Crystallographic Information File (CIF) Format
A Crystallographic Information File (CIF) is a standard file format used to describe the crystal structure of materials. It is a text-based format that contains a set of data items that describe the crystal structure in detail. The CIF format was developed by the International Union of Crystallography (IUCr) to provide a standardized way of representing crystallographic data. Today, the CIF format is widely used in crystallography and materials science research. It is used to store and exchange crystallographic data between researchers, and it is also used by crystallographic databases to store and distribute crystal structure data. This atomic format is supported by many software packages that are used for crystallographic analysis, making it a versatile and widely adopted format. The CIF format consists of a series of data items, each of which describes a specific aspect of the atomic structure. The data items are arranged in a logical order that reflects the hierarchical structure of the crystallographic information. The data items are grouped into categories, each of which represents a different aspect of the atomic samples. For example, there are categories for unit cell parameters, atomic positions, symmetry operations, and other aspects of the crystal structure. The common representation of CIF files depicted in below figure.
Each data item in the CIF format consists of a tag and a value. The tag is a short text string that identifies the data item, and the value is the actual data that is being described. For example, the data item that describes the unit cell parameters might look like this:
_cell_length_a 5.43
This data item has a tag of “_cell_length_a” and a value of “5.43”, which represents the length of the unit cell along the a-axis. The CIF format also allows for the use of loops, which are used to describe repeating sets of data items. Loops are useful for describing large numbers of atoms or symmetry operations in a compact and efficient way. A loop consists of a set of tags that define the columns of the loop, followed by a series of rows that contain the actual data. For example, a loop that describes the atomic positions might look like this:
loop_
_atom_site_label
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
Si 0.0000 0.0000 0.0000
O1 0.3020 0.3020 0.3020
O2 0.6980 0.6980 0.6980
This loop describes the positions of three atoms in a crystal: one silicon atom and two oxygen atoms. The columns of the loop are defined by the tags “_atom_site_label”, “_atom_site_fract_x”, “_atom_site_fract_y”, and “_atom_site_fract_z”. The rows contain the labels and fractional coordinates of each atom. The CIF format also includes support for comments and whitespace, which can be used to make the file more readable and easier to understand. Comments begin with a “#” and continue to the end of the line. Whitespace can be used to separate data items and make the file easier to read. In addition to describing crystal structures, the CIF format can also be used to describe other types of scientific data, such as spectroscopic data or electron density maps. The flexibility and versatility of the CIF format make it a valuable tool for scientists in many fields.