Superconductivity versus structural phase transition in the closely related Bi2Rh3.5S2 and Bi2Rh3S2

Single crystals of Bi2Rh3S2 and Bi2Rh3.5S2 were synthesized by solution growth, and the crystal structures and thermodynamic and transport properties of both compounds were studied. In the case of Bi2Rh3S2, a structural first-order transition at around 165 K is identified by single-crystal diffraction experiments, with clear signatures visible in resistivity, magnetization, and specific heat data. No superconducting transition for Bi2Rh3S2 was observed down to 0.5 K. In contrast, no structural phase transition at high temperature was observed for Bi2Rh3.5S2; however, bulk superconductivity with a critical temperature, T-c approximate to 1.7 K, was observed. The Sommerfeld coefficient. and the Debye temperature (Theta(D)) were found to be 9.41 mJ mol(-1) K-2 and 209 K, respectively, for Bi2Rh3S2, and 22 mJ mol(-1) K-2 and 196 K, respectively, for Bi2Rh3.5S2. Study of the specific heat in the superconducting state of Bi2Rh3.5S2 suggests that Bi2Rh3.5S2 is a weakly coupled, BCS superconductor.