Holes, Electrons, and Valence

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. 

2There are a number of ways in which hopping conductivity can be treated theoretically. In semiconductor 

In the earliest models it was believed that at temperatures more or less below 1220 K the situation could be represented by the following equation  

It is now suggested that a more realistic possibility is given by 

The situation with zinc oxide, ZnO, a material that has been investigated for a similar number of years, is comparable. Usually, nonstoichiometric ZnO is an n-type semiconductor. In the past it has been generally accepted that this is due to an excess of Zn in the form of Zn+ interstitials  

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