Random point defect chemistry

The crystal chemistry of scheellte-type structures Is further complicated by defect stoichiometries with extensive vacancies at cation sites. In general, random cation vacancies are examples of point defects. However, at high concentrations, an ordering of vacancies can occur at which time the vacancy can no longer be considered as a point defect and a new periodic lattice (or unit cell) will have been generated. 

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