Symmetry-restricted phase transitions in two-dimensional solids

Group-theoretical criteria which determine the possibility of group-subgroup phase transitions have been implemented on a computer. Lower-symmetry groups are determined by the subduction and chain criteria. We list all possible such symmetry-restricted transitions in two dimensions corresponding to $\overrightarrow{\mathrm{k}}$ points of symmetry. We indicate relative origins and orientations of the prototype group and subgroup sufficient to obtain a wide class of useful experimental information.