Abstract
The idea of looking for metallic states in hydrides rather than in pure hydrogen has remarkably increased the options for attaining the high-temperature superconductivity predicted by Ashcroft in 1968. This article presents a few viewpoints which might serve as a practical guide to searching new metallic hydrides which possess high superconducting transition temperatures. According to the Goldhammer-Herzfeld criterion, it is important to pay attention to unstable compounds at one atmospheric pressure because they can be good metals at high pressures. It is also helpful to make use of possible structural similarities of hydrides between the diagonally adjacent elements in the periodic table in order to control insulator-to-metal transition. The effects of anharmonic proton zero-point energy on stable structures are also discussed by applying the self-consistent harmonic approximation to solid hydrogen.
