The crystal structure of a newly synthesized compound Na2Ba2[B10O17(OH)2] has been determined (Syntex \(P\bar 1\) diffractometer, MoKα radiation, 1784 crystallographically nonequivalent reflections, anisotropic approximation, R = 1.7%). The parameters of the monoclinic unit cell are a = 11.455(7), b = 6.675(4), c = 9.360(7) Å, β = 93.68(5)°, Z = 2, sp. gr. C2. The structure consists of double pseudohexagonal layers built by BO4-tetrahedra and BO3-triangles forming three-membered rings of two mutually orthogonal orientations. The neighboring layers along the [001] direction are bound by Na-polyhedra and hydrogen bonds with participation of OH groups. The interlayer tunnels along the [100] direction are filled with columns of Ba-polyhedra. The crystallochemical characteristics of a number of synthetic Ba-borates (to which the structure of new decaborate is related) are considered in terms of borate building blocks singled out in the structure.

The crystal structure of the new synthetic compound Ba5[B20O33(OH)4] ⋅ H2O was established by the methods of X-ray diffraction (a Stoe IPDS diffractometer, λMoKα?, 1860 independent reflections, anisotropic refinement, R = 1.95%, localization of hydrogen atoms): a = 9.495(2) Å, b = 6.713(1) Å, c = 11.709(2) Å, β = 95.09(1)°, sp. gr. P2, Z = 1. The structure is based on double pseudohexagonal layers consisting of BO4-tetrahedra and BO3 triangles linked into three-membered rings in two mutually perpendicular directions. The double layers adjacent along the [100] direction are linked together through the Ba-polyhedra and hydrogen bonds with the participation of the OH-groups occupying the “end” vertices of two B-triangles. The interlayer space is also filled with a sheet of Ba-polyhedra. The structure of the compound is compared to the structures of topologically similar Ba and Ca borates and hydroborates.

The crystal structure of strontium hilgardite, CaSr[B5O9]Cl ⋅ H2O, was established by the method of X-ray diffraction analysis (synchrotron radiation; diffractometer equipped with a position-sensitive detector; λ = 0.688 Å; 3191 reflections with F < 4σ(F); R = 0.045 in the anisotropic approximation). The triclinic unit-cell parameters are as follows: a = 6.5732(6) Å, b = 6.4445(6) Å, c = 6.3693(6) Å, α = 60.995(2)°, β = 61.257(2)°, γ = 77.191(2)°, sp. gr. P1; Z = 1. The Ca and Sr atoms were found to be disordered over two positions. The structures of strontium hilgardite and hilgardite-1A differ in the configurations of the seven-vertex Sr-and Ca(2)-polyhedra. The structure solved in this work is consistent with two series of borates studied previously. One of these groups involves pentaborates with different degrees of hydration of borate complexes, and the second group includes Sr-containing borates.

Two new Ba borates, Ba[B5O8(OH)]H2O (I) and LiBa2[B10O16(OH)3] (II), were synthesized in the hydrothermal systems Cs2CO3–BaO–B2O3–H2O (T=550 K, P=100 bar, 20 days). The crystal structures of I and II were established from X-ray single crystal diffraction; I, space group P-1, a=6.785(5), b=6.831(5), c=10.629(6) Å, α=100.07(4), β=91.98(6), γ=119.46(7)°, V=417.01(58) Å3, Rhkl=0.069 for 1328Fo>4σ(Fo); II, space group P-1, a=6.732(1), b=11.369(2), c=11.581(2) Å, α=119.31(3), β=90.11(3), γ=73.08(3)°, V=728.87(42) Å3, Rhkl=0.048 for 1166Fo≥4σ(Fo). The topology of the borate sheet of phase I is practically the same as that in the monoclinic structure of biringuccite, Na2[B5O8(OH)]·H2O. The gaps between the sheets in synthetic Ba-biringuccite contain Ba atoms and water molecules. Phase II is characterized by a new boron sheet, formed by five crystallographically independent BO4 tetrahedra and by five BO3 triangles.

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Two new Ba borates Ba-2[B5O9]Cl . 0.5H(2)O (I) and Ba-2[B5O8(OH)(2)](OH) (II) were synthesized in the hydrothermal systems ACl (A(2)CO(3))-BaO-B2O3-H2O, where A=Li, Na, K, Rb, Cs, NH4 cations (T=550 K, P=100-150 bar, 20-25 days). The crystal structures of I and II were established by X-ray single crystal diffraction; I: space group Pnn2; a=11.716(2), b=11.574(2), c=6.700(1)Angstrom: V=908.5(2) Angstrom(3), R-hkl=0.057 for 831 Fo>4 sigma(Fo); II: space group P2(1)/c; a=6.713(2), b=16.480(5), c=8.387(3)Angstrom, beta=106.80(3)degrees, V=888.2(4)Angstrom(3), R-hkl=0.092 for 857 Fo greater than or equal to 4 sigma(Fo). The topology of the borate framework of phase I is the same as compared with that revealed in the orthorhombic structure of Ca2B5O9Br that is an orthorhombic modification of natural hilgardite. The pentaborate framework [B5O9](3-) is formed by three crystallographically independent BO4 tetrahedra and by two BO3 triangles. II is a sheet borate: its pentaborate [B5O8(OH)(2)](3-) sheets are characterized by the same ratio between BO4 tetrahedra and BO3 triangles as compared with the pentaborate framework in I and in all natural hilgardites. (C) 2000 Elsevier Science S.A. All rights reserved.

Two new Ba borates Ba2[B5O9]Cl·0.5H2O (I) and Ba2[B5O8(OH)2](OH) (II) were synthesized in the hydrothermal systems ACl (A2CO3)–BaO–B2O3–H2O, where A=Li, Na, K, Rb, Cs, NH4 cations (T=550 K, P=100–150 bar, 20–25 days). The crystal structures of I and II were established by X-ray single crystal diffraction; I: space group Pnn2; a=11.716(2), b=11.574(2), c=6.700(1)Å, V=908.5(2) Å3, Rhkl=0.057 for 831 Fo>4σ(Fo); II: space group P21/c; a=6.713(2), b=16.480(5), c=8.387(3) Å, β=106.80(3)°, V=888.2(4) Å3, Rhkl=0.092 for 857 Fo⩾4σ(Fo). The topology of the borate framework of phase I is the same as compared with that revealed in the orthorhombic structure of Ca2B5O9Br that is an orthorhombic modification of natural hilgardite. The pentaborate framework [B5O9]3− is formed by three crystallographically independent BO4 tetrahedra and by two BO3 triangles. II is a sheet borate: its pentaborate [B5O8(OH)2]3− sheets are characterized by the same ratio between BO4 tetrahedra and BO3 triangles as compared with the pentaborate framework in I and in all natural hilgardites.