Ru₉Zn₇Sb₈: A structure with a 2 ×- 2 ×- 2 supercell of the half-Heusler phase

The title compound Ru₉Zn₇Sb₈was synthesized via a high-temperature reaction from the elements in a stoichiometric ratio, and its structure was solved by a single-crystal X-ray diffraction method. The structure [cubic, space group Fm3̄m, Pearson symbol cF96, a = 11.9062(14) Å (293 K), and Z = 4] adopts a unique 2a _hh× 2a_hh× 2a_hhsupercell of a normal half-Heusler phase and shows abnormal features of atomic coordination against the Pauling rule. The formation of this superstructure was discussed in light of the valence electron concentration per unit cell. It is a metallic conductor [ρ(300 K) = 16 μΩ·m], and differential scanning calorimetry revealed that Ru₉Zn₇Sb₈undergoes a transformation at 1356(1) K and melts, by all indications, congruently at 1386 K. At room temperature, its thermal conductivity is about 3 W/m·K, which is only one-quarter of that of most normal half-Heusler phases. Ru ₉Zn₇Sb₈as well as its analogues of iron-, cobalt-, rhodium-, and iridium-containing compounds are expected to serve as a new structure type for exploring new thermoelectric materials.