Analysis of the ferroelectric phase transition in BaAl2O4 by group theoretical methods and first-principles calculations -: art. no. 064105

Group theoretical methods are applied to a ferroelectric phase transition in barium aluminate (BaAl2O4). 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.