posted on 2004-08-10, 00:00authored bySimone Fabbrici, Erica Montanari, Lara Righi, Gianluca Calestani, Andrea Migliori
Transition metal fluorides with tetragonal tungsten bronze (TTB) structure, having general
formula KxMIIxMIII1-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 K0.53FeF3, 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.