Peculiarities of the low-symmetry phase structure near the phase-transition point

Abstract

The structure of the low-symmetry phase near the transition point depends on the number of arbitrary parameters, which is smaller than that determined by the space group GD of this phase. Such a feature gives rise to some nontrivial structural effects in the vicinity of the transition point: there are numerical relationships among the displacements of the atoms belonging to different orbits of the GD group. In some cases, atoms may be displaced in a direction not singled out by symmetry in any of the 230 space groups. These effects are revealed by group-theoretical methods and the Landau concept of one irreducible representation.