La${}_2$Rh${}_{3+\delta }$Sb${}_4$: A new ternary superconducting rhodium-antimonide

Rhodium-containing compounds offer a fertile playground to explore novel materials with superconductivity and other fantastic electronic correlation effects. A new ternary rhodium-antimonide La${}_2$Rh${}_{3+\delta }$Sb${}_4$ ($\delta \approx 1\mathrm{/}8$) has been synthesized by a Bi-flux method. It crystallizes in the orthorhombic Pr${}_2$Ir${}_3$Sb${}_4$-like structure, with the space group $Pnma$ (No. 62). The crystalline structure appears as stacking the two dimensional RhSb${}_4$- and RhSb${}_5$-polyhedra networks along $\mathbf{b}$ 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-4$d$ and Sb-5$p$ characters. The calculations also imply that the redundant Rh acts as charge dopant. Superconductivity is observed in this material with onset transition at $T_c^{on}\approx$ 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 }$Sb${}_4$ ($Ln$=Rare earth, $Tm$=Rh, Ir…) family may host new material bases where new superconductors, quantum magnetism and other electronic correlation effects could be found.