Predicted High-Pressure Hot Superconductivity in Li$_2$CaH$_{16}$ and Li$_2$CaH$_{17}$ Phases that Resemble the Type-II Clathrate Structure
Abstract: High-temperature high-pressure superconducting hydrides are typically characterized by cage-like hydrogenic lattices filled with electropositive metal atoms. Here, density functional theory based evolutionary crystal structure searches find two phases that possess these geometric features and are related to the Type-II clathrate structure. In these $Fd\overline{3}m$ Li$2$CaH${16}$ and $R\overline{3}m$ Li$2$CaH${17}$ phases the calcium atom occupies the larger cage and the lithium atom the smaller one. The highest superconducting critical temperatures predicted within the isotropic Eliashberg formalism, 330~K at 350~GPa for $Fd\overline{3}m$ Li$2$CaH${16}$ and 370~K at 300~GPa for $R\overline{3}m$ Li$2$CaH${17}$, suggest these structures fall in the class of high-energy-density quantum materials known as hot superconductors. As pressure is lowered the cage-like lattices distort with the emergence of quasimolecular hydrogenic motifs; nonetheless Li$2$CaH${17}$ is predicted to be superconducting down to 160~GPa at 205~K.
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