Thermoelectric Properties of Na₂ZnSn₅ Dimorphs with Na Atoms Disordered in Tunnels

Ingots of dimorphs (two polymorphs), hP-Na₂ZnSn₅ (metastable phase) and tI-Na₂ZnSn₅ (stable phase), were prepared from the melt of the constituent elements with the stoichiometric composition by furnace cooling from 773 to 500 K within 1 h (4.6 K min^-1 on average) and slow cooling at a rate of 0.5 K min^-1 from 703 K to room temperature, respectively. From the electrical conductivities, Seebeck coefficients, and thermal conductivities measured at 295 K for the ingots, the dimensionless figures of merit (ZT) of hP-Na₂ZnSn₅ and tI-Na₂ZnSn₅ were calculated to be 0.21 and 2.8 × 10^-2, respectively. The lattice components of the thermal conductivities were estimated to be 1.10 W m^-1 K^-1 (hP-Na₂ZnSn₅) and 0.61 W m^-1 K^-1 (tI-Na₂ZnSn₅). Results of the crystal structure analysis by single-crystal X-ray diffraction of both phases demonstrated that the Na atoms in the tunnels of Zn/Sn frameworks had discrete (static) and large continuous (dynamic) positional disorder, which could play a role in reducing the lattice thermal conductivity due to phonon scattering. The disorder of ²³Na nuclei was also evidenced by the solid-state nuclear magnetic resonance spectroscopy.