Synthesis, Crystal Structure, and Thermoelectric Properties of Na_2+xAl_2+xSn_4-x (x = -0.38, -0.24)

Single crystals of novel compounds, Na_2+xAl_2+xSn_4-x with x = -0.38 and -0.24, were prepared by slow cooling from the melts of the constituent element metals from 1023 and 1123 K, respectively. Single-crystal X-ray diffraction analysis revealed that the compounds crystallize in hexagonal cells (space group P6₁22) (x = -0.38, a = 6.4050(6) and c = 6.1427(6) Å x = -0.24, a = 6.3984(3) and c = 6.1529(3) Å). In the crystal structures, four-bonded Al and Sn atoms form frameworks in which helical tunnels are contained. Na atoms are statistically arranged at positions of a 6b site in the tunnels with occupancies of approximately 0.3. Polycrystalline bulk samples with a nominal composition X = -0.24 were prepared by heating compacts of a NaSn, Al, and Sn powder mixture at 623 K. The electrical conductivity and Seebeck coefficient of the polycrystalline samples changed from 3.09 × 10³ to 1.03 × 10⁴ S m^-1 and from -222 to -185 μV K^-1, respectively, with increasing temperature from 295 to 472 K. The thermal conductivity was 0.29-0.36 W m^-1 K^-1 at 295-371 K. The dimensionless figures of merit ZT were 0.15-0.21 at 295-371 K.