Phase competition and effect of chemical ordering in ferroelectric relaxor PbSc0.5Nb0.5O3 from first principles

ABSTRACTWe present results of first principles calculations for ferroelectric relaxor PbSc0.5Nb0.5O3. An ordered supercell with Sc and Nb atoms alternating in all directions (NaCl-like superstructure) has a ferroelectric ground state with R3 spacegroup. It is characterized by a tilt system a−a−a− as well as different sizes of adjacent oxygen octahedra. Analysis of phonon instabilities in cubic Fm3‾m reveals similarity with an antiferroelectric PbZrO3. This is further confirmed by the fact that possibly antiferroelectric P21/b structure of PbSc0.5Nb0.5O3 (symmetry lowered from Pbam due to Sc/Nb superstructure) is energetically very close to the ground state. The ferroelectric order is shown to be less sensitive to deviations in the B-site distribution than the antiferroelectric one.