Theory on the Possibility of Incommensurate Modulation along an Axis Perpendicular to the x Axis in K2SeO4

Following a previous paper, the present paper also investigates incommensurate modulation in K 2 SeO 4 . While the direction of modulation previously was assumed // x , it presently is assumed ⊥ x . Directions ⊥ x are represented by the y axis for simplicity's sake. In view of the number of equivalent wavevectors, the transition parameter system adopted for the x axis cannot be entirely analogous to that to be adopted for y ; this new system is determined. The thermodynamic potential function is found. A certain number of possible ferroic phases are deduced, which include phases incommensurate along one or both of the x and y axes. The problem “To which of these ferroic phases should the prototypic phase change if the transition is assumed second order?” is solved.