Synthesis and structural characterization of Na(x)Si136 (0 < x ≤ 24) single crystals and low-temperature transport of polycrystalline specimens.

Na(x)Si(136) clathrate-II single crystals with x = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na(24)Si(136) single crystals were grown from the precursor Na(4)Si(4) by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na(24)Si(136) single crystals were subjected to thermal decomposition in a nitrogen atmosphere at 10 Torr and 405 °C. The Na content was controlled by the duration of thermal decomposition. The structural properties were investigated using single-crystal X-ray diffraction and compared with those of single-crystal Na(24)Si(136). The quality of the obtained products also allowed for low-temperature transport property measurements on agglomerates of crystals allowing for an investigation into the low-temperature electrical and thermal properties as a function of Na content.