High-TCSuperconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions Between Hydrogen and Central-Atom Electrons

The uniquely characteristic macrostructures of binary hydrogen-clathrate compoundsMH(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 temperaturesT(C)have thus far been measured for syntheses withM = La, Y, and Th. In compressed LaH10, independent studies reportT(C)of 250 K and over 260 K, a maximum inT(C)with pressureP, and normal-state resistance scaling with temperature (suggesting unconventional pairing). According to reported band structure calculations ofFm3 over bar m-phase LaH10, 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 forMH(n)clathrates is derived asT(C0) = k(B)(-1)?[(n + v)/2A](1/2)e(2)/zeta, where ? is a universal constant, (n + v) is the chemical valence sum per formula unit, taking unity for H andvfor atomM,Ais the surface area of the H-polyhedron cage, and zeta is the mean distance between theMsite and the centroids of the polyhedron faces. Applied toFm3 over bar mLaH(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 inT(C)ofFm3 over bar mLaH(10)andIm3 over bar mH(3)S are discussed, along with a generalized prospective for Coulomb-mediated superconductivity inMH(n).