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.