LOW SYMMETRY CRYSTAL FIELD IN LITHIUM SPINEL

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LOW SYMMETRY CRYSTAL FIELD IN LITHIUM SPINEL

M. Pietrov, H. Szymczak, R. Wadas, W. Wardzynski

To cite this version:

M. Pietrov, H. Szymczak, R. Wadas, W. Wardzynski. LOW SYMMETRY CRYSTAL FIELD IN LITHIUM SPINEL. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-847-C1-849.

�10.1051/jphyscol:19711296�. �jpa-00214326�

JOURNAL DE PHYSIQUE Colloque C I , supplkment au no 2-3, Tome 32, Fe'vrier-Mars 1971, page C 1

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847

LOW SYMMETRY CRYSTAL FIELD IN LITHIUM SPLNEL

M. P. PIETROV (**)

Institute of Semiconductors, Leningrad, USSR, H. SZYMCZAK, R. WADAS, W. WARD~YNSKI

Institute of Electron Technology, Warsaw, Poland

Resume.

-

Nous avons mesurk l'absorption optique et le spectre de luminescence du spinelle LiAlsOa : Cr3+ &

293 OK, 77 OK et 4,2OK. Les valeurs obtenues ainsi que les rksultats de mesures de RPE et RMN concordent avec une thkorie du champ cristallin qui comprend le couplage spin-orbite et l'action d'un champ cristallin de faible symktrie sur un ion du site B dans les spinelles. Les formules des parambtres g, D et E de l'hamiltonien de spin et de la skparation du doublet ont kt6 dkrivh.

Abstract.

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The optical absorption and luminescence spectra of spinel LiAls08 : Cr3+ have been measured at 293 OK, 77 OK and 4.2 OK. The data together with the results of EPR and NMR measurements have been fitted to a crystal field theory which includes spin-orbit coupling and low symmetry crystal field acting on ion in the B site in spinels.

Formulae for g, D, E parameters of spin-hamiltonian and 2E doublet splitting have been derived.

I. Introduction.

-

Spectroscopic measurements of the magnetically dilute spinels have been the subject of intense investigations for last few years. The purpose of this research is to determine the crystal field acting on magnetic ion. Such investigations are especially important to the one-ion theory of the magnetic properties of the ferrimagnetic spinels. The charac- teristic crystal structure of these materials [I] is the main reason for the interpretative and experimental difficulties. Therefore up to the present the full infor:

mation about the crystal field has been obtained [2]

only for spinels MgA120, and ZnAl,O, with chromium ions. The chromium ions in these spinels occupy B positions which have sixfold distorted octahedral coordination belonging to the D,, point group.

In the present work the absorption and luminescence spectra due to chromium ions LiAl,08 spinel were measured. In this spinel the chromium ion occupies B position, but the crystal field acting on this ion has a lower symmetry (C,) than in MgA1204 and ZnAl,04.

LiA150, is inverse spinel with 113 octahedral ordering (as its ferrimagnetic analog LiFe508).

11. Results. - The absorption and luminescence spectra were measured a t temperatures of 293 OK, 77 OK and 4.2 O K . In absorption the single peak at 17 540 cm-l, double peak (resolved at 4.20K) a t 24 850 cm-' and 26 490 cm-I as well as band edge in the region of 38 500 cm-I were found. Figure 1 shows the short-wavelength absorption band at various temperatures. Luminescence was measured on single crystal and on polycrystalline samples. The two R lines in Li-spinel are at 14 041 cm- and 14 308 cm-

'.

On the vibrational sidebands there are some lines connected with Cr3+

-

Cr3+ pairs. To obtain the full information about the crystal field, especially about its low symmetry components the above mentioned optical measurements should be completed by the EPR and NMR data. The necessary investigations have been done on the samples used in the optical measurements. The results are similar to those obtained previously by Anselm et al. [3] and by Stauss [4].

FIG. 1. -The short-wavelength absorption band at various temperatures : I / 293 OK, 2/ 77 OK, 3/ 4.2 OK.

The EPR spectrum of a chromium ion in the Li-spinel can be described by means of the spin harniltonian 131 :

where g = 1.97,

I

D = 0.378 cm-', E/D = 0,0635 S = 312. The AlZ7 NMR spectrum in the B-site of Li-spinel can be described by the hamiltonian :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711296

C 1

-

⁸⁴⁸ M. P. PIETROV, H. SZYMCZAK, R. WADAS, W. WARDZYNSKI

x [3 1:

-

I(I

+

^{1) +ey(l: - I;)]} (2) where

I

e2 qQ

I

= 4.56 MHz y = 0.347 I =

5 .

The non zero value of parameters E and y indicates that there is a low symmetry field acting on the ion in the B position. Existence of a low symmetry field results also from the large R-doublet splitting A = 267 cm-l. We would like to point out that in Li-spinels besides large values of A and y the parameter E is relatively small.

111. Discussion.

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The analysis of the experimental results will be carried out on the base of the crystal field theory. In the calculations the strong cubic crystal field approximation was used. The following form of the one-electron wave functions have been chosen :

The strength of the octahedral and trigonal crystal field is described by the parameters :

where V,,,, V,, are the crystal field potential of the octahedral and trigonal symmetry, respectively.

The potential of the rhombic field is assumed to be in the form :

and the strength or the rhombic field can be described by the parameter A :

The wide absorption bands observed in the experiment are attributed to the following transitions :

R-doublet observed in luminescence is attributed to the transition within the tz configuration :

a a 2 ~ ( t ; ) + 4 ~ 2 ( t ; )

.

From the position of the terms 4T2, a 4T1, a 'E it is

possible to determine the Racah's parameters B and C and crystal field parameter Dq :

The fine structure of the optical and EPR spectra provide the valuable information about the low symme- try crystal field parameters. Formulae for g,

D,

E parameters of spin hamiltonian (1) and a '~(t2)'level splitting A have been derived :

where

5

is given by expression (4) from Macfarlane's calculations [5]

5

⁼ spin-orbit coupling constant (2S+1ri) = energy separation between

'" ri

and 4A2 levels.

where ('Ti) = energy separation between 'Ti and a 'E levels. In the expressions for D and E one should include interatomic spin-spin interactions V,, which give the contribution to D and E in the form :

<

^4A2

^{1 Ks} I

^a 4Tl

> <

^{a 4 ~ 1}

I K,, K, 1

^{4 ~}

>

²

(a ^{4 ~ 1}

-

4 ~ 2 )

Marvin's radial integrals M(") in this expression were taken equal to :

From the expressions (5) and (6) one can estimate parameters A, v' and

t.

Parameter v is small and there- fore it is inconvenient to evaluate it from above expressions. The most exact way to estimate v is to use the formula of the splitting of the a 4 ~ 1 level.

Finally :

where k = orbital reduction factor determined from the expression for g-factor.

The signs of v' and A were determined by the comparison with MgA1,04 spinel for which v'

<

0.

Assuming that the chromium ions in Li-spinel enter precisely into the place of the aluminium ions the above given results can be additionally verified through the comparison with the results of A12' NMR measu- rements.

LOW SYMMETRY CRYSTAL mELD I N LITHIUM SPINEL C 1

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849 From the model proposed in this paper it results : assuming :

we find e2 qQ and q close to the experimental values from [4].

References

[I] SMIT (J.), WIJN (H. P. J.), Ferrites, John Wiley and [3] ANSELM (L. N.), BIR ( G . L.), M ~ N I K O V A (J. E.), Sons, New-York, 1959. PIETROV (M. P.), Fiz. Tverd. Tela, 1966, 8, 1013.

[2] WOOD (D. L.), IMBUSH (G. F.), MACFARLANE (R. M.), [4] STAUSS ( G . H.), J. Chem. Phys., 1964, 40, 1988.

KISLIUK (P.), LARKIN @. M.), J. Chem. Phys., [ 5 ] MACFARLANE (R. M.), J. Chem. Phys., 1967, 47,2066.

1968, 48, 5255.