Homo- and Heterovalent
Substitutions in the New Clathrates
I Si<sub>30</sub>P<sub>16</sub>Te<sub>8–<i>x</i></sub>Se<sub><i>x</i></sub> and Si<sub>30+<i>x</i></sub>P<sub>16–<i>x</i></sub>Te<sub>8–<i>x</i></sub>Br<sub><i>x</i></sub>: Synthesis, Crystal Structure,
and Thermoelectric Properties
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Abstract
The new cationic clathrates I Si<sub>30</sub>P<sub>16</sub>Te<sub>8–<i>x</i></sub>Se<sub><i>x</i></sub> and
Si<sub>30+<i>x</i></sub>P<sub>16–<i>x</i></sub>Te<sub>8–<i>x</i></sub>Br<sub><i>x</i></sub> were synthesized by the standard ampule technique. The Si<sub>30</sub>P<sub>16</sub>Te<sub>8–<i>x</i></sub>Se<sub><i>x</i></sub> (<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<sub>30+x</sub>P<sub>16–<i>x</i></sub>Te<sub>8–<i>x</i></sub>Br<sub><i>x</i></sub> (<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<sup>–6</sup> to 1.0 × 10<sup>–5</sup> K<sup>–1</sup> 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<sub>32.1(2)</sub>P<sub>13.9(2)</sub>Te<sub>6.6(2)</sub>Br<sub>1.0(1)</sub> at 750 K