Spontaneous Magnetodielectric Effect and Its Coupling to the Lattice Dynamics in Fluoroperovskites

Experimental results on temperature dependences of the low-frequency dielectric permittivity of the group of magnetic fluoroperovskites with different crystal and magnetic structures are presented. Orthorhombic NaCoF3 and NaNiF3, cubic RbFeF3, hexagonal RbNiF3 and tetragonal K2CoF4 and K2NiF4 were investigated. The analysis of experimental results in combination with those of our previous studies of other fluoroperovskites was carried out taking into account the influence of the spontaneous magnetodielectric effect on lattice dynamics. It revealed the role of the spin-phonon coupling and the anharmonic contribution, which leads an increase of dielectric permittivity at heating, and contribution of the hidden structural instability manifested as an increase of the dielectric permittivity at cooling. It was established that the relative contributions of these three main mechanisms to the temperature dependence of dielectric permittivity are significantly different in all fluoroperovskites under study but they are well correlated with the tolerance factor t, which characterizes a relation between ionic radii and is a measure of stability of AMF3 perovskite crystal structure. The obtained results and their analysis reliably demonstrate that the low-frequency dielectric spectroscopy is a highly sensitive method to study particular features of the lattice dynamics of fluoroperovskites at magnetic and structural phase transitions.