**Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions between Hydrogen and Central-Atom Electrons**, D. R. Harshman and A. T. Fiory [arXiv]

The uniquely characteristic macrostructures of binary hydrogen-clathrate compounds MH_{n} formed at high pressure, a cage of hydrogens surrounding a central-atom host, is theoretically predicted in various studies to include structurally stable phonon-mediated superconductors. High superconductive transition temperatures *T*_{C} have thus far been measured for syntheses with *M* = La, Y, and Th. In compressed LaH_{10}, independent studies report *T*_{C} of 250 K and over 260 K, a maximum in *T*_{C} with pressure *P*, and normal-state resistance scaling with temperature (suggesting unconventional pairing). According to reported band structure calculations of *Fm*3*m*-phase LaH_{10}, the La is anionic, with the chemical valence electrons appearing evenly split between La and H_{10}. Thus, compressed LaH$_{10} contains the combination of structure, charge separation, and optimal balanced allocation of valence electrons for supporting unconventional high-*T*_{C} superconductivity mediated by Coulomb interactions between electronic charges associated with La and H_{10}. A general expression for the optimal superconducting transition temperature for *M*H_{n} clathrates is derived as *T*_{C0} = *k*_{B}^{-1}Λ[(n + *v*)/2*A*]^{1/2}*e*^{2}/ζ, where Λ is a universal constant, (n + *v*) is the chemical valence sum per formula unit, taking unity for H and *v* for atom *M*, *A* is the surface area of the H-polyhedron cage, and ζ is the mean distance between the *M* site and the centroids of the polyhedron faces. Applied to *Fm*3*m* LaH_{10}, *T*_{C0} values of 249.8(1.3) K and 260.7(2.0) K are found for the two experiments. Associated attributes of charge allocation, structure, effective Coulomb potential, and H-D isotope effect in *T*_{C} of *Fm*3*m* LaH_{10} and *Im*3*m* H_{3}S are discussed, along with a generalized prospective for Coulomb-mediated superconductivity in *M*H_{n}.

Dale R. Harshman and Anthony T. Fiory, Journal of Superconductivity and Novel Magnetism **33**, 2945 (2020).