Structure determination corundum

The structures determined for hematite and corundum show that these crystals consist of a compact arrangement of approximately, but not exactly, spherical ions of oxygen and of iron or aluminum, held together by inter-ionic forces which are prob- atoms in the units of structure of ably electrostatic in nature. No evidence hematite and corundum small cir-... 

The structure of the insulating low-temperature phase is a monoclinic distortion of the corundum structure. Structure determination (Dernier and Marezio 1970) shows that the V-V distance increases from 2.697 to 2.745 A, which across shared octahedral edges increases from 2.882 to 2.907. The average V-O distance remains constant. The material is antiferromagnetic, with a moment of 1.2/ B on... 

The Appendix gives calculated patterns based on the results of X-ray structure determinations for the clinker phases. For each pattern, a Reference Intensity Ratio (H23) is included. This is the integrated intensity of the strongest individual reflection (which may be a component of an overlap) relative to that of the strongest peak of corundum in a 1 1 mixture by... 

Cr203 has the same bulk crystal structure as a-Al203, namely corundum. Of its several low Miller index surfaces only one, (0001), has been employed for adsorbate structural determinations so far. To overcome sample charging problems a thin film has been utilised for these studies, rather than a single crystal. The surface structure of this (0001) oriented thin film has been investigated by LEED-IV [112]. Simulations of the experimental data evidence a chromium terminated surface with large vertical interlayer relaxations, reaching down five or six layers. 

TEM analysis of the nanocomposite with an A1 content beyond the percolation threshold reveals spherical pure metal nanoparticles with a mean diameter of about 10 nm (Fig. 6a), while below the percolation threshold the composite contains agglomerates of rhombohedral AI2O3 (corundum) with a mean size of 55 nm (Fig. 6c). A sample with a metal content just at the percolation threshold contains metal nanoparticles of 10nm and alumina aggregates of 28 nm in diameter (Fig. 6b). The inorganic phase is homogeneously dispersed within the polymeric matrix in all of the investigated samples. It has been shown that the nanocomposite structure determines the oxidation behaviour of A1 nanoparticles within the polymeric matrix under air exposure. 

The crystal structures of hematite and corundum have been determined through the use of Taue and spectral photographs, interpreted with the aid of the theory of space groups. The unit of structure is a rhombohedron with a = 55° 17 and a = 5.420 = = 0.010 A. for hematite, and with a = 55° 17 and a = 5.120 = = 0.010 A. for corundum. The space group underlying the atomic arrangement is D. ... 

Acta Cryst. B39 165-170 Pauling, L. Hendricks, S.B. (1925) The crystal structures of hematite and corundum. J. Am. Chem. Soc. 47 781-790 Pauling, L. (1929) The principles of determining the structure of complex ionic crystals. J. Amer. Chem. Soc. 51 289-296 Payne, J.E. Davis, J.A. Waite,T.D. (1996) Uranium adsorption on ferrihydrite — effect of phosphate und humic add. Radiochemica Acta 74 239-243... 

Iron oxide with a corundum-type structure, that is, aFe203, is a good catalyst for reactions such as the catalytic oxidation of S02 [53], Studies carried out on this reaction show that S02 adsorption is the rate-determining step it contributes electrons to the conduction band and the adsorption of 02 withdraws the conduction electrons from an oxygen vacancy [53],... 

The structures of three forms of Rh203 have been determined, the normal (corundum) form, a high-temperature orthorhombic form, and a high-pressure polymorph made at 1200°C under 65 kbar. In this high-pressure form there are pairs of face-sharing octahedra (as in corundum) but a different selection of shared edges the structure may be regarded as built from slices of the corundum structure. 

Our conclusion is, therefore, that the Weiss constant is a measure, in these systems, of the coordination number with respect to paramagnetic ions of the same charge. In the corundum structure, thanks to the sharp change in direction shown by the plot of paramagnetic neighborhood against number of atom layers, it is possible to determine when an oxide layer is only three or four atom layers thick. 

Exactly the same procedure as outlined above is used in appUcations to crystals. Bonds between pairs of different atoms are considered to have separate valences to be determined by valence sums. Atoms are considered different if they are distinct in the crystallographic sense. A difficulty that often arises in practice is that bonds between pairs of atoms that are related by symmetry are not necessarily themselves related by symmetry a familiar example is that of the corundum (a-Al203) structure in which there is only one kind of A1 atom and one kind of O atom but two different Al-0 bonds that are of unequal lengths. The method of analysis used above would ascribe equal bond valences to these two bonds, but, because they are of unequal lengths, different apparent bond valences would be ascribed to them. 

Those lines have been previously ascribed to Eg luminescence of Cr " " in an intermediate crystal field site (Tolstoy and Shinfue 1960 Wojtowicz 1991). Nevertheless, several contradictions prevent us from accepting such an interpretation. Excitation spectra of two types of enussion lines are very similar to those pubhshed earlier and determined as a and b environments (Platonov et al. 1998). Crystal field parameters calculated based on their excitation and polarized absorption spectra gave Dq = 1720 cm and B = 730 cm for a and Dq= 1600 cm and B = 570 cm for b environments. As was already mentioned, it was concluded that these great differences in the crystal field parameters cannot be explained by a distribution of Cr between two or more of the four crystallographicaUy different octahedral sites in the kyanite structure. The presence of a corundum precursor in kyanite was confirmed by our experiments. Nevertheless, those lines have long decay times typical for Cr in strong crystal field. 

As an example, the joint analysis of IR and Raman spectra provided evidence of the partial ordering of cations in a Fe-Cr corundum-type mixed sesquioxides, which are used industrially as high temperature water-gas shift catalysts, but are also active in olefin oxidative dehydrogenation. X-ray diffraction (XRD) patterns of these solids indicate the conmdum-type structure without any superstructure. This implies that iron and chromium ions are randomly distributed. IR and Raman spectra instead definitely show that cations are at least partially ordered in layers such as in the ilmenite-type superstructure. Similarly, XRD analysis shows a cubic (non-ferroelectric) structure of nanometric BaTi03, while vibrational spectroscopies reveal microscopic asymmetry of this structure. Similarly, IR spectroscopy allowed the determination of the state of vanadium in solid solution in Ti02 anatase catalysts, and the presence of Ti" + in the silicalite framework of TSl catalysts, " used for the selective oxidation of phenol and the ammoximation of cyclohexanone with hydrogen peroxide. 

CICPs are metal oxides, which can have a number of possible structures. The biggest determining factor in the structure is the oxygen/metal (0/M) ratio (Table 5-1). As long as the metal ions are of a comparable size, it is the 0/M ratio that largely determines what the structure will be. Two crystal structures dominate the class of CICPs - those of rutile and spinel. The hematite and corundum structures are also observed, but are much less common. 

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