Synthesis, Crystal Structure, and Thermoelectric Properties of Na2+xAl2+xSn4–x (x = −0.38, –0.24)

Single crystals of novel compounds, Na2+xAl2+xSn4–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.