On the foundation of the vibronic origin of structural (ferroelectric) phase transitions in crystals

Abstract

The basic pseudo Jahn-Teller model of the vibronic theory of ferroelectricity is given here additional proof. It is shown that the vibronic mixing of the ground electronic state with appropriate excited ones is the only source of dynamic instability of polyatomic system, and hence in the microscopic formulation of the problem based on the Schrödinger equation the structural instability resulting in spontaneous distortion of the lattice can take place only in the presence of certain electronic configurations satisfying the criterion of vibronic instability and vibronic anharmonicity