OPTICAL ABSORPTION OF V--TYPE CENTERS IN MgO AND CaO : SMALL POLARONS BOUND IN OCTAHEDRAL SYMMETRY

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OPTICAL ABSORPTION OF V–TYPE CENTERS IN

MgO AND CaO : SMALL POLARONS BOUND IN

OCTAHEDRAL SYMMETRY

O. Schirmer

To cite this version:

JOURNAL DE PHYSIQUE Colloque C7, supplément au n° 12, Tome 37, Décembre 1976, page C7-197

OPTICAL ABSORPTION OF V - T Y P E CENTERS

IN MgO AND CaO : SMALL POLARONS BOUND

IN OCTAHEDRAL SYMMETRY

O. F. SCHIRMER

Institut fiir Angewandte Festkorperphysik der Fraunhofer-Gesellschaft, Eckerstr. 4, D 7800 Freiburg, FRG

Résumé. — On explique l'absorption des centres V- (antimorphes des centres F+) et des centres

affins dans les oxydes d'alcalino-terreux comme un transfert des trous entre sites 02~ équivalents

près des défauts du type lacune de cation. Le transfert est induit par la lumière et le trou est déplacé contre le potentiel piégeant. La compétition entre l'interaction trou-phonon, qui abaisse la symétrie, et l'interaction de résonance, qui maintient la symétrie cubique, est formulée comme un effet Pseudo-Jahn-Teller. Le modèle explique la largeur, la puissance d'oscillateur, la position et le dédoublement par résonance des bandes optiques. Les paramètres qui expliquent ces caractéristiques repro-duisent la position d'un état relaxé et excité et l'interaction de résonance entre les états fonda-mentaux vibratoires, connus pour MgO : V~. Les transitions entre orbitales non équivalentes aux sites équivalents O2 - conduisent à des bandes d'énergie élevée, observées pour tous les centres du

type V-. La généralisation du traitement de ces systèmes simples à l'absorption de petits polarons

libres sont donnés. La version détaillée de ce travail est publiée dans Zeitschrift fiir Physik B [1]. Abstract. — The optical absorption of V~-type centers (F+ antimorphs and related defects) in the

alkaline-earth oxides is explained as a light induced transfer of holes between equivalent 02~ sites next to cation vacancy type defects, the excitation acting against the self-induced trapping potentials. The interplay between symmetry-breaking hole-phonon- and symmetry-restoring resonance-interaction is formulated as a Pseudo-Jahn-Teller effect. The model explains the width, oscillator strength, position and resonance splitting of the bands. The parameters explaining these features also reproduce the position of a relaxed stable excited state and the resonance interaction between the vibrational ground states, known for MgO : V-. Transitions between nonequivalent orbitals at the

equivalent O2 - sites lead to bands at higher energies, observed for all V-. -type centers.

Generalisa-tions of the treatment of these simple systems to optical absorption of free small polarons are given. The detailed version of the paper is published in Zeitschrift fur Physik B [1 ].

Reference

[1] SCHIRMER, O. F., Z. Phys. B 24 (1976) 235.

DISCUSSION

M. J. NORGETT. — We have recently completed calculations for the V-centre in MgO at Harwell (Norgett, Pathak et Stoneham). These seem to confirm that the 2.3 eV adsorption does correspond to the polaron like transition of the hole hopping about the cation vacancy. The energy we calculate, after a num-ber of approximations, is in very good agreement with the observed energy. It is perhaps more satis-factory that we can predict accurately the displace-ment of the band to lower energy when the cation vacancy is replaced by a N a+ ion in the V-(Na+

)-center.

T. M. WILSON. — Calculations of the electronic

structure of the V-center in MgO are underway at the Oak Ridge National Laboratory and at Oklahoma State University. Our preliminary results for the 2A

ground state of the V-center indicate significant mixing of the 3s- and 3p-like orbitals with the 2s and 2p to form the occupied O- -orbitals centered

on the O " ion adjacent to the vacancy. Theoretical predictions for the electric dipole moment of the center and the oscillator strength for the 2AX -* 2E

transition should be quite sensitive to this mixing. 14