Phase transitions in fluoride KFe2F6 with tetragonal tungsten bronze structure

•KFe2F6 undergoes two structural P4/mbm → Pbam → G2 and magnetic transformations.•No polar phase and any doubling of unit cell were observed.•Thermal prehistory effects on Fe2+ and Fe3+ distribution and distorted phases sequence.

Heat capacity, thermal dilatation, structure, Mössbauer spectra and dielectric permittivity of fluoride KFe2F6 with tetragonal tungsten bronze crystal structure were studied. The as-made sample undergoes two structural phase transitions P4/mbm (T1 ≈ 340 K) → Pbam (T2 ≈ 250 K) → G2 and magnetic phase transformation at Tm ≈ 133 K. Heating up to 600–700 K and subsequent cooling in helium atmosphere leads to a change of phase transition temperatures and diffused anomalies in thermal expansion. The results obtained and influence of thermal prehistory of the sample on its physical properties are discussed in the context of previous studies on related fluorides KFe2F6, which sometimes suggest conflicting structural details.

Graphical abstractKFe2F6 with tetragonal tungsten bronze structure undergoes two structural phase transitions between nonpolar phases P4/mbm → Pbam → G2 without any doubling of unit cell and one magnetic phase transition.Figure optionsDownload full-size imageDownload as PowerPoint slide