Pressure-induced phonon softenings and the structural and magnetic transitions in CrO2

Abstract

To investigate the pressure-induced structural transitions of chromium dioxide (CrO2), phonon dispersions and total-energy band structures are calculated as a function of pressure. The observed structural transition has been theoretically reproduced at P approximate to 10 GPa from the ground-state tetragonal CrO2 (t-CrO2) of the rutile type to orthorhombic CrO2 (o-CrO2) of the CaCl2 type. The half-metallic property is found to be preserved in o-CrO2. The softening of the Raman-active B-1g phonon mode, which is responsible for this structural transition, is demonstrated. The second structural transition is found to occur for P >= 61.1 GPa from ferromagnetic (FM) o-CrO2 to nonmagnetic monoclinic CrO2 (m-CrO2) of the MoO2 type, which is related to the softening mode at q = R(1/2, 0, 1/ 2). The third structural transition has been identified at P = 88.8 GPa from m-CrO2 to cubic CrO2 of the CaF2 type that is a FM insulator.