= Vacancy defect
A missing atom at a lattice site that would be occupied in the reference crystal. A <vacancy defect> is a point defect and can contribute <configurational entropy> because many choices of empty sites can have the same macroscopic energy. Formation energy and pressure-volume work oppose this entropy gain. Vacancies can be present in <thermal equilibrium>; they need not all be preparation mistakes. Their allowed number and creation mechanism depend on whether the lattice size, atom number and surface reservoirs are fixed.
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