Arrangement Of Oxygen Atoms Research Articles

Ordered Arrangement of Oxygen in the Interstitial Solid Solution of Zirconium-Oxygen System

The crystal structure of interstitial solid solution of zirconium-oxygen system was investigated by means of X -ray, electron and neutron diffraction on specimens in the composition range between 11 and 27 at% 0. The specimens were prepared by melting appropriate mixtures of zirconium metal and zirconium dioxide in an arc-furnace. In addition to a superstructure found by Holmberg and Dagerhamn in the composition ZrO y (0.33< y <0.40), a new superstructure having a different ordered arrangement of oxygen was found in the range of ZrO x (0.25< x <0.33). The atomic arrangement of oxygen in these superstructures can be expressed in terms of the stacking sequence of octahedral interstitial sites in the lattice of α-Zr, i.e. , ABCABC … and ABAB … stackings in ZrO x and ZrO y respectively. However, the limiting composition ZrO 0.333 has an intermediate structure in which the arrangement of oxygen atoms is basically described as ABCBCACAB … stacking with nine layers period. The actual distribution of oxygen in sp...

Crystal Structure of the Transition-Metal Molybdates and Tungstates. IV. Paramagnetic CuMoO4

Paramagnetic CuMoO4 is triclinic, space group P1̄, with lattice constants a = 9.903 ± 0.028, b = 6.783 ± 0.015, c = 8.359 ± 0.0019 Å, α = 101.08° ± 0.20°, β = 96.88° ± 0.20°, and γ = 107.05° ± 0.20° at 298°K, with six formula weights in the unit cell. The integrated intensities of 6310 reflections within a reciprocal hemisphere of radius (sinθ) / λ = 1.02 Å−1 were measured with PEXRAD; of these, 3081 were independent and significantly above background. The atomic arrangement is similar to that in ZnMoO4 but with corresponding atoms displaced by as much as 0.4 Å. Refinement by the method of least squares gave a final agreement factor between measured and calculated structure factors of 0.064. Two of the independent copper atoms occupy axially distorted octahedra of oxygen atoms; the four short Cu–O bond distances in each octahedron range between 1.933 ± 0.010 and 1.980 ± 0.012 Å, with an average value of 1.953 Å. The axial Cu–O bonds range in length between 2.213 ± 0.012 and 2.619 ± 0.011 Å, with an average value of 2.364 Å. The third copper atom occupies a distorted tetragonal pyramidal arrangement of oxygen atoms, with basal Cu–O distances ranging from 1.894 ± 0.013 to 1.985 ± 0.010 Å, the average being 1.928 Å; the apical Cu–O bond length is 2.341 ± 0.009 Å. The three independent molybdenum atoms are surrounded by somewhat distorted oxygen tetrahedra with Mo–O distances varying between 1.701 ± 0.010 and 1.862 ± 0.008 Å, the average being 1.772 Å. All atoms in CuMoO4 vibrate with significant thermal anisotropy.

Crystal Structure of the Transition-Metal Molybdates and Tungstates. III. Diamagnetic α-ZnMoO4

Diamagnetic α-ZnMoO4, the stable phase at STP, is triclinic with lattice constants a=9.625±0.015 Å, b=6.965±0.010 Å, c=8.373±0.015 Å, α=103.28°±0.15°, β=96.30°±0.15°, and γ=106.72±0.15° at 298°K, with six formula weights in the unit cell. The integrated intensities of 6489 reflections in a reciprocal hemisphere with (sinθ)/λ≤1.02 Å−1 were measured with PEXRAD; 3269 independent Fmeas were significantly above background. The structure was solved in Space Group P1; the subsequent atomic arrangement, obtained using a minimum interatomic contact requirement together with three-dimensional Fourier series and least-squares model fitting, closely approximated a centrosymmetric distribution. Refinement by the method of least squares in Space Group P1̄ gave a final agreement factor between measured and calculated structure factors of 0.0726. Each of the three crystallographically independent Mo atoms is surrounded by distorted tetrahedra of oxygen atoms; the Mo–O distances vary between 1.707±0.009 and 1.842±0.009 Å, the average being 1.764 Å. Two of the independent zinc atoms occupy distorted octahedra of oxygen atoms; the octahedral Zn–O distances range from 1.977±0.011 to 2.243±0.010 Å, with an average of 2.078 Å. The remaining zinc atom is in a distorted ``square'' pyramidal arrangement of oxygen atoms with Zn–O distances that vary from 1.954±0.012 to 2.106±0.007 Å, the average being 2.025 Å. The thermal vibration of the metal atoms is significantly anisotropic.

Question of the transmission of electronic influences in systems with a ?-arrangement of oxygen atoms

Question of the transmission of electronic influences in systems with a ?-arrangement of oxygen atoms

Structure of Vanadium Dioxide

GOLDSCHMIDT1 came to the conclusion that vanadium dioxide had a rutile structure in which, in its idealized form, each vanadium would be surrounded by a regular octahedral arrangement of oxygen atoms. In fact, in actual rutile structures, the regular octahedral arrangement must always be lost2 (as is in fact observed), though the unit cell will remain tetragonal, because four of the metal oxygen (or halogen) distances will remain equal. The other two could be different without any loss of symmetry, but it is found that usually they have virtually the same length as the other four (for example, in titanium dioxide four of 1.94 Å. and two of 1.99 Å. and in magnesium fluoride four of 2.00 Å. and two of 1.98 Å.)3. The arrangement of the six cations around a metal atom in a tetragonal rutile structure may therefore be visualized as “one at the north pole, one at the south, and four on the equator as a rectangle”.

Societies and Academies

LONDON Royal Society, Mar. 3.—W. L. Bragg and J. West: The structure of certain silicates. Each series of silicates classified together as a mineral species, although with wide variation in chemical composition, is regarded as based on a characteristic type of oxygen assemblage. A number of structures are based on cubic or hexagonal arrangements of closest packing of oxygen atoms, the metal and silicon atoms being inserted into this framework. The dimensions of the unit cell are related to the fundamental spacings of this simple background of oxygen atoms, on which the complex pattern formed by the other atoms is embroidered. Closest-packing is found for an extended series of compounds, ranging from BeO, Al2O2, BeA2O4, and MgA2O4 to cyanite Al2SiO5, olivine (Mg, Fe)2SiO4, monticellite MgCaSiO4, and the chondrodite [(MgOH)2Mg3(SiO4)2] group. The arrangement of oxygen atoms is in general more complex, closestpacking being an exceptionally simple case, but the packing of oxygen atoms seems to remain the predominant feature of the structure.

The structure of beryl, Be 3 Al 2 Si 6 O 18

1. The structure of beryl, described in this paper, has some interesting features. In the first place, the ratio of silicon to oxygen atoms in the molecule is that of a metasilicate. As far as we know, no other example of a metasilicate has as yet been completely analysed (Wyckoff has made some measurements on diopside MgCa(SiO 3 ) 2 , but has only succeeded in determining its space group). Although the ratio of silicon to oxygen is as one to three, we find no SiO 3 groups in beryl. We find instead an arrangement of oxygen atoms around a silicon atom of exactly the same conformation as the groups (SiO 4 ) == in typical orthosilicates such as olivine and garnet. In this metasilicate the ratio of one silicon to three oxygen atoms is effected by a sharing of two oxygen atoms of each tetrahedral SiO 4 group with neighbouring groups. Thus the structure forms an intermediate stage between the orthosilicates and the structures of quartz and christobalite where every oxygen atom is shared by silicon atoms. This interesting feature may throw some light on the structure of other metasilicates. In the second place, we have analysed the structure by making careful quantitative measurements of the absolute intensity of X-ray reflexion, and interpreting these measurements by the formulæ which have proved to hold for simpler cases. The structure is one of moderate complexity, depending on seven parameters. We hope to show that all these parameters can be directly and accurately fixed. The analysis is greatly simplified when quantitative data are available, and there appears to be no reason why structures with many more parameters should not be attacked in the same direct manner. 2. Beryl belongs to the holosymmetric class of the hexagonal system. Groth ('Chemische Krystallographie’) quotes the axial ratio— a : c = 1 : 0·4989, and gives the density as being between 2·6 and 2·7. We have found the following values for the axes, measured by the X-ray spectrometer— c = 9·17 ± 0·01 Å. a = 9·21 ± 0·01 Å.