On the question of the incommensurate phase at the cooperative Jahn–Teller ordering in KDy(MoO4)2

It is shown that the temperature anomalies in the fine structure of the absorption spectra and birefringence of KDy(MoO4)2, observed during the phase transition in the region of cooperative Jahn–Teller ordering, correlate with different types of ultrasonic anomalies. These anomalies show us that cooperative ordering may be accompanied by the existence of an intermediate phase. Here, we discuss symmetry considerations to show that an incommensurate phase at cooperative ordering in the layered compound KDy(MoO4)2 is possible. The activity of the irreducible representations of Au-type phonons of the rhombic D2h point group may lead to an incommensurate phase in the temperature region of 12–17 K.