Probing the electronic properties of ternary AnM3n−1B2n (n = 1: A = Ca, Sr; M = Rh, Ir and n = 3: A = Ca, Sr; M = Rh) phases: observation of superconductivity

• Published in: Science and technology of advanced materials 2013-05, Vol.14 (3), p.035003
• Main Authors: Takeya, Hiroyuki, ElMassalami, Mohammed, Terrazos, Luis A, Rapp, Raul E, Capaz, Rodrigo B, Fujii, Hiroki, Takano, Yoshihiko, Doerr, Mathias, Granovsky, Sergey A
• Format: Article
• Language: English
• Subjects: 10.14; 20.07; 30.01; 3n−1; A=Ca; band structure; CaRh; Crystal structure; Density of states; Electronic properties; heat capacity; Homology; Ir; M=Rh; Magnetoresistivity; Mathematical analysis; Specific heat; Strontium; Superconductivity; ternary superconductors; type structures
• ISSN: 1468-6996; 1878-5514
• DOI: 10.1088/1468-6996/14/3/035003

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.