Synthesis and Structural Characterization of Na_<i>x</i>Si₁₃₆ (0 < <i>x</i> ≤ 24) Single Crystals and Low-Temperature Transport of Polycrystalline Specimens

Na_<i>x</i>Si₁₃₆ clathrate-II single crystals with <i>x</i> = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na₂₄Si₁₃₆ single crystals were grown from the precursor Na₄Si₄ by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na₂₄Si₁₃₆ 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 Na₂₄Si₁₃₆. 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