ELECTRONIC INSTABILITY OF THE GAMMA-15-PHONON IN BATIO3

The valence-band-energy lowering of BaTiO3 accompanying the 15-phonon freezing at 120° C is calculated using Kahn-Leyendecker linear combination of atomic orbitals (LCAO) band theory. The total stabilized energy per unit cell is about 151 meV at room temperature; sufficiently large for phonon freezing. Copper-doped BaTiO3 shows a Curie temperature TC higher than for the undoped sample. The electron-phonon-interaction theory developed in this paper can easily explain this surprising phenomenon, whereas ordinary electrostatic theory could not explain it. It is also shown by an ESR experiment that the Cu ion in ABO3 compounds is a good trap for another impurity's d electron, this being compatible with the rise in TC for Cudoped BaTiO3. © 1979 The American Physical Society.