Polyhedral transformation and phase transition in TcO2

Abstract

By using ab initio random structure search, we have predicted a stable low pressure phase (phase-LP) of TcO₂, which shares the same space group with the ambient condition phase (phase-AC) of TcO₂ but exhibits very different crystallographic features. A novel polyhedral transformation from the TcO₆-octahedrons in phase-AC to TcO₅-hexahedrons in phase-LP has been observed in the phase transition. Large voids along the (100) and (001) directions in phase-LP protect its low density stability, which can serve as the transport channel for protons and β particles. We show that the TcO₅-hexahedrons in phase-LP are more distorted and show stronger anisotropy than the TcO₆-octahedrons in phase-AC. According to the analysis on the electronic structures and chemical bonding of TcO₂, the various origins of the metallic nature of phase-AC and phase-LP have been explored and discussed as well.