Figure 1: Atomic structure and diversity of FE phases of Sr7Ti6O19.
From: Atomic scale imaging of competing polar states in a Ruddlesden–Popper layered oxide
(a) DFT-calculated paraelectric unit cell structure, black outline, of Sr7Ti6O19 with the rock salt layer (SrO)2 in the middle. (b) Enlarged view of the rock salt region showing the correlated rumpling δ and the cation and oxygen interplanar distances, uC and uO, respectively due to the presence of the rock salt layer. The polar displacements of the cations ΔxC and oxygen ΔxO atoms away from their paraelectric state positions are shown. All distortions are exaggerated for clarity. (c) DFT-calculated layer dipole moment for the SrO (blue bars) and TiO2 (green bars) layers along the [100]PC axis for the predicted FE, FiE and AFE unit cells. The unit cell net polarization for each nanoslab lies along one of the 〈110〉PC directions. The three metastable phases, Eu1, Eg1, and Eu2, arise from the unstable Γ-point phonons, and the A-Eu1, A-Eg1, and A-Eu2 phases, are obtained by zone boundary (kz=π/c) phonons. (d) Calculated energy as a function of displacement (mode amplitude) for the various phonons. Note that the Γ-point modes are degenerate to those of the corresponding zone boundary instabilities.
