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  • Author: Reginald Gruehn x
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The hybrid phases Nb59O147F and Nb65O161F3 were prepared at high temperature and investi­gated by high resolution transmission electron microscopy. Both phases have block structures. The sample of the phase Nb59O147F is very well ordered, whereas the phase Nb65O161F3 shows some defects. In a sample of Nb65O161F3 microdomains of a new 2:1 hybrid phase Nb96O238F4 were found. Structural differences of the systems Nb2O5/NbO2F and Nb2O5/WO3 are discussed.

Abstract

A simple, quick and reliable method is derived to calculate values of [(O,F)/Me] for block structures. The method of calculation is exemplified using ideal and real (defect) block structures and inconsistent published data. The method represents a simple means to systematically classify block structures.

Abstract

Thermal Behaviour, Crystal Structures, Modifications Using the high temperature Guinier technique the transformation of the room temperature form N-PbSO4 to the cubic high-temperature form H-PbSO4 was observed. The nonquenchable H-PbSO4 modification crystallizes in the α-NaClO4-type with a(900°) = 7,23 Å, Z = 4 and dX-ray (900 °C) = 5,33 g/cm3 . The thermal dilatation of N-and H-PbSO4 was measured.

In the quaternary system H–Nb2O5–WO3–NbO2F the phase WNb15O40F was prepared in form of light-yellow needles. The compound crystallizes in the monoclinic system with the unit cell dimensions a = 22.254 A, b = 3.829 A, c = 20.298 A and β = 114.29°, space group C 2. Electron optical investigations showed the crystal structure containing 5 x 3 x co-blocks of Nb—(O, F)-octahedra, with metal atoms in tetrahedral positions located at the junctions of every four blocks. X-ray investigations showed a range of homogeneity between (O, F)/ (W, Nb) = 2.556 < 2.563 (WNb15O40F) < 2.571.

The hypothetical phase WNb18O47F2 with 6 x 3-blocks was found as a microdomain by electron microscopy.

Three different polymorphs of FeNbO4 were prepared by chemical transport. One of them is a new modification, which crystallisizes in the GaNbO4-type, monoclinic, space group C2, a = 12.52 Å; b= 3.83 Å; c = 6.67 A; β = 107.5°. The detailed structure, composed of 2 x 2 ‘blocks’ of corner shared metal-oxygen octahedra, was investigated by electron-diffraction and two-dimensional high resolution electron microscopy.

Abstract

Well shaped crystals of Cr2(SO4)3 and Al2(SO4)3 could be prepared by chemical transport reactions using HCl (Cr2(SO4)3) or SOCl2 (Al2(SO4)3) as transport agent. Cr2(SO4)3 and Al2(SO4)3 are isostructural to the rhombohedral modification of Fe2(SO4)3 and crystallize in the trigonal space group R[unk] with a = 8.1245(7) (8.0246(4)) Å and c = 21.944(3) (21.357(1)) Å, Z = 6 (hexagonal setting). The crystal structures were refined to a residual R = 0.035 using 828 unique reflections with |F|≥3 σ|F| for Cr2(SO4)3 and to R = 0.026 (772 unique reflections with |F|≥3 σ|F|) for Al2(SO4)3.

Main building units of the two compounds are MO6-octahedra and SO4 -tetrahedra which are linked via common corners to a 3-dimensional network.

Abstract

We describe the synthesis and crystallisation by means of chemical vapour transport reactions of TiPO4 and VPO4. TiPO4 (a = 5.2985(5) Å, b = 7.911(1) Å, c = 6.3473(5) Å) and VPO4 (a = 5.2316(5) Å, b = 7.7738(7) Å, c = 6.2847(5) Å) adopt the crystal structure of chromiumvanadate CrVO4 (Cmcm, Z = 4). The results of the refinements of both crystal structures from single crystal data are given. TiPO4 (VPO4): 335 (223) symmetry independent reflections from 1260 (1439) reflections, 22 (22) variables, residuals. R = 0.062 (0.049).

By using chemical transport reactions with various transporting agents (HgCl2, NbCl5, Nb3O7Cl) a slightly substoiehiometric NbO2-phase, β-NbO2, was obtained from samples with O/Nb ∼ 1.5 (source; T > 1373 K) and with deposition temperatures > 1273 K (sink). The rango of composition of β-NbO2 was found to exist from NbO1.990 to NbO1.998.

The structure of the tetragonal, column-shaped black crystals was determined by X-ray diffraction. It crystallizes tetragonally in the space group I41 with lattice constants a = 9.693(3) Å, c = 5.985(1) Å and Z = 10 formula units.

The crystal structure of β-NbO2 is shown to be a deformed rutile type. As in α-NbO2 the Nb-atoms are grouped in pairs. However, both oxides are different with respect to their long-range order.