Volume 167, Nº 3 (2025)

Using terahertz (THz) and radio-frequency (RF) spectroscopy, spectra of the complex dielectric permittivity ε∗(ν) = ε'(ν) + iε''(ν) of pressed pellets of metallofullerene Li@C60PF6 are measured in the frequency ranges ν =0.3–3 THz (10–110 cm−1) (T =5–300 K) and 1 Hz–1 MHz (T =0.3–60 K). In the THz region, a temperature-unstable absorption band is observed and two possible interpretations of its origin are presented, based either on the quantized rotation of the Li ion within the C60 cage or on the hopping of Li over minima in the multi-well localizing potential. A pronounced maximum in the temperature dependence of the real RF permittivity ε'(T ) is observed at around the temperature TC =24 K of a previously discovered phase transition of presumably antiferroelectric type. The maximum is attributed to the temperature evolution of a soft relaxational band lying outside our working frequency intervals — at 1 MHz < ν < 0.3 THz. It is shown that the frequency of this band has a V-shaped temperature dependence around the transition temperature TC and that it does not vanish at TC, both features typically observed for the ferroelectric order-disorder phase transitions. It is concluded that dielectric measurements of Li@C60PF6 in the microwave frequency range are required in order to obtain detailed information on the parameters and on the temperature behavior of the soft relaxation, which will provide information on the microscopic origin of the phase transition.

Using the single-particle approximation we calculate the envelopes of the spectral lines of terahertz absorption and inelastic neutron scattering spectra corresponding to magnetic dipole transitions between the sublevels of Tb3+ ions in the Tb2Ti2O7 crystal, split by the field of random deformations induced by point defects of the crystal lattice upon violation of the stoichiometric composition of the crystal.