Oxygen-Excess Phases

In a material in which the cations can take on a higher valence, the holes might be located on cations giving them ionic charges of M3+ as opposed to M2+  

The holes can reside on normal cations converting them from M2+ to M3+ or M4+  

If the holes are able to gain enough energy to move from a cation when illuminated, the materials are photoconducting. Thermal energy may also be able to liberate the holes and the solids are p-type semiconductors. The transition-metal monoxides NiO and CoO represent this behavior (Sections 1.11.4 and 4.3.2). 

As in the preceding examples, the holes can reside on cations, converting them to a lower valence state. In the present example, the charge on the lanthanum is fixed as La3+, and any holes must associate with the Ni2+ cations to convert them to Ni3+  

The material would be expected to be a hole (p-type) semiconductor. However, in this compound the interstitial oxygen ions can diffuse fairly quickly, and the oxygen diffusion coefficient is higher than normal, so that the compound shows both high oxygen diffusivity and electronic conductivity, a situation referred to as mixed conductivity (Section 8.7). 

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Nickel oxide is a classical nonstoichiometric oxide that has been studied intensively over the last 30-40 years. Despite this, there is still uncertainty about the electronic nature of the defects present. It is well accepted that the material is an oxygen-excess phase, and the structural defects present are vacancies on cation sites. Although it is certain that the electronic conductivity is by way of holes, there is still hesitancy about the best description of the location of these charge carriers. 

The analysis of oxygen-excess oxides is similar to that for metal-rich phases just given. For example, the creation of oxygen excess by cation vacancies can be written ... 

For most polymers, the yield of hydroperoxides is relatively low even in the presence of oxygen excess. The relatively high values were, e.g., obtained during oxidation of atactic polypropylene [79], In the initial phases of oxidation, the yield of hydroperoxide related to 1 mol of oxygen absorbed is 0.6 at 130 °C when passing the maximum concentration it decreases considerably. In isotactic polypropylene, the maximum yield of hydroperoxides attains the value 0.2, only [80]. This may be probably related with a local accumulation of hydroperoxides in domains of defects in the crystalline structure which leads to an increased ratio of participation of hydroperoxide groups in the chain reaction of an oxidation process (induced decomposition of hydroperoxides) and finally to a lower yield of hydroperoxides... 

Dezanneau et al. reported Lai cMn03+a nanocrystalline powders prepared by an acylamide polymerization sol-gel method (Dezanneau et al., 2002a,b, 2003). The composition analysis revealed that for La/ Mn < 0.9 the Mna04 phase was present, while for La/Mn > 0.9, the high oxygen excess led to considerable vancancies on cationic sites. The Curie temperature remained constant at 295 K for the former case, while decreased Curie temperatures were observed for the latter case, due to the increasing amoimt of Mn vacancies. 

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

The method of heterogeneous isotopic exchange was used to determine the self-diffusion coefficient of oxygen in Pr70j2 (iota phase) as a function of temperature and pressure. The results indicated that there are two types of hyperstoichiometric material depending on the degree of oxygen excess. When the compound is close to stoichiometry, the diffusion coefficient can be written... 

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