PECULIARITIES OF THE LOW-SYMMETRY PHASE-STRUCTURE NEAR THE PHASE-TRANSITION POINT

The structure of the low-symmetry phase near the transition point depe nds on the number of arbitrary parameters, which is smaller than that determined by the space group G(D) of this phase. Such a feature gives rise to some nontrivial structural effects in the vicinity of the tra nsition point: there are numerical relationships among the displacemen ts of the atoms belonging to different orbits of the G(D) group. In so me cases, atoms may be displaced in a direction not singled out by sym metry in any of the 230 space groups. These effects are revealed by gr oup-theoretical methods and the Landau concept of one irreducible repr esentation.