The crystal structure of Ga2—xFexO3, with 0.7≤x≤1.4, has been determined at 298°K by x-ray diffraction. Ga2—xFexO3 crystallizes in the orthorhombic system, space group Pc21n, with lattice constants a = 8.7512±0.0008, b = 9.3993±0.0003, c = 5.0806±0.0002 Å and eight formula weights in the unit cell. The complete scattering pattern was measured with PEXRAD to (sinθ)/λ≤1.02 Å−1. One gallium atom in the asymmetric unit is in a tetrahedral site, the other gallium and the two iron atoms are in octahedral sites. For x>1, the excess iron partly occupies the octahedral gallium site: the iron sites are to a smaller extent occupied by gallium. The gallium tetrahedron is very nearly regular, with Gatetr−0 = 1.846±0.005 Å. The gallium octahedron is irregular, with average Gaoct−0 = 2.026±0.004 Å; the iron octahedra are somewhat more irregular, with average Feoct−0 = 2.036±0.004 and 2.049±0.004 Å in the two crystallographically independent octahedra. The oxygen atoms pack in pseudohexagonal layers with a double-hexagonal close-packed repeat. The absolute configuration has been determined by the anomalous scattering: the positive sense of the polar b-axis is essentially parallel to an 0→Gatetr bond. The thermal vibrations in the crystal are isotropic, with a characteristic temperature Θ of 534°K. The magnetic spin structure has been deduced, and an atomic mechanism postulated for the piezoelectric effect, in terms of the crystal structure.
Paramagnetic alpha-MnMoO4, the stable phase at STP, is monoclinic with lattice constants a=10.469±0.005, b=9.516±0.005, c=7.143±0.005 Å, and β=106°17′±6′, and space group C2/m. The crystal structure has been solved and refined by a combination of three-dimensional Patterson and Fourier series and by the method of least squares. 1873 independent integrated intensities were measured by PEXRAD; the final agreement factor between measured and calculated structure factors is 0.0574. Two crystallographically independent Mn atoms are surrounded by highly distorted oxygen octahedra; the Mn–O distance varies between 2.091±0.006 and 2.252±0.003 Å, with average length 2.164 Å. Two crystallographically independent Mo atoms are surrounded by slightly distorted oxygen tetrahedra, with Mo–O distances ranging from 1.724±0.005 to 1.851±0.004 Å, the average being 1.761 Å. The thermal vibrations are significantly anisotropic. An analysis is given of various current methods for estimating the standard deviation in the measured structure factor.
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