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

Abstract

We report a complete structural study of CoF 2 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 CoF 2 and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF 2 but different to ZnF 2 and MgF 2 , despite that the Co 2+ size (ionic radius) is similar to Zn 2+ and Mg 2+ . We found that the complete phase-transition sequence is tetragonal rutile (P 4 2 /mnm) → CaCl 2 type (orthorhombic P nnm) → distorted PdF 2 (orthorhombic P bca) + PdF 2 (cubic P a3) in coexistence → fluorite (cubic F m3m) → cotunnite (orthorhombic P nma). 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 I 4/mmm) instead. Although the first rutile → CaCl 2 -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 CaCl 2 -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 CoF 2 as a function of pressure.

The Jahn-Teller Distortion at High Pressure: The Case of Copper Difluoride

“…Interestingly, the lowest-frequency Ag mode (not observed experimentally) is predicted to soften upon compression. This behavior resembles the one found in compounds adopting at ambient conditions the rutile aristotype, for example ZnF2 [24], CoF2 [28,31], FeF2 [30], and MnF2 [33]. In these systems the pressure-induced softening of a low-frequency B1g mode leads to a second order phase transition from the P42/mnm structure to a CaCl2-type polymorph (Pnnm symmetry, Z = 2).…”

“…This compound is one of the simplest binary connections containing the Cu 2+ cation. It belongs to the family of metal difluorides, which have been extensively studied at high pressure [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. Due to the operation of the JT effect CuF 2 adopts at ambient conditions a rarely-encountered crystal structure found only in one other compound (CrF 2 ) [38].…”

High-Pressure Phase Transitions of Zinc Difluoride up to 55 GPa

“…For rutile structures (TiO2, CaCl2) it is equal to about 9 meV/GPa, while it is larger for the fluorite structures (21 meV/GPa) and PbCl2 (18 meV/GPa). These values are larger than reported for CoF2 (dEg/dp < 7 meV/GPa), 20 but smaller than for MgF2 (dEg/dp ≈ 40 meV/GPa). 17 These differences most likely stem from the different electronic configuration between Zn 2+ (3d 10 ), Co 2+ (3d 7 ) and Mg 2+ (2s 2 2p 6 ).…”

“…Numerous experimental and theoretical investigations were conducted in order to establish the phase sequence for these compounds. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] The motivation for these studies was the fact that the behaviour of difluorides at these conditions can serve as an analogue of the high-pressure transformations of dioxides, in particular SiO2. 7,8 Moreover high-pressure studies on MF2 systems revealed a surprisingly wide range of structures adopted by these compounds.…”

“…18 For transition metal difluorides it was found that the high-pressure phase transition sequence differs considerably between ions of similar size, but with different d electron count, as witnessed by comparing MnF2 (d 5 system) 8,19 and CoF2 (d 7 electron count). 20 Moreover for d 9 cations (Cu 2+ , Ag 2+ ) the high-pressure polymorphism is strongly influenced by structural distortion arising from an uneven occupation of the eg orbitals (often referred to as the Jahn-Teller effect) 28 as well as strong spin-spin magnetic interactions. 24 In this context the phase transitions of zinc difluoride are of interest, as the Zn 2+ cation can be viewed as a non-magnetic (3d 10 electronic configuration) counterpart of Mn 2+ , Co 2+ , and Cu 2+ .…”