Crystal structures and superconductivity of technetium hydrides under pressure
Guided by a simple strategy in search of new superconducting materials, we have performed extensive simulations on crystal structures and electronic properties of Tc–H compounds at high pressures. Three stoichiometries, namely TcH, TcH2, and TcH3, were predicted to be stable under high pressure. The chemical bonding characters of Tc–H compounds were investigated by introducing the electron localization function and electronic projected density of states, which show that the stable Tc–H compounds including TcH, TcH2 and TcH3 are all classified as ionic crystals as a result of Tc → H charge transfer. Based on the Bardeen–Cooper–Schrieffer theory and estimated from the Allen–Dynes modified McMillan equation, the superconducting critical temperature (Tc) values of the I4/mmm phase for TcH2 are 5 K at 100 GPa and 11 K at 200 GPa. Moreover, it is found that Cmcm-TcH2 and P42/mmc-TcH3 at 300 GPa have Tc values of 9 K and 10 K, respectively. Our current results significantly enrich the crystal structures of the Tc–H system and provide a further understanding of their structural features and physical properties.