Vacancy

As was written, the crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances. That is, long-range order exists. Upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest neighbor atoms. But the reality is different, and real crystals are never perfect. There are always defects. The influence of these defects is not always adverse, and specific characteristics are often deliberately fashioned by introducing controlled amounts or numbers of particular defects.

Vacancy defects result from a missing atom in a lattice position. The stability of the surrounding crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. The vacancy type of defect can result from imperfect packing during the crystallization process, or it may be due to increased thermal vibrations of the atoms brought about by elevated temperature. All crystalline solids contain vacancies, and it is not possible to create such a material that is free of these defects. A vacancy (or pair of vacancies in an ionic solid) is sometimes called a Schottky defect. This point defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units to maintain an overall neutral charge in the ionic solid.

References:

Materials Science:

U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
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Gaskell, David R. (1995). Introduction to the Thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
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J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

See above:

Crystallographic Defects