Unusual Helical Disorder of Na Atoms in the Tunnel Structure of Thermoelectric Compound Na_2+xGa_2+xSn_4–x at High Temperature

Abstract

A ternary intermetallic phase, Na2+x Ga2+x Sn4–x (space group P6122, a ∼ 6.33 Å and c ∼ 6.16 Å), which has helical tunnels containing Na atoms in the framework structure constructed by tetrahedrally coordinated Ga/Sn atoms, exhibits a high thermoelectric property at x = 0.19 and at room temperature. An unusual disordered Na atom distribution in Na2+x Ga2+x Sn4–x , which is represented in one crystallographic Na site with an occupancy of ∼1/3, was investigated by high-temperature synchrotron powder diffraction and solid-state 23Na MAS NMR spectroscopy. In the samples of x = 0.0 and x = 0.19, lattice constants of both a and c and unit-cell volumes were increased monotonically in the temperature range from R.T. to 723 K. Equivalent atomic displacement parameters, U eq, for the Na sites of Na2+x Ga2+x Sn4–x with x = 0.0 and 0.19 were unusually large, ca. 0.20 and 0.12 Å2 at 373 K, and increased to ca. 0.30 and 0.22 Å2 at 673 K, respectively, in contrast to a slight increase of those for the Ga/Sn sites, ca. 0.02–0.04 Å2. Electron density distribution (EDD) obtained by the maximum entropy method for the sample with x = 0.19 shows a consecutive helical electron distribution by statistical arrangement of Na atoms, reflecting an unusual static and/or dynamic disorder structure. The EDD of the Na atom was elongated along the c axis with increasing temperature in the tunnel. A solid-state 23Na MAS NMR spectrum showed a single broad signal (fwhm of ca. 13 ppm), indicating the presence of an intrinsic inhomogeneous distribution of Na atoms around the Na site. Additionally, the analysis of spin–lattice relaxation time, T 1, of 23Na nuclei suggested the presence of two types of Na nuclei having different mobility. This finding would be related to the ununiform local distribution of Ga atoms in the framework.

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

“…Na 2+x X 2+x Sn 4−x (X = Al, Ga, In) and two polytypes of Na 2 ZnSn 5 were synthesized according to previously reported procedures. [27][28][29][30] Because Na 2+x Al 2+x Sn 4−x could only be obtained as a solid solution with a less-than-stoichiometric Na content, [28] the most Na-rich composition (x = −0.18, Na 1.82 Al 1.82 Sn 4.18 ) was chosen because the rattling of Na is an important factor in the reduction of the lattice thermal conductivity. The Na-rich sample (x = 0.19 in Na 2.19 Ga 2.19 Sn 3.81 ), which gave the highest thermoelectric properties in the previous work on Na 2+x Ga 2+x Sn 4−x , was prepared for the same reason.…”

“…[18][19][20][21][22] In addition to the cage frameworks found in clathrates and skutterudites, several Sn-based intermetallic compounds (Na 2+x X 2+x Sn 4−x (X = Al, Ga, Sn) and Na 2 ZnSn 5 ) exhibit unique "helical tunnel frameworks" comprising Sn atoms and X or Zn atoms that host Na atoms in quasi 1D open spaces (Figure 1; Figures S1 and S2, Supporting Information). [23][24][25][26][27][28][29][30] Polycrystalline samples of Na 2+x Ga 2+x Sn 4−x (x = 0.19) exhibit excellent thermoelectric properties, with a figure of merit (ZT = 0.98, 295 K) comparable to that of practical Bi 2 Te 3 -based thermoelectric materials near room temperature (ZT = 1). [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).…”