High-Temperature Superconductivity in Alkaline and Rare Earth Polyhydrides at High Pressure: A Theoretical Perspective

TL;DR

This paper reviews theoretical predictions of high-temperature superconductivity in alkaline and rare earth polyhydrides under high pressure, highlighting novel structures and potential room-temperature superconductors.

Contribution

It provides a comprehensive overview of computational methods and structural motifs associated with high-$T_c$ polyhydrides, including recent predictions and synthesis prospects.

Findings

High $T_c$ predicted for cage-like hydrogenic structures.

Diverse hydrogen motifs correlate with $T_c$ levels.

Some phases may exhibit room-temperature superconductivity.

Abstract

The theoretical exploration of the phase diagrams of binary hydrides under pressure using \emph{ab initio} crystal structure prediction techniques coupled with first-principles calculations has led to the \emph{in silico} discovery of numerous novel superconducting materials. This Perspective article focuses on the alkaline earth and rare earth polyhydrides whose superconducting critical temperature, $T_c$, was predicted to be above the boiling point of liquid nitrogen. After providing a brief overview of the computational protocol used to predict the structures of stable and metastable hydrides under pressure, we outline the equations that can be employed to estimate $T_c$. The systems with a high $T_c$ can be classified according to the motifs found in their hydrogenic lattices. The highest $T_c$s are found for cages that are reminiscent of clathrates, and the lowest for systems that contain atomic and molecular hydrogen. A wide variety of hydrogenic motifs including 1- and 2-dimensional lattices, as well as H$_{10}^{\delta-}$ molecular units comprised of fused H$_5^{\delta-}$ pentagons are present in phases with intermediate $T_c$s. Some of these phases are predicted to be superconducting at room temperature. Some may have recently been synthesized in diamond anvil cells.