Concentration dependence of the modulation parameter with the lock-in phase transition in the system K₂Mo_xW_1−xO₄

An improved method for synthesizing the compounds K₂Mo_xW_1−xO₄ has been applied. The phase diagram, both on heating and cooling, is presented. The observed nearly linear character of the concentration dependence of the modulation parameter p at 693 K (both after heating and cooling) indicates the existence of a continuous series of incommensurate structures in the concentration interval 0 ≤ x ≤ 1. Either at x = 0 exactly or close to this value a lock-in phase transition takes place, making the structure commensurately modulated.