Unusual structural evolution in KCuF$_3$ at high temperatures by neutron powder diffraction*

High-resolution neutron powder diffraction has been performed to study the structural evolution of the perovskite KCuF$_3$ at temperatures up to 900 K. Results of the Rietveld refinement reveal an unusual site distortion that increases as temperature increases. In contrast to the widely accepted assumption that a cooperative Jahn-Teller transition may occur at 800 K, no phase transition was observed up to 900 K. We have made a comparative study of the Jahn-Teller distortion in fluorides and oxides with variables such as temperature, pressure, and the dilution by non-Jahn-Teller active ions in these compounds.