A material’s thermoelectric efficiency is represented by its figure of merit, zT. In order to maximize zT, the electrical conductivity and Seebeck coefficient of a material must be simultaneously increased. Materials with anisotropic crystal structures are of particular interest as they present a method of decoupling the Seebeck coefficient from the electrical conductivity.
The focus of the research are Ca5M2Sb6 (M = Al, Ga, or In) Zintl compounds, containing covalently-bonded MSb4 tetrahedral polyanions that resemble infinite double chains. Density functional theory predicts light band mass and improved thermoelectric performance in the direction parallel to the MSb4 chains. Verifying this effect experimentally requires single crystals of sufficient size. Synthesis of pure-phase polycrystalline Ca5M2Sb6 (M= Ga, or In) and subsequent flux growth using either Sn, GaSb, or InSb flux was used to obtain single crystals up to 2 mm in length. The crystals were found to grow preferentially along the c-axis (parallel to the tetrahedral chains), leading to needle-like morphologies. The grown crystals were analyzed using single crystal X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy to determine phase purity and crystal structure. The electronic and thermal properties were measured parallel to the long axis of the needles, in the direction predicted to have highest zT.