Electronic Structure and Polarization of Polar BiOIO3

Newly discovered polar BiOIO3 with orthorhombic stability (Pca2(1) phase) was analyzed using first-principles calculations. To clarify the physics of BiOIO3, we assumed that a phase transition occurs from a fictitious tetragonal P4/nmm phase to an orthorhombic Pca2(1) phase based on group theory and a symmetry mode analysis. The energetics and the order parameters involved in the transition from the tetragonal to the orthorhombic phase were analyzed. The primary order parameter of the phase transition is closely related to two tilting modes of the oxygen polyhedral (M-4 and M-1 modes). We also present the relationship of the distortion modes to the structural phase transition and the physical properties of the polar phase.