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The present paper reports the first stage of the study by the present group on the thermal decomposition of La2Mo2O9 under lower oxygen partial pressure. The stability of La2Mo2O9 at 1473 K in an oxygen partial pressure $$ (p_{{{\rm O}_{2} }} ) $$ ranges from 2.44 × 10 to 4.33 × 10 Pa was studied by XRD. The results showed that La2Mo2O9 could remain stable in a $$ p_{{{\rm O}_{2} }} $$ range greater than 2.44 × 10Pa. While below this $$ p_{{{\rm O}_{2} }} $$, the formation of several reduced and crystallized molybdates were found. By means of XRD, TEM, SAED, and EDS, the decomposition process of La2Mo2O9 at 1473 K under $$ p_{{{\rm O}_{2} }} $$ of 9.75 × 10Pa was investigated. The intermediate and final products formed during the thermal decomposition were characterized. It was found that the La2Mo2O9 decomposition consisted of two steps. La2Mo2O9 first decomposed to intermediate products like LaMo2O5, La5Mo4O14, and La12Mo6O35 and then to final products like La5Mo3O16 and a new Mo-rich compound. Tilt series of SAED patterns of this new structure were recorded. These measurements were used for the reconstruction of the reciprocal space. The cell attained was monoclinic with parameters: a ≈ 1.195 nm, b ≈ 4.133 nm, c ≈ 1.314 nm, and β ≈ 95°. [ABSTRACT FROM AUTHOR]

Abstract: LaMnO3+δ and La0.7Ca0.3MnO3 were synthesized from La2O3(La(OH)3), CaCO3 and MnO2 powder mixture with solid-state reaction technique. X-ray diffraction and thermal analysis were employed in the present study on the process of synthesizing of the two compound powders. The kinetic study on solid-state reaction between La2O3 and MnO2 in the powder mixture was isothermally carried out for LaMnO3+δ formation. The result showed that the reaction process was controlled by three-dimensional solid-ionic diffusion. Both Jander and Ginstling-Brounstein model can be used to describe the reaction kinetics satisfactorily. The relevant apparent activation energy values obtained were as great as 205±11 and 189±8kJ/mol. The reaction process of La0.7Ca0.3MnO3 preparation from La2O3, SrCO3 and MnO2 powder mixture was also studied using X-ray diffraction and thermal analysis. The result indicated that the following steps composed the overall reaction: the decomposition of the reactant; the formation of La1−x Ca x MnO3+δ ; La2O3 and Mn3O4 reacted with La1−x Ca x MnO3 to form final La0.7Ca0.3MnO3 phase. The latest step was the most time-consuming one among others in the overall reaction. [Copyright &y& Elsevier]

Abstract: The effect of oxygen partial pressure () on the electrical conductivity of La2Mo2− x W x O9−δ (x = 0.0, 0.5, 0.7) was investigated from 873 to 1073 K using AC impedance spectroscopy in oxygen, air, argon, and CO–CO2 mixtures with CO volume percentages, 0.5%, 9%, 33% and 50% used for producing lower oxygen partial pressures. The results have shown that the electrical conductivity increases slowly with the decreasing when the CO content is not greater than 0.5 vol.%. has more significant influence on the electrical conductivity of W-doped La2Mo2O9 than on that of undoped La2Mo2O9. For La2Mo1.3W0.7O9, the logσ attained in all atmospheres except oxygen and CO–CO2 mixture with 50 vol.% CO, exhibits a good linear relationship with log . The corresponding slopes attained are as large as 0.019–0.021, much lower than 1/4, suggesting that the enhancement of total electrical conductivity could be mainly due to the increase of ionic conductivity when decreases. XRD and XPS analyses for La2Mo2−x W x O9−δ have confirmed that a small amount of Mo6+ has been reduced to Mo4+ during the electrical conductivity measurements in CO–CO2 mixture with 9 vol.% CO, and further decreasing causes the formation of La7Mo7O30. This may lead to the remarkable changes in their electrical conductivities. A critical value for each compound at a specified temperature between 873 and 1073 K has been found, and below this value the compound would become unstable. The critical values attained at 973 K are as follows: 10−12, 10−15 and 10−17 Pa for La2Mo2O9, La2Mo1.5W0.5O9 and La2Mo1.3W0.7O9 respectively. [Copyright &y& Elsevier]