Antiferrodistortive structural phase transition in the Jahn-Teller system DyKMo2O8. I. Raman scattering and infrared absorption studies

A cooperative Jahn-Teller effect is observed in the orthorhombic layered rare-earth compound DyKMo2O8 at TD=14.3 K. Raman and infrared spectra show a mode near 30 cm-1 at 2 K which decreases in energy in a mean-field fashion with increasing temperature up to TD. The Raman spectra show also a well defined mode at approximately 22 cm-1 which becomes overdamped at 10 K and whose intensity decreases continuously to zero at TD. This soft mode is neither Raman or infrared active above TD. Estimates of the ion-lattice coupling show that a B-symmetry local distortion is favoured, and a comparison of the spectroscopic data with group theoretical predictions suggest that the transition is antiferrodistortive. A two-sublattice Ising model which includes intra- and inter-sublattice effective exchange interactions, J(k) and H(k), is developed and is found to display the two-mode behaviour observed: the lower-energy soft mode follows the order parameter (Sz) whereas the upper mode has the energy of the mean field. A fit to the Raman data yields the result J(0) approximately=H(0)=7.5 cm-1.