The cooperative Jahn-Teller ordering investigation by absorption spectra in $KDy(MoO_4)_2$

Absorption spectra fine structure of $KDy(MoO_4)_2$ in the region of cooperative Jahn-Teller type ordering was studied. Temperature anomalies in the spectra occurring at phase transformation correlate with the ultrasound peculiarities observed earlier. Based on the symmetry approaching, possible activity of the irreducible representations of the rhombic $D_{2h}$ point group was discussed, which lead to the incommensurate phase at cooperative ordering. It was supposed, that coupled $A_u$-type phonon mode may lead to the incommensurate phase existence, which is possible at least in the temperature region 17-12 K.