Search for Pnma materials with high-temperature structural phase transitions

A systematic search for structures having a high-temperature structural phase transition can be carried out through the identification in the structural databases of those structures that can be considered pseudosymmetric. Pseudosymmetry in a crystal structure indicates the possibility of a similar configuration of higher symmetry. If the distortion relating both structures is small enough, it can be expected that the crystal acquires the more symmetric configuration through a Landau-type phase transition at a higher temperature. Here, we present the results of such a search among inorganic structures with space group Pnma retrieved from the Inorganic Crystal Structure Database. Pseudosymmetry has indeed been detected in those compounds with a known (displacive) Landau-type phase transition at higher temperatures. This is measured by a parameter Δ, which measures the maximal atomic displacement relating the pseudosymmetry-transformed structure and the original one. In most of these compounds with a known phase transition, this parameter was smaller than 1.0 Å 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 Δ distributions in both sets of compounds suggests a smaller Δ window (with 0.7 Å as maximal value) for selecting the materials having maximal probability of exhibiting a phase transition at higher temperatures. A set of 58 compounds fulfils this criterion.