First-principles calculations of lattice dynamics in CdTiO3 and CaTiO3: Phase stability and ferroelectricity

Abstract

First-principles calculations of various phases of CdTiO3 carried out with the aim of obtaining insights into the mechanism of the ferroelectric phase transition and the structure of the low-temperature ferroelectric phase are reported. The results indicate that the preferred symmetry of the low-temperature phase is Pna2_1, rather than P21ma, corresponding to a small relative shift of the Ti and O ions in the paraelectric Pnma phase with the polarization axis parallel to the long axis. Calculated phonon dispersion curves show a distinct soft mode at the Γ point of the Pnma phase, which vanishes in the Pna2_1 phase, confirming that the transition to the ferroelectric phase is of the soft-mode displacive type. Calculations of perovskite CaTiO3, which also has an orthorhombic Pnma structure at room temperature but, unlike CdTiO3, does not exhibit a ferroelectric phase transition down to 4.2 K, were also carried out to help characterize the factors controlling ferroelectric phase transitions in perovskite titanates.