K Vidyasagar

Four new A2CdSn2S6 (A = Cs(1), Rb(2), K(3-β) and K(3-α)) compounds have been synthesized and characterized by X-ray diffraction and spectroscopic methods. The isostructural cesium(1) and rubidium(2) compounds have noncentrosymmetric three-dimensional tetrahedral [CdSn2S6]2- framework structure and show second-harmonic-generation response. The layered potassium compounds, 3-β and 3-α, have centrosymmetric tetrahedral and octahedral [CdSn2S6]2- framework structures respectively and the compound 3-β, on heating, transforms to compound 3-α. The compound 3-α undergoes facile potassium ion-exchange in aqueous medium with various mono-, di- and tri- valent cations at room temperature.

The novel compounds Sr13NbAs11 and Eu13NbAs11 have been synthesized from SrAs, Eu5As4, Sr, Nb and As in niobium ampoules at 1173-1273 K. The tetragonal tI 200 phases are defect variants of the Ca14AlSb11 structure (space group I41/acd (no. 142); Sr13□NbAs11: a = 1649.8(2) and c = 2214.1(3); Eu13□NbAs11: a = 1632.9(8) and c = 2197.3(8) pm; Z = 8). The structures are built from the cations Sr2+, and Eu2+, respectively, and from the anions [NbAs4]7-, As3-, and the linear polyanion [As3]7-. This polyanion (isosteric to I3-) is asymmetric with d(As-As) = 273.0 and 346.0pm (Sr) and 274.7 and 335.6pm (Eu), respectively. The bond lengths in the tetrahedral anions are d(Nb-As) = 250.8 and 251.1 pm. The complete structural arrangement is related to that of Cu2O by forming two interpenetrating networks. The oxygen atoms are substituted by niobium centered As4 tetrahedra, and the Cu atoms are substituted by As6 octahedra (centered by Sr, Eu). The central As atoms of the polyanions connect the nets. Both As networks are enveloped by the remaining cations forming cubes, tetragonal antiprisms and capped trigonal prisms. © Johann Ambrosius Barth 1996.

Manganese perovskites of the CaSbxMn1-xO3 (x=0.1, 0.2, 0.25, 0.33, 0.5) family have been synthesized by solid state reactions and structurally characterized by Rietveld refinement of the powder X-ray diffraction data. CaSb0.1Mn0.9O3 crystallizes in Pnma space group with a GdFeO3-type orthorhombic structure, whereas the other members of the series crystallize in the space group P21/m. Unit cell volumes of these members increase linearly with antimony content. Divergences between ZFC and FC magnetic susceptibilities were observed for CaSb0.1Mn0.9O3, CaSb 0.33Mn0.67O3 and CaSb0.5Mn 0.5O3 at low temperatures. Antiferromagnetic transitions at 125K and 113K were found for CaSb0.2Mn0.8O3 and CaSb0.25Mn0.75O3, respectively. © 2003 Elsevier Inc. All rights reserved.

New quaternary A2MSn3S8 (A ​= ​alkali metals; M ​= ​Zn, Cd) sulfides were synthesized and characterized by X-ray diffraction and spectroscopic methods. α-Cs2ZnSn3S8(1), α-Rb2ZnSn3S8(2) and α-Cs2CdSn3S8(3) have layered tetrahedral [MSn3S8]2- frameworks, in which Zn2+, Cd2+ and Sn4+ ions are ordered. No such ordering is present in three-dimensional tetrahedral [MSn3S8]2- framework of β-Cs2ZnSn3S8(4), β-Rb2ZnSn3S8(5), β-Cs2CdSn3S8(6) and β-Rb2CdSn3S8(7) compounds. The β-modifications undergo thermal transformation to α–modifications. K2CdSn3S8(8) and Na2CdSn3S8(9) have a new type of layered structure, in which Cd2+ and Sn4+ ions are octahedrally coordinated and disordered in [CdSn3S8]2- framework. Their potassium and sodium ions undergo facile ion-exchange with mono-, di- and trivalent cations at room temperature. The Raman spectra and band gap values of compounds 1–9 corroborate their structural differences.

New isostructural tellurates, A2[Te3O 8(OH)4] (A = Cs, Rb, K, Na), have been synthesized under hydrothermal conditions and characterized by X-ray diffraction, thermal and infrared spectroscopic studies. These compounds contain, as determined from single crystal X-ray diffraction study of the potassium compound, linear Te 3O8(OH)4]2- anions and, for charge compensation, A+ ions. Alkali metal ions of these tellurates undergo a partial ion exchange with one another.

Three new isostructural quaternary antimony(III) thiostannates(IV), A2Sb2Sn3S10 (A=K, Rb, Cs) have been synthesized by using alkali metal thiosulfate flux and structurally characterized by X-ray diffraction. Their structures contain A+ ions around the [Sb2Sn3S10]2- chains, which are built from SbS3 pyramids, SnS6 octahedra and SnS4 tetrahedra. Raman and Mössbauer spectroscopic measurements corroborate the oxidation states and coordination environments of Sb(III) and Sn(IV). All three compounds are wide band gap semiconductors. Potassium compound undergoes partial exchange with strontium, cadmium and lead ions.

Five isomorphous AM2V2O11 vanadates of niobium and tantalum, namely, BaNb2V2O11, BaTa2V2O11, SrNb2V2O11, SrTa2V2O11, and PbTa2V2O11, were prepared by solid-state reactions and structurally characterized by single-crystal and powder X-ray diffraction techniques. Barium and strontium compounds, respectively, have centrosymmetric and noncentrosymmetric types of layered structure, wherein [M2V2O11]2- anionic layers are interleaved with A2+ cations. Both types of layered structure are found for lead compound. The strontium and lead compounds are type I phase-matching materials with second-harmonic-generating efficiencies of 33-50% of LiNbO3, and their dielectric properties were evaluated. A three-dimensional structural variant was also identified for strontium compounds, which crystallize in noncentrosymmetric orthorhombic space group C2221.

The title compound, tetraethylammonium hexa-μ3-oxo- hexaμ2-oxo-tetradecaoxooctamolybdate, contains discrete centro-symmetric α-[Mo8O26]4- anions and, for charge compensation, four tetraethylammonium cations per anion. The compound reported by Lu, Haung, Huang & Haung [(1989). Jiegou Huaxue, 8, 23-26] is evidently isostructural with the title compound, although Lu et al. formulated their phase as having a composition of {[(C2H 5)4N]+}2·2H +-[Mo26O26]4-. © 2005 International Union of Crystallography Printed in Great Britain - all rights reserved.

Twenty new molybdenum phosphonates and diphosphonates have been synthesized and structurally characterized by single crystal and powder X-ray diffraction, CHN analyses, spectroscopic and thermal studies. Four of them are molecular phenyl- and benzyl-phosphonates containing discrete [(MoO3)5(O3PR)2]4- (R = Ph or CH2Ph) cyclic anions. The sixteen non-molecular compounds are layered isostructural phenylphosphonates, A2[Mo2O5(O3PPh)2] (A = NH4, Tl, Rb, Cs) and K1.5(H3O)0.5[Mo2O5(O3PPh)2] and the corresponding diphosphonate compounds with pillared anionic layers, A2[Mo2O5(O3P(CH2)3PO3)], A2[Mo2O5(O3P(CH2)4PO3)] and A2[Mo2O5(O3P(C6H4)PO3)]. The A+ ions reside in the interlayer region as well as in the cavities within the anionic layers.

Three new sulfides Li2CdSnS4 (1), Na2CdSnS4 (2) and Na6CdSn4S12 (3), have been synthesized using a molten flux of alkali-metal polysulfides and structurally characterized by single crystal X-ray diffraction studies. Both the layered compound 1 and the three-dimensional compound 2 have tetrahedral anionic frameworks of (CdSnS4)2- and, for charge compensation, alkali-metal cations. The two-dimensional compound 3 has Na+ ions interleaved between anionic (CdSn4S12)6- layers with an octahedral framework. Compounds 1, 2 and 3 are structurally related to Cu2CdGeS4, LiAlSe2 and Na2SnS3 respectively. Compound 2, as determined from its diffuse reflectance spectrum, is a semiconductor with a band gap of 1.52 eV.