During attempts of preparing yttrium oxotellurates(IV) using Y 2 O 3 and TeO 2 in YCl 3 fluxes, the occasional reaction of these educts with the walls of the evacuated silica ampoules led to colourless, lath-shaped single crystals of Y 2 [Si 2 O 7 ] in the new ζ -type structure as a minor by-product which was investigated by X-ray diffraction. The title compound crystallizes monoclinically in the space group P2 1 /m (a = 503.59 (5)

About Lanthanide Oxotantalates with the Formula MTaO4 (M = La – Nd, Sm – Lu)Besides being a by‐product of solid state syntheses in tantalum ampoules the lanthanide(III) oxotantalates of the formula MTaO4 can be easily prepared by sintering lanthanide sesquioxide M2O3 and tantalum(V) oxide Ta2O5 with sodium chloride as flux. Under these conditions two structure types emerge depending upon the M3+ cationic radius. For M = La – Pr the MTaO4‐type tantalates crystallize in the space group P21/c with lattice constants of a = 762(±1), b = 553(±4), c = 777(±4) pm, β = 101(±1)° and four formula units per unit cell. With M = Nd, Sm – Lu, the monoclinic cell dimensions (space group P2/c) shrink to the lattice constants like a = 516(±9), b = 551(±9), c = 534(±9) pm, β = 96.5(±0.3)° and there are only two formula units present. Both structures show a coordination sphere of eight oxygen atoms for the lanthanide trications shaped as distorted square antiprism for the structure with the larger lanthanides (in the following referred to as A‐type) and as trigonal dodecahedron for the structure with the smaller ones (called as B‐type in the following). The coordination environment about the Ta5+ cations can be described as a slightly distorted octahedron (CN = 6) for the A‐type structure of MTaO4 and a heavily distorted one (CN = 6) for the B‐type. The difference between the two types results from the interconnection of these [TaO6]7− octahedra. Whereas they are connected via four vertices to form corrugated layers according to $^{2}_{\infty}\rm[TaO^{v}_{4/2}O^{t}_{2/2}]^{3-}$ parallel the bc‐plane in the A‐type, the octahedra of the B‐type MTaO4 structure share edges to built up zig‐zag chains $^{1}_{\infty}\rm[TaO^{e}_{4/2}O^{t}_{2/2}]^{3-}$ along the c axis.

Z = 2) entstehen durch Umsetzung von Holmiumsesquioxid mit Tellurdioxid in geeigneten molaren Verha È ltnissen (Ho 2 O 3 : TeO 2 = 1 : 4 bzw. 1 : 5) innerhalb von acht Tagen in evakuierten Quarzglasampullen bei 800°C. Der Einsatz von Caesiumchlorid (CsCl) als Fluûmittel in etwa fu È nffach molarem Ûberschuû sorgt fu È r schnelle und quantitative Reaktionen zu einkristallinen Zielprodukten. In der Kristallstruktur von Ho 2 Te 4 O 11 erfolgt die Verknu È pfung von [HoO 8 ]-Poly-edern u È ber Sauerstoffkanten, wodurch sich ein Netz 2 1 {[Ho 2 O 10 ] 14± } parallel zur (001)-Ebene ausbildet. Der Kristallstruktur von Ho 2 Te 5 O 13 liegen dagegen nur noch von sauerstoffverknu È pften [(Ho1)O 8 ]-und [(Ho2)O 7 ]-Polyedern aufgebaute Ba È nder 1 1 {[(Ho1) 2 (Ho2) 2 O 20 ] 28± } zugrunde, die la È ngs [100] verlaufen. Gemeinsam ist beiden Strukturen allerdings die stereochemische Aktivita È t der nicht-bindenden Elektronenpaare (¹lone pairsª) sa È mtlicher Te 4+ -Kationen (Te1 und Te2 in Ho 2 Te 4 O 11 ; Te1±Te5 in Ho 2 Te 5 O 13 ), die w 1 -polyedrische Koordinationsfiguren mit 3 + 1, 4 bzw. 3 + 2 Sauerstoffatomen um die Zentralteilchen bewirkt.Abstract. Ho 2 Te 4 O 11 (monoclinic, C2/c; a = 1240.73(8), b = 511.21(3), c = 1605.84(9) pm, b = 106.142(7)°; Z = 4) and Ho 2 Te 5 O 13 (triclinic, P1; a = 695.67(5), b = 862.64(6), c = 1057.52(7) pm, a = 89.057(6), b = 86.825(6), c = 75.056(6)°; Z = 2) are obtained by the reaction of holmium sesquioxide with tellurium dioxide in appropriate molar ratios (Ho 2 O 3 : TeO 2 = 1 : 4 and 1 : 5, respectively) in evacuated silica tubes within eight days at 800°C. The application of cesium chloride (CsCl) as flux in about five times molar excess secures fast and complete reactions to the single-crystalline products aimed at. In the crystal structure of Ho 2 Te 4 O 11 [HoO 8 ] polyhedra are connected via oxygen edges thereby building up a network 2 1 {[Ho 2 O 10 ] 14± } (001). On the other hand, the crystal structure of Ho 2 Te 5 O 13 exhibits oxygen-linked [(Ho1)O 8 ] and [(Ho2)O 7 ] polyhedra, which form ribbons 1 1 {[(Ho1) 2 -(Ho2) 2 O 20 ] 28± } running along [100]. Common to both structures, however, is the stereochemical activity of the nonbonding electron pairs (ªlone pairsº) of all the of the Te 4+ cations (Te1 and Te2 in Ho 2 Te 4 O 11 , Te1±Te5 in Ho 2 Te 5 O 13 ) causing w 1 -polyhedral figures of coordination with 3 + 1, 4 and 3 + 2 oxygen atoms, respectively, around the central atoms.

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The present work is the first comprehensive account of the knowledge acquired from single crystals of the isotopic series M 2 Te 4 O 11 (M = La -Nd, Sm -Yb). In the crystal structure, the M 3+ cation is coordinated by eight oxygen atoms in the shape of a distorted square antiprism. Out of these polyhedra a mesh-like [M 2 O 16 ] 14− layer parallel to the (001) plane is built via three common edges. The [Te 4 O 11 ] 6− double layers in turn build two tellurium-oxygen chains crosswise to each other. The construction of the tellurium-oxygen partial structure is, however, only possible taking the secondary Te-O contacts into consideration. In most oxotellurates(IV), three oxygen atoms are covalently bound to the Te atoms (d(Te-O) ≈ 180 − 200 pm; ψ 1 tetrahedron). Another oxygen atom is found in the near vicinity at a distance of 230 to 280 pm. The significance of such secondary interactions for the stability of the crystal structures was recognized recently in theoretical as well as experimental investigations. All oxygen atoms with distances smaller than 280 pm are counted to the secondary coordination sphere. This limit may seem somewhat arbitrary but it accounts very well for the Te-O partial structure. The coordination sphere for the tellurium center is a ψ 1 trigonal bipyramid including the stereochemically active electron pair ("lone pair"). A description of the crystal structure is also possible without this partial structure, however in the [TeO 3+1 ] 4− polyhedra above and below the meshes of [M 2 O 10 ] 14− layers are linked via Te 2 -O 6 -Te 2 contacts only.