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

Abstract

Single crystals of novel compounds, Na2+x Al2+x Sn4–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.

Thermoelectric Properties of Na2ZnSn5 Dimorphs with Na Atoms Disordered in Tunnels

“…Na 2+ x Tr 2+ x Sn 4– x (Tr = Ga, Al) are isostructural with hP -Na 2 ZnSn 5 and the values of κ total – κ carrier measured for the polycrystalline sintered samples with relative densities of ca. 75% have been reported to be 0.26 W m –1 K –1 (Tr = Ga) and 0.27 W m –1 K –1 (Tr = Al). , In these cases, the κ total – κ carrier values did not simply correspond to the κ lattice values because of the contribution of other factors such as grain boundaries and pores in the samples. The thermal conductivities κ total of the Na 2 ZnSn 5 dimorphs measured for the ingot disks with a relative density of 94–98% are close to the intrinsic values.…”

“…75% have been reported to be 0.26 W m −1 K −1 (Tr = Ga) and 0.27 W m −1 K −1 (Tr = Al). 23,24 In these cases, the κ total − κ carrier values did not simply correspond to the κ lattice values because of the contribution of other factors such as grain boundaries and pores in the samples. The thermal conductivities κ total of the Na Low-Temperature Single-Crystal XRD and Lattice Dynamical Analysis.…”

“…Recently, ternary Zintl compounds, Na 2+ x Tr 2+ x Sn 4– x (Tr = Ga, Al), were synthesized and their crystal structures and thermoelectric properties were characterized. , In the crystal structures, (Ga or Al)/Sn atoms are tetrahedrally coordinated and form a 3D framework structure including helical tunnel spaces, and Na atoms are statistically located at a Na site in the tunnels with an occupancy of around 1 / 3 . Sintered samples with relative densities of ca.…”

“…The thermal conductivities of the samples at room temperature were reported to be 0.52–0.68 W m –1 K –1 (Tr = Ga, x = 0.19) and 0.29 W m –1 K –1 (Tr = Al, x = −0.24). The possibility that large dynamic and static disorders of Na atoms in the tunnels of the compounds play a role in the reduction of the thermal conductivities has been pointed out. , …”

“…The possibility that large dynamic and static disorders of Na atoms in the tunnels of the compounds play a role in the reduction of the thermal conductivities has been pointed out. 23,24 Dimorphs, hP-Na 2 ZnSn 5 (metastable phase) and tI-Na 2 ZnSn 5 (stable phase), have been synthesized and their In the present study, polycrystalline ingots of hP-Na 2 ZnSn 5 and tI-Na 2 ZnSn 5 were prepared and their thermal properties were characterized by the measurement of electrical conductivity, Seebeck coefficient, and thermal conductivity under an Ar atmosphere. Lattice dynamical analyses for Zn and Sn atoms which form the frameworks and for Na atoms in the tunnels were performed with atomic displacement parameters obtained by low-temperature single-crystal X-ray diffraction (XRD).…”