Uniaxial pressure induced phase transitions in multiferroic materials BiCoO3

Abstract

The crystallographic structure stability, spin state and electronic structure variation in tetragonal multiferroic material BiCoO₃ under uniaxial pressure are investigated by means of first-principles density functional theory calculations. The lattice parameters, atomic internal coordinates and magnetic moment change abruptly under c axis compression of 9 GPa. A first-order structural phase transition occurs with a unit cell volume collapse of 9.5%, accompanied by the Co–O coordination polyhedron changing from CoO₅ pyramid to the distorted CoO₆ octahedron. A spin state transition of the Co³⁺ ions from the high-spin configuration in the CoO₅ pyramidal coordination to the nonmagnetic low-spin configurations in the distorted CoO₆ octahedron coordination has been explored. Contrasted electronic structure calculations are performed with PBE Generalized Gradient Approximation (GGA) and B3LYP hybrid functional. The hybrid functional drastically improves the band gap of the ground state. A controversial electronic structure has been predicted by GGA-PBE and B3LYP hybrid functional for the high pressure phase BiCoO₃. We propose that the high pressure phase BiCoO₃ is a nonmagnetic insulator.