pseudoorthorhombic

Pseudoorthorhombic describes crystals that exhibit approximate orthorhombic symmetry in their geometry or diffraction pattern, but whose true crystal structure does not belong to the orthorhombic system. It is a form of pseudo-symmetry used in crystallography and mineralogy to indicate that the observed symmetry is only near or superficially orthorhombic, with underlying lattice parameters or atomic arrangement deviating from true orthorhombic rules.

Causes and contexts. Pseudo-orthorhombic situations commonly arise when a lower-symmetry lattice (often monoclinic or tetragonal) is distorted slightly. For example, a monoclinic cell with a beta very close to 90 degrees or nearly equal a and b can produce lattice metrics that mimic orthorhombic symmetry. Twinning, microdomain structure, or subtle displacements of atoms can also yield an apparent orthorhombic diffraction pattern even though the true space group is not orthorhombic.

Diagnostic features. Distinguishing pseudoorthorhombic cases requires precise crystallographic analysis. Indicators include small but systematic deviations of angles from 90 degrees, refinement improvements when adopting a lower-symmetry model, and occasional reflection splits or anomalous intensity patterns inconsistent with true orthorhombic symmetry. High-quality single-crystal or powder diffraction, complemented by electron microscopy, helps reveal the underlying lower-symmetry structure.

Relation to practice. Pseudoorthorhombic is not a distinct crystal system but a descriptor that guides correct structure solution and interpretation. Recognizing pseudo-symmetry is important for accurate determination of lattice parameters, space groups, and derived properties, and it often signals subtle distortions, tilting, or phase behavior relevant to the material’s chemistry and physics.