Origin and evolution of ferroelectricity in the layered rare-earth-titanate, R2Ti2O7, Carpy-Galy phases

We report a systematic first-principles study based on density functional theory (DFT) of the structural and ferroelectric properties of the R2Ti2O7 perovskite-related oxides with R = La, Ce, Pr, and Nd. We show that, in all cases, the ferroelectric distortion to the ground-state polar P2(1) structure from its parent centrosymmetric P2(1)/m phase is driven by a single polar soft mode consisting of rotations and tilts of the TiO6 octahedra combined with displacements of the R ions. A secondary centrosymmetric distortion, which is stable in the parent structure, contributes substantially to the energy lowering of the ground state. We evaluate the trends in structure and polarization across the series as a function of R and reconcile discrepancies in reported values of polarization in the literature. Our results confirm that the family of R2Ti2O7 materials belong to the class of proper geometric ferroelectrics. (C) 2019 The Authors. Published by Elsevier Ltd.