Secondary order parameters in the structural phase transitions

In the Landau type phase transitions the primary order parameter is related with irreducible representation which subduces the high symmetry space groupG_o to the low symmetry space subgroupG andG_o G. We propose to define the secondary order parameters as irreducible representations which subduceG_o to the subgroupsH_l, whereH_l are simultaneously supergroups forG, i.e.G_o H_l G. From this statement general conclusions are drawn. In particular, for some phase transitions with elargement of the elementary cell, in addition to the superlattice reflections from the primary order parameter, the superlattice reflections from the secondary order parameters can arise in another non-equivalent high symmetry point of the reciprocal space.