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.