- Email: [email protected]
Luminescence study of LiGa5−xFexO8
Journal of Luminescence 28 (1983) 41—52 North-Holland Publishing Company
41
LUMINESCENCE STUDY OF LiGa5.~Fe~Os C. McSHERA, P.J. COLLERAN, T.J. GLYNN and G.F. IMBUSCH Physics Department, University College, Ga/way, Ire/and
J.P. REMEIKA Bell Laboratories, Murray Hill, New Jersey 07974, USA
Received 29 August 1982
Absorption, excitation, and luminescence measurements are presented for LiGa5 — Fe~O8 with concentrations x from 0.0 15 to 0.26. 6A The 6S)~—4T,(4G) emission and excitation indicate that the transitiondata in the d5 configurared luminescence tion for Fe3* in tetrahedral observed is sites. due to Single-crystal the 1( samples allow direct absorption measurements for the first time and these indicate that most of the Fe3 + substitutes for Ga3 + in octahedral sites. Fine structure in the zero-phonon line and in the sideband is reported.
I. Introduction Luminescence from Fe3~, incorporated as an impurity in many host materials, has been reported in the literature [1—5].In each case, the luminescence is assigned to the transition 6A 6S) ~—4T 4G)between the lowest crystal 1( these levels 1( belong to different strong field levels in the d5 configuration. Since field configurations, a broad band transition is expected and the occurrence of a zero-phonon line depends on the difference in coupling to the lattice in the ground and excited states. In a review of several Fe3~-doped oxides [5], it has been shown that the emission consists of a broad band in the red or near-infrared region when the Fe3 + ion is tetrahedrally coordinated by oxygen ions. On the other hand, the crystal field is usually larger when the Fe3~ ion occupies an octahedral site and the luminescence would be expected at longer wavelengths. There have been only a few reports of emission from Fe3 + in octahedral sites, and these occur, generally, in the region of 1 ~m [6]. Much work has been done on Fe3~-doped LiAl 5O8’and LiGa5O8 which are unusual in that a strong zero-phonon line is observed in the luminescence at low temperatures [1,2,7]. We have investigated single-crystal samples of the system LiGa5~Fe~O8(x = 0.015, 0.03, 0.05, 0.12, 0.26) over a wide 3 + temperature ions occupy range. LiGa5 08 has an inverse spine! structure in which the Ga two types of site, tetrahedrally-coordinated A sites and octahedrally-coordi0022-231 3/83/0000—0000/$03.O0 © 1983 North-Holland
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Luminescence study of LiGaç —
nated B sites. Previous studies of various mixed spinels and garnets [8.9] have shown that, despite their similar ionic sizes, Fe3 + has a much greater preference for octahedral sites than Ga3~and on this basis one might expect Fe3~ when present as a dilute dopant, to occupy only octahedral sites in LiGa 5O8. However, there is much evidence that the visible luminescence in 3~is due to Fe3~on tetrahedral sites. The results of our studies LiGa5O8: Fe are presented below and are compared with other investigations of this system.
2. Materials preparation Single crystals were grown by exsolution from a slowly cooled matter solvent. Li 2CO3 1.837 g and component oxides, Ga203 23.408, Fe,03 0.0200; with B203 8.000 and PbO 50.000 as the flux are weighed into a 100 c.c. Pt crucible. The covered crucible is heated in a temperature-controlled electric furnace to 1300°C and held at that temperature for 8—lU h to effect solution. Controlled cooling is then begun at a rate of 2°C/h and continued to 400°C. The crucible is removed from the furnace and allowed to cool to room temperature. The crystals are extracted from the solidified flux by immersing the uncovered crucible in a hot solution of HNO3: H,0. (1:3 by volume). =
=
=
=
=
3. Experimental Luminescence spectra were recorded using an argon ion laser for excitation. a Spex 14018 double spectrometer, and an EM! 9863 B photomultiplier (S-20 response) with photon-counting electronics. A 650-W tungsten iodide lamp and ~m monochromator were used for excitation experiments, and suitable filters were used to select the luminescence. For lifetime measurements the laser was chopped and synchronised to a Tracor 1706 multichannel analyser with a minimum channel width of 100 ~ss.A variable temperature cryostat was used to obtain data between 10 and 300 K. Temperatures were measured with a gold—iron thermocouple mounted on the sample block.
4. Results Luminescence spectra of LiGa495Fe00508 at 10, 80 and 300 K, excited by 3~ the argon laser tuned oftoa457.9 nm, are line shown fig.cm 1. Atand 10 K the Fe luminescence consists zero-phonon at 15in108 a structured phonon sideband extending to 13500 cm ‘The peak of the zero-phonon line in the 10 K spectrum is approximately 5 times the intensity of the strongest
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Luminescence study of LiGa
WAVELENGTH
5—
658 WAVELENGTH (nrn)~
~6~3
664
~~E~0K
10K 15600
43
Fe~O8
14800
14000
ENERGY (cnY)
Fig. 1. Luminescence spectrum of LiGa4 95Fe0 0508 at 10, 80, and 300 K. The inset shows the zero-phonon line at 4.2 K.
I 15200 ~—
15150
15100
10K1 15050
ENERGY(cm’)
Fig. 2. Luminescence spectrum in the region of the zero-phonon line of LiGa495Fe00508 at temperature between 10 and 80 K.
sideband feature. The full-width at half-maximum of this zero-phonon line at 10 K was measured to be 12.5 cm’. At 80 K, the zero-phonon line appears to be split into two components with a separation of 28 cm’ and total half-width of 75 cm ‘. Much of the structure observed in the sideband at 10 K has disappeared at 80 K, leaving only four broad features, labelled A, B, C, D in fig. 1. At room temperature, the zero-phonon line is no longer observable and the luminescence consists of a broad featureless band. The sharp feature R which 3~impurity appears in all theSince spectra 13948 cm ‘ is is approximately attributed to trace amounts [10]. the at Fe3~emission one hundred of Cr weaker than the Cr3~ emission from LiGa times containing 31, trace amounts of Cr5O8 give samples rise to appreciable similar concentrations of Cr luminescence in our samples. The luminescence in the region of the zero-phonon line was recorded with higher resolution at several temperatures between 10 and 80 K. From these data, shown in fig. 2, it is clear that the zero-phonon line has four components with estimated intensities and separations as shown in fig. 2. They are observable, but not completely resolved, at temperatures between 10 and 45 K. However, they broaden with increasing temperature so that at 80 K the
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~ENERGY 25 20
30
(c)
300
Luminescence study of LiGa
—
~Fe ~
3c~’) 15
(10
~ ~
400
/
26
500 600 700 WAVELENGTH (om)
630
800
640
650
660
WAVELENGTH (nm) Fig. 3. (a) Excitation spectrum at 77 K of LiGa 495Fe00508 using a tungsten source for excitation and filters to select the luminescence, (b) Excitation spectrum at 77 K of the same sample using a commercial spectrofluorimeter (Perkin—Elmer MPF-3). (c) The absorption spectrum of LiGa495Fe00508 at 77 K. The weaker absorption bands at longer wavelength were recorded on a more heavily doped sample LiGa474 Fe0 2608
Fig. 4. Detail of the excitation spectrum shown in fig. 3a at 77 K in the region of the zero-phonon line.
zero-phonon line appears to consist of two components. The inset in fig. I shows the zero-phonon line recorded at 4.2 K with the sample immersed in liquid helium. The symmetric shape of this line may be compared with the 10 K spectrum shown in fig. 2 where the zero-phonon line has a definite asymmetry, suggesting that two levels are contributing to the luminescence at 10 K. It is clear from these spectra that the structure in the zero-phonon line is due to a splitting in the excited state. For iron concentration x less than 0.05, the luminescence intensity increased with concentration. The luminescence, however, was very weak for x 0.12 and no luminescence was for x 0.26.quenching Similar behaviour ob3~ observed where luminescence occurred was for iron served in a-Gagreater 203 : Fethan 300 ppm [6]. The cause of this quenching is not concentrations known. The radiative lifetime of the Fe3~ emission in LiGa 3~ was : Fe 6 ms measured at room temperature, 80 and 10 K and was found5O8 to be irrespective of temperature. Two excitation spectra, recorded at 77 K under different experimental conditions, are shown in fig. 3a, b. The spectrum of fig. 3a was obtained by using a filter to select the emission above 680 nm. Since the samples contained Cr3~impurities in addition to Fe3~,and since Cr3~also emits above 680 nm. =
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Luminescence study ofLiGa
5 — ~Fe~O8
45
3~ bands [11]. This phase-sensitive detection was used to null out any Cr spectrum is uncorrected for the variation in intensity of the tungsten light source with wavelength. The spectrum shown in fig. 3b was recorded on a commercial spectrofluorimeter. In this instrument a monochromator is used to select only the Fe3~emission, and the output in this case is corrected for variations in the intensity of the light source. As a result, this spectrum provides a more accurate indication of the relative strengths of the absorption bands. However, because of the large bandwidth required on the exit monochromator, the zero-phonon line does not appear in excitation. We attempt to assign spectral features to the energy levels of the Fe3~ion in a tetrahedral field. It should be noted that for the d5 system both the tetrahedral and octahedral crystal field levels are ordered similarly and both are identified by the same crystal field parameter Dq [1]. The assignment of the excitation features to tetrahedral crystal field levels is shown in fig. 3b. The 4T,(4G) zero-phonon line is clearly seen in excitation at 662 nm in fig. 3a. The excitation spectrum in the region of the zero-phonon line is shown in more detail in fig. 4. The peaks marked A’, B’, C’ correspond (to within 30 cm ‘) to the peaks A, B, C in the luminescence sideband at 80 K (fig. 1). The 4T,(4G) and 4T 4G) bands appear to overlap in the excitation spectrum shown in fig. 2( could be due to a splitting of these levels by the lower symmetry 3a. This crystal field in the tetrahedral sites (see below) or it could be due to some remaining Cr3~absorption. An absorption spectrum of LiGa 3~,recorded at 77 K using a Cary 5O8 : Fe model 118 spectrophotometer, is shown in fig. 3c. In previous studies of this system only powder samples were available and direct absorption experiments were not possible [7]. On close examination the absorption spectrum is found to be significantly different from the excitation spectra in fig. 3a, b, which suggests that the absorption may not be due to Fe3~in tetrahedral sites. As fig. 3c shows, the absorption spectrum extends beyond the region where luminescence is observed in emission, and indeed appears to extend beyond the range of our Cary spectrophotometer (800 nm). This would seem to suggest that any luminescence associated with this absorption will occur in the infrared were luminescence from octahedrally-coordinated Fe3 + is generally found.
5. Analysis of results and discussion The luminescence observed in this material is similar to that observed by Neto et al. in the same system [7] and to that observed in the isomorphous material LiA1 3~by several authors [1,2,12].This luminescence is due to 5O8 : Fe transition and is interpreted as originating on Fe3~’ions the 4T 4G) —~6A,(6S) 1( in tetrahedral sites. The detailed structure uncovered in the zero-phonon line
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Luminescence study of LiGa,
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by our variable-temperature studies has not been observed in previous investigations. In the emission spectra of transition metal ions in crystals the shape and intensity of the zero-phonon line and sideband are determined by the difference in coupling of the ion to the lattice in the ground and excited states. When this difference in coupling is small a sharp zero-phonon line, accompanied by a weak one-phonon sideband reflecting the phonon density of states, is usually observed. When the difference in coupling is large, as in F-centre systems, the ground and excited states show a large relative shift on a configurational coordinate diagram and the zero-phonon line is unobservable. It should be noted that in many cases the zero-phonon transition occurs via a magnetic dipole process and may be obscured by intense vibronic sidebands occurring via the much stronger electric dipole mechanism. In the case of LiGa5O8: Fe3’1 the difference in coupling between the ground and excited states appears to correspond to a situation intermediate between these two cases. The sideband appears to be mainly single-phonon in origin and the Stokes shift between the peak of the sideband in emission and the corresponding absorption feature is — 800 cm It is of interest to compare the Fe3~emission with that due to Mn2’ ions which have the same d5 configuration. In MnF 2~ion occupies a site of 2 the Mn inversion symmetry and the 4T 4G)—6A 6G) transition consists of a weak 1( no-phonon line accompanied by 1( a much stronger vibronic sideband [13]. Since only a small fraction of the emission occurs in the zero-phonon line (less than 1%), the measured radiative lifetime of 35 ms indicates that the intrinsic lifetime of the zero-phonon transition is several seconds, which is consistent with a spin-forbidden magnetic dipole process on the Mn ion in a site of inversion symmetry. For Fe3~in LiGa 5O8 the zero-phonon line accounts for 15% of the luminescence at 10 K, which, taken with the radiative lifetime of 6 ms, indicates that the intrinsic lifetime of the zero-phonon line is 40 ms and suggests that the emission occurs via the spin-forbidden electric dipole 3~ion being in a tetrahedral mechanism. This is entirely consistent with the Fe site which lacks inversion symmetry. The energy differences (in cm I) between the zero-phonon line and the phonon sideband features observed in the 10 K luminescence spectrum shown in fig. la are listed in table 1. It also lists for comparison the frequencies of the vibrational modes of LiGa• 5O8 which were measured by Raman scattering [14] and the frequencies of the infrared-active modes of LiGa5O8 as given by Keramides et al. [15]. Since the infrared and Raman techniques measure the host material phonon frequencies at k 0, it is understandable that the sideband peaks do not agree exactly with the vibrational frequencies of the lattice. Due to dispersion in the curves relating phonon frequency and wave vector, the lattice phonon frequencies at other points in the Brillouin zone, where the coupling to the impurity ion may be stronger, will differ from those =
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Luminescence study of LiGa
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Table I 3~and the frequencies of the observed vibrational of LiGa Comparisonmodes between the sideband features of LiGa5O8 : Fe 5O8 Sideband features (cm_i) 108 141 204
IR modes [15] (cm~)
Raman modes [14] (cm_i) 129.5 136.0
198 224 250
243 276 306 342
344 358
393 426 462 483 510
388
238.5 274.1 303.8 322.9 348.7 373.2 403.0 428.3 466.5
486
546
540 592
610
515.5 525.8 534.4 583.5 609.0 633.2 645.6 658.0 742.0 766.0
at k 0. Also, if the interaction which couples the dopant ion to the lattice is short-range, then the vibrations of the lattice in the vicinity of the dopant ion will dominate the sideband and thus the sideband will not reflect the vibrational frequencies of the host lattice. Neto et a!. [7] treat the sideband of this transition (at 25 K) as being predominantly multiphonon with vibronic structure characterised by a fundamental energy of 182 ±31 cm The additional structure resolved in the 10 K spectrum (fig. Ia) is not consistent with this assignment and the agreement between the sideband peaks and the Raman and infrared frequencies is consistent with the single-phonon picture suggested above. We now come to a consideration of the zero-phonon line itself and the splitting observed in fig. 2. The width of this line at 4.2 K (inset, fig. 1) is probably a result of homogeneous broadening due to residual strains in the crystal and the narrow line width observed (0.5 nm) is an indication of the =
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Luminescence study of LiGa
5
_
~Fe ~
4T 6A quality of the single-crystal samples. When one considers that the 1—T 1 transition energy shows a large variation with crystal field, the line width of the zero-phonon line is surprisingly the line width of 2E—4Asmall, being comparable with 3~ ions in the same 0.2 nm observed 2 transition to only Cr slightly with crystal material, although for the the energy of this transitiondue varies field. If the Fe3~ion were in a site of pure tetrahedral symmetry (Td). the 4T 1 level would be split by spin—orbit coupling only, resulting in three components a doublet, a quartet, and a sextet, the doublet lying lowest. However, when the near neighbour cations are considered the actual site symmetry is C3 (and in the “octahedral” site, the would 4T 4G) level will site splitsymmetry into 4E and 4A be C2). Group theory predicts that the 1( 2 in C3 symmetry and that spin—orbit coupling will lift any remaining degeneracy. 6A 6S) ground state except degeneracy, in the by excited state. The 1( This splitting. is split for intoKramers a quartet and a doublet spin—orbit coupling. which has been measured by EPR techniques for many d5 systems [16]. is generally less than 0.1 cm’ and thus can be ignored. Calculations of the splitting of the lowest 4T 1 state, to spin—orbit only. have 5 systems usingdue typical values of interaction the cubic crystal fieldbeen and carried out parameters for d spin—orbit [17,18]. The overall splitting is typically 100 cm ‘. However, in this case the C 3-symmetry field is expected to be significant to 3 due ions. the large charge difference between the due neighbouring Li +symmetry and Ga field is Therefore, we assume that the splitting to the lower greater than that due to spin—orbit coupling; the spin—orbit splitting being seen as a fine structure on the C 3 crystal field levels. This is consistent with the 4E observation four the components in the zero-phonon line, provided that4Etheterm level is lowerofthan 4A 2 level, since shown spin—orbit this into four levels. The excitation spectra in fig.coupling 3 are notsplits inconsistent with such a splitting in the 4T 4T 4G) transition is only In fig. 3b, the 1( weakly observed while in 1fig.level. 3a the specrum is distorted somewhat by the variation in intensity of the light source and possibly by the presence of some residual Cr3~bands. Such a splitting should also occur in 4T levels 4T 4D) level shown in2 fig. 3. and would explain the observed splitting in the The energy levels for a d5 ion in an2(octahedral or tetrahedral crystal field were assigned to the excitation and absorption spectra as shown in fig. 3. Although these spectra are very similar between 400 and 500 nm there is a definite shift between the energies of the 4E(4D) and 4T 4D) levels as measured in excitation and ‘in absorption. Therefore, in view of2(the known site preferences of Fe3~and Ga3~ions, we interpret the excitation and absorption spectra as being due to Fe3~in tetrahedral and octahedral sites, respectively. The difference between the absorption and excitation traces, especially for the x 0.26 sample, together with the weakness of the luminescence, indicate that only a small fraction of the Fe3~ions occupy tetrahedral sites. The energy level assignments indicated in fig. 3 were used to calculate the crystal field —
~‘
=
Absorption: Dq=800cm~t B=5l0 cm_i C=32l0cm~’
Dq=700cm B=565 cm_i C=3000cm’
Excitation: i)
15300 15380
theor(cm_i)
15830
15790
exp(cm~t)
theor(cm_i)
exp(cm
Energy level
4T 4G) 1(
18310
18690
18610
18480
4T 4G) 2(
21150
21150
20670
20740
4E, 4A 4G) 1(
5O8
Table 2 3~in LiGa Calculated and measured energy levels for the excitation and absorption spectra of Fe
23190
22220
23140
21640
4T 4D) 2(
24790
24720
24630
24630
4E(4D)
28810
31850
4T 4P) 1(
~0
000
a
N
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Luminescence study of LiGa,
Fe ,O,~
parameter Dq and the Racah parameters B and C using the energy matrices for a d5 ion in a cubic crystal field. It was found that the best fit to the excitation data was obtained using B 565 cm’’, C 3000 cm~ and Dq 700 cm 1; on the other hand, the best fit to the absorption data was obtained with B 510 cm ‘, C 3210 cm and Dq 800 cm~’.Table 2 shows, for comparison, the experimental energy levels in excitation and in absorption, together with the theoretical values calculated using the above parameter values. The crystal field levels below 380 nm in fig. 3(c) are obscured by the strong absorption in this region, which is presumably due to charge transfer transitions, and the absorption peaks marked B could not be fitted to the crystal field levels of Fe3~in an octahedral field. Our assignment of the d5 energy levels to the excitation spectrum is different from that of Neto et al. [7], who assign the peak at 24620 cm~‘(406 nm) to the 4A 4E(4G) energy level and fit all other levels accordingly. These 1, observe the part of the excitation spectrum beyond 550 nm. authors did not Since a portion of this (e.g. the A’, B’, C’ features) is the mirror image of the luminescence spectrum, there is no doubt that this excitation band represents the 6A 6S) —s4T,(4G) absorption band. Their assignment of the band at 1( 18350 cm’(548 nm) to the 4T,(4G) level indicates a very large Stokes shift of 3600 cm ‘. Such a large Stokes shift is not consistent with the observation of a zero-phonon line in emission. Pott and McNicol [II, in their study of LiAl 3~,observed an excitation spectrum similar to that shown in fig. 5O8: Fe assignment of the d5 energy levels to this spectrum is the same as 3b and their that proposed here for LiGa 3t Telfer and Walker [4] examined the 5O8: Fe number of minerals in the plagioclase excitation spectrum of Fe3~in a large series. In these minerals, Fe3~is always tetrahedrally coordinated and emission is generally observed at 700 nm. The excitation spectra reported for these materials are similar to that observed in this study for LiGa 3~and they 5O8: Fe employed also interpret their data using an energy assignment similar to that here. Since the absorption spectrum, which we are attributing to octahedrally-coordinated Fe3~,extends to 800 nm, any emission associated with this absorption would be expected to occur at longer wavelengths. Attempts were made to observe luminescence out to 1.1 p.m, using an RCA 7102 photomultiplier (S-I response) and using xenon arc lamp, argon-ion laser and nitrogen laser excitation. No luminescence was observed. However, the photomultiplier has a very low quantum efficiency and the response is falling off between 0.9 and 1.1 ~tm. Neto et al. [7] report luminescence from octahedrally-coordinated Fe3~in LiGa 3~ on 5O8 atsites 720wasnm. However, to find Fe some octahedral observed by usnoin luminescence this region; weattributable did, however, luminescence due to trace amounts of Cr3~ in all our samples (indicated by R in fig. I). Luminescence has been observed at 950 nm in a-Ga 3~ fromtoFethe in octahedral sites [6] in which the average Ga—O distance 203 is close =
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Luminescence study of LiGa
5 _ 0008
51
octahedral Ga—O distance in LiGa5O8 and a value of 860 cm was obtained for3~ theincrystal field parameter The value greater suggested herethat (800 I) for octahedral sites is notDq. significantly than forcmFe3~ in Fe tetrahedral sites (700 cm I) This is consistent with point-charge calculations, taking into account the different distances to the neighbouring ligands in the two cases.
6. Conclusion The excitation and absorption spectra of LiGa have The been lifetime associ3’1~ in tetrahedral and octahedral sites,5~Fe~O8 respectively. ated intensity with Fe of the luminescence, as well as its relationship to the excitation and spectrum, confirm that the emission is due to a small fraction of the Fe3~ions in tetrahedral sites. The absorption spectrum suggests that the bulk of the Fe34 occupy octahedral sites in agreement with the established site preferences of Fe3~and Ga3~ions. Although lower-symmetry fields in both sites will be expected to split the cubic crystal field levels, values of Dq, B, and C have been obtained which give reasonable agreement between calculated and experimental energy levels. In addition, the detailed fine structure at low temperature in the zero-phonon line and in the vibrational sideband have been resolved for the first time.
Acknowledgements We acknowledge helpful communications with Dr. G. Walker and Dr. D.L. Wood. This work was supported by the Irish National Board for Science and Technology under grant No. URG-l8-77. Two of us (CMcS and PJC) wish to acknowledge the support of the Irish Department of Education through its Maintenance Grant Scheme.
References [I] G.T. Polt and B.D. McNicol, J. Chem. Phys. 56 (1972) 5246. [21 N.T. Melamed, F. de S. Barros, P.J. Vicarro and JO. Artman, Phys. Rev. B5 (1972) 3377. [3] D.J. Telfer and G. Walker, J. Lumin. 11(1976) 315. [4] D.J. Telfer and G. Walker, Modern Geology 6 (1978) 199. [51W.H.J. Stork and G.T. Pott, J. Phys. Chem. 78 (1974) 2496. [6) G.T. Pott and B.D. McNicol, J. Lumin, 6 (1973) 225. [7] J. Mario Neto, T. Abritta, F. deS. Barros and NT. Melamed, J. Lumin. 22(1981)109; 24/25 (1981) 249. [8] J.A. Schulkes and G. Blasse, J. Phys. Chem. Sol. 24(1963)1651.
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Luminescence study of LiGa
0_~Fe,05
[9] M. Marezio, J.P. Remeika and PD. Dernier, Acta. Cryst. B24 (1967) 1670. [10] T.J. Glynn, J.P. Larkin, G.F. Imbusch, DL. Wood and J.P. Remeika. Phys. Lett. 30A (1969) 189. [II] M.O. Henry, J.P. Larkin and G.F. lmbusch, Phys. Rev. Bl3 (1976) 1893. [12] D.T Palumbo, J. Lumin. 4 (1971) 89. [13] R.L. Greene, D.D. Sell, R.S. Feigelson. G.F. lmbusch and Hi. Guggenheim. Phys. Rev. 171 (1968) 600. [14] C. McShera, T.J. Glynn and J.P. Remeika. to be published. [15] V.G. Keramides, BA. DeAngelis and W.B. White, J. Sol. St. Chem. 15 (1975) 223. [16] W. Low, Paramagnetic Resonance in Solids (Academic Press. New York, 1960). [17) D.H. Goode, J. Chem. Phys. 43 (1965) 2830. [18] W. Low and G. Rosengarten, J. Molec. Spectr. 12 (1964) 346.
41
LUMINESCENCE STUDY OF LiGa5.~Fe~Os C. McSHERA, P.J. COLLERAN, T.J. GLYNN and G.F. IMBUSCH Physics Department, University College, Ga/way, Ire/and
J.P. REMEIKA Bell Laboratories, Murray Hill, New Jersey 07974, USA
Received 29 August 1982
Absorption, excitation, and luminescence measurements are presented for LiGa5 — Fe~O8 with concentrations x from 0.0 15 to 0.26. 6A The 6S)~—4T,(4G) emission and excitation indicate that the transitiondata in the d5 configurared luminescence tion for Fe3* in tetrahedral observed is sites. due to Single-crystal the 1( samples allow direct absorption measurements for the first time and these indicate that most of the Fe3 + substitutes for Ga3 + in octahedral sites. Fine structure in the zero-phonon line and in the sideband is reported.
I. Introduction Luminescence from Fe3~, incorporated as an impurity in many host materials, has been reported in the literature [1—5].In each case, the luminescence is assigned to the transition 6A 6S) ~—4T 4G)between the lowest crystal 1( these levels 1( belong to different strong field levels in the d5 configuration. Since field configurations, a broad band transition is expected and the occurrence of a zero-phonon line depends on the difference in coupling to the lattice in the ground and excited states. In a review of several Fe3~-doped oxides [5], it has been shown that the emission consists of a broad band in the red or near-infrared region when the Fe3 + ion is tetrahedrally coordinated by oxygen ions. On the other hand, the crystal field is usually larger when the Fe3~ ion occupies an octahedral site and the luminescence would be expected at longer wavelengths. There have been only a few reports of emission from Fe3 + in octahedral sites, and these occur, generally, in the region of 1 ~m [6]. Much work has been done on Fe3~-doped LiAl 5O8’and LiGa5O8 which are unusual in that a strong zero-phonon line is observed in the luminescence at low temperatures [1,2,7]. We have investigated single-crystal samples of the system LiGa5~Fe~O8(x = 0.015, 0.03, 0.05, 0.12, 0.26) over a wide 3 + temperature ions occupy range. LiGa5 08 has an inverse spine! structure in which the Ga two types of site, tetrahedrally-coordinated A sites and octahedrally-coordi0022-231 3/83/0000—0000/$03.O0 © 1983 North-Holland
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Luminescence study of LiGaç —
nated B sites. Previous studies of various mixed spinels and garnets [8.9] have shown that, despite their similar ionic sizes, Fe3 + has a much greater preference for octahedral sites than Ga3~and on this basis one might expect Fe3~ when present as a dilute dopant, to occupy only octahedral sites in LiGa 5O8. However, there is much evidence that the visible luminescence in 3~is due to Fe3~on tetrahedral sites. The results of our studies LiGa5O8: Fe are presented below and are compared with other investigations of this system.
2. Materials preparation Single crystals were grown by exsolution from a slowly cooled matter solvent. Li 2CO3 1.837 g and component oxides, Ga203 23.408, Fe,03 0.0200; with B203 8.000 and PbO 50.000 as the flux are weighed into a 100 c.c. Pt crucible. The covered crucible is heated in a temperature-controlled electric furnace to 1300°C and held at that temperature for 8—lU h to effect solution. Controlled cooling is then begun at a rate of 2°C/h and continued to 400°C. The crucible is removed from the furnace and allowed to cool to room temperature. The crystals are extracted from the solidified flux by immersing the uncovered crucible in a hot solution of HNO3: H,0. (1:3 by volume). =
=
=
=
=
3. Experimental Luminescence spectra were recorded using an argon ion laser for excitation. a Spex 14018 double spectrometer, and an EM! 9863 B photomultiplier (S-20 response) with photon-counting electronics. A 650-W tungsten iodide lamp and ~m monochromator were used for excitation experiments, and suitable filters were used to select the luminescence. For lifetime measurements the laser was chopped and synchronised to a Tracor 1706 multichannel analyser with a minimum channel width of 100 ~ss.A variable temperature cryostat was used to obtain data between 10 and 300 K. Temperatures were measured with a gold—iron thermocouple mounted on the sample block.
4. Results Luminescence spectra of LiGa495Fe00508 at 10, 80 and 300 K, excited by 3~ the argon laser tuned oftoa457.9 nm, are line shown fig.cm 1. Atand 10 K the Fe luminescence consists zero-phonon at 15in108 a structured phonon sideband extending to 13500 cm ‘The peak of the zero-phonon line in the 10 K spectrum is approximately 5 times the intensity of the strongest
C. McShera et al.
640
/
Luminescence study of LiGa
WAVELENGTH
5—
658 WAVELENGTH (nrn)~
~6~3
664
~~E~0K
10K 15600
43
Fe~O8
14800
14000
ENERGY (cnY)
Fig. 1. Luminescence spectrum of LiGa4 95Fe0 0508 at 10, 80, and 300 K. The inset shows the zero-phonon line at 4.2 K.
I 15200 ~—
15150
15100
10K1 15050
ENERGY(cm’)
Fig. 2. Luminescence spectrum in the region of the zero-phonon line of LiGa495Fe00508 at temperature between 10 and 80 K.
sideband feature. The full-width at half-maximum of this zero-phonon line at 10 K was measured to be 12.5 cm’. At 80 K, the zero-phonon line appears to be split into two components with a separation of 28 cm’ and total half-width of 75 cm ‘. Much of the structure observed in the sideband at 10 K has disappeared at 80 K, leaving only four broad features, labelled A, B, C, D in fig. 1. At room temperature, the zero-phonon line is no longer observable and the luminescence consists of a broad featureless band. The sharp feature R which 3~impurity appears in all theSince spectra 13948 cm ‘ is is approximately attributed to trace amounts [10]. the at Fe3~emission one hundred of Cr weaker than the Cr3~ emission from LiGa times containing 31, trace amounts of Cr5O8 give samples rise to appreciable similar concentrations of Cr luminescence in our samples. The luminescence in the region of the zero-phonon line was recorded with higher resolution at several temperatures between 10 and 80 K. From these data, shown in fig. 2, it is clear that the zero-phonon line has four components with estimated intensities and separations as shown in fig. 2. They are observable, but not completely resolved, at temperatures between 10 and 45 K. However, they broaden with increasing temperature so that at 80 K the
C. McShera et aL
44
~ENERGY 25 20
30
(c)
300
Luminescence study of LiGa
—
~Fe ~
3c~’) 15
(10
~ ~
400
/
26
500 600 700 WAVELENGTH (om)
630
800
640
650
660
WAVELENGTH (nm) Fig. 3. (a) Excitation spectrum at 77 K of LiGa 495Fe00508 using a tungsten source for excitation and filters to select the luminescence, (b) Excitation spectrum at 77 K of the same sample using a commercial spectrofluorimeter (Perkin—Elmer MPF-3). (c) The absorption spectrum of LiGa495Fe00508 at 77 K. The weaker absorption bands at longer wavelength were recorded on a more heavily doped sample LiGa474 Fe0 2608
Fig. 4. Detail of the excitation spectrum shown in fig. 3a at 77 K in the region of the zero-phonon line.
zero-phonon line appears to consist of two components. The inset in fig. I shows the zero-phonon line recorded at 4.2 K with the sample immersed in liquid helium. The symmetric shape of this line may be compared with the 10 K spectrum shown in fig. 2 where the zero-phonon line has a definite asymmetry, suggesting that two levels are contributing to the luminescence at 10 K. It is clear from these spectra that the structure in the zero-phonon line is due to a splitting in the excited state. For iron concentration x less than 0.05, the luminescence intensity increased with concentration. The luminescence, however, was very weak for x 0.12 and no luminescence was for x 0.26.quenching Similar behaviour ob3~ observed where luminescence occurred was for iron served in a-Gagreater 203 : Fethan 300 ppm [6]. The cause of this quenching is not concentrations known. The radiative lifetime of the Fe3~ emission in LiGa 3~ was : Fe 6 ms measured at room temperature, 80 and 10 K and was found5O8 to be irrespective of temperature. Two excitation spectra, recorded at 77 K under different experimental conditions, are shown in fig. 3a, b. The spectrum of fig. 3a was obtained by using a filter to select the emission above 680 nm. Since the samples contained Cr3~impurities in addition to Fe3~,and since Cr3~also emits above 680 nm. =
=
C. McShera et al.
/
Luminescence study ofLiGa
5 — ~Fe~O8
45
3~ bands [11]. This phase-sensitive detection was used to null out any Cr spectrum is uncorrected for the variation in intensity of the tungsten light source with wavelength. The spectrum shown in fig. 3b was recorded on a commercial spectrofluorimeter. In this instrument a monochromator is used to select only the Fe3~emission, and the output in this case is corrected for variations in the intensity of the light source. As a result, this spectrum provides a more accurate indication of the relative strengths of the absorption bands. However, because of the large bandwidth required on the exit monochromator, the zero-phonon line does not appear in excitation. We attempt to assign spectral features to the energy levels of the Fe3~ion in a tetrahedral field. It should be noted that for the d5 system both the tetrahedral and octahedral crystal field levels are ordered similarly and both are identified by the same crystal field parameter Dq [1]. The assignment of the excitation features to tetrahedral crystal field levels is shown in fig. 3b. The 4T,(4G) zero-phonon line is clearly seen in excitation at 662 nm in fig. 3a. The excitation spectrum in the region of the zero-phonon line is shown in more detail in fig. 4. The peaks marked A’, B’, C’ correspond (to within 30 cm ‘) to the peaks A, B, C in the luminescence sideband at 80 K (fig. 1). The 4T,(4G) and 4T 4G) bands appear to overlap in the excitation spectrum shown in fig. 2( could be due to a splitting of these levels by the lower symmetry 3a. This crystal field in the tetrahedral sites (see below) or it could be due to some remaining Cr3~absorption. An absorption spectrum of LiGa 3~,recorded at 77 K using a Cary 5O8 : Fe model 118 spectrophotometer, is shown in fig. 3c. In previous studies of this system only powder samples were available and direct absorption experiments were not possible [7]. On close examination the absorption spectrum is found to be significantly different from the excitation spectra in fig. 3a, b, which suggests that the absorption may not be due to Fe3~in tetrahedral sites. As fig. 3c shows, the absorption spectrum extends beyond the region where luminescence is observed in emission, and indeed appears to extend beyond the range of our Cary spectrophotometer (800 nm). This would seem to suggest that any luminescence associated with this absorption will occur in the infrared were luminescence from octahedrally-coordinated Fe3 + is generally found.
5. Analysis of results and discussion The luminescence observed in this material is similar to that observed by Neto et al. in the same system [7] and to that observed in the isomorphous material LiA1 3~by several authors [1,2,12].This luminescence is due to 5O8 : Fe transition and is interpreted as originating on Fe3~’ions the 4T 4G) —~6A,(6S) 1( in tetrahedral sites. The detailed structure uncovered in the zero-phonon line
46
C. McShera et at.
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Luminescence study of LiGa,
-
by our variable-temperature studies has not been observed in previous investigations. In the emission spectra of transition metal ions in crystals the shape and intensity of the zero-phonon line and sideband are determined by the difference in coupling of the ion to the lattice in the ground and excited states. When this difference in coupling is small a sharp zero-phonon line, accompanied by a weak one-phonon sideband reflecting the phonon density of states, is usually observed. When the difference in coupling is large, as in F-centre systems, the ground and excited states show a large relative shift on a configurational coordinate diagram and the zero-phonon line is unobservable. It should be noted that in many cases the zero-phonon transition occurs via a magnetic dipole process and may be obscured by intense vibronic sidebands occurring via the much stronger electric dipole mechanism. In the case of LiGa5O8: Fe3’1 the difference in coupling between the ground and excited states appears to correspond to a situation intermediate between these two cases. The sideband appears to be mainly single-phonon in origin and the Stokes shift between the peak of the sideband in emission and the corresponding absorption feature is — 800 cm It is of interest to compare the Fe3~emission with that due to Mn2’ ions which have the same d5 configuration. In MnF 2~ion occupies a site of 2 the Mn inversion symmetry and the 4T 4G)—6A 6G) transition consists of a weak 1( no-phonon line accompanied by 1( a much stronger vibronic sideband [13]. Since only a small fraction of the emission occurs in the zero-phonon line (less than 1%), the measured radiative lifetime of 35 ms indicates that the intrinsic lifetime of the zero-phonon transition is several seconds, which is consistent with a spin-forbidden magnetic dipole process on the Mn ion in a site of inversion symmetry. For Fe3~in LiGa 5O8 the zero-phonon line accounts for 15% of the luminescence at 10 K, which, taken with the radiative lifetime of 6 ms, indicates that the intrinsic lifetime of the zero-phonon line is 40 ms and suggests that the emission occurs via the spin-forbidden electric dipole 3~ion being in a tetrahedral mechanism. This is entirely consistent with the Fe site which lacks inversion symmetry. The energy differences (in cm I) between the zero-phonon line and the phonon sideband features observed in the 10 K luminescence spectrum shown in fig. la are listed in table 1. It also lists for comparison the frequencies of the vibrational modes of LiGa• 5O8 which were measured by Raman scattering [14] and the frequencies of the infrared-active modes of LiGa5O8 as given by Keramides et al. [15]. Since the infrared and Raman techniques measure the host material phonon frequencies at k 0, it is understandable that the sideband peaks do not agree exactly with the vibrational frequencies of the lattice. Due to dispersion in the curves relating phonon frequency and wave vector, the lattice phonon frequencies at other points in the Brillouin zone, where the coupling to the impurity ion may be stronger, will differ from those =
C. McShera et al.
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Luminescence study of LiGa
5 — ~Fe~O8
47
Table I 3~and the frequencies of the observed vibrational of LiGa Comparisonmodes between the sideband features of LiGa5O8 : Fe 5O8 Sideband features (cm_i) 108 141 204
IR modes [15] (cm~)
Raman modes [14] (cm_i) 129.5 136.0
198 224 250
243 276 306 342
344 358
393 426 462 483 510
388
238.5 274.1 303.8 322.9 348.7 373.2 403.0 428.3 466.5
486
546
540 592
610
515.5 525.8 534.4 583.5 609.0 633.2 645.6 658.0 742.0 766.0
at k 0. Also, if the interaction which couples the dopant ion to the lattice is short-range, then the vibrations of the lattice in the vicinity of the dopant ion will dominate the sideband and thus the sideband will not reflect the vibrational frequencies of the host lattice. Neto et a!. [7] treat the sideband of this transition (at 25 K) as being predominantly multiphonon with vibronic structure characterised by a fundamental energy of 182 ±31 cm The additional structure resolved in the 10 K spectrum (fig. Ia) is not consistent with this assignment and the agreement between the sideband peaks and the Raman and infrared frequencies is consistent with the single-phonon picture suggested above. We now come to a consideration of the zero-phonon line itself and the splitting observed in fig. 2. The width of this line at 4.2 K (inset, fig. 1) is probably a result of homogeneous broadening due to residual strains in the crystal and the narrow line width observed (0.5 nm) is an indication of the =
48
C. McShera et at.
/
Luminescence study of LiGa
5
_
~Fe ~
4T 6A quality of the single-crystal samples. When one considers that the 1—T 1 transition energy shows a large variation with crystal field, the line width of the zero-phonon line is surprisingly the line width of 2E—4Asmall, being comparable with 3~ ions in the same 0.2 nm observed 2 transition to only Cr slightly with crystal material, although for the the energy of this transitiondue varies field. If the Fe3~ion were in a site of pure tetrahedral symmetry (Td). the 4T 1 level would be split by spin—orbit coupling only, resulting in three components a doublet, a quartet, and a sextet, the doublet lying lowest. However, when the near neighbour cations are considered the actual site symmetry is C3 (and in the “octahedral” site, the would 4T 4G) level will site splitsymmetry into 4E and 4A be C2). Group theory predicts that the 1( 2 in C3 symmetry and that spin—orbit coupling will lift any remaining degeneracy. 6A 6S) ground state except degeneracy, in the by excited state. The 1( This splitting. is split for intoKramers a quartet and a doublet spin—orbit coupling. which has been measured by EPR techniques for many d5 systems [16]. is generally less than 0.1 cm’ and thus can be ignored. Calculations of the splitting of the lowest 4T 1 state, to spin—orbit only. have 5 systems usingdue typical values of interaction the cubic crystal fieldbeen and carried out parameters for d spin—orbit [17,18]. The overall splitting is typically 100 cm ‘. However, in this case the C 3-symmetry field is expected to be significant to 3 due ions. the large charge difference between the due neighbouring Li +symmetry and Ga field is Therefore, we assume that the splitting to the lower greater than that due to spin—orbit coupling; the spin—orbit splitting being seen as a fine structure on the C 3 crystal field levels. This is consistent with the 4E observation four the components in the zero-phonon line, provided that4Etheterm level is lowerofthan 4A 2 level, since shown spin—orbit this into four levels. The excitation spectra in fig.coupling 3 are notsplits inconsistent with such a splitting in the 4T 4T 4G) transition is only In fig. 3b, the 1( weakly observed while in 1fig.level. 3a the specrum is distorted somewhat by the variation in intensity of the light source and possibly by the presence of some residual Cr3~bands. Such a splitting should also occur in 4T levels 4T 4D) level shown in2 fig. 3. and would explain the observed splitting in the The energy levels for a d5 ion in an2(octahedral or tetrahedral crystal field were assigned to the excitation and absorption spectra as shown in fig. 3. Although these spectra are very similar between 400 and 500 nm there is a definite shift between the energies of the 4E(4D) and 4T 4D) levels as measured in excitation and ‘in absorption. Therefore, in view of2(the known site preferences of Fe3~and Ga3~ions, we interpret the excitation and absorption spectra as being due to Fe3~in tetrahedral and octahedral sites, respectively. The difference between the absorption and excitation traces, especially for the x 0.26 sample, together with the weakness of the luminescence, indicate that only a small fraction of the Fe3~ions occupy tetrahedral sites. The energy level assignments indicated in fig. 3 were used to calculate the crystal field —
~‘
=
Absorption: Dq=800cm~t B=5l0 cm_i C=32l0cm~’
Dq=700cm B=565 cm_i C=3000cm’
Excitation: i)
15300 15380
theor(cm_i)
15830
15790
exp(cm~t)
theor(cm_i)
exp(cm
Energy level
4T 4G) 1(
18310
18690
18610
18480
4T 4G) 2(
21150
21150
20670
20740
4E, 4A 4G) 1(
5O8
Table 2 3~in LiGa Calculated and measured energy levels for the excitation and absorption spectra of Fe
23190
22220
23140
21640
4T 4D) 2(
24790
24720
24630
24630
4E(4D)
28810
31850
4T 4P) 1(
~0
000
a
N
50
C. McShera et at,
/
Luminescence study of LiGa,
Fe ,O,~
parameter Dq and the Racah parameters B and C using the energy matrices for a d5 ion in a cubic crystal field. It was found that the best fit to the excitation data was obtained using B 565 cm’’, C 3000 cm~ and Dq 700 cm 1; on the other hand, the best fit to the absorption data was obtained with B 510 cm ‘, C 3210 cm and Dq 800 cm~’.Table 2 shows, for comparison, the experimental energy levels in excitation and in absorption, together with the theoretical values calculated using the above parameter values. The crystal field levels below 380 nm in fig. 3(c) are obscured by the strong absorption in this region, which is presumably due to charge transfer transitions, and the absorption peaks marked B could not be fitted to the crystal field levels of Fe3~in an octahedral field. Our assignment of the d5 energy levels to the excitation spectrum is different from that of Neto et al. [7], who assign the peak at 24620 cm~‘(406 nm) to the 4A 4E(4G) energy level and fit all other levels accordingly. These 1, observe the part of the excitation spectrum beyond 550 nm. authors did not Since a portion of this (e.g. the A’, B’, C’ features) is the mirror image of the luminescence spectrum, there is no doubt that this excitation band represents the 6A 6S) —s4T,(4G) absorption band. Their assignment of the band at 1( 18350 cm’(548 nm) to the 4T,(4G) level indicates a very large Stokes shift of 3600 cm ‘. Such a large Stokes shift is not consistent with the observation of a zero-phonon line in emission. Pott and McNicol [II, in their study of LiAl 3~,observed an excitation spectrum similar to that shown in fig. 5O8: Fe assignment of the d5 energy levels to this spectrum is the same as 3b and their that proposed here for LiGa 3t Telfer and Walker [4] examined the 5O8: Fe number of minerals in the plagioclase excitation spectrum of Fe3~in a large series. In these minerals, Fe3~is always tetrahedrally coordinated and emission is generally observed at 700 nm. The excitation spectra reported for these materials are similar to that observed in this study for LiGa 3~and they 5O8: Fe employed also interpret their data using an energy assignment similar to that here. Since the absorption spectrum, which we are attributing to octahedrally-coordinated Fe3~,extends to 800 nm, any emission associated with this absorption would be expected to occur at longer wavelengths. Attempts were made to observe luminescence out to 1.1 p.m, using an RCA 7102 photomultiplier (S-I response) and using xenon arc lamp, argon-ion laser and nitrogen laser excitation. No luminescence was observed. However, the photomultiplier has a very low quantum efficiency and the response is falling off between 0.9 and 1.1 ~tm. Neto et al. [7] report luminescence from octahedrally-coordinated Fe3~in LiGa 3~ on 5O8 atsites 720wasnm. However, to find Fe some octahedral observed by usnoin luminescence this region; weattributable did, however, luminescence due to trace amounts of Cr3~ in all our samples (indicated by R in fig. I). Luminescence has been observed at 950 nm in a-Ga 3~ fromtoFethe in octahedral sites [6] in which the average Ga—O distance 203 is close =
=
=
=
=
=
C. McShera et at.
/
Luminescence study of LiGa
5 _ 0008
51
octahedral Ga—O distance in LiGa5O8 and a value of 860 cm was obtained for3~ theincrystal field parameter The value greater suggested herethat (800 I) for octahedral sites is notDq. significantly than forcmFe3~ in Fe tetrahedral sites (700 cm I) This is consistent with point-charge calculations, taking into account the different distances to the neighbouring ligands in the two cases.
6. Conclusion The excitation and absorption spectra of LiGa have The been lifetime associ3’1~ in tetrahedral and octahedral sites,5~Fe~O8 respectively. ated intensity with Fe of the luminescence, as well as its relationship to the excitation and spectrum, confirm that the emission is due to a small fraction of the Fe3~ions in tetrahedral sites. The absorption spectrum suggests that the bulk of the Fe34 occupy octahedral sites in agreement with the established site preferences of Fe3~and Ga3~ions. Although lower-symmetry fields in both sites will be expected to split the cubic crystal field levels, values of Dq, B, and C have been obtained which give reasonable agreement between calculated and experimental energy levels. In addition, the detailed fine structure at low temperature in the zero-phonon line and in the vibrational sideband have been resolved for the first time.
Acknowledgements We acknowledge helpful communications with Dr. G. Walker and Dr. D.L. Wood. This work was supported by the Irish National Board for Science and Technology under grant No. URG-l8-77. Two of us (CMcS and PJC) wish to acknowledge the support of the Irish Department of Education through its Maintenance Grant Scheme.
References [I] G.T. Polt and B.D. McNicol, J. Chem. Phys. 56 (1972) 5246. [21 N.T. Melamed, F. de S. Barros, P.J. Vicarro and JO. Artman, Phys. Rev. B5 (1972) 3377. [3] D.J. Telfer and G. Walker, J. Lumin. 11(1976) 315. [4] D.J. Telfer and G. Walker, Modern Geology 6 (1978) 199. [51W.H.J. Stork and G.T. Pott, J. Phys. Chem. 78 (1974) 2496. [6) G.T. Pott and B.D. McNicol, J. Lumin, 6 (1973) 225. [7] J. Mario Neto, T. Abritta, F. deS. Barros and NT. Melamed, J. Lumin. 22(1981)109; 24/25 (1981) 249. [8] J.A. Schulkes and G. Blasse, J. Phys. Chem. Sol. 24(1963)1651.
52
C McShera clot.
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Luminescence study of LiGa
0_~Fe,05
[9] M. Marezio, J.P. Remeika and PD. Dernier, Acta. Cryst. B24 (1967) 1670. [10] T.J. Glynn, J.P. Larkin, G.F. Imbusch, DL. Wood and J.P. Remeika. Phys. Lett. 30A (1969) 189. [II] M.O. Henry, J.P. Larkin and G.F. lmbusch, Phys. Rev. Bl3 (1976) 1893. [12] D.T Palumbo, J. Lumin. 4 (1971) 89. [13] R.L. Greene, D.D. Sell, R.S. Feigelson. G.F. lmbusch and Hi. Guggenheim. Phys. Rev. 171 (1968) 600. [14] C. McShera, T.J. Glynn and J.P. Remeika. to be published. [15] V.G. Keramides, BA. DeAngelis and W.B. White, J. Sol. St. Chem. 15 (1975) 223. [16] W. Low, Paramagnetic Resonance in Solids (Academic Press. New York, 1960). [17) D.H. Goode, J. Chem. Phys. 43 (1965) 2830. [18] W. Low and G. Rosengarten, J. Molec. Spectr. 12 (1964) 346.