Partial Molar Enthalpy Of Oxygen Research Articles

Defect-Induced Properties and Thermodynamics of La0.5Ba0.5CoO3–δ

The defect structure model of La0.5Ba0.5CoO3–δ was successfully verified using a combined set of coulometric and thermogravimetric data. The results were used to model the dependences of Seebeck coefficient, measured vs T and pO2 simultaneously with the total conductivity, and chemical strain of this oxide. The partial molar enthalpy of oxygen in La0.5Ba0.5CoO3–δ was calculated from the defect structure model and used to evaluate the enthalpy increments and high-temperature heat capacity of an oxide with constant chemical composition, La0.5Ba0.5CoO2.99, from the drop calorimetric measurement results in air. The standard enthalpies of formation of cubic La0.5Ba0.5CoO2.95 and double LaBaCo2O5.90 perovskites, and the enthalpy of cation ordering in A-sublattice of La0.5Ba0.5CoO2.95, were found using the results of solution calorimetry at 298.15 K. With the data obtained, we predicted the chemical compatibility of La0.5Ba0.5CoO3–δ with solid oxide fuel cell (SOFC) electrolyte materials.

Defect chemistry and high-temperature thermodynamics of PrBaCo2O6-δ

Using the drop calorimetry and differential scanning calorimetry (DSC), the enthalpy increments of pure PrBaCo2O6-δ double perovskite in air in the temperature range from 365.3 K to 1272.8 K were obtained. As-measured enthalpy increments corresponded to the constant pO2, but variable oxygen content in PrBaCo2O6-δ (isodynamical conditions). The defect structure analysis yielded the analytical expressions for the equilibrium oxygen partial pressure, log(pO2) = f(δ, T), and the partial molar enthalpy of oxygen, Δh¯O = f(T, δ) in PrBaCo2O6-δ, both functions being valid in relatively wide temperature range. The value of Δh¯O at 1273 K was assessed independently by means of direct oxidation of PrBaCo2O6-δ in the DSC setup, and was found to agree well with Δh¯O calculated from the defect structure model and pO2 – T – δ diagram. The knowledge of Δh¯O = f(T, δ) allowed recalculating the enthalpy increments for the sample with constant oxygen content, Δ298.15TH∘(PrBaCo2O5.77), which were fitted with a modified Einstein function. The smoothed values of Δ298.15TH∘, heat capacity Cp(T), and entropy increments Δ298.15TS∘ were evaluated and tabulated for PrBaCo2O5.77 in the temperature range 350 ≤ T/K ≤ 1300. The results obtained can be used for calculating Δ298.15TH∘, Cp(T), and Δ298.15TS∘ for PrBaCo2O6-δ with an arbitrary constant oxygen content.

Defect structure and redox energetics of NdBaCo2O6-δ

The oxygen content in the double perovskite NdBaCo2O6-δ was measured as a function of oxygen partial pressure (1–10−5 atm) at different temperatures (623–1323 K) using thermogravimetry analysis (TG) and coulometric titration technique. The defect structure model of the oxide studied was proposed and successfully verified using the pO2 – T – δ data, yielding the enthalpies and entropies of the defect reactions involved. Standard enthalpy of formation from elements (ΔfH298∘) was measured for quenched NdBaCo2O6-δ samples with different oxygen content. Regarding the ΔfH298∘(6-δ) dependences, two important observations were made. First, the ΔfH298∘(6-δ) consists of two linear fragments with different slopes reflecting different modes of oxygen exchange, accompanied by either reduction of cobalt from Co+3 to Co+2 or from Co+4 to Co+3 in the ranges 5.0 ≤ 6-δ ≤5.5 and 5.5 < 6-δ < 5.7, respectively. Second, when corrected for the quenching effect arising from the oxygen vacancy disordering, the experimental ΔfH298∘(6-δ) values and those calculated using the partial molar enthalpy of oxygen, which was evaluated using the defect structure model, agree well with each other. This can be regarded as an independent verification of both the model and the related defect reaction thermodynamics.

Redox energetics and enthalpy increments of GdBaCo2O6-δ

The enthalpy increments for the GdBaCo2O6-δ double perovskite were determined by means of drop calorimetry in air in the temperature range between 365.4 K and 1272.4 K. The heats of orthorhombic I→orthorhombic II phase transition at 347.2 K and orthorhombic II→tetragonal transition at 745.8 K are equal to (2.95 ± 0.07) kJ·mol−1 and (2.40 ± 0.09) kJ·mol−1, respectively, as measured with differential scanning calorimetry for GdBaCo2O6-δ. These calorimetric data were used to determine the heat capacity of GdBaCo2O6-δ and the high-temperature ‘chemical’ contribution to the enthalpy increments, caused by the oxygen exchange. The differentiation of the ‘chemical’ contribution with respect to oxygen nonstoichiometry yielded the partial molar enthalpy of oxygen in the GdBaCo2O6-δ, which agrees well with the previously reported results. The enthalpy increment data of nonstoichiometric oxides have proven itself as a valuable source of redox energetics information.

Composition Dependent Anomaly in Oxygen Vacancy Formation Energy of La0.6Sr0.4Co1-YFey O3-Δ Thin Films

Introduction Oxygen nonstoichiometry is an important parameter for perovskite type (ABO3) oxides such as La0.6Sr0.4Co1-yFeyO3-δ (LSCF) in considering their electrochemical or mechanical properties. Because LSCF is sometimes used as a thin film, it is necessary to understand the oxygen nonstoichiometry in the film. Previous reports [1][2] claimed that oxygen nonstoichiometry in a film of LSCF deposited by pulsed laser deposition (PLD) did not always match with that of the bulk. It is considered that partial molar enthalpy of oxygen, h O, was different between the bulk and the film, but its mechanism was not clarified. Careful comparison of the previous results suggests that the deviation of the enthalpy (=hO,film –hO,bulk ) is dependent on Co to Fe ratio as well as the deposition process. In this study, we have made systematic measurements of the oxygen nonstoichiometry in LSCF films deposited by PLD with the process parameters unchanged but changing the Co to Fe ratio. The results will be compared with those in the bulk, and the composition dependence of the deviation of h O will be discussed. Method The oxygen vacancy concentrations in La0.6Sr0.4Co1-yFeyO3-δ (y=0,0.2,0.4,0.6,0.8,1) films were determined from the electrochemical capacitance of the LSCF film electrode deposited on sintered Ce0.9Gd0.1O1.95 electrolyte. LSCF dense films were prepared by pulsed laser deposition with 4th harmonic of Nd:YAG laser (266 nm) under pure oxygen atmosphere of 1 Pa on heated substrates at 973K for 3h. The resulting thickness was about 500-750 nm. Counter electrode (CE) was porous La0.6Sr0.4CoO3-δ sintered before the film deposition. Reference electrode (RE) was porous Pt sintered after PLD. Electrochemical impedance spectroscopy measurements were performed at 773-973K in O2-Ar mixed gases (p(O2) = 101-105Pa). The results were well fitted to R-C parallel equivalent circuit. Results and discussions Kuhn et al. [3] modeled the nonstoichiometry of Co-rich La0.6Sr0.4Co1-yFeyO3-δ (LSCF) with an itinerant electron model [4], and Fe-rich LSCF, with a localized electron model taking into account the disproportionation of Fe [5]. The itinerant electron model is characterized by partial molar enthalpy of oxygen, h O, and the deviation from ideal solution, a. The localized electron model is characterized by the standard enthalpy change of oxygen incorporation and charge disproportionation reaction, Δ HOX and Δ Hi . The same model was applied to LSCF films in this study. Fig.1 shows the partial molar enthalpy of oxygen in terms of the difference from the standard state,Δ hO =hO -h0 O , for Co-rich LSCF, and the standard enthalpy change of oxygen incorporation reaction,Δ hO , for Fe-rich LSCF. TheΔ hO andΔ HOX of LSCF bulk are taken from Kuhn et al [3]. In Co-rich LSCF,Δ hO of the film was 20~30 kJ/mol greater than that of bulk. On the other hand, in Fe-rich LSCF,Δ HOX of the film was almost the same as that of bulk. This result suggests that Co ion in B site may play an important role in making difference between the film and the bulk. The mechanism of the increase in theΔ hO in the LSCF film will be discussed.

Interrelation of transport properties, defect structure and spin state of Ni3+ in La1.2Sr0.8Ni0.9Fe0.1O4+δ

The total conductivity, Seebeck coefficient and oxygen non-stoichiometry for La1.2Sr0.8Ni0.9Fe0.1O4+δ have been measured vs temperature and oxygen partial pressure P(O2). The measurements were carried out at 800, 850, 900 and 950 °C within the P(O2) range of 10−5–0.21 atm. La1.2Sr0.8Ni0.9Fe0.1O4+δ was shown to be oxygen deficient in all temperature and P(O2) ranges studied. The calculated values of the partial molar enthalpy of oxygen depend very slightly on oxygen content (δ), indicating that La1.2Sr0.8Ni0.9Fe0.1O4+δ with the oxygen deficiency can be considered an ideal solution. The model of point defect equilibria in La1.2Sr0.8Ni0.9Fe0.1O4+δ has been proposed and fitted to experimental dependencies. Subsequent joint analysis of the defect structure and transport properties revealed that electron holes can coexist in both localized and quasi-delocalized states in the oxide: the former corresponded to high-spin state Ni3+ and the latter – to low-spin state Ni3+. The mobilities of localized electron holes were shown to be significantly lower in comparison to quasi-delocalized ones. The behavior of localized electron holes was explained in terms of a small polaron conduction mechanism; in contrast, quasi-delocalized electron holes were described in terms of a band conduction approach. The small polaron conduction mechanism was shown to be predominant in the Sr- and Fe-co-doped lanthanum nickelate.

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides LaxSr3−xFe2O7−δ (LSFO7‐x) (x = 0, 0.25, 0.5) was measured by the high‐temperature gravimetry and the coulometric titration. In the composition series, the P(O2) dependencies exhibited typical plateaus at δ = (2−[])/2. Meanwhile, La0.5Sr2.5Fe2O7−δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O2), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr‐site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen‐deficient and the lower decomposition P(O2). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, , and the partial molar entropy of oxygen, , calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.

Oxygen nonstoichiometry and thermodynamic quantities in the Ruddlesden–Popper oxides LaxSr3 − xFe2O7 − δ

Oxygen nonstoichiometry in the Ruddlesden–Popper type oxides LaxSr3−xFe2O7−δ (x=0, 0.25 and 0.5) was determined by means of the high temperature gravimetry and the coulometric titration. The measurements were carried out in the temperature range between 773 and 1073K and the P(O2) range between 10−40 and 1bar. The P(O2) dependencies of the oxygen nonstoichiometry exhibited typical plateaus at δ~1.0 for Sr3Fe2O7−δ, δ~0.875 for La0.25Sr2.75Fe2O7−δ and δ~0.75 for La0.5Sr2.5Fe2O7−δ, where the average oxidation state of iron is 3+. In the LaxSr3−xFe2O7−δ series, La0.5Sr2.5Fe2O7−δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against on P(O2), temperature, and the La content. Thermodynamic quantities of the partial molar enthalpy of oxygen, hO−hO°, and the partial molar entropy of oxygen, sO−sO°, were calculated from measured nonstoichiometric data. The behavior of oxygen nonstoichiometry and thermodynamic quantities indicates that the ideal-solution-like defect formation is established (La,Sr)3Fe2O7 in the oxygen deficient region.

Oxygen nonstoichiometry, the defect equilibrium model and thermodynamic quantities of the Ruddlesden-Popper oxide Sr3Fe2O(7-δ).

Oxygen nonstoichiometry of the Ruddlesden-Popper oxide Sr3Fe2O7-δ was measured at intermediate temperatures (773-1073 K) by coulometric titration and high temperature gravimetry. The oxygen nonstoichiometric behavior was analyzed using the defect equilibrium model with localized electrons. From the defect chemical analysis, estimated oxygen vacancy concentration at the O3 sites increases and at the O1 sites decreases with the increasing temperature. This characteristic behavior is considered to be caused by the redistribution of oxygen and vacancies between the O1 and O3 sites. The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, h(O) - h°(O), and the partial molar entropy of oxygen, s(O) - s°(O), calculated from the Gibbs-Helmholtz equation are in good agreement with those from the statistical thermodynamic calculation based on the defect equilibrium model, indicating that the proposed defect equilibrium model is reasonable.

Defect chemical and statistical thermodynamic studies on oxygen nonstoichiometric Nd 2 − xSr xNiO 4 + δ

In order to elucidate how oxygen content changes in Nd 2 − x Sr x NiO 4 + δ ( x = 0, 0.2, 0.4), defect chemical and statistical thermodynamic analyses were carried out. The relationship among δ, P(O 2), and T were analyzed by a defect equilibrium model. Since Nd 2 − x Sr x NiO 4 + δ shows metal like band conduction at high temperatures, chemical potential of hole is expressed by the integration of the Fermi-Dirac distribution function and the density of state. The nonstoichiometric variation of oxygen content in Nd 2 − x Sr x NiO 4 + δ can be explained by the defect equilibrium model with a regular solution approximation. Partial molar entropy and partial molar enthalpy of oxygen are calculated from the nonstoichiometric data and Gibbs–Helmholtz equation. The relationship among defect structure, defect equilibrium, and thermodynamic quantities is elucidated by the statistical thermodynamic model. Thermodynamic quantities are calculated by the statistical thermodynamic model with the results of defect chemical analysis and compared with those obtained from experimental results. Thermodynamic quantities calculated by the statistical thermodynamic model can explain rough tendency of those obtained from the δ– T– P(O 2) relationship.

Oxygen nonstoichiometry and chemical stability of Nd 2−xSr xNiO 4+δ

Nonstoichiometric variation of oxygen content in Nd 2− x Sr x NiO 4+ δ ( x=0, 0.2, 0.4) and decomposition P(O 2) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range from 873 to 1173 K and the P(O 2) range from 10 −20 to 1 bar. Nd 2− x Sr x NiO 4+ δ shows the oxygen excess and the oxygen deficient composition depending on P(O 2), temperature, and the Sr content. To evaluate the characteristics of oxygen nonstoichiometric behavior, partial molar enthalpy of oxygen was calculated. The value of partial molar enthalpy of oxygen slightly approaches zero as δ increases in the oxygen excess region while that is independent of δ in the oxygen deficient region. Discussion was made by comparing data of this study with nonstoichiometric and thermodynamic data of La 2− x Sr x NiO 4+ δ : Nd 2− x Sr x NiO 4+ δ show more oxygen excess than La 2− x Sr x NiO 4+ δ in the higher P(O 2) region, while the nonstoichiometric behavior in the oxygen deficient composition is almost the same. The variation of partial molar enthalpy of oxygen with δ for Nd 2− x Sr x NiO 4+ δ in the oxygen excess region is much smaller than that of La 2− x Sr x NiO 4+ δ . The oxygen nonstoichiometric behavior of Nd 2− x Sr x NiO 4+ δ is more ideal-solution-like than that of La 2− x Sr x NiO 4+ δ .

Thermodynamic quantities and defect equilibrium in La 2−xSr xNiO 4+δ

In order to elucidate the relation between thermodynamic quantities, the defect structure, and the defect equilibrium in La 2− x Sr x NiO 4+ δ , statistical thermodynamic calculation is carried out and calculated results are compared to those obtained from experimental data. Partial molar enthalpy of oxygen and partial molar entropy of oxygen are obtained from δ- P(O 2)– T relation by using Gibbs–Helmholtz equation. Statistical thermodynamic model is derived from defect equilibrium models proposed before by authors, localized electron model and delocalized electron model which could well explain the variation of oxygen content of La 2− x Sr x NiO 4+ δ . Although assumed defect species and their equilibrium are different, the results of thermodynamic calculation by localized electron model and delocalized electron model show minor difference. Calculated results by the both models agree with the thermodynamic quantities obtained from oxygen nonstoichiometry of La 2− x Sr x NiO 4+ δ .

Thermogravimetric Analysis and Defect Models of the Oxygen Nonstoichiometry in La2–xSrxCuO4–y

The oxygen nonstoichiometry, y, of La2–xSrxCuO4–y (0 ≤x≤ 1) was determined as a function of temperature (800–1050°C) and oxygen partial pressure (10–4 to 1 atm) by a thermogravimetric technique. The results were interpreted in terms of a number of defect models. It was found that a model based on the defect reaction 4Cu×Cu+ 2V¨o+ O2(g)= 4Cu.Cu+ 2O×o described the results well when the x and y dependences of the partial molar enthalpy of oxygen in the solid were considered. The large defect concentrations observed in this system lead to defect interactions, possibly ordering on the oxygen sublattice.

A defect structure study of nonstoichiometric uranium dioxide by statistical-mechanical models

In order to clarify the defect structure of UO 2 + x two types of defect clusters, the 2:2:2 cluster and the cuboctahedral cluster, are explored by comparing the oxygen potential and the partial molar enthalpy of oxygen calculated from the statistical-mechanical model formulated for each cluster with the measured ones. The model for the cuboctahedral clusters in equilibrium between various charge states gives good agreements with both the oxygen potential and the partial molar enthalpy of oxygen, whereas the models of the clusters which assume a single charge state do not.

Thermodynamic stability field of the 123 and 124 phases in the Y2O3–BaO–Cu–O system

The partial molar entropy of solution of oxygen in YBa2Cu3Ox, δsO, has been computed using equilibrium data and the recently measured partial molar enthalpy of oxygen. While δhO is independent of oxygen content (6 &lt; x &lt; 7), δsO depends on x. The po2, T location of phase equilibrium between the 123 and 124 superconducting phases is calculated. The standard free energies of interactions of the superconducting phases with carbon dioxide (CO2) and water vapor (H2O) have been assessed. The free energies of carbonation of the fully oxidized 123 and 124 phases are −323 ± 22 kJ mol−1 and −291 ± 32 kJ mol−1, respectively. The free energies of hydration of the 123 and 124 are −397 ± 22 kJ mol−1 and −469 ± 32 kJ mol−1, respectively. These large exothermic values show that degradation of these superconductors at ambient conditions is thermodynamically favorable.

Crystal Chemical and Thermodynamic Study on CaUO 4- x, (Ca 0.5Sr 0.5)UO 4- x, and α-SrUO 4- x ( x = 0 ∼ 0.5)

A crystallographic and thermochemical study of CaUO 4- x , (Ca 0.5Sr 0.5)UO 4- x , and αSrUO 4- x with x values ranging from nearly 0 to 0.5 was carried out. As the crystal radius of the divalent cations increases, a (rhomb) of nearly stoichiometric compounds increases, accompanied by an increase in the cell volume, whereas α(rhomb) decreases. With nonstoichiometry, the lattice parameters of (Ca 0.5Sr 0.5)UO 4- x and α-SrUO 4- x discontinuously change at 4 ∼ x = 3.79 and 3.77, respectively, although each compound is a single phase in the entire range from 4 - x = 4.00 to 3.50. For CaUO 4- x , the products were two-phase mixtures between 4 ∼ x = 3.98 and 3.70. These crystal structures are discussed taking into account the lattice parameters and interatomic distances. In the nonstoichiometric crystals, oxygen vacancies exist on the O II sites but not on the O I sites. The enthalpy of formation of α-SrUO 4- x was measured for several specimens with different x values by solution calorimetry, and was expressed in a second-order polynomial of x by least-squares calculation. This quantity and its derivative, i.e., partial molar enthalpy of oxygen, suggest the change of defect species with x and the existence of repulsive vacancy-vacancy interaction.

Investigation of defect equilibria in YBa 2Cu 3O x by a solid state electrochemical method

The partial pressure of oxygen, P(O 2), in equilibrium with YBa 2Cu 3O x was determined as a function of oxygen composition, x, and temperature, T, in the range 6.35> x>5.97 and 1120> T>825 K. A solid state electrochemical method was used allowing for accurate control of composition by Coulometric titration and determination of P(O 2) by open circuit EMF measurements. Decomposition of YBa 2Cu 3O x was found to occur at x values as high as 6.1. Evidence for additional possible phase transitions for x>6.1 are discussed. Three relevant defect models are considered in detail and fitted to these and other data. The outcome of this analysis is a model that is consistent with all the available data and assumes that the dominant defects are neutral oxygen interstitials, O t ∗ and that the concentration of charged defects is small compared to [ O i ∗ ]. The hole concentration, p, follows a P( O 2) 1 2 dependence over a significant range of x. The relative change in the position of the Fermi energy upon reduction from x=6.35 to x=6.1 is 0.2 eV. The partial molar enthalpy of oxygen in YBa 2Cu 3O 6.1 with respect to pure oxygen is 179 kJ/mol. It is suggested that the chemical and tracer diffusion coefficient are independent of the diffusivity determined from conductivity. Information concerning the thermodynamic factor and the enthalpy of oxidation is presented.

Thermodynamic Study of Reduced Phases in the BaLa4Cu5O13.1−x System

The thermodynamic stabilities of reduced, perovskiterelated cuprates with the BaLa4Cu5O13.1−x structure were investigated using differential scanning calorimetry techniques. For 0 &lt; x 0.6 the partial molar enthalpy of oxygen in the structure is constant at ‐187.4 kJ/mol O2. This value is almost identical to those reported for other perovskite cuprates. These results suggest that −190 kJ is a reliable value for the oxidation of Cu from 2+ to 3+ and can be used with confidence in calculations of the stabilities of the p‐doped perovskite cuprates. Anomalously large enthalpies measured for samples prepared with higher values of x under forming gas probably result from trace contaminants of Cu metal.

Electrical Properties and Phase Stability of a Zinc Ferrite

The electrical conductivity and theromoelectric power of a zinc ferrite of nominal composition 0.46ZnO·0.54Fe2O3 were examined as a function of oxygen partial pressure under equilibrium conditions at elevated temperatures. The isothermal variations of both transport properties were found to be closely correlated with PO2‐induced phase transitions from wustite to ferrite spinel to hematite with successively increasing PO2's. The region of thermodynamic stability of each phase was determined from either of the two property isotherms. Within the single‐phase field of the spinel ferrite, both properties are PO2 dependent. Applying the small polaron hopping model to these results, the PO2 and temperature dependencies of the octahedral ferrous and ferric ion concentrations were extracted and interpreted in terms of the buffering capability of the nominal A‐site cation and the variability of the partial molar enthalpy of oxygen with the progress of the oxidation reaction of Fe2+ to Fe3+.

Thermodynamic treatment of coulomb interactions in non-stoichiometric oxides

Due to the presence of charged defects, Coulomb interactions play a major role in a number of non-stoichiometric oxides. Various treatments of these interactions have been discussed. It has been shown that even for relatively high values of the dielectric constant, only two varieties of Monte-Carlo treatment, the Ewald summation method and minimum image with dodecahedron cells, provide reliable results. A typical consequence of these interactions is a strong increase of the partial molar enthalpy of oxygen near the stoichiometry due to the globally attractive nature of electrostatic interactions in dilute solutions.