Tetrahedral tilting and ferroelectricity in Bi2AO5 (A=Si, Ge) from first principles calculations

The properties of a tetrahedron containing Bi(2)AO(5) (A=Si, Ge) are examined using Ab initio calculations and symmetry mode analysis. Stabilization of the polar phase is observed in both compounds with a monoclinic Cc phase. In the monoclinic ground state, the tilting angle (phi(1)) of tetrahedron is 7.21 degrees and 21.94 degrees for the Si and Ge compound, respectively. The relationship between a primary order parameter and the tetrahedral tilting is identified and an analytical formula between them is proposed by analyzing the structure. The detailed layer-by-layer polarization calculations shows that the main polarization component originates from the tetrahedron tilting of the AO(4) unit, and the analytical relationship between the primary order parameter and spontaneous polarization is also calculated. This B3LYP hybrid functional calculation provides a band gap of 4.44 eV and 4.18 eV for Bi2SiO5 and Bi2GeO5, respectively. The main difference between the two compounds is also analyzed based on the electronic structure and electron localization function analysis. (C) 2015 Elsevier Inc. All rights reserved.