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

Abstract

A ternary intermetallic phase, Na2+xGa2+xSn4–x (space group P6122, a ∼ 6.33 A and c ∼ 6.16 A), 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 A2 at 373 K, and increased to ca. 0.30 and 0.22 A2 at 673 K, respectively, in contrast to a slight increase of those for the Ga/Sn sites, ca. 0.02–0.04...