Oxygen ionic oxides

There are many other ionic oxides with structures which are more complicated than these. We will not go into them here. But it is worth knowing that most can be thought of as a dense (f.c.c. or c.p.h.) packing of oxygen, with various metal ions arranged, in an orderly fashion, in the octahedral or the tetrahedral holes. 

To determine oxidation numbers, we assign each valence electron to a specific atom in a compound. This means that the oxidation number of an atom is the charge it would have if the compound were composed of ions. Ionic FeO, for example, would contain Fe cations and 0 anions. Thus, in FeO we assign iron an oxidation number of +2 and oxygen an oxidation number of-2. 

A normal oxide is a binary (two element) compound containing oxygen in the -2 oxidation state. BaO is an example of an ionic oxide and S02 is an example of a molecular (covalent) oxide. 

Taking as an example an ionic oxide MO, this material can be made into a metal-excess nonstoichiometric material by the loss of oxygen. As only neutral oxygen atoms are removed from the crystal, each anion removed will leave two electrons behind, which leads to electronic conductivity. The oxygen loss can be incorporated as oxygen vacancies to give a nonstoichiometric oxide with a formula MOi v, or the structure can assimilate the loss and compensate by the introduction of cation interstitials to give a formula M1+xO. 

Oxygen Diffusion Coefficient (D ), Oxygen Surface Exchange Coefficient (Ar), and Oxygen Ionic Conductivity (tr,) of LaMn03-Based Oxides Measured by SIMS Technique... 

The simplest of structures is the rock salt structure, depicted in Figure 2.2a. Magnesium oxide is considered to be the simplest oxide for a number of reasons. It is an ionic oxide with a 6 6 octahedral coordination and it has a very simple structure — the cubic NaCl structure. The structure is generally described as a cubic close packing (ABC-type packing) of oxygen atoms in the (111) direction forming octahedral cavities. This structure is exhibited by other alkaline earth metal oxides such as BaO, CaO, and monoxides of 3d transition metals as well as lanthanides and actinides such as TiO, NiO, EuO, and NpO. 

Hyperfine coupling constant, 22 267, 269 Hyperfine interaction, ESR data for, 22 274 Hyperfine parameters for O, 32 128-130 Hyperfine splitting, 31 81 Hyperfine structure, trimer species, 31 98-99 Hyperfine tensor, 22 267, 273-279, 336, 340 constants, 32 20-21 dioxygen species, 32 18-25 equivalent oxygen nuclei, 32 18-21 ionic oxides, 32 40... 

It is important to emphasize that the atomic oxygen anion-radical plays a role in catalytic oxidation occnrring on varions oxide surfaces. For instance, O reacts with methane at room temperatnre over varions metal oxides (Lee and Gralowsky 1992). On solid catalysis, Q- is more reactive toward alkanes and alkenes than other ionic oxygen species. Iwamoto and Lnnsford (1980) assumed that O is the active oxygen species oxidizing benzene to phenol on with 70% selectivity at... 

A In fully ionic oxides the oxygen species present is O therefore the oxidation slate of the oxygen is equal to the charge, i.e. 2. This value is taken to he the oxidation stale of oxygen in compounds that are not necessarily ionic. The oxide ion has the fully occupied valence orbitals equivalent to the electronic eonliguralion ol neon. 2j 2p<. the Group IS element at the end of the same period. 

The oxides in Group IV do not show an octet structure Te02 has a distorted rutile structure, in which each Te atom is surrounded by six oxygens and thus resembles the more ionic oxides Ti02 and Sn02. 

Oxygen forms ionic oxides, such as Li20 and MgO, with active metals, and covalent oxides, such as P4OK) and SO3, with nonmetals. Oxides can also be classified according to their acid-base properties. Basic oxides are ionic, and acidic oxides are covalent. Amphoteric oxides, such as AI2O3, exhibit both acidic and basic properties. 

BIMEVOX — Figure. Oxygen ionic conductivity of solid oxide electrolytes at atmospheric oxygen pressure. See Ref. [ii] for details... 

Because intermetallic systems undoubtedly display certain special features that follow from their metallic binding forces, considerable importance attached to the growing evidence that the chalcogenides, the essentially ionic oxides, the nitrides, and other representative binary compounds of the transition metals were, not infrequently, both variable and irrational in composition. Schenck and Ding-mann s equilibrium study of the iron-oxygen system (39) was notable in this connection They showed that stoichiometric ferrous oxide, FeOi 000, the oxide of an important and typical valence state, did not exist. It lay outside the broad existence field of a nonstoichiometric phase. It is, perhaps, still not certain... 

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