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 (T_c) 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 T_c 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.