Spectroscopic data on crystals of rare-earth compounds having structural instability due to the cooperative Jahn–Teller effect are reviewed. Based on an analysis of these data, it is inferred that the dynamic coupling of low-frequency electronic excitations of the rare-earth ions with crystal lattice vibrations plays an essential role in the formation of the low-energy spectra of layered crystals. The role of this coupling in the formation of anomalies of various physical properties of crystals of rare-earth compounds with highly anisotropic structure is examined.

We report a systematic study of low-energy lattice vibrations in the layered systems KY(MoO), KDy(MoO), KEr(MoO), and KTm(MoO). A layered crystal structure and low symmetry of the local environment of the rare-earth ion cause the appearance of vibrational and electronic excitations in Terahertz frequencies. The interaction between these excitations leads to sophisticated dynamical properties, including non-linear effects in paramagnetic resonance spectra. The THz study in magnetic field allows for the clear distinction between lattice vibrations and electronic excitations. We measured the THz transmission spectra and show that the low energy lattice vibrations in binary molybdates can be well described within the quasi-one-dimensional model. The developed model describes the measured far-infrared spectra, and results of our calculations agree with previous Raman and ultrasound studies.

Electron paramagnetic resonance spectra of KTm(MoO 4 ) 2 were measured as a function of magnetic field between 3 and 11.5 cm −1 at T = 2 K. We found that in addition to the absorption line caused by the electronic excitation of Tm 3+ ions, the spectra contain sidebands. Far-infrared transmission measured with polarized light from 10 to 75 cm −1 revealed vibration modes at 16.7 and 25.7 cm −1 for polarizations E ω a and E ω c, respectively. We show that sidebands in the spectra of paramagnetic resonance result from a parametric resonance between the electronic excitations of the Tm 3+ ions and the acoustic vibrations of the crystal lattice.