First Tin Pnictide Halides Sn₂₄P_19.3I₈ and Sn₂₄As_19.3I₈:  Synthesis and the Clathrate-I Type of the Crystal Structure

Abstract

Sn(24)P(19.3)I(8) (I) and Sn(24)As(19.3)I(8) (II) have been prepared by a standard ampule synthesis. I crystallizes in a cubic space group Pm&thremacr;n, a = 10.9540(10) Å, z = 1. The crystal structure of I is built of the 3D net composed of tin and phosphorus atoms, while iodine atoms occupy large polyhedral holes of two different types, pentagonal dodecahedral and tetrakaidodecahedral. An arrangement of such polyhedra follows that of the clathrate-I type. The 3D net has vacancies at one of the phosphorus atoms positions. The vacancies cause the split of the tin atomic position into two, having different coordination, which is reflected in the (119)Sn Mössbauer spectrum. The vacancy concentration correlates well with the occupancy factors of the split tin atomic positions, and in accordance with the Zintl-Klemm formalism for valence compounds, I is a narrow-gap semiconductor. Powder diffraction data shows that II belongs to the same clathrate family, but has an 8 times larger face-centered cubic unit cell.

Novel Compounds Sn10In14P22I8 and Sn14In10P21.2I8 with Clathrate I Structure: Synthesis and Crystal and Electronic Structure

“…The spectrum recorded from Sn In P I shows one doublet in accordance with one unique tin site in the ideal clathrate I structure. The chemical isomer shift of 1.77 mm s\ is close to the value of 1.84 mm s\ recorded from the four-coordinated tin in the parent phase Sn P I (12). A slight broadening of the resonance line ( "1.4 mm s\) can be attributed to a superposition of two doublets having very close values, re#ecting two statistically probable types of tin coordination, 1Sn #3P and 1In #3P.…”

“…The latter position is partially occupied in the structure of Sn P I (12), and this is interrelated with the distribution of tin atoms among two partially occupied 24-fold sites. Filling the vacancies in the phosphorus site might remove the splitting of the metal position, observed in the case of the Sn In P I structure with the fully occupied 24k metal site.…”

“…The "rst object is a hypothetical clathrate constructed by "lling all the framework positions in the space group Pm3 n. The second object is a real derivative achieved by a random distribution of 14 indium atoms among the 24 metal atom positions. The third object has an almost real composition which is only 0.3 P atoms per formula short (12). It is constructed in such a way that there are two types of Sn atoms, 4-and 3#3-coordinated, in equal numbers, compared to the observed 55:45 ratio.…”

K7.62(1)Si46 and Rb6.15(2)Si46: Two Structure I Clathrates with Fully Occupied Framework Sites

“…Following the Zintl-Klemm concept, the formation of two vacancies in the framework structure, i.e., A ¹t ) , would allow the accommodation of the eight extra electrons from the alkali metals (A) onto the framework, while maintaining electroneutrality (35,36 &&fully occupied'' framework stoichiometries (27}32). In the same vein, the inclusion of pnictides (Pn"P, As) is also permitted in the framework when halogen atoms (Br, I) are incorporated as guest atoms (33,34). We have recently examined the Structure I clathrate (Na Si ) in the Na}Si system using Rietveld analysis of powder X-ray di!raction data combined with quantitative Si MAS NMR spectroscopy, and have demonstrated that the framework contained no vacancies (38,39).…”

Thermoelectric Power Generation by Clathrates

“…sample shows much higher conductivity of 6.5 × 10 3 S m −1 [17,26]. Temperature-dependent impedance spectroscopy measurements showed that for SPS-compacted sample of high density (92% of theoretical), intergrain contacts start to contribute signiicantly to total impedance only below 75 K, while above this temperature only activation part could be detected [34].…”