Material Candidates and Structures II: Skutterudites, Zintl Phase, and Clathrates

Skutterudite compounds are potential thermoelectric materials at high temperature. The “phonon-glass electron-crystal” system, i.e., materials with very low thermal conductivity such as glass and materials with good electronic transport properties such as crystalline materials, can lead to efficient thermoelectrics in skutterudite compounds due to their crystal structure. Generally, these are composites of metal elements and pnictogen elements in the form of MX₃, where, in general, Co, Fe, Rh, and Ir represent M and P, As, and Sb are X, respectively. This class of compound consists of 32 atoms, having 8 cubic sublattices, composed of metal elements; 6 of them are filled with pnictogen square planar rings and form octahedral structure with metal elements. Such type of compounds belongs to cubic space group Im3 ¯ crystal structure [1]. Approaches have been made in the literature to improve ZT primarily by lowering the thermal conductivity in skutterudites. Their complex lattice structure, due to the composite of large unit cell by heavy atomic masses, is the main reason for the low thermal conductivity. Most of the explanation is based on CoSb₃, as it is one of the most common candidates of binary skutterudite with the body-centered cubic crystal structure, as shown in Fig. 6.1, having two interstitial voids at the 2a positions in the crystal lattice.