Thermoelectric properties and rattling phenomena in tunnel-structured Zintl compound Na₂CdSn₅

This work comprised detailed assessments of the thermoelectric properties and crystal structure of the Zintl compound Na 2 CdSn 5 . A polycrystalline sample having a relative density of 98% was prepared by pressure sintering, and was found to exhibit p-type semiconductor characteristics with a low thermal conductivity of 0.58 W m –1 K –1 at room temperature. Single-crystal X-ray diffraction analyses at 90, 200, and 300 K showed that Na atoms within tunnel structures had large atomic displacement parameters, indicating that a rattling phenomenon occurred to a significant degree, promoting phonon scattering. A phase transition from tetragonal (space group: I4̅2d) to orthorhombic (space group: P2 1 2 1 2 1 ) symmetry was observed by cooling from 200 K to 90 K, accompanied by Na atomic ordering. Partial replacement of Cd atoms with Ag to produce Na 2 Cd 1− x Ag x Sn 5 ( x = 0.01–0.05) resulted in hole doping and a 30 to 50-fold increase in electrical conductivity, while maintaining low thermal conductivity. An optimally doped sample ( x = 0.02) had a dimensionless figure of merit, ZT , of 0.23 at 393 K, representing a fourfold enhancement relative to the undoped compound.