Influence of Thermally Activated Solid-State Crystal-to-Crystal Structural Transformation on the Thermoelectric Properties of the Ca5–xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) System

Abstract

The solid-solution Zintl compounds with the mixed cations of Ca2+and Yb2+ in the Ca5–xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) system have been synthesized by high-temperature solid-state reactions. Two slightly different crystal structures of the Ba5Al2Bi6-type and Ca5Ga2Sb6-type phases have been characterized for seven compounds with 2.5 ≤ x ≤ 5.0 and three compounds with 1.0 ≤ x ≤ 2.0, respectively, by both powder and single-crystal X-ray diffraction analyses. The two title phases adopt the orthorhombic space group Pbam (Z = 2, oP26) with seven independent asymmetric atomic sites and share certain structural similarities, including infinite one-dimensional [Al2Sb8] double chains and isolated space-filling Ca2+/Yb2+ cations. Interestingly, we reveal the crystal-to-crystal solid-state structural transformation of the Yb-rich compound Ca1.5Yb3.5Al2Sb6 from the Ba5Al2Bi6-type to the Ca5Ga2Sb6-type phase through the postannealing process, which can be rationalized as the phase transition from the kinetically more stable s...