Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Mixed Valence Chemistry-A Survey and Classification

The oxidation of CO by O2 and by NO on supported chromium oxide and other metal oxide catalysts

Effect of alloying on the oxidation behaviour of Co–Al–W superalloys

In situ Raman spectroscopic investigation of chromium surfaces under hydrothermal conditions

The characterization of CrOxSiO2 catalysts by photoelectron spectroscopy (XPS), X-ray and optical measurements

The formation of Cr3O8, Cr2O5, CrO2, and Cr2O3 by the decomposition of anhydrous CrO3 under a high oxygen pressure and their physical and chemical properties are presented. The crystal structure of CrO2 is a rutile type and its lattice constants do not vary with the ratio O/Cr. The specific gravity of CrO2 decreases with an increase in the ratio O/Cr. This is explained by assuming that the crystal structure of CrO2 contains an excess of Cr ions at interstitial sites. This crystal model is supported by the low electrical resistivity of CrO2. An electron microscopic examination showed an abnormal grain growth of CrO2 during the decomposition of Cr2O5 to CrO2. The activation energies of the decomposition of Cr2O5 and CrO2 to Cr2O3 in air are 61.7 and 48.2 kcal. per mole, respectively. The pressure-temperature diagram of various chromium oxides has been determined. From the pressure-temperature curve of CrO2-Cr2O3 the heat of decomposition is found to be 26.3 kcal. per mole.

Decomposition of Higher Oxides of Chromium Under Various Pressures of Oxygen

Several mixtures in the system Te-CrO2 were prepared by the decomposition of anhydrous chromic acid, CrO3, and telluric acid (H6TeO6) under high oxygen pressures; the physical and chemical properties were then studied. When Te was added, the grain size of CrO2 decreased rapidly from 5 to 10μ to 0.1 to 1.0μ and the magnetic coercive force increased from 20 to 120 oersteds. The magnetic Curie temperature and the lattice constants of CrO2 did not vary, however, with additions of Te up to 20 atomic %. X-ray analysis showed that the CrO2 plus Te was not a solid solution but a mixture of CrO2 and TeCr2O6 that was nonmagnetic.

Buichi Kubota

Papers

Decomposition of Higher Oxides of Chromium Under Various Pressures of Oxygen

Modified Ferromagnetic Chromium Dioxide