3 years ago

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

Unusual Helical Disorder of Na Atoms in the Tunnel Structure of Thermoelectric Compound Na2+xGa2+xSn4–x at High Temperature
Takuji Ikeda, Hisanori Yamane, Takahiro Yamada
A ternary intermetallic phase, Na2+xGa2+xSn4–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+xGa2+xSn4–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, Ueq, for the Na sites of Na2+xGa2+xSn4–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, T1, 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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b05727

DOI: 10.1021/acs.jpcc.7b05727

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