Pressure-induced phase-transition sequence in CoF2: An experimental and first-principles study on the crystal, vibrational, and electronic properties

Article

关键词: INITIO MOLECULAR-DYNAMICS;    TOTAL-ENERGY CALCULATIONS;    AB-INITIO;    RUTILE-TYPE;    GERMANIUM DIOXIDE;    TRANSFORMATION;    SPECTRA;    STRAIN;    RUBY;    SIO2;   

DOI  :  10.1103/PhysRevB.88.214108

来源: SCIE

【 摘 要 】

We report a complete structural study of CoF2 under pressure. Its crystal structure and vibrational and electronic properties have been studied both theoretically and experimentally using first-principles density functional theory (DFT) methods, x-ray diffraction, x-ray absorption at Co K-edge experiments, Raman spectroscopy, and optical absorption in the 0-80 GPa range. We have determined the structural phase-transition sequence in CoF2 and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF2 but different to ZnF2 and MgF2, despite that the Co2+ size (ionic radius) is similar to Zn2+ and Mg2+. We found that the complete phase-transition sequence is tetragonal rutile (P4(2)/mnm) -> CaCl2 type (orthorhombic Pnnm) -> distorted PdF2 (orthorhombic Pbca) + PdF2 (cubic Pa (3) over bar) in coexistence -> fluorite (cubic Fm (3) over barm) -> cotunnite (orthorhombic Pnma). It was observed that the structural phase transition to the fluorite at 15 GPa involves a drastic change of coordination from sixfold octahedral to eightfold cubic with important modifications in the vibrational and electronic properties. We show that the stabilization of this high-pressure cubic phase is possible under nonhydrostatic conditions since ideal hydrostaticity would stabilize the distorted-fluorite structure (tetragonal I4/mmm) instead. Although the first rutile -> CaCl2-type second-order phase transition is subtle by Raman spectroscopy, it was possible to define it through the broadening of the E-g Raman mode which is split in the CaCl2-type phase. First-principles DFT calculations are in fair agreement with the experimental Raman mode frequencies, thus providing an accurate description for all vibrational modes and elastic properties of CoF2 as a function of pressure.

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