Probing the electronic properties of ternary A(n)M(3n−1)B(2n) (n = 1: A = Ca, Sr; M = Rh, Ir and n = 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 CaRh(2)B(2)-type (Fddd), while that of n = 3 is Ca(3)Rh(8)B(6)-type (Fmmm); the latter can be visualized as a stacking of structural fragments from AM(3)B(2) (P6/mmm) and AM(2)B(2). 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 Ca(3)Rh(8)B(6), T(c) = 4.0 K and H(c2) = 14.5 kOe, while for Sr(3)Rh(8) B(6), 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 Ca(3)Rh(8)B(6) lies at almost the top of a pronounced local DOS peak, while that of CaRh(2)B(2) 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.