A study on La₂Mo₂O₉ decomposition process

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 × 10⁻⁵ to 4.33 × 10⁻⁹ Pa was studied by XRD. The results showed that La₂Mo₂O₉ could remain stable in a \( p_{{{\rm O}_{2} }} \) range greater than 2.44 × 10⁻⁵ Pa. 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 × 10⁻⁷ Pa 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°.