Novel Structures and Superconductivity of Silane under Pressure

Following the suggestion that hydrogen-rich compounds, and, in particular, silane (SiH4), might be high-T₂ superconductors at moderate pressures, very recent experiments have confirmed that silane metallises and even becomes superconducting at high pressure. In this article, we present a structural characterization of compressed silane obtained with an ab initio evolutionary algorithm for crystal structure prediction. Besides the earlier molecular and chainlike structures of P2₁/c and I4₁/a symmetries, respectively, we propose two novel structures with space groups Fdd2 and Pbcn, to be stable at 25-55 and 220-250 GPa, respectively. According to our calculations, silane becomes metallic and superconducting at 220 GPa in the layered Pbcn structure, with a theoretical T₂ of 16 K. Our calculations also show that the imaginary phonons of the recently proposed P6₃ generate the Pbcn structure.