Phase Separation and Phase Transitions in Multiferroic K_0.58FeF₃ with the Tetragonal Tungsten Bronze Structure

The tetragonal tungsten bronze (TTB) structure multiferroic fluoride K058FeF3 (alternatively K3-xFe5F15 (x approximate to 0.1)) has been studied using variable temperature high-resolution synchrotron powder X-ray diffraction. The as-made sample occurs as a mixture of two closely related TTB phases, one tetragonal (a(TTB) approximate to 12.68 angstrom, C-TTB approximate to 3.98 angstrom) and one orthorhombic (a approximate to 12.70 angstrom, b approximate to 12.66 angstrom, c approximate to 3.98 angstrom). Heating above 340 K produces a single metrically tetragonal phase, but the phase separation reproducibly reappears on cooling. Below 300 K the tetragonal phase undergoes a structural phase transition to a second orthorhombic phase (with a approximate to b approximate to root 2 a(TTB)) and these two different orthorhombic phases continue to coexist down to at least 125 K. These results are discussed in the context of previous studies, which sometimes suggest conflicting structural details within this system.