Oxygen order-disorder phase transition in layered GdBaCo2O 5+δ perovskite: Thermodynamic and transport properties

Abstract

The effects of the orthorhombic (O) to tetragonal (T) phase transition around 450/500 C on the thermodynamic and transport properties of the layered cobaltite GdBaCo2O5+δ have been investigated in detail using complementary techniques such as thermo-gravimetry (TG), differential scanning calorimetry (DSC), dilatometry, angle dispersive as well as energy dispersive high temperature X-ray diffraction (HT-XRD and ED-XRD). Experimental data show that the O/T transition temperature shifts to higher values when the pO2 of the experiment increases. The equilibrium pO2 data shows the presence of a narrow two-phase field around 5 + δ ≈ 5.42. The presence of a two-phase field together with an enthalpy change (â̂†H ≈ 4 J/g) and HT-XRD data showing coexistence of both phases confirms that this phase transition is first-order type. Oxygen partial molar enthalpy and entropy of both phases, the "ordered" O and the "disordered" T, are obtained from TG under controlled pO 2, and yield lower entropy and enthalpy values for the ordered O phase. The O/T phase transition at constant temperature involves a volume contraction and a density change of 0.3%. Isotherms of electrical conductivity as a function of pO2 show that this compound exhibits itinerant charge carrier behavior regardless of the crystal symmetry. The charge carrier mobility decreases for the T phase, which might be due to the shortening of CoOCo distance along a-direction.