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

Abstract

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.

Enhancement of superconductivity on the verge of a structural instability in isovalently doped $β$-ThRh$_{1-x}$Ir$_{x}$Ge

“…5(c) that the superconducting transition moves towards higher temperatures and becomes sharpened. The overall behavior is very similar to that observed in undoped β-ThRhGe [27], and corroborates that superconductivity and structural transition are still competing with each other in lightly Ir-doped samples. For x = 0.5, whose results are displayed in Figs.…”

“…In summary, we have studied systematically the effect of isovalent Ir doping on the structural and superconductivity of β-ThRh 1−x Ir x Ge. The undoped β-ThRhGe displays an incomplete structural transition at T s = 244 K and a superconducting transition at T c = 3.36 K [27]. With increasing Ir content x, the structural transition is gradually smeared out and disappears at x = 0.5, where T c exhibits a maximum of 6.88 K. This maximum is accompanied by a small jump in γ, a maximum in λ ep , a minimum in Θ D and a resistivity temperature exponent n ≈ 1.6.…”

“…Very recently, a new ternary equiatomic germanide ThRhGe has been synthesized and characterized [27]. Depending on the annealing temperature, two orthorhombic polymorphs are obtained at ambient condition.…”

Finite Temperature String with Order Parameter as Collective Variables for Molecular Crystal: A Case of Polymorphic Transformation of TNT under External Electric Field

“…However, despite extensive experimental [12][13][14][15][16][17][18][19][20] and theoretical [21][22][23] investigations on the polymorphic transformation of the molecular crystals, at present it is still unclear how the molecules transform into the new crystal forms under the external electric field [1,[3][4][5][6]. Due to the much lower time scale than that of real occurrence [24,25], it is difficult to converge free energy in the polymorphic transformation, as it is a rare event [26,27] in theoretical simulation [28].…”

“…Exploring the polymorphic transformation mechanism at the molecular level is essential for effectively introducing an external electric field to control the transformation products and improve crystal qualities [ 1 , 5 , 7 ]. However, despite extensive experimental [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ] and theoretical [ 21 , 22 , 23 ] investigations on the polymorphic transformation of the molecular crystals, at present it is still unclear how the molecules transform into the new crystal forms under the external electric field [ 1 , 3 , 4 , 5 , 6 ]. Due to the much lower time scale than that of real occurrence [ 24 , 25 ], it is difficult to converge free energy in the polymorphic transformation, as it is a rare event [ 26 , 27 ] in theoretical simulation [ 28 ].…”