Alexandra Navrotsky

Thermodynamics of manganese oxides: Sodium, potassium, and calcium birnessite and cryptomelane

Significance Manganese oxides with layer and tunnel structures occur widely in the natural environment and inspire technological applications. In addition to having variable compositions, these structures often are found as small particles (nanophases). This study explores, using experimental thermochemistry, the role of composition, oxidation state, structure, and surface energy in the thermodynamic stability of synthetic birnessites and cryptomelane. These quantitative thermodynamic findings help explain and support qualitative observations of phase-transformation patterns gathered over the past five decades. Manganese oxides with layer and tunnel structures occur widely in nature and inspire technological applications. Having variable compositions, these structures often are found as small particles (nanophases). This study explores, using experimental thermochemistry, the role of composition, oxidation state, structure, and surface energy in the their thermodynamic stability. The measured surface energies of cryptomelane, sodium birnessite, potassium birnessite and calcium birnessite are all significantly lower than those of binary manganese oxides (Mn3O4, Mn2O3, and MnO2), consistent with added stabilization of the layer and tunnel structures at the nanoscale. Surface energies generally decrease with decreasing average manganese oxidation state. A stabilizing enthalpy contribution arises from increasing counter-cation content. The formation of cryptomelane from birnessite in contact with aqueous solution is favored by the removal of ions from the layered phase. At large surface area, surface-energy differences make cryptomelane formation thermodynamically less favorable than birnessite formation. In contrast, at small to moderate surface areas, bulk thermodynamics and the energetics of the aqueous phase drive cryptomelane formation from birnessite, perhaps aided by oxidation-state differences. Transformation among birnessite phases of increasing surface area favors compositions with lower surface energy. These quantitative thermodynamic findings explain and support qualitative observations of phase-transformation patterns gathered from natural and synthetic manganese oxides.

Structure and Thermochemistry of Perrhenate Sodalite and Mixed Guest Perrhenate/Pertechnetate Sodalite

Treatment and immobilization of technetium-99 (99Tc) contained in reprocessed nuclear waste and present in contaminated subsurface systems represents a major environmental challenge. One potential approach to managing this highly mobile and long-lived radionuclide is immobilization into micro- and meso-porous crystalline solids, specifically sodalite. We synthesized and characterized the structure of perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, and the structure of a mixed guest perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)2-x(TcO4)x. Perrhenate was used as a chemical analogue for pertechnetate. Bulk analyses of each solid confirm a cubic sodalite-type structure (P4̅3n, No. 218 space group) with rhenium and technetium in the 7+ oxidation state. High-resolution nanometer scale characterization measurements provide first-of-a-kind evidence that the ReO4- anions are distributed in a periodic array in the sample, nanoscale clustering is not observed, and the ReO4- anion occupies the center of the sodalite β-cage in Na8[AlSiO4]6(ReO4)2. We also demonstrate, for the first time, that the TcO4- anion can be incorporated into the sodalite structure. Lastly, thermochemistry measurements for the perrhenate sodalite were used to estimate the thermochemistry of pertechnetate sodalite based on a relationship between ionic potential and the enthalpy and Gibbs free energy of formation for previously measured oxyanion-bearing feldspathoid phases. The results collected in this study suggest that micro- and mesoporous crystalline solids maybe viable candidates for the treatment and immobilization of 99Tc present in reprocessed nuclear waste streams and contaminated subsurface environments.

Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scattering study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5u2009kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7 fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. The results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination.

Thermal annealing of natural, radiation-damaged pyrochlore

Abstract Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400–1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1·1018 α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6·1018 dpg), and Blue River/Canada (10.5 wt% U, 115.4·1018 dpg), are compared with a crystalline reference pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28%, Blue River 85% and Miass 100% according to XRD), as the recrystallization temperature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm−1 are sensitive to radiation damage (M. T. Vandenborre, E. Husson, Comparison of the force field in various pyrochlore families. I. The A2B2O7 oxides. J. Solid State Chem. 1983, 50, 362, S. Moll, G. Sattonnay, L. Thomé, J. Jagielski, C. Decorse, P. Simon, I. Monnet, W. J. Weber, Irradiation damage in Gd2Ti2O7 single crystals: Ballistic versus ionization processes. Phys. Rev. 2011, 84, 64115.), the degree of local order was deduced from the ratio of the integrated intensities of the sum of the Raman bands between 605 and 680 cm−1 divided by the sum of the integrated intensities of the bands between 810 and 860 cm−1. The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short- (Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In contrast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm−1 resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon annealing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravimetry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1%, Blue River ca. 1.5%, Miass ca. 2.9%).

Structure and thermodynamic stability of UTa3O10, a U(V)-bearing compound

Heating a mixture of uranyl(vi) nitrate and tantalum(v) oxide in the molar ratio of 2u2009:u20093 to 1400 °C resulted in the formation of a new compound, UTa3O10. The honey colored to yellow brown crystals of UTa3O10 crystallize in an orthorhombic structure with the space group Fddd (no. 70), lattice parameters a = 7.3947(1), b = 12.7599(2), c = 15.8156(2) Å, and Z = 8. Vertex sharing [TaO6]7- octahedra of two crystallographically distinct Ta cations form a three dimensional tantalate framework. Within this framework, six membered rings of [TaO6]7- octahedra are formed within the (001) plane. The center of these rings is occupied by the uranyl cations [UO2]+, with an oxidation state of +5 for uranium. The pentavalence of U and Ta was confirmed by X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy. The enthalpy of formation of UTa3O10 from Ta2O5, β-U3O7, and U3O8 has been determined to be 13.1 ± 18.1 kJ mol-1 using high temperature oxide melt solution calorimetry with sodium molybdate as the solvent at 700 °C. The close to zero enthalpy of formation of UTa3O10 can be explained by closely balanced structural stabilizing and destabilizing factors, which may also apply to other UM3O10 compounds.

Thermodynamic Properties of Polymorphs of Fluorosulfate Based Cathode Materials with Exchangeable Potassium Ions

FeSO4 F-based frameworks have recently emerged as attractive candidates for alkali insertion electrodes. Mainly owing to their rich crystal chemistry, they offer a variety of new host structures with different electrochemical performances and physical properties. In this paper we report the thermodynamic stability of two such K-based FeSO4 F host structures based on direct solution calorimetric measurements. KFeSO4 F has been reported to crystallize in two different polymorphic modifications-monoclinic and orthorhombic. The obtained enthalpies of formation from binary components (KF plus FeSO4 ) are negative for both polymorphs, indicating that they are thermodynamically stable at room temperature, which is very promising for the future exploration of sulfate based cathode materials. Our measurements show that the low-temperature monoclinic polymorph is enthalpically more stable than the orthorhombic phase by ≈10u2005kJu2009mol-1 , which is consistent with the preferential formation of monoclinic KFeSO4 F at low temperature. Furthermore, observed phase transformations and difficulties in the synthesis process can be explained based on the obtained calorimetric results. The KMnSO4 F orthorhombic phase is more stable than both polymorphs of KFeSO4 F.