“Frozen” phonon calculations: Lattice dynamics and -Instabilities

A general description of the “frozen” phonon concept is given, and the relation between ground state electronic structure calculations and finite temperature phase transitions is discussed. On the basis of linear-muffin-tin-orbital calculations for copper halides it is exemplified which kind of statements about lattice dynamics can be obtained from such calculations. The application of the method to SrTiO3 and BaTiO3 demonstrates that the ferroelectric instability is the result of a delicate balance between forces produced by bonding and nonbonding states. Calculations with different unit cell volumes have shown that the difference between SrTiO3 and BaTiO3 is mainly due to the different sizes of the oxygen octahedra in the two materials.