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Weil M.,Institute for Chemical Technologies and Analytics | Stoger B.,Institute for Chemical Technologies and Analytics
Acta Crystallographica Section B: Structural Science | Year: 2010

The structures of the 3d divalent transition-metal diarsenates M 2As2O7 (M = Mn, Co, Ni, Zn) can be considered as variants of the monoclinic (C2/m) thortveitite [Sc2Si2O7] structure type with a ≃ 6.7, b ≃ 8.5, c ≃ 4.7 Å,α ≃ 90, β ≃ 102,γ ≃ 90° and Z = 2. Co 2As2O7 and Ni2As2O 7 are dimorphic. Their high-temperature (β) polymorphs adopt the thortveitite aristotype structure in C2/m, whereas their low-temperature (α) polymorphs are hettotypes and crystallize with larger unit cells in the triclinic crystal system in space groups P1̄ and P1, respectively. Mn2As2O7 undergoes no phase transition and likewise adopts the thortveitite structure type in C2/m. Zn2As 2O7 has an incommensurately modulated crystal structure [C2/m(α,0,γ)0s] with q = [0.3190 (1), 0, 0.3717 (1)] at ambient conditions and transforms reversibly to a commensurately modulated structure with Z = 12 (I2/c) below 273 K. The Zn phase resembles the structures and phase transitions of Cr2P2O7. Besides descriptions of the low-temperature Co2As2O7, Ni 2As2O7 and Zn2As2O 7 structures as five-, three- and sixfold superstructures of the thortveitite-type basic structure, the superspace approach can also be applied to descriptions of all the commensurate structures. In addition to the ternary M 2As2O7 phases, the quaternary phase (Ni,Co) 2As2O7 was prepared and structurally characterized. In contrast to the previously published crystal structure of the mineral petewilliamsite, which has the same idealized formula and has been described as a 15-fold superstructure of the thortveitite-type basic structure in space group C2, synthetic (Ni,Co)2As2O7 can be considered as a solid solution adopting the α-Ni2As 2O7 structure type. Differences of the two structure models for (Ni,Co) 2As2O7 are discussed. © 2010 International Union of Crystallography. Source


Stoger B.,Institute for Chemical Technologies and Analytics | Weil M.,Institute for Chemical Technologies and Analytics | Zobetz E.,Institute for Chemical Technologies and Analytics
Zeitschrift fur Kristallographie | Year: 2010

The orthotellurates(VI) of strontium and barium were synthesized at 1373 K in air by oxidation of TeO2 in a melt of SrBr2 and BaBr2, respectively. Their unprecedented structures were determined from room temperature single crystal X-ray diffraction data. Sr 3TeO6, [F1̄, Z = 32, a = 16.5338(2) Å, b = 16.6308(2) Å, c = 16.7112(3) Å, a = 90.031 (2)°, β = 90.035(2)°, γ = 90. 1370(10)°, R[F2 > 2σ(F2)] = 0.035, 19195 F2 data, 375 variable parameters! and Ba3TeO6 [IA1/a, Z = 80, a = 19. 3878(10) Å, c = 34.909(2) Å, R[F2 > 2σ(F2)] = 0.034, 19576 F2 data, 453 variable parameters] are eight- and 40-fold superstructures which can be described as a pseudo-cubic close-packing of [TeO6]6- octa- hedra with M2+ (M = Ba, Sr) atoms located in the tetra- hedral and octahedral voids. Sr3TeO6 and Ba3TeO6 are hettotypes of the cubic aristotype with the double perovskite structure crystallizing in space group Fn3̄m. A comparative discussion of the crystal structures and their group-subgroup relations are given, including different approaches for the structure descriptions. © by Oldenbourg Wissenschaftsverlag, München. Source

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