Lattice dynamics and microscopic mechanisms of the spontaneous magnetodielectric effect in the antiferromagnetic fluoroperovskites KCoF3 and RbCoF3

Experimental studies demonstrate that the low-frequency dielectric permittivity of many magnetic materials is sensitive to the spin ordering, but the microscopic mechanisms of the observed phenomena still remain poorly understood. Here we report on the study of lattice dynamics and the spontaneous magnetodielectric effect using far-infrared and low-frequency dielectric spectroscopy in the model fluoroperovskites ${\mathrm{KCoF}}_{3}$ and ${\mathrm{RbCoF}}_{3}$ in the temperature range of 5--300 K, which includes the antiferromagnetic transition at ${T}_{N}=115$ K and 101 K, respectively. We show that the dielectric permittivity is mainly defined by the transverse TO and longitudinal LO infrared-active phonons and their specific contributions were determined. The anomalous growth of the low-frequency dielectric permittivity observed in ${\mathrm{KCoF}}_{3}$ with cooling is explained by the increase of the $\text{LO-TO}$ splitting of the lowest frequency phonon caused by about $7\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ softening. An important conclusion is that, microscopically, the spontaneous magnetodielectric effect is caused by the frequency shifts of only those TO and LO phonons which change the ${180}^{\ensuremath{\circ}}$ angle of the superexchange ${\mathrm{Co}}^{2+}\phantom{\rule{0.16em}{0ex}}\ensuremath{-}\phantom{\rule{0.16em}{0ex}}{\mathrm{F}}^{1\ensuremath{-}}\phantom{\rule{0.16em}{0ex}}\ensuremath{-}\phantom{\rule{0.16em}{0ex}}{\mathrm{Co}}^{2+}$ pathway, thus resulting in its modulation due to the spin-phonon coupling. The observed anomalous softening of the lowest frequency phonon and increase of the low-frequency dielectric permittivity were interpreted as a manifestation of the ferroelectric instability in cubic fluoroperovskites.