Synthese, Struktur und Eigenschaften der tantalreichen Silicidchalkogenide Ta15Si2QxTe10x (Q = S, Se)

Abstract

Professor Herbert Jacobs zum 65. Geburtstag gewidmet Inhaltsu È bersicht. Die quaterna Èren Tantalsilicidchalkogenide Ta 15 Si 2 Q x Te 10±x (Q = S, Se) sind durch Umsetzung entsprechender Gemenge aus Dichalkogeniden, Silicium und Tantal bei 1770 K in zugeschweiûten Molybda Èntiegeln herstellbar. Ihre Struktur wurde aus den intensita È tssta Èrksten Reflexen von ca. 3 lm dicken faserigen Einkristallen bestimmt und u È ber Rietveld-Profilanpassungen von Ro È ntgen-Pulverdiffraktogrammen phasenreiner Pra È parate besta Ètigt. Ta 15 Si 2 S 3.5 Te 6.5 bzw. Ta 15 Si 2 Se 3.5 Te 6.5 kristallisiert monoklin in der Raumgruppe C2/m mit Z = 2, a = 2393.7(1) pm, b = 350.08(2) pm, c = 1601.2(1) pm, b = 124.700(4)°bzw. a = 2461.3(2) pm, b = 351.70(2) pm, c = 1601.7(1) pm, b = 124.363(5)°. Dreifach u È berdachte trigonal-prismatische Ta 9 Si-Cluster, die durch eingeschlossene Siliciumatome stabilisiert sind, ko È nnen als charakteristische Baugruppe der Struktur angesehen werden. Die Cluster sind zu Doppelstra È ngen kondensiert. Weitere Tantalatome verknu È pfen die Stra Ènge zu Schichten. Die verbleibenden Valenzen auf deren Oberfla Èchen werden von Chalkogenatomen abgesa Èttigt, die entsprechend einseitig von drei, vier oder fu È nf Ta-Atomen koordiniert sind. Der Zusammenhalt der ca. 1 nm dicken, von Chalkogenatomen bedeckten Lamellen wird durch einzelne verbru È ckende Ta±S±Ta-bzw. Ta±Se±Ta-Bindungen und Dispersionswechselwirkungen zwischen den Chalkogenatomen vermittelt. Die Silicidchalkogenide sind ma Èûig gute metallische Leiter, die trotz ausgepra Ègter Anisotropie der Struktur zwischen 10 und 310 K keine elektronischen Instabilita Èten erkennen lassen.Synthesis, Structure, and Properties of the Tantalum-rich Silicide Chalcogenides Ta 15 Si 2 Q x Te 10±x (Q = S, Se).Abstract. The quaternary tantalum silicide chalcogenides Ta 15 Si 2 Q x Te 10±x (Q = S, Se) are accessible from proper, compacted mixtures of the respective dichalcogenides, silicon and elemental tantalum at 1770 K in sealed molybdenum tubes. The structures were determined from the strongest Xray intensities of fibrous crystals with cross sections of about 3 lm and confirmed by fitting the profile of single phase Xray diffractograms. The phases Ta 15 Si 2 S 3.5 Te 6.5 and Ta 15 Si 2-Se 3.5 Te 6.5 crystallize in the monoclinic space group C2/m with two formula units per unit cell, a = 2393.7(1) pm, b = 350.08(2) pm, c = 1601.2(1) pm, b = 124.700(4)°, and a = 2461.3(2) pm, b = 351.70(2) pm, c = 1601.7(1) pm, b = 124.363(5)°, respectively. Tri-capped trigonal prismatic Ta 9 Si clusters stabilized by encapsulated Si atoms can be seen as the characteristic unit of the structure. The clusters are fused into twin columns which are connected by additional Ta atoms, thus forming corrugated layers. The remaining valences at the surfaces of the layered Ta±Si substructure are saturated by those of chalcogen atoms which are coordinated only from one side by three, four or five Ta atoms. Few bridging covalent Ta±S±Ta and Ta±Se±Ta bonds and, otherwise, dispersive interac...

RE5Co4Si14 (RE = Ho, Er, Tm, Yb): Silicides Grown from Ga Flux Showing Exceptional Resistance to Chemical and Thermal Attack

“…Si-rich compounds are of interest for a variety of applications where extreme environments would be prevalent because they will likely be less dense than the Ni super alloys, which are currently used in the bulk of aerospace propulsion components. Despite these considerations exploratory research in new complex silicides has been relatively limited. − …”

New Barium Copper Chalcogenides Synthesized Using Two Different Chalcogen Atoms: Ba2Cu6–xSTe4 and Ba2Cu6–xSeyTe5–y

“…That no mixed S/Te occupancies were identified in the structure of Ba 2 Cu 6– x STe 4 has its precedence in the structure of Zr 6 STe 2 . Such mixed occupancies are possible, however, as is the case in Ta 6 S 1+ x Te 3– x (with up to 4% S on Te sites and 7% Te on S sites) and Ta 15 Si 2 S x Te 10– x (with one pure S site and four mixed S/Te sites).…”

“…The strategy to use mixed anions, such as S/Te and Se/Te, for creation of new structures with mixed occupied chalcogen sites was demonstrated to be successful with the synthesis of Ta 6 S 1+ y Te 3– y (0 ≤ y ≤ 0.8) and Ta 15 Si 2 S y Te 10– y (2.5 ≤ y ≤ 4) and Ta 15 Si 2 Se y Te 10– y (3 ≤ y ≤ 7) . The general strategy dates back some 20 years, when new ternary Nb–Ta sulfides with differential fractional site occupancies on the cation sites were uncovered, namely, Nb 1.72 Ta 3.28 S 2 , Nb 0.95 Ta 1.05 S, Nb 4.92 Ta 6.08 S 4 , and Nb 6.74 Ta 5.26 S 4 .…”

“…While the former has no counterpart among the selenides or the tellurides, the telluride Ba 2 Cu 4Àx Te 5 adopts a type similar to Ba 2 Cu 4Àx Se y Te 5Ày , namely, with the same structure motifs but different interconnection of these motifs to the threedimensional structure.The strategy to use mixed anions, such as S/Te and Se/Te, for creation of new structures with mixed occupied chalcogen sites was demonstrated to be successful with the synthesis of Ta 6 S 1þy Te 3Ày (0 e y e 0.8) 11 and Ta 15 Si 2 S y Te 10Ày (2.5 e y e 4) and Ta 15 Si 2 Se y Te 10Ày (3 e y e 7). 12 The general strategy dates back some 20 years, when new ternary NbÀTa sulfides with differential fractional site occupancies on the cation sites were uncovered, namely, Nb 1.…”

Crystal Structure and Physical Properties of the New Chalcogenides Ba3Cu17–x(S,Te)11 and Ba3Cu17–x(S,Te)11.5 with Two Different Cu Clusters

“…While no mixed S:Te occupancies were identified in the sulfide-tellurides Ba 2 Cu 6– x STe 4 and Zr 6 STe 2 , each of the four crystals investigated here has at least one mixed S:Te site, with ratios of up to 30:70 and 80:20, respectively. Other examples for mixed S:Te occupancies, while rare, do exist, including Ta 6 S 1+ x Te 3– x (with up to 4% S on Te sites and 7% Te on S sites) and Ta 15 Si 2 S x Te 10– x (with one pure S site and four mixed S:Te sites, albeit with very high standard deviations). It is most probable that, because of the large difference in atom sizes, the S and Te atoms do not occupy the same absolute position.…”