Charge Order and Tilt Modulation in Multiferroic K_xM^II_xM^III_1-xF₃ (0.4 < x < 0.6) Transition Metal Fluorides with Tetragonal Tungsten Bronze Structure

Transition metal fluorides with tetragonal tungsten bronze (TTB) structure, having general formula K_xM^II_xM^III_1-xF₃ with 0.4 < x < 0.6 and M belonging to transition metals, have been studied over the past decades because of the frustrated magnetic ordering arising at low temperature. We investigated the structural properties of TTB fluorides by transmission electron microscopy (TEM). The results, coupled with an accurate structure analysis performed by single-crystal XRD on K_0.53FeF₃, revealed ferroelectricity and a unified structural description of these materials, where charge order and tilt modulations coexist, each one with its own periodicity differently commensurate to the conventional TTB cell. Whereas the charge order is peculiar of the mixed valence fluorides, the nature of the tilt modulation is clearly associable to the structural features of TTB niobates, suggesting also for the fluoride bronzes the existence of a generalized ferroelectric-ferroelastic behavior at room temperature.