the Ni concentration
was too low to get reliable measu~ments of the absorption bands. The room temperature (RT) spectrum of the Cd sample given in Fig. 3 shows three main bands centered at about 450, 940 and 1650 nm (22,300, 10,650 and 6100 cm-‘) together with a small band at about 680 in (14,700 cm-‘) and a shoulder close to 51Onm (19,60Ocm-‘). These absorptions can be assigned to the electronic transitions from the ?A&(F) ground level of Ni2+ in an octahedral environment (corresponding to a divalent cation substitutional position) to the excited levels given in the figure. Using a crystal field approximation, the calculated bands positions can be fitted to the centroids of the
20500 WAVENUMBER
22000
23500
(cm-‘)
Fig. 4. NI‘*+ absorption spectra measured at 10 K in CsCdF,:Ni. The bands correspond to the transitions from the ‘A, to the levels indicated in the figure. Arrows in (a) show the spin-orbit level positions calculated with I = -280cm-‘.
B. VILLACAMPA et al.
268
ones with the following crystal field and
experimental
Racah parameters: A=lODq=6l00cn-‘,B=970cm-I,
C=3875cm-‘. (10)
When measured at LNT, the A value for CsCdF, obtained from the position of the ‘A%(F)-+ ‘Z’%(F) absorption band is -6250cm-‘. In the case of KMgF, this band appears at 7620 cm-’ at LNT [l I]. From this value and the distance Ni2+-F- in KMgF, (Table l), we can estimate the constant K in the approximate relation Dq = K/r ‘. Using this expression and the calculated K value, we obtain for CsCdF,:Ni a Ni2+-F- distance d w 2.07 A, which agrees reasonably with that calculated from the unpaired spin density. The absorption spectrum of CsCdF,:Ni also has been measured at 10 K. We give in Fig. 4 the spectra corresponding to the main transitions from the 3Aa(F) ground state. A structure can be observed in each of the bands. Photoluminescence measurements in both Cd and Ca crystals have been performed at different temperatures exciting either with 450 nm or 1500 nm light corresponding to the 3A2(F)- ‘T,(P) and
3A2(F) - ‘T,(F) electronic transitions, respectively. The results corresponding to 10 K are given in Figs 5 and 6. The transitions corresponding to each of the bands are given in the figures, As in the case of the low tem~rature absorption, a structure has been observed in all the emission bands. We will try to analyze now the low temperature absorption and luminescence spectra. Let us begin with the absorption data. The band at N 6000 cm-’ is associated with the 3A2(F)-+ ‘T,(F) magnetic dipole allowed electronic transition and the narrow line at the lowest energy is a true zero phonon (ZP) line. This band shows two groups of lines separated by about lOOOcm-‘. The structure of this band in other compounds has been usually explained [4] as due to the transitions to the four SO components of the 3T2(F) level and the corresponding vibronic transitions due to the interaction with odd phonon modes and the replicas associated with even modes. Trying to check if this explanation can be used in our case, where no clear phonon sequences have been found in the spectrum, we have calculated the positions of the transitions to these SO levels using a value of 1 = - 280 cm-’ as derived from our EPR measurements. These positions are shown by sticks in
b
* 1
ZPL * 4
/~
z
w
I
12400
18100
I
I
13200
18500
18900
I
J
I
14ooo
9f)o
19300
4
WAVENUMBER
9400
9800
(cm-‘)
Fig. 5. Luminescence spectra of CsCdFJ :Ni’+ crystals measured at 10 K. The calculated ZP line position of the ‘Tz(D) -+ ‘R,(F) transition is shown in Fig. SC (see the text).
EPR and optical study of Ni2+ ions
269
b :PL
12600
_..;‘w ,
, 14200
13400
9400
‘T,O+‘A,O
C
d
9800
QIW’A,O
41
1 ZPL
WAVENUMBER
(cm-‘)
Fig. 6. Luminescence spectra of CsCaF,:Ni2+ crystals measured at 10 K. The calculated ZP line position of the ‘T,(D)‘A,(F) transition is shown in Fig. 6c (see the text).
Fig. 4a. It can be seen that the splitting between the SO levels is much smaller than the experimental one (and remains smaller even if we take the free ion SO parameter 1 = -324 cm-‘). This was not observed in
other similar cases [4] although a splitting of the 3A,(F) + )7’*(F) absorption band in two has been reported for Ni*+ in CsCdCl, [21] and BrCdF, [22], but in these cases the splitting has been associated with the trigonal distortion of the Ni*+ environment in these lattices. However, in our case Ni*+ ions are in a cubic site, as shown by EPR measurements, and no crystal field splitting of the 3T2(F) level should appear. A possible explanation for the structure of the 3A,(F) -+ 3T2(F) band could be that the intensity of the ZP transition corresponding to the two highest energy SO levels were small and thus the structure in the 6000-6500 cm-’ region would be due to vibronic transitions associated with these two SO levels. There is however another possibility related with a dynamical J-T interaction of the ‘T,(F) level that we want to point out in order to explain the two groups of lines and the splitting bigger than the SO one. Some years ago the optical absorption line shapes due to transitions from orbital singlets to triplet states of
defect centers with cubic symmetry were calculated by Cho [23] using a semi-classical approach under the Frank-Condon approximation and taking into account SO and J-T interactions with A,,, Eg and Ta vibrational modes of the lattice. The shapes depend on the coupling constants but shapes similar to that of Fig. 4a can be obtained when comparable SO and EB coupling are considered. In this case the overall splitting in the band is bigger than the one predicted by SO alone. Also in the case of Co*+ in ZnS the assignment of the line structure of the 4A2(F)-4T,(F) transition studied by Weakliem [24] required a SO coupling constant larger than that of the free ion, if the J-T interaction was not taken into account. Koidl et al. [25] were able to explain the observed spectrum using a quantum mechanical calculation including a weak J-T coupling with up to four phonon states of both E and T modes. Some years later Uba and Baranowski [26] extended these calculations to the same transition in ZnSe (with a spectrum very similar to ours) considering up to 20 phonon states whose frequencies corresponded to maxima of the phonon density of states of the host crystal. Their results were in good agreement with the experimental spectrum.
270
VILLACAMPA et at.
Because of the extension of the calculations and since we do not know the phonon density of states of our crystals we have not performed a quantitative determination of the levels due to the J-T interaction and consequently we cannot make a precise assignment of the different peaks. However, we think that, in order to understand the absorption band corresponding to the 3_42(F)- 3T,(F) transition of Ni2+ in CsCdF, the dynamical J--T interaction among the ‘T,(F) and the phonons of the matrix has to be taken into account. A second i.r. band (Fig. 4b) is due to the transition from the ground state ‘A,(F) to the 3r,(F) level. Again in this case we have calculated the SO levels positions but the structure in the experimental spectrum is not clear and, since in this case no ZP line is observed, it is difficult to compare the calculated lines positions with the measured ones. The only point to mention is the groups of lines (marked with stars in the figure) whose structure seems to be the same as the one observed in the ‘7’,(D) -+ ‘T,(F) emission spectrum (see below) as expected because in both cases the final level of the transition is the same. The next absorption band in order of increasing energies is the one corresponding to the ‘A,(F)“T,(D) (Fig. 4c). This transition is magnetic dipole forbidden and the ZP line is not observed. However, we can obtain its position adding the ZP energies corresponding to the ‘T,(D ) --+ 3T’2(F) and 3r,(F)--+3A2(f;) emissions (see below) both magnetic dipole allowed. In this way we get an energy of 19,15Scmfor the ZP line of the 3.42(F)‘7’,(D) transition. Several lines have been observed in the absorption band, the three main ones at about 191, 308 and 428cm-’ from the ZP line but these energies do not match with those obtained from the structure of other absorption and emission lines. Again in this case a dynamical J-T interaction of the ‘T,(D ) level could be the reason for this mismatching. Finally the highest energy absorption band shown in Fig. 4d, that is due to the ‘A,(F) -+ 3T, (P) electronic transition, does not show any clear structure. The SO level positions have also been estimated and again it seems that, even if we use the free ion SO parameter, their total splitting (w7OOcm-I) is smaller than the one corresponding to the shoulders in the measured spectrum. As in the other cases it is likely that both J-T and phonon replicas should be considered to account for the results. We will comment now on the luminescence spectra given in Figs 5 and 6 corresponding to Cd and Ca compounds, respectively. Beginning with the ‘T2(D)--+ 3r#,F) transition (Fig. Sa) it can be
observed that’ the general shape of the emission coincides with that of the absorption band 3A,(F) -+ 3T,(F) (Fig. 4a). The structure of the high energy group in emission coincides with that at the lowest energies in absorption (the corresponding lines in both figures have the same labels), except for two strong and broad lines that appear in the emission spectrum and not in the absorption one. These two lines marked with a star in Fig. 5a are at 180 cm-’ of lines 1 and 3, respectively. Trying to check if these two lines are associated with transitions from the ‘T,(D) level, we have performed lifetime measurements (see below) detecting either in the ZP line or in these broad lines. In both cases we got the same lifetime value indicating that both emissions come from the same level. It seems that the transitions from the ‘T,(D) to the vibronic levels of the ‘T,(F) corresponding to these bands are more allowed than those from the 3A2(F) to the same vibronic levels. Concerning the ’ Tz (D )--+ ‘T, (F) emission (Fig. 5b) only a smafl region can be measured because the Si avalanche photodiode response decays to zero at energies lower than 9000cm-‘. The assignment of this transition is based on the energy levels positions and the lifetime measurements that coincides with that of the ‘T,(D) level measured in other emission bands. Besides, a similar emission has been previously reported in other Auoroperovskites 161.As we have already pointed out, we can only compare this structure with those barely observed in the absorption spectrum. The splitting among neighbout peaks are about 50, 65 and IOOcm-’ going from low to high energies. Finally we will make some comments on the structure of the two emissions to the 3A2(F) level (Figs SCand Sd). The one from the 3T2(F) is magnetic dipole allowed and so, the line at the highest energy in Fig. 5d is the ZP one. On the other hand, the ’ T2(D ) --* ‘A,(F) emission is magnetic dipole forbidden and the observed lines are of electric dipole origin and due to interactions with odd vibrational modes. As we have explained, the true origin of this band can be placed at 19,155cm-‘. In the same way, for CsCaF, we can also calculate the ZP line energy and the value obtained is 19,305 cm ‘. As can be seen in Figs Sd and 6d, the 37’2(F) --+ ‘A,(F) emission shows phonon replicas corresponding to one and two phonons of 260 and 275cm-’ in the Cd and Ca compounds, respectively. Similar replicas appear in the ‘T,(D ) -+ ‘A,(F) emission associated with the false origins 2, 3 and 4 (at -215, 280 and 315 cm-’ from the ZP line) in Fig. 5c and with the false origins 2, 3,4 and 5 (at -205, 290, 325 and 4OOcm-’ from the ZP line) in Fig. 6c.
BPR and optical study of &I?+ ions As a conclusion of this part we can say that the experimental results in our crystals cannot be explained with the SO levels plus vibronie transitions and phonon replicas using the same frequencies for all the transitions as reported in other ~uoro~rovskites. On the contrary, it seems that the phonons involved using this explanation are different for all the levels, Besides, the positions of the calculated SO levels are very far, in some of the bands, from the observed lines. A similar situation, not so clear, was reported in a recent paper co~s~nding to Ni*+ in RbCdF, and RbCaF, [3] although in these cases the influence of the structural phase transitions experienced by these compounds at 124 and 195 K, respectively, could be involved. In the Cs compounds the Ni2+ environment remains cubic down to 1OK as shown by our EPR results and so we propose that in order to account for the detaih of the optical properties of N?+ ions, at least in Rb and Cs ~uoro~rovskites, the dynamical J-T effect has to be considered. We have also measured the decay of the luminescent emissions of the ‘T&D) level after pulsed excitation with 450nm light. An exponential decay has been found at all the temperatures for crystals with a low Ni”+ content. The lifetime (5) obtained from these measurements are given in Fig. 7. The analysis of these results is difficult because there are several ~ansitions from the ‘T2(D) emitting level to the 3T1(F), ‘T,(F), ‘E(D) and 3A2(F) levels (the transition to the ‘E (D ) has not been observed in the photoIum~ne~en~ m~s~remen~). We have tried to fit the lifetime thermal evolution to the approximate expression used when only two levels are involved in the emission process:
(11) where the first term gives the purely electronic transition probability, the second one accounts for the 8@J . 7OD3
l
‘r#++‘A$V
l
*
*. 500-A
3 F 4OQE 300. 200
*
A
d
*
A
CsCdF,
C&F3
a
+ A A
loo
l A
l A
06
0
50
loo
150 T(K)
200
*
2.50
*
300
Fig. 7. Lifetime of the ‘T,(D) level of Ni2+ in CsCdF, (A) and CsCaF, (+) as a fw~~t~on of temperature.
271
vibronic contributions and the last one represents the non-radiative processes in the Mott approximation [271. We could apply this expression to our case with z ;’ being the sum of the purely electronic transition probabilities from the *T2 level, if the frequency dependence of all the vibronic ~nt~butions were the same (and in this case the second term would be the sum of all these ~nt~butions) and if one of the non-radiative processes were dominant. However we have not heen able to get a good fit among the thermal evolution predicted by eqn (11) and our ex~~mental data indicating that the situation in this case is more complicated than the one explained above, Although we cannot make a quantitative analysis of our lifetime results there is a point that we want to stress. As in the case of the Rb compounds [28], it is found that the lifetime of the ‘T&l ) level is much shorter in the Cd crystals than in the Ca ones. Besides the vacation in this lifetime is smaller when we substitute Rb by Cs than when we go from Ca to Cd although the changes in the lattice constant and cons~uently in the crystal field parameters are bigger in the first case. At the moment we cannot give a clear explanation for the observed behaviour. Finally we will make some ~mmen~ on the influence of RT X-irradiation on the photoluminescence of Ni2* in the two compounds. In the case of CsCaF,, Ni+ and Ni3+, centers have been detected by EPR in irradiated crystals. Also several strong bands which can be due to radiation~~~du~ intrinsic defects are observed after irradiation. The optical properties of the irradiated crystals are dominated by these defects and are the same as those of the undoped crystals [29] except for the Ni2+ photolumine~n~ (see below). In CsCdF, no strong absorption is induced by X-irradiation but a small band is produced at about 300 nm similar to the one observed in RbCdF, . Upon excitation with light in this region no emission is found before irradiation while the Ni” luminescent bands are observed in irradiated samples. At the same time the.3OOnm band bleaches out. The same behaviour is observed in the irradiated Ca compound exciting with light in the same region. in contrast with CsCaF, neither Ni+ nor Ni3+ ions have been observed by EPR in the irradiated Cd crystals. So we cannot explain the emission observed under 300 nm excitation as due to Ni’+ excited ions produced by direct ionization of Ni+ or N?+ or by the recombination of these ions with holes (or electrons) produced by optical ionization of the defects responsible of the 300 nm band. It has been found in Eu doped BaFBr [30,31] that the ELI*+emission at 390nm is observed in X-irradiated samples excited with light of 470 or 575 nm that do not correspond
B. V~LLACAMPA ef al.
272
The mechanism for this emission investigated using optical and optically detected magnetic resonance (ODMR) [32,33] techniques is as follows. Some electron and hole centers are produced close to Eu*+ ions during irradiation. The subsequent irradiation in the F center absorption bands produces ionization of the electron centers followed by electron-hole recombination and the recombination energy is transferred to the near Eu2+ that goes to an excited state. The subsequent relaxation of the EuZ+ ions gives out the observed emission. A similar mechanism could be responsible for the Ni*+ emission found in our crystals and, in order to check it, ODMR measurements are in progress. to any
Eu2+ absorption.
~ckno~le~ge~enrs-We wish to thank Dr V. Rodriguez (Univeridad de La Laguna, Tenerife, Spain) for infrared photoluminescence measurements and Dr M. Moreno (Universidad de Cantabria, Santander, Spain) for some ootical absorption measurements. This work-has ‘been supportkd by the Direccibn General de Investigaciirn Cientifica y T&mica (Spain) under contract #MAT92-1279.
REFERENCES 1 Alonso P. J., Alcalh R. and Spaeth J. M., Phys. Rev. 841, 10902 (1990). 2. Alcala R., Casas-Gonzalez J., Villacampa B. and Alonso P. J., J. Lamin. 48 & 49, 569 (1991). 3. ViIlacampa B., Alcal& R., Alonso P. J. and Spaeth J. M., J. Phys.: Condens. Matter 5, 747 (1993). 4. Ferguson J., Progress in Inorganic Chemistry, (Edited by S. J. Lippard), Vol. 12, p. 260. Wiley, New York (1970). 5. Vehse W. E., Lee K. H., Yun S. I. and Sibley W. A., J. Lumin. 10, 149 (1975). 6. Sturge M. D., Solid State Comm. 9, 899 (1971). 7. May P. S. and Gudel H. U., C/tern. Phys. L&t&s 175, 488 (1990). 8. Modine F. A., Sonder E., Unruh W. P., Finch L. B. and Westbrook R. D., Phys. Rev. BlO, 1623 (1974).
9. Rousseau M., Gesland G. Y., Julliard J., Nouet J., Zarembowitch J. and Zarembowitch A., Phys. Reu. B12, 1579 (1975). 10. Iverson M. V. and Sibley W. A., J. Lumin. 20, 311 (1979). Il. Knox K., Shulman R. G. and Sugano S., Phys. Rev. 130, 512 (1963). 12. Hall T. P. P., Hayes W., Stevenson R. W. H. and Wilkens J., J. Che& Phys. 38, 1977 (1963). 13. Tomono Y.. Kato T. and Tanokura Y.. J. Phvs. _ Sot. Jpn 53, 773.(1984). 14. Ziaei M. E., Can. J. Phys. 59, 298 (1981). 15. Abragam A. and Bleaney B., Electron Paramagnetic Resonance. Dover, New York (1970). 16. Smith S. R., Dravnieks F. and Wertz J. E., Phys. Rey. 178, 471 (1969). 17. McMahon D. H., Phys. Rev. A128, I34 (1964). 18. Barriuso M. T. and Moreno M., Phys. Rev. BB, 3623 (1984). 19. Moreno G., J. Phys. Chem. Solids 51, 835 (1990). 20. Liehr A. D. and Ballhausen C. J., Ann. Phys. (NY.) 2, 134 (1959). 21. May P. S. and Gudel H. LJ., J. Lumin. 46,277 (1990). 22. de Viry D., Tercier N., Denis J. P., Blanzat B. and Pell6 F., J. Chem. Phys. 97, 2263 (1992). 23. Cho K., J. Phys. Sot. Jup. 25, 1372 (1968). ^. 24. Weakliem H. A., J. Chem. Phys. 30, 2117 (1962). 2s. Koidl P., Schirmer 0. F. and Kaufmann U., Phys. Rev. B&4926 (1973). 26. Uba S. M. and Baranowski J. M., Phys. Reu. Bl7,69 (1978). 27. Mott N. F., Proc. R. Sot. London Ser. A167,384 (1938). 28 Alea% R., Zorita E. and Alonso P. J., Phys. Reu. B38, 11156 (1988). 29 Seretlo J. R., Martin J. J. and Sonder E., Phys. Rev. B14, 5404 (1976). 30, Sonoda M., Takano M., Miyahara J. and Kato H., RadiaZogy, 148, 833 (1983). 31. Takahashi K., Kohoha K:, Miyanodai M., Kanemitsu Y., Amitani K. and Shionoya, S., J. Lwmin. 31-32,266 (1984). 32. von Seggern H., Voigt T., Kniipfer W. and Lange G., J. A@. Phys. 64, 1405 (1988). 33. Hangleiter T., Koschnick F. K., Spaeth J. M., Nuttall R. H. D. and Eachus R. S., J. Phis.: Condens. Matter, 2, 6837 (1990). 34. Koschnick F. K., Spaeth J. M., Eachus R. S., McDugle W. G. and Nuttall R. H., Phys. Rev. Left. 67, 3571 (1991).
Pergamon
EPR AND OPTICAL
STUDY OF Ni’+ IONS IN CsCaF, AND CsCdF,
B. VILLACAMPA, R. CASES, V. M. ORERA and R. ALCAL;Z~ Instituto de Ciencia de Materiales de Aragcin, Universidad de Zaragoza-CSIC, Plaza San Francisco s/n, 50009 Zaragoza, Spain (Received 24 September 1993; accepted 15 December 1993) Ahatract-Ni2+ ions in CsCaFj and CsCdFS have been studied by optical and magnetic resonance techniques. The EPR spectrum of Ni*+ in an octahedral en~iro~ent with small distortions due to random strains in the crystal has been measured. g and superhyperfine (SHF) tensors have been obtained and discussed. From the analysis of the SHF constants the IQ‘*+-F- distances are estimated and compared with those in other fluoroperovskites. The optical absorption and photoluminescence spectra have been measured at different temperatures. A dynamical Jahn-Teller (J-T) coupling of the excited electronic levels of Niz+ to the lattice vibrations is tentatively proposed in order to account for some of the results. Luminescence decay measurements show that there is a strong change in the lifetime of the ‘I;a@ ) level going from Ca to Cd crystals as previously observed in similar fluoroperovskites having Rb Instead of Cs. Finally, a new excitation for the NI‘*+ luminescence has been observed in 300 K X-irradiated crystals and the possible mechanisms involved in this process are discussed. 1yeywords: Ni’+, fluoroperovskite, EPR, luminescence.
1. ~RODU~ON In some recent papers [l-3] we have reported the spectroscopic properties of Ni2+ ions in RbCaF, and RbCdF3. Nickel impu~ties enter these crystals in a divalent cation substitutional position and most of their optical properties agree with those given in the literature for Ni’+ ions in similar environments. There are, however, some results that cannot easily be understood. So, the structure observed in the low temperature (10 K) optical spectra of Ni2+ in octahedral symmetry has usually been associated with the transitions among the energy levels calculated in a static approximation including crystal field and spin-orbit (SO) interactions and with the phonon replicas corresponding to the vibrational modes of the matrix 14-71, but in our case we have not been able to account for our experimental results using these approximations. This can be due to the influence of the dynamic Jahn-Teller (J-T) effect that has not been taken into account in these calculations. However, since the two Rb compounds show some structural phase transitions at temperatures higher than 10 K [8,9] it also could happen that the disagreement among the observed lines positions and those calculated following the usual procedure were related with the lowering of the point symmetry of Ni2+ due
TAuthor to whom correspondence should be addressed.
to the phase transition. To check this point it seems interesting to perform similar measurements in other fluoroperovskites such as CsCdF, and CsCaF, that do not present these structural phase transitions. Resides, although the crystal field (10 Dq) and Racah parameters (B and C) of Ni*+ were very similar in both RbCaF, and RbCdF, crystals, as expected, due to the simiIarity of the two Ni2+ environments, the lifetime of the ‘Yf%level measured at 10 K was much smaller in the Cd (490 p(s) compound than in the Ca one (830~s) [2]. Also, it was found that room temperature (RT) X-irradiation was able to produce Ni+ and Ni3+ ions in the Ca compound but not in the Cd one [3]. At the same time a new radiation induced absorption band was observed at about 3OOnm in both matrices such that after excitation with light in this band the Ni2+ emissions were found [2]. To test if these results are general or only appear in the Rb crystals we should perform similar studies in other fluoro~rovs~tes. Finally it is known that there is an approximate dependence of the cubic crystal field parameter Dq for divalent 3d ions in similar octahedral environments, with the fifth power of their distances to the nearest neighbour anions. Taking the Dq values obtained for N?+ in RbCdF, and RbCaF, and those given in the literature corresponding to the same ion in KZnF, [lo] and KMgF, ill], as a function of the F-Ni2+ distance, no agreement with the fifth 263
B. VILLACAMPAetal.
264
power law can be found unless we assume a strong relaxation of the lattice around Ni*+ ions in the Rb compounds, This relaxation expected because of the difference among the Ni*+ ionic radius (0.69 A) and those of Ca2+ (0.99A;) and Cd*+ (0.97~~ can be estimated from the superhyperfine (SHF) interaction parameters obtained from magnetic resonance measurements but for the Rb crystals the SHF structure was not resolved. Again in this case a comparison with the Cs compounds seems to be interesting. We present in this paper a spectroscopic study of NiZC in CsCdF, and CsCaF, single crystals. Optical absorption, photoluminescence, lifetime and EPR measurements have been performed at different temperatures. The structure of the optical spectra as well as the lifetime of the ‘Tk level have been measured and the results are discussed and compared with those in other fluoroperovskites. The changes in the optical properties of these crystals induced by Xirradiation have also been studied. Finally we have analyzed the spin-Hamiltonian (SH) parameters derived from the EPR experiments, including the SHF interactions with F- ions, and approximate values for the Ni**-F- distances have been obtained. 2. EXPERIMENTAL Single crystals of Ni-doped CsCdF, and CsCaF,, were grown by the Bridgman technique using a RF heated furnace and vitreous carbon crucibles. The nominal NiFz content ranged from 0.1 to 1%. Optical absorption measurements were taken with a Hitachi U3400 spectrophotometer. Photoluminescence spectra corresponding to transitions from the *T, level were obtained after excitation with the 457.9 nm line of a 20 W CW Coherent Ar+ laser (Innova 200) and detecting through a 0.5 m Spex monochromator with either a Hamamatsu R-928 photomultiplier or a Si avalanche photodiode (C30955E) from RCA. The ‘T2-+ 3A, luminescence was excited with light from a 300 W Xe lamp and detected with a cooled PbS detector. Lifetime measurements of the ‘T2 level were performed exciting with a pulsed dye laser (pulse width 2 ns) and using a Tektronix 2430 digital oscilloscope. The lifetime of the 3T, level was not measured because the response time of the PbS detector was too slow. Sample temperatures between 10 and 300 K were achieved with a CTI-Cryogenics close cycle cryorefrigerator. EPR spectra were obtained in a Varian E-112 spectrometer working in the X-band. For measurements at low temperatures a continuous flow helium cryostat (ESR900} from Oxford Inst~ments was
used. Magnetic field values were determined with a NMR gaussmeter model ER035 from Bruker. The diphenyl-picryl-hydrazyl (DPPH) resonance line (g = 2.0037 f 0.0002) was used to calibrate the microwave frequency. 3. EXPERIMENTAL RESULTS AND DISCUSSION
EPR results The EPR spectra of “as grown” Ni doped CsCaF, and CsCdF, crystals have been measured at different tem~ratures between 300 and 10 K for different orientations of the static magnetic field. The results are almost identical for both compounds except for small displacements of the line positions, Thus we will refer mainly to the data corresponding to the Cd compound. At 300 K the lines are very broad and it is difficult to determine their positions. At liquid nitrogen temperature (LNT) an isotropic spectrum of three lines is observed (Fig, 1): a broad line at g x 2.36 (-280 mT) with a peak to peak width of about 75 mT (this is an average value because the linewidth is sample dependent), a narrower one (peak to peak width -2 mT), centered in the same g-value as the broad line but with the phase inverted and another line (width N 15 mT) at g x 4.7 f_ 140 mT). The two first lines, that will be called normal line (NL) and inverted line (IL), respectively, are symmetric. The other line, labeled half field line (HFL), is strongly asymmetric being more extended towards the low field side. There is also a structure of six lines centered at _ 325 mT due to a small concentration of unwanted Mn2+ impurities. This spectrum is very similar to the one previously reported for Ni2+ ions in the Rb fluoroperovskites [I, 31 and it has been explained as being due to Ni’+ ions in an octahedral environment. In this symmetry the ground state of Ni’+ is a non-Kramer S = 1 state (3.42,(3F)), The broad line corresponds to the I----NLI
*
L
175
275
RfmT)
3 5
Fig. 1. EPR spectrum of Ni2+ in CsCdF, measured at LNT with S/j{ 100). The structure at about 325 mT corresponds to traces of Mn*+ impurities.
EPR and optical study of Ni2+ ions
265
10) 0 ]_+1) magnetic dipole transitions. Residual strains in the lattice produce a distribution of zero field splitting and this accounts for the large width of the NL. The g-value obtained from the position of this line using the following SH: H=g/YSB
T=lOK
(11
cannot be accurately determined due to the width of the line. The approximate value is g x 2.36 for both CsCdF, and CsCaF, . These values can be compared with those reported in the literature for Ni*+ in other fluoroperovskites [12-141 (Table 1). It can be seen that the g-shift with respect to the free electron value (g, = 2.0023) is bigger when the lattice parameter increases. This is in qualitative agreement with the predictions of the approximate expression: g = g, - 8dlA,
265
275
285
295
B(mT)
T=IOK 1
(2)
where 1 is the SO coupling constant and A = 10 D, is the octahedral crystal field parameter that decreases when the NiZ+-F- distance increases. The approximate value of A obtained from the optical absorption spectra measured at LNT (see below) is A = 6250 cm-’ in CsCdF, . Using this value and that of the experimental g-factor, we obtain from eqn (2): 1 = -280 cm-’ for CsCdF, that corresponds to a reduction of about 15% with respect to the free ion value I = -324cm-‘. Similar reductions have been found for Ni2+ ions in other fluoroperovskites and can be associated with bonding effects [15]. The inverted line has been explained by Smith et al. [16] as due to a broadening (due to cross relaxation between the I- 1) o 10) and 10) 9 11) transitions) of the absorption lines corresponding to Ni2+ centers with a minimum strain, in such a way that the shift of the resonance lines due to zero field splitting is smaller than the homogeneous width of the EPR line. Finally, the HFL is associated with the spin forbidden transition 11) o I - 1) that becomes partly allowed because of the strain-spin coupling that induces a mixing of the 10) state with both 11) and I-- 1). The asymmetry of this line can be also explained taking into account these effects [17].
I
1
125
I
I
135
I
I
145 B(mT)
Fig. 2. EPR spectrum of Nir+ in CsCdF, measured at 10K. Superhyperbne structure appears superimposed to both the DQ transition (a) and the HFL (b) (see the text). g//(100).
When measured at 10 K the EPR spectrum shows some new features superimposed on the lines observed at LNT. In both the g x 2.36 and g z 4.7 regions, a sharp structure appears which depends on the orientation of the static magnetic field B with respect to the crystal axes. These two regions, measured with B parallel to a (100) direction, are shown in Fig. 2. The structure corresponding to the high field region (Fig. 2a) can be understood as follows [15]. In a S = 1 system, double quantum (DQ) transitions can be observed. This happens in our case for those Ni2+ ions in an undistorted octahedral environment and for them the corresponding linewidth is narrow so it can easily be observed superimposed to the broad line due to distorted centers. The
Table 1. Bonding parameters in several fluoroperovskites. A constants are given in x 1O-4cm-’ Sample
KM@, KZnF, CsCdF, CsCaF,
g 2.28 2.29 2.36 2.36
An 65.9t 56.7 55.38 54.65
tData from Hall et al. [12]. $Data from Tomono et al. [13]. pur measurements.
‘4, 25.8t 21.1 19.15 17.95
A, 39.2 37.0$ 31.2 30.1
1 155
&b-f,% A 3.6 10.1 2.5 l.OS$ 3.1 9.3 3.2 9.45
f,%
&A)
a(A)
0.55 0.53 0.45 0.43
1.99 2.01 2.05 2.06
3.973 4.055 4.525 4.562
266
~.VILLACAMPA
SHF interaction with the surrounding fluorine nuclei gives the observed structure. The lines correspond to the I- 1) C+ 10) and 10) e I+ 1) transitions and the positions can be obtained using the following SH: H=gBSB+?
,=I
[A,(S,I:.,+S,.i:,,)+A,,S,II]
(3)
with S = 1 and I’= l/2. A,, and A, are the SHF interaction parameters and the zj axes are along the Ni2+-F- bonding directions. The values of the SH parameters obtained by fitting the line positions calculated using eqn (3) to the observed ones are given in Table 1 together with other values reported in the literature for Ni2+ in similar environments. The values found for CsCdF, are very close to the ones previously obtained by Ziaei [14] using ENDOR. With respect to the structure that appears in the g z 4.7 region (Fig. 2b), it is due to the SHF splitting of the I - 1) o (+ 1) line because of the interaction with the octahedron of fluorines. The values of the SHF constants can be used in order to get some information on the unpaired spin densities in the 2s (f,) and 2p (,f, and jn) fluorine orbitals. We will follow the arguments given by Hall et al. [12]. The SHF interaction of the magnetic electrons of Ni2+ and the nuclei of the six surrounding fluorines can be described using orthogonalized molecular orbitals. The (T antiboding orbitals are given by: Qi.*~,.,=N[d,*_?*-(1/2)1’(Y,
f Yy,- VI?- Y,)],
CD 3:2~.2 = NV,:2 ,2 - (A ‘/(12)‘“)(2Yy, + 2Ye - Y, - y‘l and the 7~antibonding
FY,- y's>1
(41
orbitals are:
?r.,, = M L&J- ( 1PV, (P1 +
@yz = Mfd,.z- (WMP, cp:,=1M[d,-(li2)A,(P,+P,
P2 - P4 - Ps
fP3
-
117
Ps - P6)lr
(5)
-ps-P6)1.
The ligand functions Y, , YJ2. . . Y, of the er orbitals are assumed to be composed by the contributions of 2s and 2pcr fluorine orbitals (neglecting the 1s contribution) and may be written as: i ‘Y = i,,s + &,prr. In the case of rc orbitals the ligand functions are 2~7~. The ligands 1,2,3 and 456 are on the positive and negative x, y and z axes, respectively. The SHF interaction parameters A,, and A, can be written as: A, = A, + 2(A, + A, - A,)
(6)
A, = A, - (A, -t- A, - A,),
(7)
et al.
where A,$is the contact interaction through the fluorine 2s orbital, A, is the dipole-dipole interaction: Ad = ggnajIn/R 3 and A, and A,, give the interactions through fluorine 2pa and 2pn orbitals. On the other hand, the spin densities are approximately given by: j;= (l/3)N2L: = 2 A,J/AzI, f,=(1/3)N21:=2A,SIA,,
(8)
fK=(l~4fMZii::=2AxS/AZII,
with S the spin of the Ni2+ ground state and AZr= @/3)ngPg,P,II1/ OhI2 = 1.503 cm ‘, A, = (2/.5)g~gJ,,(r
-‘&
= 0.0429 cm-‘,
(91
where g,,, & are the nuclear g-factor and the nuclear magneton, ]+ (0)2sI is the value of the wavefunction of the 2s fluorine orbital at the fluorine nucleus and (r -3)2p is the average value of (r -“) for the 2p orbital. Using these equations together with some corrections discussed in [12] and [14], the spin densities can be obtained if we give a value to the Ni’+-F- distance in order to calculate A,. We have used the average value between the divalent cation-anion spacing in the pure lattice and the Ni*“-F- distance in CsNiF,, but the values of the spin densities are almost the same if the distance used to calculate A, is changed within reasonable limits. We also give in Table 1 the spin densities corresponding to Ni2+ ions in other cubic fluoroperovskites, obtained from the data given in the literature [li, 19. It can be seen that j, decreases when the lattice parameter of the host crystal increases. We have also tried to get a quantitative relationship among the spin densities and the Ni*+-Fdistances. It is known that in a purely ionic bonding approximation I, is related with the standard overlap integral (2.r Id,) by: ;C,= (3)“*(2s ]cf,). In some recent papers [IS, 191 it has been shown that covalency effects can be approximately taken into account assuming that 1, = c (3)“*(2s Id,), where c is a constant with a value close to 1 for different 3d ions. With this approximation we have obtained the value of the constant c for Ni2+ from the .f, value in KMgF, assuming that the Ni2+-F- distance is I .99 ,&, the mean value of the Mg2+-F- in KMgF, and the Ni’+-F- distance in KNiF,. In this way we have found c = 1.45. Using this value and the SHF interaction parameters given in Table 1 we have estimated the Ni*‘-F- distance for the different matrices. The results are also given in Table 1. We can see that the estimated Ni’+-F- distance increases with the lattice
EPR and optical study of Ni*+ ions
WAV~MBER
267
(cm“)
Fig. 3. RT absorption spectra of Ni2+ m CsCdF,. The transitions are from the ‘A2 ground state to the levels indicated in the figure.
parameter but more slowly than the distance between the fluorine ions and the divalent cation in the pure crystal. This indicates that in CsCdF, and CsCaF, there is an inward relaxation of the fluorine octahedron towards the Ni2+ ion, as expected because of the smaller ionic radius of Ni*+ as compared with Ca2+ and Cd*+. These results are in agreement with those derived from the optical measurements.
Optical rest&s The optical absorption spectrum of Ni doped CsCdF, single crystals has been measured at different temperatures. In the Ca crystal the Ni concentration
was too low to get reliable measu~ments of the absorption bands. The room temperature (RT) spectrum of the Cd sample given in Fig. 3 shows three main bands centered at about 450, 940 and 1650 nm (22,300, 10,650 and 6100 cm-‘) together with a small band at about 680 in (14,700 cm-‘) and a shoulder close to 51Onm (19,60Ocm-‘). These absorptions can be assigned to the electronic transitions from the ?A&(F) ground level of Ni2+ in an octahedral environment (corresponding to a divalent cation substitutional position) to the excited levels given in the figure. Using a crystal field approximation, the calculated bands positions can be fitted to the centroids of the
20500 WAVENUMBER
22000
23500
(cm-‘)
Fig. 4. NI‘*+ absorption spectra measured at 10 K in CsCdF,:Ni. The bands correspond to the transitions from the ‘A, to the levels indicated in the figure. Arrows in (a) show the spin-orbit level positions calculated with I = -280cm-‘.
B. VILLACAMPA et al.
268
ones with the following crystal field and
experimental
Racah parameters: A=lODq=6l00cn-‘,B=970cm-I,
C=3875cm-‘. (10)
When measured at LNT, the A value for CsCdF, obtained from the position of the ‘A%(F)-+ ‘Z’%(F) absorption band is -6250cm-‘. In the case of KMgF, this band appears at 7620 cm-’ at LNT [l I]. From this value and the distance Ni2+-F- in KMgF, (Table l), we can estimate the constant K in the approximate relation Dq = K/r ‘. Using this expression and the calculated K value, we obtain for CsCdF,:Ni a Ni2+-F- distance d w 2.07 A, which agrees reasonably with that calculated from the unpaired spin density. The absorption spectrum of CsCdF,:Ni also has been measured at 10 K. We give in Fig. 4 the spectra corresponding to the main transitions from the 3Aa(F) ground state. A structure can be observed in each of the bands. Photoluminescence measurements in both Cd and Ca crystals have been performed at different temperatures exciting either with 450 nm or 1500 nm light corresponding to the 3A2(F)- ‘T,(P) and
3A2(F) - ‘T,(F) electronic transitions, respectively. The results corresponding to 10 K are given in Figs 5 and 6. The transitions corresponding to each of the bands are given in the figures, As in the case of the low tem~rature absorption, a structure has been observed in all the emission bands. We will try to analyze now the low temperature absorption and luminescence spectra. Let us begin with the absorption data. The band at N 6000 cm-’ is associated with the 3A2(F)-+ ‘T,(F) magnetic dipole allowed electronic transition and the narrow line at the lowest energy is a true zero phonon (ZP) line. This band shows two groups of lines separated by about lOOOcm-‘. The structure of this band in other compounds has been usually explained [4] as due to the transitions to the four SO components of the 3T2(F) level and the corresponding vibronic transitions due to the interaction with odd phonon modes and the replicas associated with even modes. Trying to check if this explanation can be used in our case, where no clear phonon sequences have been found in the spectrum, we have calculated the positions of the transitions to these SO levels using a value of 1 = - 280 cm-’ as derived from our EPR measurements. These positions are shown by sticks in
b
* 1
ZPL * 4
/~
z
w
I
12400
18100
I
I
13200
18500
18900
I
J
I
14ooo
9f)o
19300
4
WAVENUMBER
9400
9800
(cm-‘)
Fig. 5. Luminescence spectra of CsCdFJ :Ni’+ crystals measured at 10 K. The calculated ZP line position of the ‘Tz(D) -+ ‘R,(F) transition is shown in Fig. SC (see the text).
EPR and optical study of Ni2+ ions
269
b :PL
12600
_..;‘w ,
, 14200
13400
9400
‘T,O+‘A,O
C
d
9800
QIW’A,O
41
1 ZPL
WAVENUMBER
(cm-‘)
Fig. 6. Luminescence spectra of CsCaF,:Ni2+ crystals measured at 10 K. The calculated ZP line position of the ‘T,(D)‘A,(F) transition is shown in Fig. 6c (see the text).
Fig. 4a. It can be seen that the splitting between the SO levels is much smaller than the experimental one (and remains smaller even if we take the free ion SO parameter 1 = -324 cm-‘). This was not observed in
other similar cases [4] although a splitting of the 3A,(F) + )7’*(F) absorption band in two has been reported for Ni*+ in CsCdCl, [21] and BrCdF, [22], but in these cases the splitting has been associated with the trigonal distortion of the Ni*+ environment in these lattices. However, in our case Ni*+ ions are in a cubic site, as shown by EPR measurements, and no crystal field splitting of the 3T2(F) level should appear. A possible explanation for the structure of the 3A,(F) -+ 3T2(F) band could be that the intensity of the ZP transition corresponding to the two highest energy SO levels were small and thus the structure in the 6000-6500 cm-’ region would be due to vibronic transitions associated with these two SO levels. There is however another possibility related with a dynamical J-T interaction of the ‘T,(F) level that we want to point out in order to explain the two groups of lines and the splitting bigger than the SO one. Some years ago the optical absorption line shapes due to transitions from orbital singlets to triplet states of
defect centers with cubic symmetry were calculated by Cho [23] using a semi-classical approach under the Frank-Condon approximation and taking into account SO and J-T interactions with A,,, Eg and Ta vibrational modes of the lattice. The shapes depend on the coupling constants but shapes similar to that of Fig. 4a can be obtained when comparable SO and EB coupling are considered. In this case the overall splitting in the band is bigger than the one predicted by SO alone. Also in the case of Co*+ in ZnS the assignment of the line structure of the 4A2(F)-4T,(F) transition studied by Weakliem [24] required a SO coupling constant larger than that of the free ion, if the J-T interaction was not taken into account. Koidl et al. [25] were able to explain the observed spectrum using a quantum mechanical calculation including a weak J-T coupling with up to four phonon states of both E and T modes. Some years later Uba and Baranowski [26] extended these calculations to the same transition in ZnSe (with a spectrum very similar to ours) considering up to 20 phonon states whose frequencies corresponded to maxima of the phonon density of states of the host crystal. Their results were in good agreement with the experimental spectrum.
270
VILLACAMPA et at.
Because of the extension of the calculations and since we do not know the phonon density of states of our crystals we have not performed a quantitative determination of the levels due to the J-T interaction and consequently we cannot make a precise assignment of the different peaks. However, we think that, in order to understand the absorption band corresponding to the 3_42(F)- 3T,(F) transition of Ni2+ in CsCdF, the dynamical J--T interaction among the ‘T,(F) and the phonons of the matrix has to be taken into account. A second i.r. band (Fig. 4b) is due to the transition from the ground state ‘A,(F) to the 3r,(F) level. Again in this case we have calculated the SO levels positions but the structure in the experimental spectrum is not clear and, since in this case no ZP line is observed, it is difficult to compare the calculated lines positions with the measured ones. The only point to mention is the groups of lines (marked with stars in the figure) whose structure seems to be the same as the one observed in the ‘7’,(D) -+ ‘T,(F) emission spectrum (see below) as expected because in both cases the final level of the transition is the same. The next absorption band in order of increasing energies is the one corresponding to the ‘A,(F)“T,(D) (Fig. 4c). This transition is magnetic dipole forbidden and the ZP line is not observed. However, we can obtain its position adding the ZP energies corresponding to the ‘T,(D ) --+ 3T’2(F) and 3r,(F)--+3A2(f;) emissions (see below) both magnetic dipole allowed. In this way we get an energy of 19,15Scmfor the ZP line of the 3.42(F)‘7’,(D) transition. Several lines have been observed in the absorption band, the three main ones at about 191, 308 and 428cm-’ from the ZP line but these energies do not match with those obtained from the structure of other absorption and emission lines. Again in this case a dynamical J-T interaction of the ‘T,(D ) level could be the reason for this mismatching. Finally the highest energy absorption band shown in Fig. 4d, that is due to the ‘A,(F) -+ 3T, (P) electronic transition, does not show any clear structure. The SO level positions have also been estimated and again it seems that, even if we use the free ion SO parameter, their total splitting (w7OOcm-I) is smaller than the one corresponding to the shoulders in the measured spectrum. As in the other cases it is likely that both J-T and phonon replicas should be considered to account for the results. We will comment now on the luminescence spectra given in Figs 5 and 6 corresponding to Cd and Ca compounds, respectively. Beginning with the ‘T2(D)--+ 3r#,F) transition (Fig. Sa) it can be
observed that’ the general shape of the emission coincides with that of the absorption band 3A,(F) -+ 3T,(F) (Fig. 4a). The structure of the high energy group in emission coincides with that at the lowest energies in absorption (the corresponding lines in both figures have the same labels), except for two strong and broad lines that appear in the emission spectrum and not in the absorption one. These two lines marked with a star in Fig. 5a are at 180 cm-’ of lines 1 and 3, respectively. Trying to check if these two lines are associated with transitions from the ‘T,(D) level, we have performed lifetime measurements (see below) detecting either in the ZP line or in these broad lines. In both cases we got the same lifetime value indicating that both emissions come from the same level. It seems that the transitions from the ‘T,(D) to the vibronic levels of the ‘T,(F) corresponding to these bands are more allowed than those from the 3A2(F) to the same vibronic levels. Concerning the ’ Tz (D )--+ ‘T, (F) emission (Fig. 5b) only a smafl region can be measured because the Si avalanche photodiode response decays to zero at energies lower than 9000cm-‘. The assignment of this transition is based on the energy levels positions and the lifetime measurements that coincides with that of the ‘T,(D) level measured in other emission bands. Besides, a similar emission has been previously reported in other Auoroperovskites 161.As we have already pointed out, we can only compare this structure with those barely observed in the absorption spectrum. The splitting among neighbout peaks are about 50, 65 and IOOcm-’ going from low to high energies. Finally we will make some comments on the structure of the two emissions to the 3A2(F) level (Figs SCand Sd). The one from the 3T2(F) is magnetic dipole allowed and so, the line at the highest energy in Fig. 5d is the ZP one. On the other hand, the ’ T2(D ) --* ‘A,(F) emission is magnetic dipole forbidden and the observed lines are of electric dipole origin and due to interactions with odd vibrational modes. As we have explained, the true origin of this band can be placed at 19,155cm-‘. In the same way, for CsCaF, we can also calculate the ZP line energy and the value obtained is 19,305 cm ‘. As can be seen in Figs Sd and 6d, the 37’2(F) --+ ‘A,(F) emission shows phonon replicas corresponding to one and two phonons of 260 and 275cm-’ in the Cd and Ca compounds, respectively. Similar replicas appear in the ‘T,(D ) -+ ‘A,(F) emission associated with the false origins 2, 3 and 4 (at -215, 280 and 315 cm-’ from the ZP line) in Fig. 5c and with the false origins 2, 3,4 and 5 (at -205, 290, 325 and 4OOcm-’ from the ZP line) in Fig. 6c.
BPR and optical study of &I?+ ions As a conclusion of this part we can say that the experimental results in our crystals cannot be explained with the SO levels plus vibronie transitions and phonon replicas using the same frequencies for all the transitions as reported in other ~uoro~rovskites. On the contrary, it seems that the phonons involved using this explanation are different for all the levels, Besides, the positions of the calculated SO levels are very far, in some of the bands, from the observed lines. A similar situation, not so clear, was reported in a recent paper co~s~nding to Ni*+ in RbCdF, and RbCaF, [3] although in these cases the influence of the structural phase transitions experienced by these compounds at 124 and 195 K, respectively, could be involved. In the Cs compounds the Ni2+ environment remains cubic down to 1OK as shown by our EPR results and so we propose that in order to account for the detaih of the optical properties of N?+ ions, at least in Rb and Cs ~uoro~rovskites, the dynamical J-T effect has to be considered. We have also measured the decay of the luminescent emissions of the ‘T&D) level after pulsed excitation with 450nm light. An exponential decay has been found at all the temperatures for crystals with a low Ni”+ content. The lifetime (5) obtained from these measurements are given in Fig. 7. The analysis of these results is difficult because there are several ~ansitions from the ‘T2(D) emitting level to the 3T1(F), ‘T,(F), ‘E(D) and 3A2(F) levels (the transition to the ‘E (D ) has not been observed in the photoIum~ne~en~ m~s~remen~). We have tried to fit the lifetime thermal evolution to the approximate expression used when only two levels are involved in the emission process:
(11) where the first term gives the purely electronic transition probability, the second one accounts for the 8@J . 7OD3
l
‘r#++‘A$V
l
*
*. 500-A
3 F 4OQE 300. 200
*
A
d
*
A
CsCdF,
C&F3
a
+ A A
loo
l A
l A
06
0
50
loo
150 T(K)
200
*
2.50
*
300
Fig. 7. Lifetime of the ‘T,(D) level of Ni2+ in CsCdF, (A) and CsCaF, (+) as a fw~~t~on of temperature.
271
vibronic contributions and the last one represents the non-radiative processes in the Mott approximation [271. We could apply this expression to our case with z ;’ being the sum of the purely electronic transition probabilities from the *T2 level, if the frequency dependence of all the vibronic ~nt~butions were the same (and in this case the second term would be the sum of all these ~nt~butions) and if one of the non-radiative processes were dominant. However we have not heen able to get a good fit among the thermal evolution predicted by eqn (11) and our ex~~mental data indicating that the situation in this case is more complicated than the one explained above, Although we cannot make a quantitative analysis of our lifetime results there is a point that we want to stress. As in the case of the Rb compounds [28], it is found that the lifetime of the ‘T&l ) level is much shorter in the Cd crystals than in the Ca ones. Besides the vacation in this lifetime is smaller when we substitute Rb by Cs than when we go from Ca to Cd although the changes in the lattice constant and cons~uently in the crystal field parameters are bigger in the first case. At the moment we cannot give a clear explanation for the observed behaviour. Finally we will make some ~mmen~ on the influence of RT X-irradiation on the photoluminescence of Ni2* in the two compounds. In the case of CsCaF,, Ni+ and Ni3+, centers have been detected by EPR in irradiated crystals. Also several strong bands which can be due to radiation~~~du~ intrinsic defects are observed after irradiation. The optical properties of the irradiated crystals are dominated by these defects and are the same as those of the undoped crystals [29] except for the Ni2+ photolumine~n~ (see below). In CsCdF, no strong absorption is induced by X-irradiation but a small band is produced at about 300 nm similar to the one observed in RbCdF, . Upon excitation with light in this region no emission is found before irradiation while the Ni” luminescent bands are observed in irradiated samples. At the same time the.3OOnm band bleaches out. The same behaviour is observed in the irradiated Ca compound exciting with light in the same region. in contrast with CsCaF, neither Ni+ nor Ni3+ ions have been observed by EPR in the irradiated Cd crystals. So we cannot explain the emission observed under 300 nm excitation as due to Ni’+ excited ions produced by direct ionization of Ni+ or N?+ or by the recombination of these ions with holes (or electrons) produced by optical ionization of the defects responsible of the 300 nm band. It has been found in Eu doped BaFBr [30,31] that the ELI*+emission at 390nm is observed in X-irradiated samples excited with light of 470 or 575 nm that do not correspond
B. V~LLACAMPA ef al.
272
The mechanism for this emission investigated using optical and optically detected magnetic resonance (ODMR) [32,33] techniques is as follows. Some electron and hole centers are produced close to Eu*+ ions during irradiation. The subsequent irradiation in the F center absorption bands produces ionization of the electron centers followed by electron-hole recombination and the recombination energy is transferred to the near Eu2+ that goes to an excited state. The subsequent relaxation of the EuZ+ ions gives out the observed emission. A similar mechanism could be responsible for the Ni*+ emission found in our crystals and, in order to check it, ODMR measurements are in progress. to any
Eu2+ absorption.
~ckno~le~ge~enrs-We wish to thank Dr V. Rodriguez (Univeridad de La Laguna, Tenerife, Spain) for infrared photoluminescence measurements and Dr M. Moreno (Universidad de Cantabria, Santander, Spain) for some ootical absorption measurements. This work-has ‘been supportkd by the Direccibn General de Investigaciirn Cientifica y T&mica (Spain) under contract #MAT92-1279.
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