Energetics of cation mixing in urania-ceria solid solutions with stoichiometric oxygen concentrations

Cation mixing energetics in urania-ceria solid solutions with stoichiometric oxygen concentrations (U(1-y)Ce(y)O(2)) have been measured by high-temperature oxide-melt drop-solution calorimetry. Measurements have been performed on eight samples with compositions spanning y = 0.119 to y = 0.815. The measured mixing enthalpies (ΔH(mix)) range from -0.6 ± 3.3 to 3.9 ± 3.0 kJ mol(-1). These values are discussed in the context of results from atomistic modeling which take into consideration the possibility of charge transfer between uranium and cerium cations to form solid solutions with mixed charge states. A comparison between measured and calculated results for ΔH(mix) suggests that such charge transfer takes place to a limited extent in the most concentrated mixtures studied.     

Pyrochlore and Perovskite Potassium Tantalate: Enthalpies of Formation and Phase Transformation

Alkali niobates and tantalates are currently important lead‐free functional oxides. The formation and decomposition energetics of potassium tantalum oxide compounds (K₂O?Ta₂O₅) were measured by high‐temperature oxide melt solution calorimetry. The enthalpies of formation from oxides of KTaO₃ perovskite and defect pyrochlores with K/Ta ratio of less than 1 stoichiometry—K_0.873Ta_2.226O₆, K_1.128Ta_2.175O₆, and K_1.291Ta_2.142O₆—were experimentally determined, and the values are (?203.63±2.92) kJ mol^?1 for KTaO₃ perovskite, and (?339.54±5.03) kJ mol^?1, (?369.71±4.84) kJ mol^?1, and (?364.78±4.24) kJ mol^?1, respectively, for non‐stoichiometric pyrochlores. That of stoichiometric defect K₂Ta₂O₆ pyrochlore, by extrapolation, is (?409.87±6.89) kJ mol^?1. Thus, the enthalpy of the stoichiometric pyrochlore and perovskite at K/Ta=1 stoichiometry are equal in energy within experimental error. By providing data on the thermodynamic stability of each phase, this work supplies knowledge on the phase‐formation process and phase stability within the K₂O?Ta₂O₅ system, thus assisting in the synthesis of materials with reproducible properties based on controlled processing. Additionally, the relation of stoichiometric and non‐stoichiometric pyrochlore with perovskite structure in potassium tantalum oxide system is discussed.     

Oxide melt solution calorimetry of rare earth oxides

Key to the evaluation of the long-term durability and reactivity of REE-materials is the accurate determination of their thermodynamic properties. High-temperature oxide melt solution calorimetry offers an effective methodology for the determination of enthalpies of formation for REE-materials. Calorimetric techniques and crosschecks are summarized that demonstrate that consistent and correct thermodynamic data can be generated using lead borate and sodium molybdate solvents. A summary of recent calorimetric studies of REE-orthophosphates and REE-oxyapatites illustrates the successful application of our techniques in quantifying formation enthalpies and elucidating energetic trends. This revised version was published online in August 2006 with corrections to the Cover Date.     

Energetics of copper diphosphates – Cu2P2O7 and Cu3(P2O6OH)2

Differential scanning calorimetry and high temperature oxide melt solution calorimetry are used to study enthalpy of phase transition and enthalpies of formation of Cu₂P₂O₇ and Cu₃(P₂O₆OH)₂. α-Cu₂P₂O₇ is reversibly transformed to β-Cu₂P₂O₇ at 338–363 K with an enthalpy of phase transition of 0.15 ± 0.03 kJ mol⁻¹. Enthalpies of formation from oxides of α-Cu₂P₂O₇ and Cu₃(P₂O₆OH)₂ are −279.0 ± 1.4 kJ mol⁻¹ and −538.8 ± 2.7 kJ mol⁻¹, and their standard enthalpies of formation (enthalpy of formation from elements) are −2096.1 ± 4.3 kJ mol⁻¹ and −4302.7 ± 6.7 kJ mol⁻¹, respectively. The presence of hydrogen in diphosphate groups changes the geometry of Cu(II) and decreases acid–base interaction between oxide components in Cu₃(P₂O₆OH)₂, thus decreasing its thermodynamic stability.     

Iron ore sintering

Differential scanning and high temperature reaction calorimetry have been used to characterize a series of natural iron ore and flux samples commonly used during iron ore sintering. Most iron ore samples were shown to contain measurable quantities of goethite, with a characteristic dehydration peak in DSC and TG between 200 and 400°C. At higher temperatures, all samples decomposed to produce magnetite with an accompanying mass loss in the TG profile due to the evolution of oxygen. High temperature reaction calorimetry has been used to measure the heat of solution of the ore in the melt formed during iron ore sintering. The dehydration and calcination of iron ore and flux samples was also examined using high-temperature reaction calorimetry. The results support the DSC/TG findings.     

Thermodynamics of CoAl2O4-CoGa2O4 solid solutions

CoAl₂O₄, CoGa₂O₄, and their solid solution Co(Ga_zAl_1−z)₂O₄ have been studied using high temperature oxide melt solution calorimetry in molten 2PbO·B₂O₃ at 973 K. There is an approximately linear correlation between lattice parameters, enthalpy of formation from oxides, and the Ga content. The experimental enthalpy of mixing is zero within experimental error. The cation distribution parameters are calculated using the O’Neill and Navrotsky thermodynamic model. The enthalpies of mixing calculated from these parameters are small and consistent with the calorimetric data. The entropies of mixing are calculated from site occupancies and compared to those for a random mixture of Ga and Al ions on octahedral site with all Co tetrahedral and for a completely random mixture of all cations on both sites. Despite a zero heat of mixing, the solid solution is not ideal in that activities do not obey Raoult's Law because of the more complex entropy of mixing.