The effect of composition on phonon softening in ABO-type perovskites: DFT modelling

The evolution of ferroelectric instability in ABO 3 perovskites is systematically investigated for tantalates, niobates and titanates at the hybrid density-functional theory level. The influence of the A cation is analysed in terms of the frequency of the lowest F 1u IR-active phonon mode at different volumes for (Cs, Rb, K, Na)TaO 3 , (Ba, Pb, Sn, Ge)TiO 3 and (Rb, K, Na, Li)NbO 3 and correlated with the ionic radius as well as the degree of hybridization in the bonds. The atomic displacement corresponding to each mode is described as a function of volume, and the static permittivity is calculated for the stable Pm 3&cmb.macr; m phases. It is shown that the amplitude of the atomic displacements associated with the soft mode linked to the ferroelectric instability increases at a given volume when the ionic radius of the cation A decreases and when the hybridization of the B-O bond increases. This provides criteria for optimizing the dielectric properties of materials and for suggesting effective solid solutions. Tantalum perovskites presenting para-ferroelectric phase transitions, some of which are close to ambient conditions, are interesting materials for high-permittivity dielectrics in view of lead-free compounds with a high static dielectric response. The influence of the A cation on the ferroelectric instability in ABO 3 perovskites, and its associated F 1u IR-active phonon mode, is systematically investigated for tantalates, niobates and titanates at the hybrid density-functional theory level.