Synthesis of rare earth oxide carbonates and thermal stability of Nd₂O₂CO₃ II

Abstract

Oxides with practical applications, such as high T_c superconductors, catalysts, solid oxide fuel cells and membranes frequently contain basic cations, which under synthesis or process conditions will be subjected to CO₂-containing atmospheres and a carbonatization degradation may be initiated. In this paper the conditions for synthesis (formation) of potential Ln₂O₂CO₃ degradation products are described for the rare earth oxides Ln₂O₃, Ln = La, Nd. Emphasis is put on describing conditions for the formation of well characterised phase-pure samples of La₂O₂CO₃ (type IA/II), Nd₂O₂CO₃ (type IA/II) and of the solid solution series La_2–xNd_xO₂CO₃ (type II), 0 ≤ x ≤ 2, by means of decomposition studies on rare earth acetates and citrates and by carbonatization studies on the corresponding rare earth oxides.For the calculation of phase stability relationships, thermodynamic data for Ln₂O₂CO₃ are required. Herein, the thermal stability of Nd₂O₂CO₃ II has been studied by means of thermogravimetry and isothermal annealing experiments in atmospheres with various partial pressures of CO₂ (30.4 to 1.01×10⁵ Pa). The experimental results were used to establish the equilibrium pressures of CO₂ for the decomposition reaction<$$>\[ {\rm Nd}_{\rm 2} {\rm O}_{\rm 2} {\rm CO}_{\rm 3}\ {\rm II}\;{\rm (s)} \to {\rm A\hbox-Nd}_{\rm 2} {\rm O}_{\rm 3} \;{\rm (s) + CO}_{\rm 2} \;{\rm (g)} \]<$$>in the temperature region 800–1100 K. The median standard molar enthalpy and entropy of the decomposition reaction are 213 ± 27 kJ mol^–1 and 195 ± 26 J K^–1 mol^–1, respectively. At 298 K Δ_dH_m^o = 221 ± 27 kJ mol^–1 and Δ_dS_m^o = 206 ± 26 J K^–1 mol^–1.

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