All isotropy subgroups (and thus all quasicontinuous symmetry changes) corresponding to k→ points of symmetry have been obtained for the 80 diperiodic space groups. The detailed information for such phase transitions is given here for the diperiodic space group P(4/m) (2/m) (2/m). Only two distinct images (sets of representation matrices) occur for this example yielding Landau-Ginzburg-Wilson (LGW) Hamiltonians corresponding to the Ising and XY models, respectively. Minimization of the LGW Hamiltonians yields those transitions which are continuous in the mean-field description.

• Received 3 December 1984

DOI:https://doi.org/10.1103/PhysRevB.31.4350