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.

The magnetostriction phenomenon, which exists in almost all magnetically ordered materials, is proved to have wide application potential in precision machinery, microdisplacement control, robotics, and other high‐tech fields. Understanding the microscopic mechanism behind the magnetostrictive properties of magnetically ordered compounds plays an essential role in realizing technological applications and helps the fundamental understanding of magnetism and superconductivity. In paramagnets, however, the magnetostriction is usually significantly smaller because of the magnetic disorder. Here, the observation of a remarkably strong magnetostrictive response of the insulator paramagnet KEr(MoO4)2 is reported on. Using low‐temperature magnetization and dilatometry measurements, in combination with ab initio calculations, employing a quasi‐atomic treatment of many‐body effects, it is demonstrated that the magnetostriction anomaly in KEr(MoO4)2 is driven by a single‐ion effect. This analysis reveals a strong coupling between the Er3+ ions and the crystal lattice due to the peculiar behavior of the magnetic quadrupolar moments of Er3+ ions in the applied field, shedding light on the microscopic mechanism behind the massive magnetostrictive response.

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.