Vacancy point defects quenched

Nonequihbrium concentrations of point defects can be introduced by materials processing (e.g. rapid quenching or irradiation treatment), in which case they are classified as extrinsic. Extrinsic defects can also be introduced chemically. Often times, nonstoichiometry results from extrinsic point defects, and its extent may be measmed by the defect concentration. Many transition metal compounds are nonstoichiometric because the transition metal is present in more than one oxidation state. For example, some of the metal ions may be oxidized to a higher valence state. This requires either the introduction of cation vacancies or the creation of anion interstitials in order to maintain charge neutrality. The possibility for mixed-valency is not a prerequisite for nonstoichiometry, however. In the alkah hahdes, extra alkah metal atoms can diffuse into the lattice, giving (5 metal atoms ionize and force an equal number... 

Up to now, our equations have been continuum-level descriptions of mass flow. As with the other transport properties discussed in this chapter, however, the primary objective here is to examine the microscopic, or atomistic, descriptions, a topic that is now taken up. The transport of matter through a solid is a good example of a phenomenon mediated by point defects. Diffusion is the result of a concentration gradient of solute atoms, vacancies (unoccupied lattice, or solvent atom, sites), or interstitials (atoms residing between lattice sites). An equilibrium concentration of vacancies and interstitials are introduced into a lattice by thermal vibrations, for it is known from the theory of specific heat, atoms in a crystal oscillate around their equilibrium positions. Nonequilibrium concentrations can be introduced by materials processing (e.g. rapid quenching or irradiation treatment). 

Point defects such as vacancies are thermodynamically stable entities 18) meaning, in effect, that their concentration increases with increasing temperature because their free energy of formation is negative. A crystal that is quenched from high temperatures will, at lower temperatures, contain a supersaturation of point defects. But the excess... 

If a crystal is annealed at a sufficiently high temperature and for long enough, then the equilibrium concentration of vacancies will increase. Abrupt quenching of the material can freeze in unusually high concentrations of point defects. 

Point defects are mainly involved in chemisorption on cleaved surfaces. On such surfaces the density of point defects is usually very small. They can be created by heating in vacuum and rapid quenching or by electron or ion bombardment. The predominant point defects are 0 vacancies (F centers) with two electrons at each vacancy in order to maintain the local electroneutrality. This localized charge partially overlaps with the adjacent cations, increasing die population of their lowest empty orbitals. The charge distribution does not affect signilicandy the anions near the vacancy defect because their already filled 2p shell cannot accept additional charge. The O vacancies affect predominandy die electronic structure in maximal... 

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