Force Constants in BaTiO₃ and SrTiO₃

Abstract

Two possible sets of force constants in BaTiO 3 and SrTiO 3 are obtained using the reported experimental data on atomic displacements in soft mode and frequencies of other optical branches. The anharmonicity is implicitly taken into account as the temperature dependence of some force constants. It is confirmed that the cancellation with each other of long range and short range parts in the force constant k 4 for Ti-O I bond, with nonzero force constant k 1 for Ti-Ba bond, is an essential origin of the lattice instability. Temperature dependence of the force constant k 4 is also discussed.