Correct choice of superspace group for an incommensurate phase transition
Abstract
The Landau theory of second-order phase transitions imposes a precise symmetry requirement on the phase at lower temperature. It is shown how this affects the assignment of superspace groups for incommensurate phases reached by one second-order transition from a disordered state. It is noted that the symmetry requirement appears to have been overlooked in several cases when superspace groups were assigned by other authors. For those materials, namely NaNO2, biphenyl and Rb2ZnBr4(CI4), the correct choice of superspace group is discussed.