  We present a detailed study on the charge ordering (CO) transition in
GdBaCo2O5 system by combining high resolution synchrotron powder/single crystal
diffraction with electron paramagnetic resonance (EPR) experiments as a
function of temperature. We found a second order structural phase transition at
TCO=247 K (Pmmm to Pmma) associated with the onset of long range CO. At Tmin =
1.2TCO, the EPR linewidth rapidly broadens providing evidence of spin
fluctuations due to magnetic interactions between Gd3+ ions and
antiferromagnetic couplings of Co2+/Co3+ sublattices. This likely indicates
that, analogously to manganites, the long-range antiferromagnetic order in
GdBaCo2O5 sets in at TCO. Pair distribution function (PDF) analysis of
diffraction data revealed signatures of structural inhomogeneities at low
temperature. By comparing the average and local bond valences, we found that
above TCO the local structure is consistent with a fully random occupation of
Co2+ and Co3+ in a 1:1 ratio and with a complete charge ordering below TCO.
Below T = 100 K the charge localization is partially melted at the local scale,
suggesting a reentrant behavior of CO. This result is supported by the
weakening of superstructure reflections and the temperature evolution of EPR
linewidth that is consistent with paramagnetic (PM) reentrant behavior reported
in the GdBaCo2O5.5 parent compound.
