Ferroelectric Phase Transition in Pb$_{5}$Cr$_{3}$F$_{19}$ and Coupling of Electric Polarization and Magnetization

The ferroelectric fluoride Pb$_{5}$Cr$_{3}$F$_{19}$ with ferroelectric/paraelectric phase transition at 545 K offers a possibility of multiferroic behavior. The paramagnetic Cr$^{3+}$ ion with electronic spin 3/2 has two inequivalent positions in the unit cell and is responsible for magnetic properties. These properties were measured with a SQUID magnetometer from 2 K to 630 K in addition to our earlier EPR measurements. At the ferroelctric/paraelectric phase transition the lattice parameters (c and a unit cell dimensions) experience relatively big changes leading to alteration of magnetic dipole-dipole and exchange interactions. The temperature dependence of magnetic susceptibility times temperature around the phase transitioin was analyzed following the usual free energy expansion. We obtained that a coupling between the electric polarization and the magnetization is quadratic. A magnetic anomaly was observed below 25 K.