Homo- and Heterovalent Substitutions in the New Clathrates I Si30P16Te8–xSex and Si30+xP16–xTe8–xBrx: Synthesis, Crystal Structure, and Thermoelectric Properties

The new cationic clathrates I Si30P16Te8–xSex and Si30+xP16–xTe8–xBrx were synthesized by the standard ampule technique. The Si30P16Te8–xSex (x = 0–2.3) clathrates crystallize in the cubic space group Pm3n with the unit cell parameter a ranging from 9.9382(2) to 9.9696(1) A. In the case of the Si30+xP16–xTe8–xBrx (x = 1–6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) A; at lower Si/P ratios (x = 1–3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group Pm3n to Pm3. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 × 10–6 to 1.0 × 10–5 K–1 in the temperature range of 298–1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of p-type...