Thermoelectric Properties of Na₂ZnSn₅ Dimorphs with Na Atoms Disordered in Tunnels

Abstract

Ingots of dimorphs (two polymorphs), hP-Na 2 ZnSn 5 (metastable phase) and tI-Na 2 ZnSn 5 (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-Na 2 ZnSn 5 and tI-Na 2 ZnSn 5 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-Na 2 ZnSn 5 ) and 0.61 W m −1 K −1 (tI-Na 2 ZnSn 5 ). 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 23 Na nuclei was also evidenced by the solid-state nuclear magnetic resonance spectroscopy.

Unusual Helical Disorder of Na Atoms in the Tunnel Structure of Thermoelectric Compound Na2+xGa2+xSn4–x at High Temperature

“…The degree for the deviation of the Na atom position in NGS 0.0 is larger than that in NGS 0.19 . Very recently, in a similar compound of Na 2 ZnSn 5 having two different structures (tetragonal phase with ordered Zn and Sn atoms (space group I 4̅2 d ) and hexagonal phase with disordered Zn and Sn atoms (space group P 6 1 22)), we presumed that the resonance peak width in the F1 axis would be correlated with the Zn/Sn atom ordering in the framework or the temperature-independent disorder term, d , of the Na atom . In NGS x , the both relationship seems to be maintained, because the peak width in the F1 axis was very large as same as the hexagonal phase of Na 2 ZnSn 5 and increased with the increase in d .…”

Correlated Rattling of Sodium‐Chains Suppressing Thermal Conduction in Thermoelectric Stannides

“…The obtained N E , N D , and characteristic temperatures remained essentially unchanged (Table S11, Supporting Information); b) Ref. [29] [29] as a function of temperature. b) Principal axes of anisotropic atomic displacement ellipsoids are along the major axis (U anis_a of the Na site in each compound against T. The of the vertical axis in (b) is twice that in (a).…”

“…Single-crystal X-ray analysis of all helical-tunnel Na-stannides reveals large, one-directional vibrations of Na atoms along the helical tunnels, as demonstrated by the elongation of thermal ellipsoids along the tunnel at the Na site (Figure 1; Figure S2, Supporting Information). [27][28][29] The characteristic Debye (Θ D ) and Einstein (Θ E ) temperatures of the host and guest atoms were estimated from the temperature dependence of the equivalent isotropic ADPs (U eq ) and anisotropic ADPs (U ij ) by fitting with Equations ( 5) and ( 6) (Debye and Einstein models, respectively) together with the temperature-independent disorder parameters (d) (Table 2, Figure 4; Tables S2-S10, Supporting Information). [13,23] eq,aniso…”

“…[ 27 ] Two polymorphs ( hP ‐ and tI ‐phases) of Na 2 ZnSn 5 with similar helical tunnel structures exhibit particularly low κ lattice values (1.1 and 0.61 W m −1 K −1 , respectively). [ 29 ] The previous studies of the helical‐tunnel Na‐stannides are summarized in the Supporting Information and the thermoelectric properties of the compounds developed herein are tabulated in Table 1 . The data indicate helical‐tunnel Na‐stannides are a promising new class of PGEC materials, despite being unstable in air.…”

“…Figure 4. a) Equivalent isotropic displacement parameters (U eq of the Na and (Al, Ga, or In)/Sn sites of Na 1.82 Al 1.82 Sn 4.24 (red squares), Na 2.19 Ga 2.19 Sn 3.81 (black diamonds), Na 2 In 2 Sn 4 (blue triangles), Na sites of hP-Na 2 ZnSn 5 (violet circles), and tI-Na 2 ZnSn 5 (green circles)[29] as a function of temperature. b) Principal axes of anisotropic atomic displacement ellipsoids are along the major axis (U anis_a of the Na site in each compound against T. The of the vertical axis in (b) is twice that in (a).…”