Pnma

Pnma is the Hermann–Mauguin symbol for space group number 62 in the International Tables for Crystallography. It describes an orthorhombic, primitive lattice with three mutually perpendicular axes (a, b, c) of unequal length. The associated point group is D2h (mmm), indicating three twofold rotation axes along the crystallographic directions, an inversion center, and mirror planes.

The symmetry elements of Pnma include an n-glide plane perpendicular to a, a mirror plane perpendicular to b, and an a glide plane perpendicular to c. These glides, together with the lattice translations and the D2h symmetry, generate the characteristic orthorhombic arrangement. The space group leads to distinct symmetry-related positions within the unit cell and governs the allowed atomic coordinates and reflection conditions observed in X-ray or neutron diffraction.

Pnma is a common setting for many oxide and perovskite-related structures, especially those featuring tilting of octahedra in ABO3 compounds. It is often encountered in materials where distortions from the ideal cubic perovskite framework stabilize an orthorhombic structure at room or moderate temperatures. Notable examples include several lanthanide- and alkaline-earth titanates and ferrites, such as CaTiO3, which adopt the Pnma structure in their room-temperature forms. The space group is also used broadly to describe related minerals and synthetic oxides that exhibit similar symmetry constraints.

In crystallographic practice, Pnma provides a convenient standard setting for indexing diffraction data and for reporting atomic positions in compounds with orthorhombic symmetry that possess the specific combination of glide planes and mirror planes indicated by the symbol.