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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 (P42/mnm) → CaCl2 type (orthorhombic Pnnm) → distorted PdF2 (orthorhombic Pbca)+PdF2 (cubic Pa3¯) in coexistence → fluorite (cubic Fm3¯m) → 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 Eg 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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Barreda-Argüeso, J. A., López-Moreno, S., Sanz-Ortiz, M. N., Romero, A. H. (2013). Pressure-Induced Phase-Transition Sequence In Cof 2 : An Experimental And First-Principles Study On The Crystal, Vibrational, And Electronic Properties. Physical Review B - Condensed Matter and Materials Physics, 88(21).



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