We report a simplification of the Landau theory, permitting more direct prediction of compatible space groups in second-order phase transitions. The theory is applied to the ${\mathrm{V}}_3$Si transition, to predict that the lower symmetry space group is either ${C_{4v}}^7-P4\mathrm{mc}$ or ${D_{2d}}^5-P\overline{4}m2\mathrm{}$. The former possibility admits a polar axis.

• Received 14 October 1966

DOI:https://doi.org/10.1103/PhysRevLett.17.1216