Homo- and Heterovalent Substitutions in the New Clathrates I Si₃₀P₁₆Te_8–xSe_x and Si_30+xP_16–xTe_8–xBr_x: Synthesis, Crystal Structure, and Thermoelectric Properties

Abstract

The new cationic clathrates I Si(30)P(16)Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x) were synthesized by the standard ampule technique. The Si(30)P(16)Te(8-x)Se(x) (x = 0-2.3) clathrates crystallize in the cubic space group Pm3̅n with the unit cell parameter a ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si(30+x)P(16-x)Te(8-x)Br(x) (x = 1-6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; 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 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 (ZT = 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.

Crystal Structure, Magnetic, Electronic, and Thermal Transport Properties of Ternary Compounds REReB4 (RE = Ce, Gd–Er, Yb)

“…Furthermore, it is in agreement with the recent chemical‐bonding analysis that revealed covalent interactions not only between the B atoms within the layers at z = 1/2 but also polar covalent bonding between the B and M atoms that interlink the layers to form three‐dimensional [MB 4 ] anionic frameworks for TmAlB 4 41 and RERhB 4 33. The obtained α i are of the same order of magnitude as those reported for some binary borides (ScB 2 and HoB 4 42) and semiconducting clathrates 43…”

Structural Properties and Thermoelectric Performance of the Double-Filled Skutterudite (Sm,Gd)y(FexNi1-x)4Sb12

“…According to the basic concept of the aforementioned theory, in fact, such materials are expected to exhibit a dramatic decrease of phonon thermal conductivity due to the rattling movement of host ions within the cage, without affecting the mobility of charge carriers. These features are actually displayed by members of several classes of intermetallic compounds, such as skutterudites [18], clathrates [19,20], and half-Heusler phases [21]. Skutterudites MX 3 [18,22] (where M is a transition metal, such as Co, Fe, Rh or Ir and X a pnicogen atom), in particular, exhibit a body-centered cubic cell (Pearson symbol cI 32, Imtrue3¯ space group, isotypic crystal: CoAs 3 ) presenting two distinct atomic sites: the 8 c (¼, ¼, ¼) and the 24 g (0, y , z ), occupied by M and X, respectively.…”