Cmcm

Cmcm is the Hermann–Mauguin notation for a crystallographic space group in the orthorhombic system. It is one of the 230 space groups defined in the International Tables for Crystallography. The group describes a C-centered lattice with a specific arrangement of mirror and glide planes that together generate its symmetry operations.

In crystals with Cmcm symmetry, the unit cell is orthorhombic, with three mutually perpendicular axes and all angles at 90 degrees. The C-centering adds lattice translations beyond the primitive lattice, differentiating Cmcm from primitive orthorhombic groups. The symbol indicates the presence of mirror planes and glide planes along the three principal directions, yielding the characteristic symmetry of the space group.

Cmcm is observed in a range of inorganic crystals, including certain oxides, fluorides, and layered materials. It appears in minerals and in crystalline phases of some transition-metal compounds under specific temperature or pressure conditions. In materials science, identifying Cmcm symmetry helps constrain atomic positions during structure refinements and guides computational modeling of crystal structures.

Notes and context: Cmcm is a standard reference in crystallography for describing high-symmetry orthorhombic structures. Determining whether a material crystallizes in this space group requires analysis of diffraction data against the full set of symmetry operations associated with the group. For detailed symmetry operations and International Tables entries, researchers consult the published crystallography references.