Thermochemistry of yavapaiite KFe(SO4)2: Formation and decomposition

Yavapaiite, KFe(SO4)2, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO4)2, ΔH°f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol−1.K−1 and ΔG°f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe2O3 + H2O has been calculated, at pH2O = 1 atm, the phase boundary lies at 219 ± 2°C

Jarosite stability on Mars

Jarosite, a potassium (sodium) iron sulphate hydrated mineral, has recently been identified on the martian surface by the Opportunity rover. Using recent thermochemical data [Drouet and Navrotsky, 2003, Geochim. Cosmochim. Acta 67, 2063–2076; Forray et al., 2005, Geochim. Cosmochim. Acta, in press], we calculate the equilibrium decomposition curve of jarosite and show that it is thermodynamically stable under most present martian pressures and temperatures. Its stability makes jarosite potentially useful to retain textural, chemical, and isotopic evidence of past history, including possible biological activity, on Mars

First-principles computational study of defect clustering in solid solutions of ThO2_{2} with trivalent oxides

The energetics of mixing and defect ordering in solid solutions of fluorite-structured ThO$_{2}$ with oxides of trivalent cations (Sc, In, Y, Nd, La) are investigated by electronic density-functional-theory (DFT). Through DFT calculations of structures enumerated by lattice-algebra techniques, we identify the lowest-energy patterns for defect clustering for four separate dopant concentrations. The most stable structures are characterized by a repulsive interaction between nearest-neighbor vacancies on the oxygen sublattice. The enthalpies of formation with respect to constituent oxides are positive for all dopants considered, and show a tendency to decrease in magnitude as the size and electronegativity of the trivalent dopant decrease. Due to the small positive formation enthalpies and low oxygen-vacancy binding energy with La dopants, La$_{2}$O$_{3}$-ThO$_{2}$ solid solutions are predicted to have relatively high ionic conductivities relative to those for the other aliovalent dopants considered. Our results are compared with those for the more widely studied ZrO$_{2}$ and CeO$_{2}$ fluorite-structured solid solutions with trivalent cations.Comment: 9 pages, 8 figure

Energetics of the Charge-Coupled Substitution Si4+ Na++ T3+ in the-Glasses NaTO2–SiO2 (T = Al, Fe, Ga, B)

Heats of solution in molten 2PbO·B2O3 at 973 K are reported for glasses xNaT3+O2–(1 –x)SiO2 for T = Fe, Ga. These measurements, combined with previous data for T = Al, B, give a relative measure of the enthalpy of the charge-coupled substitution Si4+→ Na++ T3+. The heats of solution become more endothermic with increasing x for x→ 0.5 and exhibit a maximum near x= 0.5. This indicates an exothermic enthalpy for the substitution and an overall stabilization of the glasses. The degree to which the glasses are stabilized decreases in the order Al > Ga > Fe > B. On the basis of molecular orbital calculations, X-ray scattering, and Raman spectroscopy, it is argued that this trend is primarily due to a decrease in the range of energetically favorable T–O–T bond angles as Al, Ga, Fe, and B are substituted for Si

Thermochemistry of iron manganese oxide spinels

Oxide melt solution calorimetry has been performed on iron manganese oxide spinels prepared at high temperature. The enthalpy of formation of (MnxFe1−x)3O4 at 298 K from the oxides, tetragonal Mn3O4 (hausmannite) and cubic Fe3O4 (magnetite), is negative from x=0 to x=0.67 and becomes slightly positive for 0.670.6) spinels of intermediate compositions. The enthalpies of formation are discussed in terms of three factors: oxidation–reduction relative to the end-members, cation distribution, and tetragonality. A combination of measured enthalpies and Gibbs free energies of formation in the literature provides entropies of mixing. ΔSmix, consistent with a cation distribution in which all trivalent manganese is octahedral and all other ions are randomly distributed for x>0.5, but the entropy of mixing appears to be smaller than these predicted values for x<0.4

Carbides and Nitrides of Zirconium and Hafnium.

Among transition metal carbides and nitrides, zirconium, and hafnium compounds are the most stable and have the highest melting temperatures. Here we review published data on phases and phase equilibria in Hf-Zr-C-N-O system, from experiment and ab initio computations with focus on rocksalt Zr and Hf carbides and nitrides, their solid solutions and oxygen solubility limits. The systematic experimental studies on phase equilibria and thermodynamics were performed mainly 40-60 years ago, mostly for binary systems of Zr and Hf with C and N. Since then, synthesis of several oxynitrides was reported in the fluorite-derivative type of structures, of orthorhombic and cubic higher nitrides Zr3N4 and Hf3N4. An ever-increasing stream of data is provided by ab initio computations, and one of the testable predictions is that the rocksalt HfC0.75N0.22 phase would have the highest known melting temperature. Experimental data on melting temperatures of hafnium carbonitrides are absent, but minimum in heat capacity and maximum in hardness were reported for Hf(C,N) solid solutions. New methods, such as electrical pulse heating and laser melting, can fill the gaps in experimental data and validate ab initio predictions

Grain Growth-Controlled Giant Permittivity in Soft Chemistry CaCu3Ti4O12 Ceramics

We report a dielectric constant of up to 5.4105 at room temperature and 1 kHz for CaCu3Ti4O12 (CCTO) ceramics, derived from multiphase powders (coprecipitation products), made by a ‘‘chimie douce’’ (coprecipitation) method, and then sintered in air. The sintered products are pure-phase CCTO ceramics. The high dielectric constant is achieved by tuning the size of grains and the thickness of grain boundaries. The grain growth is controlled by varying the concentration of excess CuO in the initial powder (calcined coprecipitation products) between 1 and 3.1 wt%. The dielectric constant of pure CCTO ceramics increases with the initial CuO concentration, reaching its maximum at 2.4 wt% of CuO. A further increase of excess CuO in powders results in a permittivity decrease, accompanied by the formation of CuO as a separate phase in the sintered products. The unusual grain growth behavior is attributed to a eutectic reaction between CuO and TiO2 present in the initial powder

Probing Capacity Trends in MLi2_2Ti6_6O14_{14} Lithium-Ion Battery Anodes Using Calorimetric Studies

Due to higher packing density, lower working potential, and area specific impedance, the MLi$_2$Ti$_6$O$_{14}$ (M = 2Na, Sr, Ba, and Pb) titanate family is a potential alternative to zero-strain Li$_4$Ti$_5$O$_{12}$ anodes used commercially in Li-ion batteries. However, the exact lithiation mechanism in these compounds remains unclear. Despite its structural similarity, MLi$_2$Ti$_6$O$_{14}$ behaves differently depending on charge and size of the metal ion, hosting 1.3, 2.7, 2.9, and 4.4 Li per formula unit, giving charge capacity values from 60 to 160 mAh/g in contrast to the theoretical capacity trend. However, high-temperature oxide melt solution calorimetry measurements confirm strong correlation between thermodynamic stability and the observed capacity. The main factors controlling energetics are strong acid–base interactions between basic oxides MO, Li$_2$O and acidic TiO$_2$, size of the cation, and compressive strain. Accordingly, the energetic stability diminishes in the order Na$_2$Li$_2$Ti$_6$O$_{14}$ > BaLi$_2$Ti$_6$O$_{14}$ > SrLi$_2$Ti$_6$O$_{14}$ > PbLi$_2$Ti$_6$O$_{14}$. This sequence is similar to that in many other oxide systems. This work exhibits that thermodynamic systematics can serve as guidelines for the choice of composition for building better batteries

Experimental techniques to study structure and thermodynamics at ultra- high temperatures

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