Superconductivity of Ternary Metal Compounds Prepared at High Pressures

Abstract Various ternary metal phosphides, arsenides, antimonides, silicides and germanides have been prepared at high temperatures and high pressures. These ternary metal compounds can be classified into four groups: [1] metal-rich compounds MM′4X2 and [2] MM′X, [3] non-metal-rich compounds MXX′ and [4] MM′4X12 (M and M′ = metal element; X and X′ = non-metal element). We have studied the electrical and magnetic properties of these materials at low temperatures, and found many new superconductors with the superconducting transition temperature (Tc) of above 10 K. The metal-rich compound ZrRu4P2 with a tetragonal structure showed the superconducting transition at around 11 K, and had an upper critical field (Hc2) of 12.2 tesla (T) at 0 K. Ternary equiatomic compounds ZrRuP and ZrRuSi crystallize in two modifications, a hexagonal Fe2P-type structure [h-ZrRuP(Si)] and an orthorhombic Co2P-type structure [o-ZrRuP(Si)]. Both h-ZrRuP and h-ZrRuSi have rather high Tc values of above 12 K; on the contrary, Tc of o-ZrRuP and o-ZrRuSi is about 4 K. The alloys ZrRu1−xRhxP were prepared at high temperatures and at high pressures. The superconductivity and the crystal structure of the alloys are very sensitive to the content of Rh atoms. The Hc2 values of h-ZrRuP, h-HfRuP, h-ZrRuSi and h-ZrRuAs are 14–16 T at 0 K. The coherence length of these compounds is below 50 Å. ZrRhSi with the Co2P-type structure and ZrRuGe with a TiFeSi-type structure showed the superconductivity at around 10.5 K. Tc’s of MoNiP with the Fe2P-type structure and MoRuP with the Co2P-type structure were over 15 K. Both compounds have the highest Tc among the metal phosphides. Non-metal-rich compound NbPS showed the Tc of 13 K and the high Hc2 of 17 T. This Hc2 value is the highest among the phosphide superconductors. The Debye temperature and the density of states at the Fermi level for NbPS are 330 K and 0.51 states/eV-atom, respectively. Filled skutterudites LnRu4X12 (Ln = La and Pr, X = P, As and Sb) with a cubic structure show interesting superconductivity behavior at low temperatures. LaRu4As12 had the highest Tc of 10.3 K among the skutterudite compounds. PrRu4As12 with the magnetic element Pr transformed to a superconductor at around 2.4 K. Superconducting properties of various ternary metal compounds prepared at high pressure are discussed.