Charge ordering transition in GdBaCo2O5: Evidence of reentrant behavior

Abstract

We present a detailed study on the charge ordering transition in a GdBaCo${}_{2}$O${}_{5.0}$ system by combining high-resolution synchrotron powder/single-crystal diffraction with electron paramagnetic resonance experiments as a function of temperature. We found a second-order structural phase transition at ${T}_{\mathrm{CO}}$ = 247 K (Pmmm to Pmma) associated with the onset of long-range charge ordering. At ${T}_{\mathrm{min}}$ \ensuremath{\approx} 1.2${T}_{\mathrm{CO},}$ the electron paramagnetic resonance linewidth rapidly broadens, providing evidence of antiferromagnetic spin fluctuations. This likely indicates that, analogously to manganites, the long-range antiferromagnetic order in GdBaCo${}_{2}$O${}_{5.0}$ sets in at \ensuremath{\approx}${T}_{\mathrm{CO}}$. Pair distribution function analysis of diffraction data revealed signatures of structural inhomogeneities at low temperature. By comparing the average and local bond valences, we found that above ${T}_{\mathrm{CO}}$ the local structure is consistent with a fully random occupation of Co${}^{2+}$ and Co${}^{3+}$ in a 1:1 ratio and with a complete charge ordering below ${T}_{\mathrm{CO}}$. Below $T$ \ensuremath{\approx} 100 K the charge localization is partially melted at the local scale, suggesting a reentrant behavior of charge ordering. This result is supported by the weakening of superstructure reflections and the temperature evolution of electron paramagnetic resonance linewidth that is consistent with paramagnetic reentrant behavior reported in the GdBaCo${}_{2}$O${}_{5.5}$ parent compound.