Search for Pnma materials with high-temperature structural phase transitions

A systematic search for structures having a high-temperature structural pha se transition can be carried out through the identification in the structur al databases of those structures that can be considered pseudosymmetric. Ps eudosymmetry in a crystal structure indicates the possibility of a similar configuration of higher symmetry. If the distortion relating both structure s is small enough, it can be expected that the crystal acquires the more sy mmetric configuration through a Landau-type phase transition at a higher te mperature. Here, we present the results of such a search among inorganic st ructures with space group Pnma retrieved from the Inorganic Crystal Structu re Database. Pseudosymmetry has indeed been detected in those compounds wit h a known (displacive) Landau-type phase transition at higher temperatures. This is measured by a parameter Delta, which measures the maximal atomic d isplacement relating the pseudosymmetry-transformed structure and the origi nal one. In most of these compounds with a known phase transition, this par ameter was smaller than 1.0 Angstrom for at least one minimal supergroup of Pnma. The database contains 144 additional structures with pseudosymmetry features under the same quantitative limit. A comparison of the Delta distr ibutions in both sets of compounds suggests a smaller Delta window (with 0. 7 Angstrom as maximal value) for selecting the materials having maximal pro bability of exhibiting a phase transition at higher temperatures. A set of 58 compounds fulfils this criterion.