OXYGEN NONSTOICHIOMETRY AND THERMODYNAMIC STABILITY OF La2-xSrxCuO4-δ SOLID SOLUTIONS IN La2CuO4 - Sr2CuO3 SYSTEMтезисы доклада

Дата последнего поиска статьи во внешних источниках: 28 мая 2015 г.

Работа с тезисами доклада


[1] Alyoshin V. A., Tiphlova L. A., Monayenkova A. S. Oxygen nonstoichiometry and thermodynamic stability of la2-xsrxcuo4-δ solid solutions in la2cuo4 - sr2cuo3 system // XIX International Conference on Chemical Thermodynamics in Russia, Moscow, June 24-28, 2013, Abstracts. — MITHT Publisher Moscow Moscow, 2013. — P. 201–201. The solid solutions La2-xSrxCuO4-δ (0 < x(Sr) <1.2) in the La2CuO4 - Sr2CuO3 system, formed by heterovalent substitution of La on Sr, have the properties of high-temperature su-perconductivity, exhibit catalytic activity and are considered as a material for low temperature fuel cells. The heterovalent substitution of La on Sr increases the copper oxidation state and led to forming of oxygen vacancies in CuO2 layer, which reduce the stability of the La2-xSrxCuO4-δ crystal lattice. The increasing of copper oxidation state is dominated at small substitutions x(Sr), and oxygen vacancies formation is preferable at higher x(Sr). These structure violations are accompanied by changing of lattice energy, and prediction of stability these solid solutions on the base of structure data and thermodynamic properties is the main purpose of the work. The determination of enthalpy of formation La2-xSrxCuO4-δ (ΔoxHo298) on oxides was performed by solution calorimetry technique. Copper oxidation states were calculated from results of iodometric analysis, interatomic distances were taken from x-ray structure data. The dependence of ΔoxHo298(La2-xSrxCuO4-δ) on copper oxidation state V(Cu), strontium content x(Sr), interatomic distances and oxygen vacancies concentration δ was revealed, and a crystal energetic model, satisfactory fitted this dependence, was proposed. In the employed model the change of lattice La2-xSrxCuO4-δ energy was described as a function of structural factors variations, such as location and concentration of oxygen vacancies δ. The energy contributions of {La2O2} layer and CuO6 octahedral layer were treated separately, and the interlayer interaction was added. The correlation coefficients in the final equation were estimated by fitting experimental data on enthalpies ΔoxHo298 to model dependence. The ΔoxHo298(La2-xSrxCuO4-δ) of x(Sr) has a minimum, corresponding to x(Sr) = 0.6, caused by lattice instability. It was revealed that the main contribution in the energy variation was made the CuO6 octahedral layer because the significant copper oxidation state changing, interatomic distances variations and oxygen vacancies concentration increasing. The discrepancy between the calculated and experimental thermodynamic data did not exceeds 4 kJ/mole, which indicates the adequacy of the used model and accuracy of obtained values of ΔoxHo298. In order to assess the thermodynamic stability of La2-xSrxCuO4-δ solid solutions, the ΔoxGo298 - x(Sr) diagram was plotted. Dependence ΔoxGo298(La2-xSrxCuO4-δ) on x(Sr) has a minimum at x(Sr) = 0.6, Sr2CuO3 was considered as a point phase. According to thermody-namic analysis of the La2CuO4 - Sr2CuO3 system, the La2-xSrxCuO4-δ solid solutions thermo-dynamically stable only at x(Sr) < 0.6 under standard conditions. In the range 0.6 < x(Sr) < 1.2 this phase unstable relatively La1.4Sr0.6CuO4-δ and Sr2CuO3 and may exist in metastable state only. The problems of synthesis of metastable La2-xSrxCuO4-δ are discussed also in regard to the thermodynamic and kinetic properties of La2CuO4 - Sr2CuO3 with recommendations for obtaining uncontaminated samples.

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