Anomalous temperature behavior of dielectric permittivity of magnetic fluoroperovskites as a prove of their intrinsic structural instability
The search for new multiferroics and magnetoelectrics with efficient susceptibilities is one of the hottest topics in the today’s condensed matter physics and material science. This task requires deeper understanding of the microscopical mechanisms of interaction between the lattice, orbital, and spin systems, and therefore combined efforts of both experimentalists and theoreticians are due. Here we report and analyze results of the dielectric permittivity studies of a group of intrinsically unstable antiferromagnetic and diamagnetic fluoroperovskites. The group includes four cobalt fluoroperovskites KCoF3, RbCoF3, NaCoF3, and K2CoF4; the first two compounds experience antiferromagnetic and structural phase transitions, the last two only antiferromagnetic transition. For comparison, we also studied KNiF3 which is antiferromagnetic below TN = 246 K; no phase transitions take place in KMgF3 and KZnF3. The dielectric permittivity was studied as a function of temperature in the range of 4 – 350 K and in frequency range of 1 kHz – 1 MHz. Remarkably, all the studies compounds reveal distinctly different behavior. In KCoF3, a noticeable increase of the dielectric permittivity of about 3.3% was observed when lowering the temperature. Moreover, only in KCoF3, this behavior was superimposed by a broad peak near TN = 115 K which we attribute to the previously observed strong magnon-phonon interaction in the vicinity of Brillouin zone center. Surprisingly, a very different temperature behavior of the dielectric permittivity was observed in RbCoF3 with magnetic and crystallographic properties very similar to those of KCoF3. In antiferromagnetic KNiF3, and diamagnetic KMgF3 and KZnF3, a similar behavior was observed at low temperature with an increase of the dielectric permittivity of about 1-3%. We attribute the steady increase of the dielectric permittivity to the growth of the lattice instability of cubic fluoroperovskites at low temperature. Distinctly different behavior was observed in the orthorhombic NaCoF3 and uniaxial K2CoF4 and this allowed us to conclude that the lattice instability is suppressed when the cubic structure of fluoroperovskites is broken. Consequently, no increase of the dielectric permittivity at low temperature found. We discuss recent ab initio calculations of potential multiferroics among fluoroperovskites. We consider that our findings may serve for better understanding of involved microscopic mechanism of multiferroicity and should help in efforts of searching pathways for creating new multiferroics among fluoroperovskites and other materials.