High-pressure phase transitions of zinc difluoride up to 55 GPa

TL;DR

This study extends the high-pressure phase diagram of zinc difluoride up to 55 GPa, revealing two new phase transitions and providing insights into structural changes under extreme conditions through experiments and calculations.

Contribution

It identifies two previously unreported high-pressure phase transitions in ZnF2 and explores their structural mechanisms using combined experimental and theoretical methods.

Findings

Discovered two new phase transitions at 30 GPa and 44 GPa.

Confirmed phase transitions with Raman spectroscopy and DFT calculations.

Highlighted differences in high-pressure behavior between ZnF2 and MgF2.

Abstract

Studying the effect of high pressure (exceeding 10 kbar) on the structure of solids allows to gain deeper insight in the mechanism governing crystal structure stability. Here we report a study on the high-pressure behaviour 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: 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 behaviour 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.