Structural properties and multiferroic phase diagram of K0.6Fe0.6IIFe0.4IIIF3 fluoride with TTB structure

Iron fluorides with tetragonal tungsten bronze (TTB) structure, having general formula of (KxFexFe1-xF3)-Fe-II-F-III with 0.4<0.6, are possible candidates for complete multiferroism (elastic, electric, and magnetic orders) coupled with charge ordering phenomena. Following the indication of a previous general work, we concentrated our attention on the potassium rich region of these bronzes where an orthorhombic distortion of the fundamental TTB structure takes place. We tried to define the complex phase diagram of K0.6FeF3, involving the different ordering phenomena in the range of 5-600 K, by using a series of characterization techniques such as transmission electron microscopy, x-ray and neutron diffractions, and magnetic measurements. The results revealed the coexistence of all these orders below the magnetic transition occurring at 118 K, showing the unique behavior of a system that can be considered a model for multiferroism.