Vacancy
Encyclopedia
In crystallography
, a vacancy is a type of point defect in a crystal
.
Crystals inherently possess imperfections, often referred to as 'crystalline defects'. A defect wherein an atom
, such as silicon
, is missing from one of the lattice sites is known as a 'vacancy' defect.
Vacancies occur naturally in all crystalline materials. At any given temperature, up to the melting point of the material, there is an equilibrium concentration (ratio of vacant lattice
sites to those containing atoms).
At the melting point of some metals the ratio can be approximately 0.1%.
The creation of a vacancy can be simply modeled by considering the energy required to break the bonds between an atom inside the crystal and its nearest neighbor atoms. Once that atom is removed from the lattice site, it is put back on the surface of the crystal and some energy is retrieved because new bonds are established with other atoms on the surface. However, there is a net input of energy because there are fewer bonds between surface atoms than between atoms in the interior of the crystal.
At any given temperature, the amount of energy needed to create a vacancy is diminished because creating a vacancy disorders the interior of the crystal. The measure of this disorder is called the entropy
of the system. Adding vacancies to the material increases the entropy, which tends to reduce the total energy required to create the vacancy. This energy is called the free energy
and this is the energy that is required to create an equilibrium concentration of vacancies at a given temperature.
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
, a vacancy is a type of point defect in a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
.
Crystals inherently possess imperfections, often referred to as 'crystalline defects'. A defect wherein an atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
, such as silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, is missing from one of the lattice sites is known as a 'vacancy' defect.
Vacancies occur naturally in all crystalline materials. At any given temperature, up to the melting point of the material, there is an equilibrium concentration (ratio of vacant lattice
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
sites to those containing atoms).
At the melting point of some metals the ratio can be approximately 0.1%.
The creation of a vacancy can be simply modeled by considering the energy required to break the bonds between an atom inside the crystal and its nearest neighbor atoms. Once that atom is removed from the lattice site, it is put back on the surface of the crystal and some energy is retrieved because new bonds are established with other atoms on the surface. However, there is a net input of energy because there are fewer bonds between surface atoms than between atoms in the interior of the crystal.
At any given temperature, the amount of energy needed to create a vacancy is diminished because creating a vacancy disorders the interior of the crystal. The measure of this disorder is called the entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
of the system. Adding vacancies to the material increases the entropy, which tends to reduce the total energy required to create the vacancy. This energy is called the free energy
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...
and this is the energy that is required to create an equilibrium concentration of vacancies at a given temperature.