Polymorphism, Structural Transition, and Superconductivity in the Equiatomic Ternary Germanide ThRhGe

We report the crystal structure and physical properties of a new actinide-containing equiatomic ternary germanide ThRhGe. This compound is found to exist in two orthorhombic polymorphs at ambient pressure, which can be obtained by quenching from different annealing temperatures. α-ThRhGe crystallizes in the YPdSi-type structure with the Pmmn space group (a = 4.381 6(3) Å, b = 14.351 8(13) Å, c = 7.506 9(6) Å) and exhibits a normal metallic behavior down to 1.8 K. In contrast, β-ThRhGe adopts the TiNiSi-type structure with the Pnma space group (a = 7.298 7(5) Å, b = 4.395 9(3) Å, c = 7.624 5(5) Å) and shows an incomplete structural transition into the monoclinic structure (space group P21/c) at 244 K, following by a bulk superconducting transition below 3.36 K. The application of hydrostatic pressure suppresses the structural transition and enhances superconductivity, whose onset temperature reaches 8.36 K at 2.8 GPa. The contrasting behavior between the two polymorphs can be explained qualitatively by theoretical calculations, which show that the Fermi level lies close to a minimum of density of states in α-ThRhGe while it is exactly at a van Hove singularity associated with large density of states in the orthorhombic phase of β-ThRhGe. Our results suggest that equiatomic ternary intermetallic compounds offer a new playground for the exploration of interplay among polymorphism, structural transition and superconductivity.