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

Mixed oxides of the type MO2 (fluorite)—M2O3—I oxygen dissociation pressures and phase relationships in the system CeO2Ce2O3 at high temperatures

Consideration of the coordination of the oxygen atoms in rare-earth oxide and oxysalt structures shows that these are generally surrounded by a tetrahedron of metal atoms. All known structures can therefore be described as tri-dimensional or bi-dimensional packing of such OM4 tetrahedra sharing edges. In A-M2O3, B-M2O3, and the oxysalts, the packed tetrahedra appear as an infinite polymeric complex cation (MO)nn+. Possible implications for the unknown structures of nonstoichiometric oxides are discussed. The results suggest that the oxygen atoms are involved in a directional bonding.

OM4 Tetrahedra linkages and the cationic group (M0)nn+ in rare earth oxides and oxysalts

Crystallographic and preparative data are given for high‐purity rare‐earth iron garnets (R3Fe2Fe3O12 and {R3—xRx′}Fe2Fe3O12, where R and R′ are rare earths). Lattice constants are obtained for hypothetical La, Pr, and Nd iron garnets. Maxima have been found for amounts of Pr substitution in Sm, Gd, Y, and Lu iron garnets which in turn lead to a principal maximum lattice constant of 12.540 A for any rare‐earth iron garnet, in good agreement with 12.538 A obtained by Geller, Williams, and Sherwood. The maximum for La substitution in YIG is also given. A plot of lattice constant vs atomic number shows some influence of crystal field on the trivalent rare‐earth ions with nonspherical electron configuration superposed on the lanthanide contraction. The importance of the size difference of the dodecahedral rare‐earth ions in the praseodymium‐substituted rare‐earth iron garnets with respect to the maximum attainable lattice constants is shown corroborating observations on the neodymium substituted rare earth iro...

Crystal Chemical Study of the Rare‐Earth Iron Garnets

Hexagonally arranged surface oxygen atoms, oxygen point defects, and multiple oxygen defects at oxygen-terminated CeO2(111) surfaces in different oxidation states were visualized by noncontact atomic force microscopy (NC-AFM). The multiple defects such as line defects and triangular defects were stabilized by a local reconstruction, where edge oxygen atoms surrounding the multiple defects were displaced and gave enhanced brightness due to a geometric reason. Successive NC-AFM measurements of the same area of a slightly reduced CeO2(111) surface revealed that hopping of surface oxygen atoms faced to metastable multiple defects was thermally activated even at room temperature. In contrast, no hopping was observed either at a point oxygen vacancy or a line defect that is stabilized by local reconstruction. It was also confirmed from atom-resolved NC-AFM observations that the surface oxygen defects were easily healed by exposure to O2 gas at room temperature.

Atom-Resolved Noncontact Atomic Force Microscopic Observations of CeO2(111) Surfaces with Different Oxidation States: Surface Structure and Behavior of Surface Oxygen Atoms

The structures of praseodymium oxide phases of composition PrO1.83, PrO1.81, PrO1.78, PrO1.71, and PrO1.69 are discussed in terms of the structures of PrO2.00 and PrO1.50 (C and A types). The way in which the existence of these several stable structures are mirrored in the physical and chemical properties is discussed.

On the Structure and Related Properties of the Oxides of Praseodymium

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On the Structure and Related Properties of the Oxides of Praseodymium