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

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 Pa3̄) in coexistence → fluorite (cubic Fm3̄m) → 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.