Quantum disproportionation: The high hydrides at elevated pressures

A quartet of hydrogens can be dynamically bound to group-14 atoms, the resulting complexes having even valence. In macroscopic assemblies and at high pressures these can give way by dynamically assisted (quantum) disproportionation to complexes with odd valence and hence, in principle, to metallic tendencies. A new extended metallic composition of, for example, GeH₃ is investigated by first-principles methods within density functional theory. Its stoichiometry and its very existence is a direct consequence of inclusion of nuclear quantum dynamical contributions to the free energy. From enthalpic comparisons GeH₃ augmented with hydrogen appears preferred beyond 175 GPa, where three candidate structures are competitive, these being A15, P4₂/mmc, and Cccm. The pressure at which GeH₃ makes its appearance is significantly influenced by zero-point energy, and quantum effects play an important role in a notable trend towards disproportionation. The ensuing systems are all metallic and the superconducting transition temperature of GeH₃ has been estimated as about 100 K or higher (a common range for the three prospective structures). The general proposition of quantum disproportionation at elevated pressures appears extendable to other high hydrides.