Pbnm

Pbnm is a space-group designation used in crystallography to describe the symmetry of certain crystalline materials. It denotes an orthorhombic, primitive lattice and is commonly associated with tilted-octahedron distortions found in many perovskite-related oxides. In practice, Pbnm is an alternative setting for the same underlying symmetry as Pnma, obtained by a permutation of the crystallographic axes. Because of this, many resources that catalog space groups name the same symmetry as Pnma (No. 62), while others use Pbnm depending on the preferred axis convention. Thus Pbnm and Pnma are symmetry-equivalent descriptions.

The Pbnm setting is widely used to describe the orthorhombic distortions of perovskites with tilted BO6 octahedra, such as certain rare-earth titanates and related oxides. The symmetry captures the splitting of the cubic parent structure into three unequal lattice parameters (a, b, c) and the associated tilting patterns of octahedra, which influence physical properties like ferroelectricity, magnetism, and electronic structure in these materials.

In terms of practical use, Pbnm appears in structural reports, databases, and CIF files, especially for compounds where the conventional perovskite cubic setting would not reflect the observed orthorhombic cell. It is one of several orthorhombic space-group settings that crystallographers choose based on conventional lattice parameters or historical reasons. When comparing literature, it is important to note whether the axis labeling corresponds to Pbnm or its Pnma equivalent to avoid misinterpretation of symmetry-related parameters.