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

Abstract

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.