Corundum sesquioxides

M.G. (1984) Estimating relative ages from iron-oxide/total-iron ratios of soils in the Western Po Valley, Italy. Geoderma 33 39-52 Artman, J.O. Murphy, J.C. Foner, S. (1965) Magnetic anisotropy in antiferromagnetic corundum type sesquioxides. Phys. Rev. 138 A912-917... 

Reasoning by analogy, it would appear not unlikely that native oxides, such as alumina (corundum, ruby, sapphire), or iron sesquioxide (haematite), may be in reality an aluminate of alumina, A1(A102)3, or ferric ferrite, Fe(Fe02)3.. . ... 

Titanium Sesquioxide. T C has the corundum structure. At room temperature it behaves as a semiconductor having a small (0.2 eV) band gap. At higher temperatures, however, it becomes metallic. This is associated with marked change in the mean Ti—Ti distance. As with TiO, titanium sesquioxide, Ti CF, may be made by heating a stoichiometric mixture of titanium metal and titanium dioxide powders at 1600°C under vacuum in an aluminum or molybdenum capsule. 

Vanadium(III) Oxide. Vanadium(III) oxide (vanadium sesquioxide, V203) is a black solid, having the corundum (A C ) structure. It can be prepared by reduction of the pentoxide by hydrogen or carbon. Air oxidation proceeds slowly at ambient temperatures, but oxidation by chlorine at elevated temperatures to give VOCl3 and V203 is rapid. 

The structure of platinum dioxide and its reactions with some di, tri, and tetravalent metal oxides have been investigated. Ternary platinum oxides were synthesized at high pressure (40 kUobars) and temperature (to 1600°C). Properties of the systems were studied by x-ray, thermal analysis, and infrared methods. Complete miscibility is observed in most PtO2-rutile-type oxide systems, but no miscibility or compound formation is found with fluorite dioxides. Lead dioxide reacts with Pt02 to form cubic Pb2Pt207. Several corundum-type sesquioxides exhibit measurable solubility in PtOz. Two series of compounds are formed with metal monoxides M2PtOh (where M is Mg, Zn, Cd) and MPt306 (where M is Mg, Co, Ni, Cu, Zn, Cd, and Hg). 

Solid rhodium sesquioxide, Rh203, was first identified in 1927,1148 and has the corundum structure1149 (hexagonally close packed O2- anions distorted by the presence of Rh3+ cations in of the octahedral interstices). Each Rh is surrounded by six oxygens, three at 2.03 A and three at 2.07 A. An orthorhombic phase forms above 750 °C.1150... 

One of the most common structmes for sesquioxides of small to medium sized cations that favor sixfold coordination with oxygen is the corundum structme (o -Al203). Here, the oxygens are placed in an HCP array but only 2/3 of the octahedral sites are occupied following the motif shown in Figme 6(a). Strong Al-O-Al cross linking takes place in three dimensions in which AlOe octahedra share not only comers... 

The sesquioxide (Rh203) adopts the corundum stracture at low temperatures and the perovskite structure at high temperatures. The latter form is usually obtained by heating the element or various compounds at 900-1000 °C. Prolonged heating of Rh(N03)3 at 730 °C yields the low-temperature form. 

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. 

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