Cationic Clathrate I Si_46-xP_xTe_y (6.6(1) ≤ y ≤ 7.5(1), x ≤ 2y): Crystal Structure, Homogeneity Range, and Physical Properties

Abstract

A new cationic clathrate I Si(46-x)P(x)Te(y) (6.6(1) < or = y < or = 7.5(1), x < or = 2y at 1375 K) was synthesized from the elements and characterized by X-ray powder diffraction, thermal analysis, scanning electron microscopy, wavelength dispersive X-ray spectroscopy (WDXS), neutron powder diffraction, and (31)P NMR spectroscopy. The thermal behaviors of the magnetic susceptibility and resistivity were investigated as well. Si(46-x)P(x)Te(y) reveals a wide homogeneity range due to the presence of vacancies in the tellurium guest positions inside the smaller cage of the clathrate I structure. The vacancy ordering in the structure of Si(46-x)P(x)Te(y) causes the change of space group from Pm3n (ideal clathrate I) to Pm3 accompanied by the redistribution of P and Si atoms over different framework positions. Neutron powder diffraction confirmed that P atoms preferably form a cage around the vacancy-containing tellurium guest position. Additionally, (31)P NMR spin-spin relaxation experiments revealed the presence of sites with different coordination of phosphorus atoms. Precise determination of the composition of Si(46-x)P(x)Te(y) by WDXS showed slight but noticeable deviation (x < or = 2y) of phosphorus content from the Zintl counting scheme (x = 2y). The compound is diamagnetic while resistivity measurements show activated behavior or that of heavily doped semiconductors. Thermal analysis revealed high stability of the investigated clathrate: Si(46-x)P(x)Te(y) melts incongruently at approximately 1460 K in vacuum and is stable in air against oxidation up to 1295 K.

Thermoelectric Power Generation by Clathrates

“…This may lead to altering the band structure by introducing donor and/or acceptor levels, which may be broad enough to cause their overlap with both conduction and valence bands, giving rise to properties of "bad metal" and, provided the optimal tuning is achieved, to metal-to-semiconductor transition. As a result of this strategy, combination of S = 170 μV × K , which is almost four orders of magnitude greater than that for ideally stoichiometric compound Si 30 P 16 Te 8 [30,32].…”

“…Interestingly, the highest room-temperature ZT values are achieved for type-VIII clathrates. For instance, Sb-doped p-type Ba 8 Ga 16 Sn 30 demonstrates ZT = 0.6 and 300 K, whereas n-type Ba 8 Ga 16 Sn 30 displays ZT = 0.5 at the same temperature [45]. At higher temperature, as both electrical conductivity and Seebeck coeicient tend to grow, whereas thermal conductivity remains essentially constant (combination that is true for the majority of semiconducting clathrates), ZT increases with increasing temperature.…”

“…For instance, compact and dense pellets of Ge 30 P 16 Se 8 could be prepared at temperature of 773 K and pressure of 60 MPa that already provided sample density of 96% relative to theoretical one [29]. Similarly, silicon-based clathrates were densiied at signiicantly harsher condition of 1100 K and 110 MPa to achieve sample density of 95% [21,30]. In both cases, no degradation of the initial sample was observed, proving that densiication does not change composition and structure of clathrates and that concomitant thermoelectric measurements are performed on the samples of desired nature.…”

“…However, its band structure can be altered upon creating vacancies in guest positions with concomitant change in the Si:P ratio. As a result, band gap was decreased from 1.24 eV to minimum of 0.12 eV and electrical conductivity was increased up to (1-4) × 10 4 S m −1 depending on actual composition of clathrate [30]. Importantly, electronic structure and, hence, conducting properties are only weakly sensitive to isovalent substitution, provided that the substituting atoms reside on similar crystallographic sites.…”

Extension of the Clathrate Family: The Type X Clathrate Ge79P29S18Te6

“…, crystal size 0.05 0.1 0.2 mm, trigonal, R " 3 3m, a = 17.120(2), c = 10.609(2) , V = 2692.9 (8) 3 , Z = 1, 1 calc = 3.994 g cm…”