Superconductivity and unexpected chemistry of germanium hydrides under pressure

Following the idea that hydrogen-rich compounds might be high-T_c superconductors at high pressures, and the very recent breakthrough in predicting and synthesizing hydrogen sulfide with record-high T_c=203 K , an ab initio evolutionary algorithm for crystal structure prediction was employed to find stable germanium hydrides. In addition to the earlier structure of germane with space group Ama2, we propose a C2/m structure, which is energetically more favorable at pressures above 278 GPa (with inclusion of zero-point energy). Our calculations indicate that the C2/m phase of germane is a superconductor with T_c=67 K at 280 GPa. Germane is found to become thermodynamically unstable to decomposition to hydrogen and the compound Ge₃H₁₁ at pressures above 300 GPa. Ge₃H₁₁ with space group I 4 ¯m 2 is found to become stable at above 285 GPa with T_c=43 K . We find that the pressure-induced phase stability of germanium hydrides is distinct from analogous isoelectronic systems, e.g., Si hydrides and Sn hydrides. Superconductivity stems from large electron-phonon coupling associated with the wagging, bending, and stretching intermediate-frequency modes derived mainly from hydrogen.