Compressed hydrides as metallic hydrogen superconductors

The 2014--2015 prediction, discovery, and confirmation of record high-temperature superconductivity above 200 K in compressed H₃S, followed by the 2018 extension to superconductivity in the 250--260 K range in lanthanum hydride, mark a new era in the long-standing quest for room-temperature superconductivity: quest achieved, at the cost of supplying 1.5--2 Mbar pressure. Predictions of numerous high-temperature superconducting metal hydrides XHₙ (X=metal atom) have appeared but are providing limited understanding of why some transition temperatures Tc are high while others are low. We make use of the small mass ratio MH/MX to obtain an atomic decomposition of the coupling strength to reveal that although the X atom provides coupling strength via λX as commonly calculated, it is irrelevant for Tc because the resulting lowering of frequency moments compensates (and sometimes overcompensates) for the increase in λ. It is important for analysis and for understanding that the X atom contribution is neglected because Tc depends more transparently on λH. Five XHₙ compounds, predicted in harmonic approximation to have Tc in the 150--300 K range, are analyzed consistently for their relevant properties, revealing some aspects that confront conventional wisdom. A phonon frequency--critical temperature (ω₂-Tc) phase diagram is obtained that reveals a common structural phase instability boundary limiting Tc at the low-pressure range of each compound. The hydrogen scattering matrix elements are obtained and found to differ strongly over the hydrides. A quantity directly proportional to Tc in these hydrides is identified, indicating that (in common notation) NH(0)IH²/ωH=ηH/ωH is the parameter combination to be maximized in hydrides.