Charge ordering transition in GdBaCo2O5: evidence of reentrant behavior

TL;DR

This study investigates the charge ordering transition in GdBaCo2O5, revealing a reentrant behavior at low temperatures through diffraction, EPR, and PDF analyses, indicating complex magnetic and structural interactions.

Contribution

It provides the first detailed evidence of reentrant charge ordering behavior in GdBaCo2O5 using combined diffraction, EPR, and local structure analysis.

Findings

Charge ordering occurs at TCO=247 K with a structural phase transition.

Reentrant charge localization is observed below 100 K, indicating partial melting of charge order.

EPR linewidth broadening suggests spin fluctuations and magnetic interactions at low temperatures.

Abstract

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.