Zwei Chloridsilicate des Yttriums: Y3Cl[SiO4]2 und Y6Cl10[Si4O12]

Two Chloride Silicates of Yttrium: Y₃Cl[SiO₄]₂ and Y₆Cl₁₀[Si₄O₁₂] The chloride‐poor yttrium(III) chloride silicate Y₃Cl[SiO₄]₂ crystallizes orthorhombically (a = 685.84(4), b = 1775.23(14), c = 618.65(4) pm; Z = 4) in space group Pnma. Single crystals are obtained by the reaction of Y₂O₃, YCl₃ and SiO₂ in the stoichiometric ratio 4 : 1 : 6 with ten times the molar amount of YCl₃ as flux in evacuated silica tubes (7 d, 1000 °C) as colorless, strongly light‐reflecting platelets, insensitive to air and water. The crystal structure contains isolated orthosilicate units [SiO₄]^4– and comprises cationic layers {(Y2)Cl}²⁺ which are alternatingly piled parallel (010) with anionic double layers {(Y1)₂[SiO₄]₂}^2–. Both crystallographic different Y³⁺ cations exhibit coordination numbers of eight. Y1 is surrounded by one Cl^– and 7 O^2– anions as a distorted trigonal dodecahedron, whereas the coordination polyhedra around Y2 show the shape of bicapped trigonal prisms consisting of 2 Cl^– and 6 O^2– anions. The chloride‐rich chloride silicate Y₆Cl₁₀[Si₄O₁₂] crystallizes monoclinically (a = 1061,46(8), b = 1030,91(6), c = 1156,15(9) pm, β = 103,279(8)°; Z = 2) in space group C2/m. By the reaction of Y₂O₃, YCl₃ and SiO₂ in 2 : 5 : 6‐molar ratio with the double amount of YCl₃ as flux in evacuated silica tubes (7 d, 850 °C), colorless, air‐ and water‐resistant, brittle single crystals emerge as pseudo‐octagonal columns. Here also a layered structure parallel (001) with distinguished cationic double‐layers {(Y2)₅Cl₉}⁶⁺ and anionic layers {(Y1)Cl[Si₄O₁₂]}^6– is present. The latter ones contain discrete cyclo‐tetrasilicate units [Si₄O₁₂]^8– of four cyclically corner‐linked [SiO₄] tetrahedra in all‐ecliptical arrangement. The coordination sphere around (Y1)³⁺ (CN = 8) has the shape of a slightly distorted hexagonal bipyramid comprising 2 Cl^– and 6 O^2– anions. The 5 Cl^– and 2 O^2– anions building the coordination polyhedra around (Y2)³⁺ (CN = 7) form a strongly distorted pentagonal bipyramid.     

Synthese und Kristallstruktur des Fluorid‐ino‐Oxosilicats Cs2YFSi4O10

Synthesis and Crystal Structure of the Fluoride ino‐Oxosilicate Cs₂YFSi₄O₁₀ The novel fluoride oxosilicate Cs₂YFSi₄O₁₀ could be synthesized by the reaction of Y₂O₃, YF₃ and SiO₂ in the stoichiometric ratio 2 : 5 : 3 with an excess of CsF as fluxing agent in gastight sealed platinum ampoules within seventeen days at 700 °C. Single crystals of Cs₂YFSi₄O₁₀ appear as colourless, transparent and water‐resistant needles. The characteristic building unit of Cs₂YFSi₄O₁₀ (orthorhombic, Pnma (no. 62), a = 2239.75(9), b = 884.52(4), c = 1198.61(5) pm; Z = 8) comprises infinite tubular chains of vertex‐condensed [SiO₄]^4? tetrahedra along [010] consisting of eight‐membered half‐open cube shaped silicate cages. The four crystallographically different Si⁴⁺ cations all reside in general sites 8d with Si–O distances from 157 to 165 pm. Because of the rigid structure of this oxosilicate chain the bridging Si–O–Si angles vary extremely between 128 and 167°. The crystallographically unique Y³⁺ cation (in general site 8d as well) is surrounded by four O^2? and two F^? anions (d(Y–O) = 221–225 pm, d(Y–F) = 222 pm). These slightly distorted trans‐[YO₄F₂]^7? octahedra are linked via both apical F^? anions by vertex‐sharing to infinite chains along [010] (?(Y–F–Y) = 169°, ?(F–Y–F) = 177°). Each of these chains connects via terminal O^2? anions to three neighbouring oxosilicate chains to build up a corner‐shared, three‐dimensional framework. The resulting hexagonal and octagonal channels along [010] are occupied by the four crystallographically different Cs⁺ cations being ten‐, twelve‐, thirteen‐ and fourteenfold coordinated by O^2? and F^? anions (viz.[(Cs1)O₁₀]^19?, [(Cs2)O₁₀F₂]^21?, [(Cs3)O₁₂F]^24?, and [(Cs4)O₁₂F₂]^25? with d(Cs–O) = 309–390 pm and d(Cs–F) = 360–371 pm, respectively).     

Darstellung und Aufbau der Lanthanoidfluorid‐catena‐Trisilicate M3F[Si3O10] (M = Dy,Ho, Er) im Fluorthalenit‐Typ (Y3F[Si3O10])

Synthesis and Constitution of Fluorothalenite‐Type (Y₃F[Si₃O₁₀]) Fluoride catena‐ Trisilicates M₃F[Si₃O₁₀] with the Lanthanides (M = Dy, Ho, Er) By the reaction of the sesquioxides M₂O₃ with the corresponding trifluorides MF₃ (M = Dy, Ho, Er), SiO₂ and CsCl as flux (molar ratio: 1 : 1 : 3 : 6; 700 °C, 7 d) in evacuated silica tubes and gastight sealed metal capsules made of platinum, niobium or tantalum, respectively, single crystals of the fluoride silicates M₃F[Si₃O₁₀] (monoclinic, P2₁/n; Z = 4; M = Dy: a = 734.06(6), b = 1116.55(9), c = 1040.62(8) pm, β = 97.281(7)°; M = Ho: a = 730.91(6), b = 1111.68(9), c = 1037.83(8) pm, β = 97.238(7)°; M = Er: a = 727.89(6), b = 1107.02(9), c = 1035.21(8) pm, β = 97.209(7)°) were obtained. The most important building groups in the crystal structures of the thalenite type are “isolated” [FM₃]⁸⁺ triangles and catena‐trisilicate anions [Si₃O₁₀]^8–, which contain three [SiO₄] tetrahedra linked to a chain fragment via common corners. This has the shape of a horseshoe where both the terminal tetrahedra show different conformations (eclipsed and staggered) relative to the central unit. Therefore a chelatizing coordination on the same M³⁺ cation via oxygen atoms of both terminal [SiO₄] groups is possible. The narrow area of existence of these fluoride silicates within the lanthanide series will be discussed and structural comparisons with other catena‐trisilicates are presented.     

Er4F2[Si2O7][SiO4]: Das erste Selten‐Erd‐Fluoridsilicat mit zwei verschiedenen Silicat‐Anionen

Er₄F₂[Si₂O₇][SiO₄]: The First Rare‐Earth Fluoride Silicate with Two Different Silicate Anions By the reaction of Er₂O₃ with ErF₃ and SiO₂ at 700 °C in sealed tantalum capsules using CsCl as flux (molar ratio 5 : 2 : 3 : 20), the compound Er₄F₂[Si₂O₇][SiO₄] (triclinic, P 1; a = 648.51(5), b = 660.34(5), c = 1324.43(9) pm, α = 87.449(8), β = 85.793(8), γ = 60.816(7)°; V_m = 148.69(1) cm³/mol, Z = 2) is obtained as pale pink platelets or lath‐shaped single crystals. It consists of disilicate anions [Si₂O₇]^6– in eclipsed conformation, ortho‐silicate anions [SiO₄]^4– and isolated [Er₄F₂]¹⁰⁺ units comprising two edge‐shared [Er₃F] triangles. Er³⁺ is surrounded by 7 + 1 (Er1) or 7 (Er2–Er4) anionic neighbors, respectively, of which two are F^– in the case of Er1 and Er4 but only one for Er2 and Er3. The other ligands recruit from oxygen atoms of the different oxosilicate groups. The crystal structure can be described as simple rowing up of the three building groups ([SiO₄]^4–, [Er₄F₂]¹⁰⁺, and [Si₂O₇]^6–) along [001]. The necessity of a large excess of fluoride for a successful synthesis of Er₄F₂[Si₂O₇][SiO₄] will be discussed.     

La2Si2O7 im I—Typ: Gemäß La6[Si4O13][SiO4]2 kein echtes Lanthandisilicat

I‐Type La₂Si₂O₇: According to La₆[Si₄O₁₃][SiO₄]₂ not a Real Lanthanum Disilicate In attempts to synthesize lanthanum telluride silicate La₂Te[SiO₄] (from La, TeO₂, SiO₂ and CsCl, molar ratio: 1 : 1: 1 : 20, 950 °C, 7 d) or fluoride‐rich lanthanum fluoride silicates (from LaF₃, La₂O₃, SiO₂ and CsCl, molar ratio: 5 : 2 : 3 : 17, 700 °C, 7 d) in evacuated silica tubes, colourless lath‐shaped single crystals of hitherto unknown I‐type La₂Si₂O₇ (monoclinic, P2₁/c; a = 726.14(5), b = 2353.2(2), c = 1013.11(8) pm, β = 90.159(7)°) were found in the CsCl‐flux melts. Nevertheless, this new modification of lanthanum disilicate does not contain any discrete disilicate groups [Si₂O₇]^6‐ but formally three of them are dismutated into one catena‐tetrasilicate ([Si₄O₁₃]^10‐ unit of four vertex‐linked [SiO₄]^4‐ tetrahedra) and two ortho‐silicate anions (isolated [SiO₄]^4‐ tetrahedra) according to La₆[Si₄O₁₃][SiO₄]₂. This compound can be described as built up of alternating layers of these [SiO₄]^4‐ and the horseshoe‐shaped [Si₄O₁₃]^10‐ anions along [010]. Between and within the layers the high‐coordinated La ³⁺ cations (CN = 9 ‐ 11) are localized. The close structural relationship to the borosilicates M₃[BSiO₆][SiO₄](M = Ce ‐ Eu) is discussed and structural comparisons with other catena‐tetrasilicates are presented.     

Einkristalle des Cer(III)‐Borosilicats Ce3[BSiO6][SiO4]

Single Crystals of the Cerium(III) Borosilicate Ce₃[BSiO₆][SiO₄] Colorless, lath‐shaped single crystals of Ce₃[BSiO₆]‐ [SiO₄] (orthorhombic, Pbca; a = 990.07(6), b = 720.36(4), c = 2329.2(2) pm, Z = 8) were obtained in attempts to synthesize fluoride borates with trivalent cerium in evacuated silica tubes by reaction of educt mixtures of elemental cerium, cerium dioxide, cerium trifluoride, and boron sesquioxide (Ce, CeO₂, CeF₃, B₂O₃; molar ratio 3 : 1 : 3 : 3) in fluxing CsCl (700 °C, 7 d) with the glass wall. The crystal structure contains eight‐ (Ce1) and ninefold coordinated Ce³⁺ cations (Ce2 and Ce3) surrounded by oxygen atoms. Charge balance is achieved by both discrete borosilicate ([BSiO₆]^5– ≡ [O₂BOSiO₃]^5–) and ortho‐silicate anions ([SiO₄]^4–). The former consists of a [BO₃] triangle linked to a [SiO₄] tetrahedron by a single vertex. The anions form layers in [001] direction alternatingly built up from [BSiO₆]^5– and [SiO₄]^4– groups while Ce³⁺ cations are located in between.     

Sm4S3[Si2O7] und NaSm9S2[SiO4]6: Zwei Sulfidsilicate mit dreiwertigem Samarium

Sm₄S₃[Si₂O₇] and NaSm₉S₂[SiO₄]₆: Two Sulfide Silicates with Trivalent Samarium The sulfide silicates Sm₄S₃[Si₂O₇] and NaSm₉S₂[SiO₄]₆ are obtained as light yellow transparent crystals by the reaction of Sm, Sm₂O₃, S, and SiO₂ with fluxing SmCl₃ or NaCl, respectively, in suitable molar ratios in fused evacuated silica tubes (850 °C, 7 d). Tetragonal crystals of Sm₄S₃[Si₂O₇] (I4₁/amd; Z = 8; a = 1186.4(1); c = 1387.0(2) pm) with ecliptically conformed [Si₂O₇]^6–‐groups of corner sharing [SiO₄]‐tetrahedra are formed. These double tetrahedra as well the sulfide anions (S^2–) coordinate two crystallographically independent metal cations. They provide coordination numbers of 8 + 1 (5 S^2– and 3 + 1 O^2–) for Sm1 and 9 (3 S^2– and 6 O^2–) for Sm2. NaSm₉S₂[SiO₄]₆ crystallizes hexagonally (P6₃/m; Z = 1; a = 975.32(9); c = 676.46(7) pm) in a modified bromapatite‐type structure. The coordination spheres about the two crystallographically different Sm³⁺ cations are built up by oxygen atoms of the orthosilicate units ([SiO₄]^4–) and sulfide anions (S^2–). As a result, Sm1 and Sm2 have coordination numbers of 9 and 8, respectively. Na⁺ and (Sm1)³⁺ occupy the position 4 f in a molar ratio of 1 : 3 whereas the lower coordinated (Sm2)³⁺ occupies the 6 h position.     

Zur Dimorphie von Nd3Cl[SiO4]2

On the Dimorphism of Nd₃Cl[SiO₄]₂ On reacting NdCl₃, Nd₂O₃, and SiO₂ (molar ratio: 1 : 4 : 6) at 850 °C in evacuated silica tubes, pale violet, hydrolysis resistant neodymium(III) chloride ortho‐silicate Nd₃Cl[SiO₄]₂ can be obtained within seven days. If equimolar amounts of NaCl are added as flux, rod‐ or platelet‐shaped, transparent single crystals of two modifications accumulate simultaneously. The one with the higher density (A‐Nd₃Cl[SiO₄]₂) crystallizes monoclinically (C2/c, no. 15; a = 1416.6(1), b = 638.79(6), c = 872.21(9) pm, β = 98.403(7)°; V_m = 117.55 cm³/mol, Z = 4), whereas the one with the lower density (B‐Nd₃Cl[SiO₄]₂) exhibits orthorhombic symmetry (Pnma, no. 62; a = 709.36(7), b = 1815.7(2), c = 631.48(6) pm; V_m = 122.45 cm³/mol, Z = 4). Two crystallographically independent Nd³⁺ cations exist in each of both crystal structures, which in the A type are surrounded by nine (1 Cl^– + 8 O^2–) and ten (2 Cl^– + 8 O^2–), whilst those in the B type by only two times eight (1 Cl^– + 7 O^2– and 2 Cl^– + 6 O^2–) anions, respectively. Thereby all oxygen atoms of both forms represent members of discrete ortho‐silicate tetrahedra ([SiO₄]^4–). Although both crystal structures are built of alternating anionic double layers {(Nd1)₂[SiO₄]₂}^2– and cationic single layers {(Nd2)Cl}²⁺, there is a higher cross‐linkage of the building units in the A‐type lattice, where the cations are coordinated by three and four tetrahedra edges of ortho‐silicate anions, compared to only two times two of them in the B type. From this an about 4% higher density of Nd₃Cl[SiO₄]₂ results for the A‐type structure (D_x = 5.55 g/cm³) in comparison with B‐type Nd₃Cl[SiO₄]₂ (D_x = 5.33 g/cm³).     

[TMPA]4[Si8O20] · 34 H2O and [DDBO]4[Si8O20] · 32 H2O are heteronetwork clathrates with 1,1,4,4-tetramethylpiperazinium (TMPA) and 1,4-dimethyl-1,4-diazoniabicyclo[2.2.2]octane (DDBO) guest cations

[TMPA]₄[Si₈O₂₀] · 34 H₂O ( 1 ) and [DDBO]₄[Si₈O₂₀] · 32 H₂O ( 2 ) have been prepared by crystallization from aqueous solutions of the respective quaternary alkylammonium hydroxide and SiO₂. The crystal structures have been determined by single-crystal X-ray diffraction. 1 : Monoclinic, a = 16.056(2), b = 22.086(6), c = 22.701(2) Å, β = 90.57(1)° (T = 210 K), space group C2/c, Z = 4. 2 : Monoclinic, a = 14.828(9), b = 20.201(7), c = 15.519(5) Å, β = 124.13(4)° (T = 255 K), space group P2₁/c, Z = 2. The polyhydrates are structurally related host-guest compounds with three-dimensional host frameworks composed of oligomeric [Si₈O₂₀]^8? anions and H₂O molecules which are linked via hydrogen bonds. The silicate anions possess a cube-shaped double four-ring structure and a characteristic local environment formed by 24 H₂O molecules and six cations (TMPA, [C₈H₂₀N₂]²⁺, or DDBO, [C₈H₁₈N₂]²⁺). The cations themselves reside as guest species in large, irregular, cage-like voids. Studies employing ²⁹Si NMR spectroscopy and the trimethylsilylation method have revealed that the saturated aqueous solutions of 1 and 2 contain high proportions of double four-ring silicate anions. Such anions are also abundant species in the saturated solution of the heteronetwork clathrate [DMPI]₆[Si₈O₁₈(OH)₂] · 48.5 H₂O ( 3 ) with 1,1-dimethylpiperidinium (DMPI, [C₇H₁₆N]⁺) guest cations.     

Pr4S3[Si2O7] und Pr3Cl3[Si2O7]: Durch weiche Fremdanionen modifizierte Derivate von Praseodymdisilicat

Pr₄S₃[Si₂O₇] and Pr₃Cl₃[Si₂O₇]: Derivatives of Praseodymium Disilicate Modified by Soft Foreign Anions For synthesizing both the disilicate derivatives Pr₄S₃[Si₂O₇] and Pr₃Cl₃[Si₂O₇], Pr, Pr₆O₁₁ and SiO₂ are brought to reaction with S and PrCl₃, respectively, in suitable molar ratios (850 °C, 7 d) in evacuated silica tubes. By using NaCl as a flux, Pr₄S₃[Si₂O₇] crystallizes as pale green, transparent single crystals (tetragonal, I4₁/amd, a = 1201.6(1), c = 1412.0(2) pm, Z = 8) with the appearance of slightly compressed octahedra. On the other hand, Pr₃Cl₃[Si₂O₇] emerges as pale green, transparent platelets and crystallizes monoclinically (space group: P2₁, a = 530.96(6), b = 1200.2(1), c = 783.11(8) pm, β = 109.07(1)°, Z = 2). In both crystal structures ecliptically conformed [Si₂O₇]^6– units of two corner‐linked [SiO₄] tetrahedra with Si–O–Si bridging angles of 131° in the sulfide and 148° in the chloride disilicate are present. In Pr₄S₃[Si₂O₇] both crystallographically independent Pr³⁺ cations show coordination numbers of 8 + 1 (5 S^2– and 3 + 1 O^2–) and 9 (3 S^2– and 6 O^2–). For Pr1, Pr2 and Pr3 in Pr₃Cl₃[Si₂O₇] coordination numbers of 10 (5 Cl^– and 5 O^2–) and 9 (2 ×; 4 Cl^– and 5 O^2– or 3 Cl^– and 6 O^2–, respectively) occur.     

La3F3[Si3O9]: Das erste Fluoridsilicat aus dem ternären System LaF3/La2O3/SiO2

La₃F₃[Si₃O₉]: The First Fluoride Silicate in the Ternary System LaF₃/La₂O₃/SiO₂ By reacting La₂O₃ with LaF₃ and SiO₂ (silica gel) using CsCl as a flux (molar ratio 1 : 1 : 3 : 6; 700 °C, 21 d) it was possible to obtain single crystals of La₃F₃[Si₃O₉] (hexagonal, space group: P 6 2c (no. 190); a = 708.32(3), c = 1089.48(6) pm; Z = 2) as colourless, hexagonal platelets. The crystal structure comprises discrete cyclic [Si₃O₉]^6– anions of three corner-linked [SiO₄] tetrahedra along with a graphite-like network of the composition {[LaF_3/3]²⁺}. Shorter reaction times even produced single crystals of tysonite-type LaF₃ (trigonal, space group: P 3 c1 (no. 165); a = 718.80(6), c = 735.94(6) pm; Z = 6) on which a X-ray structure analysis was achieved, too. The structures of both compounds, each of which show an elevenfold anionic coordination (CN = 9 + 2) for the La³⁺ cations, are compared. The influence of the reactivity of the educts and the temperature on the reaction as well as the difficulties in the X-ray differentiation of fluorine and oxygen will be discussed.     

CsSc3F6[SeO3]2: A New Rare‐Earth Metal(III) Fluoride Oxoselenate(IV) With Sections Of The ReO3‐Type Structure

A new representative of rare‐earth metal(III) fluoride oxoselenates(IV) derivatized with alkali metals could be synthesized via solid‐state reactions. Colorless single crystals of CsSc₃F₆[SeO₃]₂ were obtained through the reaction of Sc₂O₃, ScF₃, and SeO₂ (molar ratio 1:1:3) with CsBr as reactant and fluxing agent. For this purpose, corundum crucibles embedded as liners into evacuated silica ampoules were applied as containers for these reactions at 700 °C for seven days. The new quintenary compound crystallizes in the trigonal space group P3m1 with a = 565.34(4) and c = 1069.87(8) pm (c/a = 1.892) for Z = 1. The crystal structure of CsSc₃F₆[SeO₃]₂ contains two crystallographically different Sc³⁺ cations. Each (Sc1)³⁺ is surrounded by six fluoride anions as octahedron, while the octahedra about (Sc2)³⁺ are formed by three fluoride anions and three oxygen atoms from three terminal [SeO₃]^2– anions. The [(Sc1)F₆]^3– octahedra link via common F^– vertices to six fac‐[(Sc2)F₃O₃]^6– octahedra forming ²_∞{[Sc₃F₆O₆]^9–} layers parallel to (001). These layers are separated by oxygen‐coordinated Cs⁺ cations (C.N. = 12), arranging for the charge compensation, while Se⁴⁺ cations within the layers surrounded by three oxygen atoms as ψ¹‐tetrahedral [SeO₃]^2– units complete the structure. EDX measurements confirmed the composition of the title compound and single‐crystal Raman studies showed the typical vibrational modes of isolated [SeO₃]^2– anions with ideal C_3v symmetry.     

Sm3Cl[SiO4]2: Ein chlorarmes Chloridorthosilicat des Samariums

Sm₃Cl[SiO₄]₂: A Chlorine‐poor Chloride Orthosilicate of Samarium Pale yellow, plate‐like single crystals of Sm₃Cl[SiO₄]₂ (orthorhombic, Pnma; a = 701.74(8), b = 1800.8(2), c = 626.63(7) pm; Z = 4) are obtained upon the reaction of SmCl₃, Sm₂O₃ and SiO₂ (”︁Kieselgel”︁”︁) in 1 : 4 : 6 molar ratios, most advantageously in the presence of substantial amounts of NaCl as fluxing agent, after seven days at 850 °C in evacuated silica ampoules. The B‐type crystal structure (isotypic with e. g. Yb₃Cl[SiO₄]₂) contains discrete orthosilicate tetrahedra [SiO₄]^4– which form anionic double layers ({(Sm1)₂[SiO₄]₂}^2–) with (Sm1)³⁺. These are alternatingly sheethed along [010] with cationic monolayers ({(Sm2)Cl}²⁺) consisting of (Sm2)³⁺ and Cl^–. Both crystallographically independent Sm³⁺ cations exhibit coordination numbers of eight (Sm1: 1 Cl + 7 O; Sm2: 2 Cl + 6 O) with respect to the involved electronegative particles.     

Über den H‐ und A‐Typ von La2[Si2O7]

On the H‐ and A‐Type Structure of La₂[Si₂O₇] By thermal decomposition of La₃F₃[Si₃O₉] at 700 °C in a CsCl flux single crystals of a new form of La₂[Si₂O₇] have been found which is called H type (triclinic, P1; a = 681.13(4), b = 686.64(4), c = 1250.23(8) pm, α = 82.529(7), β = 88.027(6), γ = 88.959(6)°; V_m = 87.223(9) cm³/mol, D_x = 5.113(8) g/cm³, Z = 4) continuing Felsche's nomenclature. It crystallizes isotypically to the triclinic K₂[Cr₂O₇] in a structure closely related to that of A–La₂[Si₂O₇] (tetragonal, P4₁; a = 683.83(7), c = 2473.6(4) pm; V_m = 87.072(9) cm³/mol, D_x = 5.122(8) g/cm³, Z = 8). For comparison, the latter has been refined from single crystal data, too. Both the structures can be described as sequence of layers of each of two crystallographically different [Si₂O₇]^6– anions always built up of two corner‐linked [SiO4] tetrahedra in eclipsed conformation with non‐linear Si–O–Si bridges (∢(Si–O–Si) = 128–132°) piled up in [001] direction and aligned almost parallel to the c axis. They differ only in layer sequence: Whereas the double tetrahedra of the disilicate units are tilted alternating to the left and in view direction ([010]; stacking sequence: AB) in H–La₂[Si₂O₇], after layer B there follow due to the 4₁ screw axis layers with anions tilted to the right and tilted against view direction ([010]; stacking sequence: ABA′B′) in A–La₂[Si₂O₇]. The extremely irregular coordination polyhedra around each of the four crystallographically independent La³⁺ cations in both forms (H and A type) consist of eight to ten oxygen atoms in spacing intervals of 239 to 330 pm. The possibility of more or less ordered intermediate forms will be discussed.     

Synthesis and Crystal Structure of the Fluoride‐Rich Rubidium Scandium Fluoride Oxosilicate Rb3Sc2F5Si4O10

The new quinary fluoride‐rich rubidium scandium oxosilicate Rb₃Sc₂F₅Si₄O₁₀ was obtained from mixtures of RbF, ScF₃, Sc₂O₃ and SiO₂ in sealed platinum ampoules after seventeen days at 700 °C. The colourless compound crystallises orthorhombically in space group Pnma with a = 962.13(5), b = 825.28(4), c = 1838.76(9) pm and Z = 4. For the oxosilicate partial structure, [SiO₄]^4– tetrahedra are connected in (001) by vertex‐sharing to form corrugated unbranched vierer single layers ${2}\atop{{\infty}}$ {[Si₄O₁₀]^4–} (d(Si–O) = 158–165 pm, ∠(O–Si–O) = 103–114°, ∠(Si–O–Si) = 125–145°) containing six‐membered rings. Similar oxosilicate layers with 6³‐net topology are well‐known for the mineral group of micas or in sanbornite Ba₂Si₄O₁₀. Regarding other systems, identical tetrahedral layers can be found in the synthetic borophosphate Mg(H₂O)₂[B₂P₂O₈(OH)₂] · H₂O. The Sc³⁺ cations are coordinated octahedrally by four F^– and two O^2– anions. These cis‐[ScF₄O₂]^5– octahedra (d(Sc–F) = 200–208 pm, d(Sc–O) = 202–205 pm) share one equatorial and two apical F^– anions with others to build up slightly corrugated ${1}\atop{{\infty}}$ {[Sc₂F${t}\atop{2/1}$ F${v}\atop{6/2}$ O${t}\atop{4/1}$ ]^7–} double chains along [010]. These are linked with the oxosilicate layers via two oxygen vertices to construct a three‐dimensional framework with cavities apt to host the three crystallographically independent Rb⁺ cations with coordination numbers of eleven, twelve and thirteen.     

Crystal Structure of the B‐type Dierbium Oxide ortho‐Oxosilicate Er2O[SiO4]

The crystal structure of B‐type Er₂O[SiO₄] has been determined by single crystal X‐ray diffraction. It crystallizes with the (Mn,Fe)₂[PO₄]F type structure in the monoclinic space group C2/c (a = 14.366(2), b = 6.6976(6), c = 10.3633(16) Å, ß = 122.219(10)°, Z = 8) and shows anionic tetrahedral [SiO₄]^4– units and non‐silicon‐bonded O^2– anions in distorted [OEr₄]¹⁰⁺ tetrahedra. The [(Er1)O₆₊₁] and [(Er2)O₆] polyhedra form infinite chains which are connected by common edges.     

Pr5F[SiO4]2[SeO3]3: Another Complex Fluoride Oxosilicate Oxoselenate(IV)

During attempts to synthesize lanthanoid(III) fluoride oxoselenates(IV) with the simple composition MF[SeO₃], not only Pr₃F[SeO₃]₄, but also Pr₅F[SiO₄]₂[SeO₃]₃ appeared as pale green crystalline by‐products in the case of praseodymium. Pr₅F[SiO₄]₂[SeO₃]₃ crystallizes triclinically in space group P$\bar{1}$ (no. 2) with a = 701.14(5), b = 982.68(7), c = 1286.79(9) pm, α = 70.552(3), β = 76.904(3), γ = 69.417(3)° and Z = 2. The five crystallographically different Pr³⁺ cations on the general positions 2i show coordination numbers of eight and nine. [(Pr1)O₈]^13– and [(Pr2)O₈]^13– polyhedra are connected to$\bar{1}$ {[(Pr1, 2)₂O₁₂]^18–} chains along the [100] direction. [(Pr3)O₇F]^12–, [(Pr4)O₈F]^14– and [(Pr4)O₈F]^14– polyhedra generate [F(Pr3, 4, 5)₃O₁₉]^30– units about their central F^– anion in triangular Pr³⁺ coordination. These units form $\bar{1}$ {[F(Pr3, 4, 5)₃O₁₆]^24–} strands, again running parallel to [100]. Their alternating connection with the $\bar{1}$ {[(Pr1, 2)₂O₁₂]^18–} chains results in $\bar{1}$ {[Pr₅O₂₀F]^26–} sheets parallel to the (001) plane. Like in the already known related compound Er₃F[SiO₄][SeO₃]₂, a three‐dimensional network $\bar{1}$ {[Pr₅O₁₇F]^20–} is achieved without the contribution of both the tetravalent silicon and selenium components. However, two Si⁴⁺ and three Se⁴⁺ cations forming tetrahedral [SiO₄]^4– and ψ¹‐tetrahedral [SeO₃]^2– units with all O^2– anions guarantee the charge balance. The formation of Pr₅F[SiO₄]₂[SeO₃]₃ was observed when praseodymium sesquioxide (Pr₂O₃: in‐situ produced from Pr and Pr₆O₁₁ in a molar ratio of ³/₁₁:⁴/₁₁),praseodymium trifluoride (PrF₃) and selenium dioxide (SeO₂) in 1:1:3 molar ratios were reacted with CsBr as fluxing agent for five days at 750 °C in evacuated fused silica (SiO₂) ampoules.     

La3OCl[AsO3]2: Ein Lanthan‐Oxidchlorid‐Oxoarsenat(III) mit “lone‐pair”‐Kanalstruktur

La₃OCl[AsO₃]₂: A Lanthanum Oxide Chloride Oxoarsenate(III) with a “Lone‐Pair” Channel Structure La₃OCl[AsO₃]₂ was prepared by the solid‐state reaction between La₂O₃ and As₂O₃ using LaCl₃ and CsCl as fluxing agents in evacuated silica ampoules at 850 °C. The colourless crystals with pillar‐shaped habit crystallize tetragonally (a = 1299.96(9), c = 558.37(5) pm, c/a = 0.430) in the space group P4₂/mnm (no. 136) with four formula units per unit cell. The crystal structure contains two crystallographically different La³⁺ cations. (La1)³⁺ is coordinated by six oxygen atoms and two chloride anions in the shape of a bicapped trigonal prism (CN = 8), whereas (La2)³⁺ carries eight oxygen atoms and one Cl^? anion arranged in the shape of tricapped trigonal prism (CN = 9). The isolated pyramidal [AsO₃]^3? anions (d(As–O) = 175–179 pm) consist of three oxygen atoms (O2 and two O3), which surround the As³⁺ cations together with the free, non‐binding electron pair (lone pair) Ψ¹‐tetrahedrally (?(O–As–O) = 95°, 3×). One of the three crystallographically independent oxygen atoms (O1), however, is exclusively coordinated by four (La2)³⁺ cations in the shape of a real tetrahedron (d(O–La) = 236 pm, 4×). These [(O1)(La2)₄]¹⁰⁺ tetrahedra form endless chains in the direction of the c axis through trans‐edge condensation. Empty channels, constituted by the lone‐pair electrons of the Cl^? anions and the As³⁺ cations in the Ψ¹‐tetrahedral oxoarsenate(III) anions [AsO₃]^3?, run parallel to [001] as well.     

New Examples for the Unexpected Stability of the 10π‐Electron Hückel Arene [Si6]10–

The compounds Ba₄Ag₂Si₆, Eu₄Ag₂Si₆, and Ca₄Ag₂Si₆, prepared from the elements at 1273 K (the components in inner corundum crucibles are enclosed in sealed quartz ampoules), are brittle semiconductors with silvery luster. They react slowly with acids liberating hydrogen. Ba₄Ag₂[Si₆] and Eu₄Ag₂[Si₆] crystallize like Ba₄Li₂[Si₆] (space group Fddd (No. 70); a = 8.613 Å, b = 14.927 Å, c = 19.639 Å, and a = 8.420 Å, b = 14.585 Å, c = 17.864 Å, respectively), whereas Ca₄Ag₂[Si₆] represents a new structure type (space group Fmmm (No. 69); a = 8.315 Å, b = 14.391 Å, c = 8.646 Å). The three compounds are Zintl phases with the formal charges M²⁺, Ag⁺ and [Si₆]^10–. The mean bond lengths d(Si–Si) = 2.335–2.381 Å in the 10π‐Hückel arene [Si₆]^10– as well as d(Ag–Si) = 2.464–2.595 Å vary with the size of the M²⁺ cations. The chemical bonding was analyzed in terms of the Electron Localization Function (ELF) and compared with the bonding in related systems (Ce₄Co₂Si₆).     

The Triclinic Lanthanoid(III) Halide Oxidoarsenates(III) Sm3Cl2[As2O5][AsO3] and Tm3Br2[As2O5][AsO3]

Pale yellow single crystals of the composition Ln₃X₂[As₂O₅][AsO₃] (Ln = Tm for X = Br and Ln = Sm for X = Cl) were obtained via solid-state reactions in the systems Ln₂O₃/As₂O₃ from sealed silica ampoules using different halides as fluxing agents. Sm₃Cl₂[As₂O₅][AsO₃] and Tm₃Br₂[As₂O₅][AsO₃] crystallize isotypically in the triclinic space group P1 with Z = 2 and cell parameters of a = 543.51(4) pm, b = 837.24(6) pm, c = 1113.45(8) pm, α = 90.084(2)°, β = 94.532(2)°, γ = 90.487(2)° for the samarium and a = 534.96(4) pm, b = 869.26(6) pm, c = 1081.84(8) pm, α = 90.723(2)°, β = 94.792(2)° γ = 90.119(2)° for the thulium compound. The isotypic crystal structure of both representatives exhibits three crystallographically different Ln³⁺ cations, each with a coordination number of eight. (Ln1)³⁺ and (Ln2)³⁺ are only coordinated by three oxygen atoms, whereas (Ln3)³⁺ shows additional contacts to halide anions in forming square [LnO₄X₄]^9– antiprisms. All As³⁺ cations are surrounded by three oxygen atoms in the shape of isolated [AsO₃]^3– ψ¹-tetrahedra. They occur either isolated or condensed as pyroanionic [As₂O₅]^4– units with a bridging oxygen atom. In both anions, non-binding lone-pair electrons are present at the As³⁺ cations with a pronounced stereochemically active function.