Electronic instability of theΓ15phonon in BaTiO3

Abstract

The valence-band-energy lowering of BaTi${\mathrm{O}}_3$ accompanying the ${\Gamma }_{15}$-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 BaTi${\mathrm{O}}_3$ shows a Curie temperature $T_C$ 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 $\mathrm{AB}{\mathrm{O}}_3$ compounds is a good trap for another impurity's $d$ electron, this being compatible with the rise in $T_C$ for Cudoped BaTi${\mathrm{O}}_3$.