Analysis of the ferroelectric phase transition inBaAl2O4by group theoretical methods and first-principles calculations

Abstract

Group theoretical methods are applied to a ferroelectric phase transition in barium aluminate $({\mathrm{BaAl}}_{2}{\mathrm{O}}_{4}).$ The atomic displacements from x-ray-diffraction data are decomposed into four modes: a primary order parameter and three secondary order parameters. The spontaneous polarization ${P}_{s}$ is shown to arise from a secondary order parameter, resulting in a very small value of ${P}_{s},$ as is typical of other improper ferroelectrics. Using a first-principles computational method, we show that the primary order parameter becomes unstable, and is driven to a nonzero value at the phase transition. Through coupling, the nonzero primary order parameter then drives the secondary order parameters to nonzero values as well.