A low-T_c superconducting modification of Th₄H₁₅ synthesized under high pressure

Among the metal hydrides, Th₄H₁₅ is the first reported superconductor with a relatively high T _c ≈ 8 K at ambient pressure. Here we report on the synthesis and characterization of a low-T _c superconducting modification of Th₄H₁₅, which is obtained via hydrogenating Th metal at 5 GPa and 800 °C by using the ammonia borane as the hydrogen source. Measurements of resistivity, magnetic susceptibility, and specific heat confirm that the obtained Th₄H₁₅ sample shows a bulk superconducting transition at T _c ≈ 6 K, which is about 2 K lower than that reported previously. Various characteristic superconducting parameters have been extracted for this compound and unusual lattice dynamics were evidenced from the specific-heat analysis. (paper)