Water uptake and energetics of the formation of barium zirconate based multicomponent oxides.

A group of multi-component oxides based on BaZrO3 have been prepared using a solid-state reaction method and examined in terms of their water uptake and thermodynamics of formation. Depending on the type and amount of acceptor substitution, the synthesized compounds exhibit various proton defect concentrations, reaching up to 0.2 mol/mol for a compound containing 10 different elements in the B-sublattice, where 50% of them are acceptors. For the most promising materials, van't Hoff plots were created and the enthalpies and entropies of hydration were calculated. At higher temperatures, these parameters do not differ from the values for the reference yttrium doped barium zirconate. However, at lower temperatures they are more negative, indicating a more exothermic process of proton incorporation.

Energetic Stability and Its Role in the Mechanism of Ionic Transport in NASICON-Type Solid-State Electrolyte Li1+xAlxTi2-x(PO4)3.

We apply high-temperature oxide melt solution calorimetry to assess the thermodynamic properties of the material Li1+xAlxTi2-x(PO4)3, which has been broadly recognized as one of the best Li-ion-conducting solid electrolytes of the NASICON family. The experimental results reveal large exothermic enthalpies of formation from binary oxides (ΔHf,ox°) and elements (ΔHf,el°) for all compositions in the range 0 ≤ x ≤ 0.5. This indicates substantial stability of Li1+xAlxTi2-x(PO4)3, driven by thermodynamics and not just kinetics, during long-term battery operation. The stability increases with increasing Al3+ content. Furthermore, the dependence of the formation enthalpy on the Al3+ content shows a change in behavior at x = 0.3, a composition near which the Li+ conductivity reaches the highest values. The strong correlation among thermodynamic stability, ionic transport, and clustering is a general phenomenon in ionic conductors that is independent of the crystal structure as well as the type of charge carrier. Therefore, the thermodynamic results can serve as guidelines for the selection of compositions with potentially the highest Li+ conductivity among different NASICON-type series with variable dopant contents.

Greigite (Fe3S4) is thermodynamically stable: Implications for its terrestrial and planetary occurrence.

Iron sulfide minerals are widespread on Earth and likely in planetary bodies in and beyond our solar system. Using measured enthalpies of formation for three magnetic iron sulfide phases: bulk and nanophase Fe3S4 spinel (greigite), and its high-pressure monoclinic phase, we show that greigite is a stable phase in the Fe-S phase diagram at ambient temperature. The thermodynamic stability and low surface energy of greigite supports the common occurrence of fine-grained Fe3S4 in many anoxic terrestrial settings. The high-pressure monoclinic phase, thermodynamically metastable below about 3 GPa, shows a calculated negative P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their magnetism may contribute to its present magnetic field.

Conductivity, structure, and thermodynamics of Y2Ti2O7-Y3NbO7 solid solutions.

The defect fluorite yttrium niobate Y3NbO7 and pyrochlore yttrium titanate Y2Ti2O7 solid solutions have been synthesized via a solid state synthesis route. The resulting stoichiometry of the oxides is Y2+xTi2-2xNbxO7, where x = 0 to x = 1. All of the samples were single-phase; however, for those with a predominant fluorite phase, a small amount of additional pyrochlore phase was detected. The volume of the solid solution unit cells linearly increases with increase in yttrium niobate content. The water uptake increases with (x) and the protonic defect concentration reaches almost 4.5 × 10-3 mol mol-1 at 300 °C. The calculated enthalpy of formation from oxides suggests strong stability for all of the compositions, with the values of enthalpy ranging from -84.6 to -114.3 kJ mol-1. The total conductivity does not have a visible dependence on Y3NbO7 content. For each compound, the total conductivity is higher in wet air. Interestingly, for samples where x < 0.5, the ratio of conductivity in hydrogen to air increases with increasing temperature, while for x > 0.5, the trend is the opposite.

Thermochemistry of cation disordered Li ion battery cathode materials, (M' = Nb and Ta, M'' = Mn and Fe).

High temperature oxide melt solution calorimetry studies on (M' = Nb5+, M'' = Mn3+ and Fe3+ and x = 0.20, 0.30 and 0.40) oxides and a new family of Ta containing Li excess disordered cathode materials, (M' = Ta5+, M'' = Fe3+ and x = 0.20, 0.30 and 0.40), synthesized by a rapid quenching method, are reported in this study. The enthalpies of formation determined from high temperature calorimetry studies reveal that the stability of compounds increases with the increasing Li content per formula unit. The reaction between more basic Li2O and acidic transition metal oxides results in the more negative enthalpies of formation for these compounds. The work reveals that the formation enthalpy term plays a more important role in the stabilization of such disordered Li ion materials at room temperature whereas configurational entropy along with lattice entropy (vibrational and magnetic) contributes to the stabilization at high temperature from which the samples are quenched.

Disorder in Ho2Ti2-x Zr x O7: pyrochlore to defect fluorite solid solution series.

Pyrochlore (A2B2O7) is an important, isometric structure-type because of its large variety of compositions and structural derivatives that are generally related to different disordering mechanisms at various spatial scales. The disordering is key to understanding variations in properties, such as magnetic behavior or ionic conduction. Neutron and X-ray total scattering methods were used to investigate the degree of structural disorder in the Ho2Ti2-x Zr x O7 (x = 0.0-2.0, Δx = 0.25) solid solution series as a function of the Zr-content, x. Ordered pyrochlores (Fd3̄m) disorder to defect fluorite (Fm3̄m) via cation and anion disordering. Total scattering experiments with sensitivity to the cation and anion sublattices provide unique insight into the underlying atomic processes. Using simultaneous Rietveld refinement (long-range structure) and small-box refinement PDF analysis (short-range structure), we show that the series undergoes a rapid transformation from pyrochlore to defect fluorite at x ≈ 1.2, while the short-range structure exhibits a linear increase in a local weberite-type phase, C2221, over the entire composition range. Enthalpies of formation from the oxides determined using high temperature oxide melt solution calorimetry support the structural data and provide insight into the effect of local ordering on the energetics of disorder. The measured enthalpies of mixing are negative and are fit by a regular solution parameter of W = -31.8 ± 3.7 kJ mol-1. However, the extensive short-range ordering determined from the structural analysis strongly suggests that the entropies of mixing must be far less positive than implied by the random mixing of a regular solution. We propose a local disordering scheme involving the pyrochlore 48f to 8a site oxygen Frenkel defect that creates 7-coordinated Zr sites contained within local weberite-type coherent nanodomains. Thus, the solid solution is best described as a mixture of two phases, with the weberite-type nanodomains triggering the long-range structural transformation to defect fluorite after accumulation above a critical concentration (50% Ti replaced by Zr).

Energetics of Formation and Disordering in Rare Earth Weberite RE3TaO7 Materials.

The recent finding of local weberite-like ordered domains in disordered and radiation damaged pyrochlore oxides has sparked interest in studying the structure, stability, and order-disorder in compounds that form in the weberite structure. In order to understand the relationships among the energetics, structure, and disordering, weberites of the formula RE3TaO7 (RE = La, Nd, Sm-Yb) were synthesized by conventional solid-state techniques. High temperature oxide melt solution calorimetry was used to determine their enthalpies of formation. Rietveld refinement of PXRD patterns shows that the La compound forms in the weberite La3NbO7 (Cmcm) structure; the Nd compound has both Y3TaO7 (C2221)-type and La3NbO7-type polymorphs; the Sm-Ho compounds crystallize in the weberite Y3TaO7 (C2221) structure; and the Ho-Yb compounds adopt the defect fluorite (Fm3̅m) disordered structure. Depending on the reaction temperature, Ho3TaO7 crystallizes in ordered Y3TaO7 (low temperature) or disordered defect fluorite (high temperature) structures. The formation enthalpy of weberites becomes more exothermic with increasing rare earth ionic radius, implying an increase in stability, i.e., La3TaO7 is most stable and Yb3TaO7 is least stable with respect to the component oxides. The calorimetric data also show that ordered Ho3TaO7 (Y3TaO7 structure) is energetically more stable by 9.2 ± 1.1 kJ/mol than disordered Ho3TaO7 (defect fluorite structure).