Probing the electronic properties of ternary A_nM_3n−1B_2n(n= 1: A = Ca, Sr; M = Rh, Ir andn= 3: A = Ca, Sr; M = Rh) phases: observation of superconductivity

We follow the evolution of the electronic properties of the titled homologous series when n as well as the atomic type of A and M are varied where for n = 1, A = Ca, Sr and M = Rh, Ir while for n = 3, A = Ca, Sr and M = Rh. The crystal structure of n = 1 members is known to be CaRh₂B₂-type (Fddd), while that of n = 3 is Ca₃Rh₈B₆-type (Fmmm); the latter can be visualized as a stacking of structural fragments from AM₃B₂ (P6/mmm) and AM₂B₂. The metallic properties of the n = 1 and 3 members are distinctly different: on the one hand, the n = 1 members are characterized by a linear coefficient of the electronic specific heat γ ≈ 3 mJ mol^-1 K^-2, a Debye temperature θ_D ≈ 300 K, a normal conductivity down to 2 K and a relatively strong linear magnetoresistivity for fields up to 150 kOe. The n = 3 family, on the other hand, exhibits γ ≈ 18 mJ mol^-1 K^-2, θ_D ≈ 330 K, a weak linear magnetoresistivity and an onset of superconductivity (for Ca₃Rh₈B₆, T_c = 4.0 K and H_c2 = 14.5 kOe, while for Sr₃Rh₈ B₆, T_c = 3.4 K and H_c2 ≈ 4.0 kOe). These remarkable differences are consistent with the findings of the electronic band structures and density of state (DOS) calculations. In particular, satisfactory agreement between the measured and calculated γ was obtained. Furthermore, the Fermi level, E_F, of Ca₃Rh₈B₆ lies at almost the top of a pronounced local DOS peak, while that of CaRh₂B₂ lies at a local valley: this is the main reason behind the differences between the, e.g., superconducting properties. Finally, although all atoms contribute to the DOS at E_F, the contribution of the Rh atoms is the strongest.