Homo- and Heterovalent Substitutions in the New Clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i>: Synthesis, Crystal Structure, and Thermoelectric Properties

The new cationic clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> were synthesized by the standard ampule technique. The Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> (<i>x</i> = 0–2.3) clathrates crystallize in the cubic space group <i>Pm</i>3̅<i>n</i> with the unit cell parameter <i>a</i> ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si_30+xP_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> (<i>x</i> = 1–6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (<i>x</i> = 1–3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group <i>Pm</i>3̅<i>n</i> to <i>Pm</i>3̅. 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 <i>p</i>-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (<i><i>ZT</i></i> = 0.1) was achieved for the Br-bearing clathrate Si_32.1(2)P_13.9(2)Te_6.6(2)Br_1.0(1) at 750 K

Homo- and Heterovalent Substitutions in the New Clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i>: Synthesis, Crystal Structure, and Thermoelectric Properties

The new cationic clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> were synthesized by the standard ampule technique. The Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> (<i>x</i> = 0–2.3) clathrates crystallize in the cubic space group <i>Pm</i>3̅<i>n</i> with the unit cell parameter <i>a</i> ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si_30+xP_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> (<i>x</i> = 1–6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (<i>x</i> = 1–3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group <i>Pm</i>3̅<i>n</i> to <i>Pm</i>3̅. 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 <i>p</i>-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (<i><i>ZT</i></i> = 0.1) was achieved for the Br-bearing clathrate Si_32.1(2)P_13.9(2)Te_6.6(2)Br_1.0(1) at 750 K