EFFECT OF ELASTIC-FORCE NONLINEARITY AND THE CHARGE-STATE ON THE TYPEOF EQUILIBRIUM DISTORTION OF DEFECTS WHOSE INITIAL ELECTRON STATE HASA T(2) SYMMETRY

In the case of the linear Jahn-Teller effect for a tetrahedral defect with electron wave functions of t(2) symmetry, a superlinearity of the elastic forces may give the defect an equilibrium configuration shape d by a simultaneous interaction with E and F-2 vibration modes. The eq uilibrium symmetry which results is no higher than orthorhombic. The s ituation is analyzed in the one-electron approximation, in a model whi ch incorporates the anharmonicity of 2 the E vibrations through the ad dition of a term alpha(Q(2)(2) + Q(3)(2))(2) to the elastic energy of the defect. That relative contribution of the anharmonic term to the t otal elastic energy of the defect, which is necessary to achieve a con figuration of lower symmetry, depends on the relative effectiveness of the interaction of bound carriers with the E and F-2 modes. If these interactions are roughly comparable, the necessary contribution may be comparatively small. When the initial t(2) State is occupied by two, three, or four electrons, the value of the anharmonicity parameter alp ha, which is required for a transition of the defect to a configuratio n shaped by interactions with both types of incompletely symmetric vib rations, is smaller than in the case of one or five electrons. It thus becomes possible to find a qualitative explanation of the progressive lowering of the symmetry of the defect from the initial t(2) state as the number of electrons trapped by this defect varies from 1 to 3. Th is lowering is similar to that observed for a vacancy in silicon. (C) 1995 American Institute of Physics.