Theoretical Study on the Structures and Electronic Properties of Tungsten Fluorides at High Pressures

The high-pressure phase diagram of tungsten fluorides has been established through first-principles swarm-intelligence based CALYPSO structure prediction method. The F-richest composition, WF₆, undergoes a series phase transitions under high pressure, from molecular crystals to condensed phases. Besides the well-known hexafluoride, WF₄ and WF₅ can also be stable under high pressure.

Abstract

Transition metal fluorides are a series of strong oxidizing agents. Tungsten (W) fluorides, particularly WF₆, have shown broad applications such as luminescence and fluorinating agent. However, other stoichiometries of W fluorides have rarely been studied. It is well-known that pressure can induce structural phase transition, stabilize new compounds, and produce novel properties. In this work, the high-pressure phases of W−F were searched systematically at the pressure range of 0–200 GPa through first-principles swarm-intelligence structural search calculations. A new stoichiometry of WF₄ has been predicted to be stable under high pressures. On the other hand, two new high-pressure phases of WF₆ with the symmetries of /m and -1 were found with decahedral structural units. The electronic properties of the W−F compounds were then investigated. The predicted stable WF₆ high-pressure phases maintain semiconducting features, since the W atom provides all its valence electrons to fluorine. We evaluated the oxidizing ability of WF₆ by calculating its electron affinity potential. The high pressure /m WF₆ molecular phase shows higher oxidation capacity than the ambient phase. The built pressure-composition phase diagram and the theoretical results of W−F system provide some useful information for experimental synthesis.

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