Rhombohedral oxides

Ghiorso MS (1997) Thermodynamic analysis of the effect of magnetic ordering on miscibility gaps in the FeTi cubic and rhombohedral oxide minerals and the FeTi oxide geothermometer. Phys Chem... 

As before, it is convenient to consider this system in order of increasing Cr content. At low Cr contents, no internal oxidation zone is seen. This is because the rate at which the external scale is formed is so rapid that the thickness of the internal oxidation zone is negligibly small. The system is best described in terms of the Fe-Cr-0 phase diagram shown schematically in Figure 5.18. The rhombohedral oxides FeiOs and Cr203 show a continuous series of solid solutions. The iron- and chromium-oxides react to form spinels which form solid solutions with Fc304. 

The reaction conditions are critical, as hydrated iron oxide, Fe202 H20, can also precipitate. The particles are either spherical or rhombohedral, depending on the nucleating material. 

Silver nitrate forms colorless, rhombic crystals. It is dimorphic and changes to the hexagonal rhombohedral form at 159.8°C. It melts at 212°C to a yellowish Hquid which solidifies to a white, crystalline mass on cooling. An alchemical name, lunar caustic, is stiU appHed to this fused salt. In the presence of a trace of nitric acid, silver nitrate is stable to 350°C. It decomposes at 440°C to metallic silver, nitrogen, and nitrogen oxides. Solutions of silver nitrate are usually acidic, having a pH of 3.6—4.6. Silver nitrate is soluble in ethanol and acetone. 

Lower Oxides. A number of hard, refractory suboxides have been prepared either as by-products of elemental boron production (1) or by the reaction of boron and boric acid at high temperatures and pressures (39). It appears that the various oxides represented as B O, B O, B22O2, and B23O2 may all be the same material ia varying degrees of purity. A representative crystalline substance was determined to be rhombohedral boron suboxide, B12O2, usually mixed with traces of boron or B2O3 (39). A study has been made of the mechanical properties of this material, which exhibits a hardness... 

Trivalent Oxides.—It is probable that the rhombohedral crystal corundum, AI2O3, consists of ions that is, the bonds are probably electrostatic. The oxygen ions are arranged in layers parallel to the base (111) within each layer the distances between adjacent ions are 2.50 and 2.89 A., the values being somewhat uncertain because of the difficulty in the accurate determination of the parameter. These values are probably compatible with the oxygen radius 1.40 A., calculated for crystals with bivalent cations. The aluminum-oxygen distances are 1.85 and 1.99 A., in satisfactory agreement with the calculated radius sum 1.90 A. 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Another issue with ScSZ is that there is a decrease in conductivity at around 580°C as indicated by broken lines in two of the curves for higher scandia contents, 10.0 to 12.0 mol%. With high scandia contents, the cubic phase transforms to a lower conductivity rhombohedral phase, the p phase, at lower temperatures [25], The phase change can be avoided by limiting the scandia content to 8 mole% [25] or by codoping with other oxides, such as those of bismuth [36] or ytterbium [37],... 

Lei and Zhu [63] found that adding 2.0 mol% Mn203 to llScSZ can inhibit the cubic-rhombohedral phase transformation in both oxidation and reduction atmospheres, and the codoped zirconia can reach nearly full density when sintered at temperatures as low as 850°C. The conductivity of 2Mn203-l IScSZ sintered at 900°C is 0.1 Scm-1 at 800°C. Figure 1.11 illustrates the conductivity of some zirconia-based ternary systems [32,42,57,63-67],... 

Undoped LaMn03 is orthorhombic at room temperature and shows an orthorhombic/ rhombohedral crystallographic transformation at 600°C [6], This transformation has been attributed to the oxidation of some Mn3+ to Mn4+ ions. Thus, the orthorhombic/... 

In another paper (Daams and Villars 1993), the results of a similar classification of the rhombohedral intermetallic structure types were shown. The 195 rhombohe-dral structure types reported in Villars and Calvert (1991) were analysed 51 types had improbable interatomic distances or correspond to oxides with no intermetallic representatives and were excluded. The remaining 144 types (corresponding to 1324 compounds) were considered. It was observed that 14 AET are greatly... 

Dark purple rhombohedral crystal density 2.703 g/cm stable in air decomposes at about 240°C moderately soluble in cold water, 6.38 g/lOOmL at 20°C, soluble in hot water, 25 g/lOOmL at 65°C decomposed by alcohol, acetone and many organic solvents causing their oxidation also decomposed by concentrated acids. 

The remaining trifluorides of the transition elements known so far apparently crystallize rhombohedrally like VFa. Prior to this structure type the cubic ReOs-type will be discussed briefly, though only oxide-fluorides rather than trifluorides seem to adapt this structure. Yet it is te simple basic type of which the others may be derived. 

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