pseudocubic

Pseudocubic describes a crystal lattice or crystal system that is nearly cubic in its metric properties but not perfectly cubic due to small distortions. In such crystals the lattice parameters are very close to equal and the interaxial angles near 90 degrees, yet the true symmetry is lower than the cubic group. The term is commonly used in the study of perovskites and related oxides where the idealized cubic ABX3 structure is distorted by tilting of the octahedra, resulting in a lower-symmetry space group such as tetragonal, orthorhombic, or rhombohedral.

In diffraction analysis the pseudocubic cell is a conventional, nearly cubic representation that facilitates comparison with the cubic perovskite archetype even when the true cell is distorted. The pseudocubic parameters are defined from the actual lattice constants through geometric relationships; a pseudocubic parameter is derived so that three such vectors form a near-cubic cube, and deviations are quantified by the differences among a, b, c and by the unit-cell angles.

Pseudocubic descriptions are particularly common for materials that crystallize in tilted perovskite–related structures, where small octahedral tilts lead to metric near-equality but lower symmetry. They are also used to report “pseudo-cubic” phases that appear cubic over certain temperature or compositional ranges, yet reveal subtle splitting of reflections upon high-resolution diffraction.

Limitations: the term is descriptive rather than a strict crystallographic designation; it signals approximate symmetry rather than a unique space group. Researchers specify the actual space group and lattice parameters alongside the pseudocubic description.