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 CO ₂ -containing atmospheres and a carbonatization degradation may be initiated. In this paper the conditions for synthesis (formation) of potential Ln ₂ O ₂ CO ₃ degradation products are described for the rare earth oxides Ln ₂ O ₃ , Ln = La, Nd. Emphasis is put on describing conditions for the formation of well characterised phase-pure samples of La ₂ O ₂ CO ₃ (type IA/II), Nd ₂ O ₂ CO ₃ (type IA/II) and of the solid solution series La _2–x Nd _x O ₂ CO ₃ (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 Ln ₂ O ₂ CO ₃ are required. Herein, the thermal stability of Nd ₂ O ₂ CO ₃ II has been studied by means of thermogravimetry and isothermal annealing experiments in atmospheres with various partial pressures of CO ₂ (30.4 to 1.01×10 ⁵  Pa). The experimental results were used to establish the equilibrium pressures of CO ₂ 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 _d H _m ^o = 221 ± 27 kJ mol ^–1 and Δ _d S _m ^o = 206 ± 26 J K ^–1  mol ^–1 .