La₂Rh${}_{3+\delta }$Sb₄: a new ternary superconducting rhodium-antimonide

Rhodium-containing compounds offer a fertile playground to explore novel materials with superconductivity (SC) and other fantastic electronic correlation effects. A new ternary rhodium-antimonide La₂Rh${}_{3+\delta }$Sb₄ ($\delta \approx 1/8$) has been synthesized by a Bi-flux method. It crystallizes in the orthorhombic Pr₂Ir₃Sb₄-like structure, with the space group Pnma (No. 62). The crystalline structure appears as stacking the two-dimensional RhSb₄- and RhSb₅-polyhedra networks along b axis, and the La atoms embed in the cavities of these networks. Band structure calculations confirm it as a multi-band metal with a van-Hove singularity like feature at the Fermi level, whose density of states are mainly of Rh-4d and Sb-5p characters. The calculations also imply that the redundant Rh acts as charge dopant. SC is observed in this material with onset transition at $T_c^{\mathrm{o}\mathrm{n}}\approx 0.8$K. Ultra-low temperature magnetic susceptibility and specific heat measurements suggest that it is an s-wave type-II superconductor. Our work may also imply that the broad $L{n}_{2}T{m}_{3+\delta }$Sb₄ (Ln = rare earth, Tm = Rh, Ir) family may host new material bases where new superconductors, quantum magnetism and other electronic correlation effects could be found.