Model for the Low-Temperature Lattice Anomaly in SrTiO3and KTaO3

The low-temperature anomaly in the lattice dynamics of SrTi${\mathrm{O}}_3$ and KTa${\mathrm{O}}_3$ is described as resulting from an anharmonic coupling between an optical and an acoustic branch. The numerical calculations performed for ${\omega }_{\mathrm{TO}}$ at the center of the Brillouin zone give an excellent fit to the available data from 4 to 300 K. It is shown that the ferroelectric phase transition in SrTi${\mathrm{O}}_3$ is prevented by the zero-point fluctuations of the acoustic mode.