Experimental estimation of the cooperative Jahn-Teller energy in orbitally ordered KCu0.8Mg0.2F3 perovskite.

High resolution synchrotron radiation X-ray powder diffraction was used to investigate the melting of the cooperative Jahn-Teller distortion (cJTd) in a perovskite of composition KCu_0.8Mg_0.2F₃. A first order phase transition relaxing the cJTd is observed at T ∼ 600 K. From the transition temperature, an estimation of kT is derived ( kT = 0.05 eV) for the cJTd in the doped compound. This is the very first observation of cJTd melting in a compound of the series KCu_{1− x} Mg _x F₃. A structural phase diagram for the Cu rich zone of the series is proposed. In principle, the extension of this experimental investigation could be used to disentangle the orbital order (OO) and cJTd energy scales in the parent compound KCuF₃, a test system for OO theories. [ABSTRACT FROM AUTHOR]