Nonstoichiometry metal excess phases

The low-pressure region is associated with the electroneutrality equation [e J = 2 [Vx ]. Electrons predominate so that the material is an n-type semiconductor in this regime. In addition, the conductivity will increase as the 7 power of the partial pressure of the gaseous X2 component increases. The number of nonmetal vacancies (and metal excess) will increase as the partial pressure of the gaseous X2 component decreases and the phase will display a nonstoichiometry opposite to that in region III. Because there is high concentration of anion vacancies, easy diffusion of anions is to be expected. 

Nonstoichiometry can be caused by oxygen deficiency (or excess) or by fractional valences of the metal components. For example, the existence of Cu " in nonstoichiometric cuprates has been widely discussed [9,10]. It is essential that in nonstoichiometric oxides the microscopic fluctuations of the composition should proceed (the so-called phase separation). The characteristic size of heterogeneities induced can exceed atomic dimensions by an order of magnitude. This phenomenon is attributable to the fact that the electron-nonuniform state of such chemically singlephase materials appears to be energetically more advantageous. 

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