How to design alloy mixtures using VCA method ||Gaurav Jhaa
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Disorder in solids
Many crystal structures possess static positional disorder. The crystal may either contain fewer atomic species than there are crystallographically equivalent sites to populate, or there might, for example, be statistical occupancy of a given crystallographic site by more than one type of atom in different unit cells. This positional disorder is typically manifested in site occupancy factors of less than unity for the average unit cell.
Materials Studio allows you to model positional disorder by specifying atom occupancy and by defining mixture atoms.
Atom occupancy
The vast majority of atoms will have an occupancy of unity. However, in the case of an atom on a partially occupied interstitial position, you can specify an occupancy ranging from 0.0 to 1.0.
Atom occupancies can be modified in the Properties Explorer.
Mixture atoms
Atomic sites in a crystal can also be described in terms of a hybrid atom that consists of two or more element types. The relative concentrations can be set for any number of atoms, but the total concentration must not exceed 100%. The mixture atoms description is the most often used representation of solid solutions, metallic alloys, disordered minerals, and so on.
Disorder and first principles calculations
There are a number of different approaches for dealing with disorder in first principles calculations:
Virtual crystal approximation (VCA): this offers technically the simplest approach, allowing calculations on disordered systems to be carried out at the same cost as calculations for ordered structures. VCA ignores any possible short range order and assumes that on each potentially disordered site there is a virtual atom which interpolates between the behavior of the actual components. This approach neglects such effects as local distortions around atoms and cannot be expected to reproduce the finer details of the disordered structures very accurately.
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