High pressure behavior of mercury difluoride (HgF2)

High Pressure-Temperature Study of MgF2, CaF2, and BaF2 by Raman Spectroscopy: Phase Transitions and Vibrational Properties of AF2 Difluorides

The phase transition of AF2 difluorides strongly depends on pressure, temperature, and cationic radius. Here, we have investigated the phase transition of three difluorides, including MgF2, CaF2, and BaF2, at simultaneously high pressures and temperatures using Raman spectroscopy and X-ray diffraction in externally heated diamond anvil cells up to 55 GPa at 300-700 K. Rutile-type difluoride MgF2 with a small cationic radius undergoes a transition to the CaCl2-type phase at 9.9(1) GPa and 300 K, to the HP-PdF2-type phase at 21.0(2) GPa, and to the cotunnite-type phase at 44.2(2) GPa. The phase transition pressure to the HP-PdF2 and cotunnite structure at 300 K for our single crystal was found to be higher than that in previous studies using polycrystalline samples. Elevating the temperature increases the transition pressure from rutile- to the CaCl2-type phase but has a negative influence on the transition pressure when MgF2 transforms from the HP-PdF2- to cotunnite-type phase. Meanwhile, the transition pressure from the CaCl2- to HP-PdF2-type phase for MgF2 was identified to be independent of the temperature. Raman peaks suspected to belong to the α-PbO2-type phase were observed at 14.6-21.0(1) GPa and 400-700 K. At 300 K, difluorides CaF2 and BaF2 in the fluorite structure with larger cationic radii transform to the cotunnite-type phase at 9.6(3) and 3.0(3) GPa at 300 K, respectively, and BaF2 further undergoes a transition to the Ni2In-type phase at 15.5(4) GPa. For both CaF2 and BaF2, elevating the temperature leads to a lower transition pressure from fluorite- to the cotunnite-type phase but has little influence on the transition to the Ni2In structure. Raman data provide valuable insights for mode Grüneisen parameters. We note that the mode Grüneisen parameters for both difluorides and dioxides vary linearly with the cation radius. Further calculations for the mode Grüneisen parameters at high pressures for MgF2, CaF2, and BaF2 yield a deeper understanding of the thermodynamic properties of the difluorides.

From Molecules to Solids: A vdW-DF-C09 Case Study of the Mercury Dihalides

The mercury dihalides show a remarkable diversity in the structural preferences in their minimum energy structure types, spanning molecular to strongly bound ionic solids. A challenge in the development of density functional methods for extended systems is to arrive at strategies that serve equally well such a broad range of bonding modes or structural preferences. The chemical bonding and the stabilities of mercury dihalides and the general utility and reliability of the van der Waals density functional with C09 exchange (vdW-DF-C09) in predicting or describing the energetics and structural preferences in these metal dihalides is examined. We show that, in contrast with the uncorrected generalized gradient approximation of the Perdew-Burke-Erzenhoff (PBE) exchange-correlation functional, qualitative and quantitative patterns in the bonding of the mercury dihalide solids are well reproduced with vdW-DF-C09 for the full series of HgX₂ systems for X = F, Cl, Br, and I. The possible existence of a low-temperature cotunnite polymorph for HgF₂ and PbF₂ is posited.

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

Studying the effect of high pressure (exceeding 10 kbar) on the structure of solids allows us to gain deeper insight into the mechanism governing crystal structure stability. Here, we report a study on the high-pressure behavior of zinc difluoride (ZnF₂)-an archetypical ionic compound which at ambient pressure adopts the rutile (TiO₂) structure. Previous investigations, limited to a pressure of 15 GPa, revealed that this compound undergoes two pressure-induced phase transitions, i.e., TiO₂ → CaCl₂ at 4.5 GPa and CaCl₂ → HP-PdF₂ at 10 GPa. Within this joint experimental-theoretical study, we extend the room-temperature phase diagram of ZnF₂ up to 55 GPa. By means of Raman spectroscopy measurements we identify two new phase transitions, HP-PdF₂ → HP1-AgF₂ at 30 GPa and HP1-AgF₂ → PbCl₂ at 44 GPa. These results are confirmed by density functional theory calculations which indicate that in the HP1-AgF₂ polymorph the coordination sphere of Zn²⁺ undergoes drastic changes upon compression. Our results point to important differences in the high-pressure behavior of ZnF₂ and MgF₂, despite the fact that both compounds contain cations of similar size. We also argue that the HP1-AgF₂ structure, previously observed only for AgF₂, might be observed at large compression in other AB₂ compounds.