ZhETF, Vol. 167,
No. 3,
p. 312 (March 2025)
(English translation - JETP,
Vol. 140, No. 3,
March 2025
available online at www.springer.com
)
Looking for spectral signatures of dipolar ordering in Li@C60PF6 endofullerene
Zhukov S.S., Melentev A.V., Savinov M., Orekhov N., Zhukova E.S., Suzuki H., Nakano M., Aoyagi S., Gorshunov B.P.
Received: October 11, 2024
DOI: 10.31857/S0044451025030022
Using terahertz (THz) and radio-frequency (RF) spectroscopy, spectra of the complex dielectric permittivity of pressed pellets of metallofullerene 6Li@C60PF6 are measured in the frequency ranges
u =0.3-3THz (10-110cm-1) (T=5-300K) and 1Hz-1MHz (T=0.3-60K). 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 is observed at around the temperature T C=24K 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<
u <0.3 THz. It is shown that the frequency of this band has a V-shaped temperature dependence around the transition temperature T C and that it does not vanish at T C, 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.
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