Evolutionary searches predicted a number of ternary phases that could be synthesized at pressures of 100-300~GPa. P63/mmc CaSH2, Pnma CaSH2, Cmc21 CaSH6, and I4ˉ CaSH20 were composed of a Ca-S lattice along with H2 molecules coordinated in a ``side-on'' fashion to Ca. The H-H bond lengths in these semiconducting phases were elongated because of H2σ→ Ca d donation, and Ca d→ H2σ∗ back-donation, via a Kubas-like mechanism. P6ˉm2 CaSH3, consisting of two-dimensional HS and CaH2 sheets, was metastable and metallic above \edit{128~GPa}. The presence of van Hove singularities increased its density of states at the Fermi level, and concomitantly the superconducting critical temperature, which was estimated to be as high as \edit{∼100~K at 128~GPa}. This work will inspire the search for superconductivity in materials based upon \edit{honeycomb HX (X=S, Se, Te), and MH2 (M=Mg, Ca, Sr, Ba) layers} under pressure.
@article{arxiv.2012.14985,
title = {A Metastable CaSH$_3$ Phase Composed of HS Honeycomb Sheets that is Superconducting Under Pressure},
author = {Yan Yan and Tiange Bi and Nisha Geng and Xiaoyu Wang and Eva Zurek},
journal= {arXiv preprint arXiv:2012.14985},
year = {2021}
}