Enthalpy of formation of Ln₂O₂CO₃ II (Ln = La, Nd, Eu) and thermodynamics of decomposition equilibria

Abstract

High temperature oxide melt solution calorimetry using molten 2PbO·B₂O₃ at 1078 K and molten 3Na₂O·4MoO₃ at 976 K has been applied to determine standard enthalpies of formation, Δ_fH_m° (298 K), of Ln₂O₂CO₃ II (Ln = La, Nd, Eu) from their oxide components Ln₂O₃ and CO₂ at 298 K. Two solvents were applied to cross-check the thermodynamic cycles. Using molten 2PbO·B₂O₃, Δ_fH_m°(298 K) for La₂O₂CO₃ II, Nd₂O₂CO₃ II and Eu₂O₂CO₃ II were found as −198.0 ± 1.7, −176.2 ± 3.1 and −151.4 ± 6.5 kJ mol⁻¹, respectively. Using molten 3Na₂O·4MoO₃ the corresponding values for Nd₂O₂CO₃ II and Eu₂O₂CO₃ II were determined as −176.6 ± 3.5 and −154.9 ± 3.2 kJ mol⁻¹, respectively. These enthalpies of formation are discussed in view of prior thermodynamic data, discrepancies are analyzed, and recommended values for enthalpy, entropy and free energy of formation for La₂O₂CO₃ and Nd₂O₂CO₃ are given.

Introduction

Alkali, alkaline earth and rare earth (Ln) metal oxides exhibit basic properties and may react at low or moderate temperatures in a CO₂-containing atmosphere to form carbonate phases. Several Ln₂O₃ bearing phases are important for high T_c superconductors, solid oxide fuel cells, and membranes. Such materials are typically synthesized or find practical applications in CO₂-containing atmospheres, e.g. air. Hence, it is important to have accurate data on the formation, thermal stability and reactivity of relevant carbonate containing phases, which may occur as undesired impurities or as CO₂ corrosion products. Rare earth oxide carbonates, Ln₂O₂CO₃, represent such potential impurities. These are reported in three crystalline modifications I, IA and II [1], [2]. At ambient pressure, type II is stable for lanthanides lighter than Gd [2], whereas types I and IA are considered metastable. For Ln heavier than Gd, only type I can be synthesized at atmospheric pressure [1].

The present contribution is our third investigation in a series dealing with thermodynamic properties of rare earth oxide carbonates Ln₂O₂CO₃ (Ln = La, Nd, Eu), with focus on the stable modification Ln₂O₂CO₃ II. In Ref. [3] we described simple synthesis routes for preparation of crystalline and phase pure samples of rare earth oxide carbonates. Further, the thermal stability of Nd₂O₂CO₃ II was studied by thermogravimetry and isothermal annealing experiments in CO₂-containing atmospheres (≈0.0003–1 atm). On this basis, the standard molar enthalpy and entropy of the decomposition reaction into its corresponding oxides was determined for the temperature region 800–1100 K [3].

From adiabatic shield calorimetry we determined the heat capacities of La₂O₂CO₃ II (12–300 K) and Nd₂O₂CO₃ II (12–930 K) [4]. From the standard entropy of the oxide carbonate and its corresponding component oxides, the entropy of the reaction:

Ln₂O₃ (s) + CO₂ (g) = Ln₂O₂CO₃ II (s)

was determined to be −170.2 ± 0.6 J K⁻¹ mol⁻¹ at 300 K for La₂O₂CO₃ II, and −177.0 ± 0.6 at 300 K and −166.9 ± 0.9 J K⁻¹ mol⁻¹ at 900 K for Nd₂O₂CO₃ II [4]. These entropy values were calculated on the assumption that there is no zero point entropy for either the component oxides or the oxide carbonates.

Based on gas equilibration experiments, Watanabe et al. [5] and Shirsat et al. [6], determined Δ_dH_m°(T) (149 and 145.0 ± 5.0 kJ mol⁻¹, respectively) and Δ_dS_m°(T) (126 and 119.2 ± 5.0 J K⁻¹ mol⁻¹, respectively), for the decomposition of La₂O₂CO₃ II into its corresponding oxides. Shirsat et al. [7], [8] reported later also Δ_dH_m°(T) and Δ_dS_m°(T) for Nd₂O₂CO₃, Sm₂O₂CO₃, Eu₂O₂CO₃ and Gd₂O₂CO₃ determined from gas equilibration studies. Patil et al. [9] and Sastry et al. [10] reported Δ_dH_m°(T) values for several oxide carbonates as derived from DTA. All published thermodynamic data for Ln₂O₂CO₃ are summarized in Table 1. For sake of clarity all data are listed as formation values, even when they are reported as decomposition enthalpies and entropies.

The aim of the present work is to provide enthalpy of formation values of Ln₂O₂CO₃ II (Ln = La, Nd, Eu) from oxide components by high temperature oxide melt solution calorimetry. Since the entropy of formation of La₂O₂CO₃ II and Nd₂O₂CO₃ II is already known from adiabatic shield calorimetry [4], one can with help of the present findings, calculate the Gibbs free energy of La₂O₂CO₃ II and Nd₂O₂CO₃ II, and compare these data with results from gas equilibration studies [3], [5], [6], [7] for the corresponding compounds. The accuracy of various sets of data is discussed and recommended values of enthalpy, entropy, and free energy of formation for La₂O₂CO₃ and Nd₂O₂CO₃ are given.