THE ELECTRONIC STRUCTURE OF THE Tl+ + CENTRE IN KCl CRYSTALS

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THE ELECTRONIC STRUCTURE OF THE Tl+ +

CENTRE IN KCl CRYSTALS

W. Dreybrodt, D. Silber

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C 4, suppliment au no 8-9, Tome 28, AoGt-Septembre 1967, page C4-98

THE ELECTRONIC STRUCTURE OF THE

T1+

+

CENTRE

IN KC1 CRYSTALS

by W. DREYBRODT and D. SILBER Physikalisches fnstitut der Universitat Frankfurt a. M.

Abstract. - The T1++ centre in KC1 crystals has been investigated by paramagnetic resonance. The coefficients of the M. 0. ground state function of the octahedral (TlC16)4- complex and the g shift have been calculated from the hyperfine structure constants using a theory of H. Watanabe [3].

RbumB.

-

Le centre T1++ dans des cristaux KC1 a Btk 6tudi6 par la resonance paramagnktique. Les coefficients de la fonction orbite moKculaire de l'Ctat fondamental de la complexe octa6dre (TIC16)4- et le dkplacement g ont 6tk calcul6 par moyen des constantes de la structure hyperfine en utilisant une theorie de H. Watanabe [3].

ESR measurements on a T1++ centre in KC1 crystals have been performed. This centre has a (6 S)'

electronic configuration and is situated at a cation lattice site.

The centre is produced by X-raying Tlf doped KC1 crystals at 77 OK and warming to room tempera- ture in order to release the holes trapped as VK centres. The ESR spectra at 77 OK and 20OK show two groups of lines, at 6 050 Gauss and at 7 250 Gauss when working at 9 GHz. The centres of the groups do not depend on the direction of the magnetic field with respect to the crystal axes. This indicates that both g value and the T1 hyperfine structure are isotro- pic. A S. h. f. Splitting due to the sixfold cubic coordi- nation of Cl- ions is observed.

The spectra have been fitted by a spin Hamiltonian of the form :

The constants of the spin Hamiltonian are :

AT' = (105.4 f 0.1) GHz A? = (72

+

5) MHz (25.5 Gauss) g = 2.010

+

0.002 A, = (20

+

1,5) MHz

(7 Gauss) A theory by H. Watanabe [3] on the g value of

(m)' state ions at cubic lattice sites has been used to

calculate the g shift from the h. f. S.-constants of the spin Hamiltonian. I n this theory, covalent bonding of the central ion to its ligands is assumed ; therefore, the alf, ground state function is written as a molecular orbital of the form :

9 N($s

-

2s xs 20 X,) "T1 T1

X = P B ~ ^ S

+

P , , ~ , B I ~ ' + SA I $- N is a normalisation constant,

$,

is the T1" 6 s

6 function, X , is a linear combination of ligand s orbitals, "Cl C1

f SAj Ij

.

and X, is a linear combination of a-bonding ligand p

j= 1 orbitals, both transforming as an a,, representation of 0,.

The h. f. S. constant of the free Tl"+ ion is 175,5 _{The coefficients}_As _and_A, _{have been calculated} GHz [l]. ESR measurements of ~ 1 + + in ZnS [2] from the h. f. S. and S. h. f. S. values. Assuming equal give a h. f. S. constant of the same order of magnitude. sign for A,, and A,, one gets :

Therefore, Breit-Rabi's formula had to be used in the

evaluation of the spectra. N = 0.77 As = 0.3 1, = 1.22.

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THE ELECTRONIC STRUCTURE OF THE T1++ C 4 - 9 9 The g shift arises from mixing of a molecular orbital A detailed report of this work has been given in state with t t , symmetry into the ground state by spin- reference [5].

orbit couphng. For the calculation, the energy diffe-

rence between the nonbonding t,, state and the anti- _References

bonding a:, ground state must be known. It has been

estimated from the optical measurements of C. J. Del- [l] CRAWFORD (M. F.) and SCHAWLOW (A. L.), Phys. becq, A. K. Gosh, and P. H. Yuster [4]. Rev., 1949, 7 6 , 1310.

The g-shift thus obtained is [2] RAUBER (A.) and SCHNEIDER (J.), Phys. stat. sol., 1966, 18, 125.

Ag = 0.015 _f 0.005. [3] WATANABE (H.), Phys. Rev., 1966, 149,402.

[4] DELBECQ (C. J.), GOSH (A. K.) and YUSTER (P. H.), The experimentally determined g-shift is Phys. Rev., 1966, 151, 599.