Solid-solid interaction in the pure and Li2O-doped MoO3-Al2O3 system

The effects of Li2O doping (1.5 and 6 mol\%) on solid-solid interactions and the phase transformation process in the MoO3-Al2O3 system were investigated using thermogravimetry, differential thermal analysis and X-ray diffraction (TG, DTA and XRD) techniques. The proportions of molybdena expressed as wt\% MoO3 were 22 and 41.4. The results obtained revealed that MoO3 interacted with Al2O3 at temperatures starting from 500 degrees C, forming orthorhombic Al-2(MoO4)(3); doping with lithium oxide was found to promote the reaction. The complete transformation of molybdenum trioxide into aluminium molybdate required heating of the mixed oxides at 700 degrees C, The molybdate produced decomposed at temperatures above 800 degrees C yielding alpha-Al2O3 and MoO3. The Li2O doping enhanced the crystallization of cc-alumina and retarded the thermal decomposition of Al-2(MoO4)(3). The MoO3 produced partly sublimed and the remaining portion dissolved in the alumina matrix forming an MoO3-Al2O3 solid solution. Li2O doping increased the amount of MoO3 sublimed at 900-1100 degrees C to an extent proportional to the concentration of the dopant. In other words, Li2O treatment decreased the solubility of MoO3 in Al2O3. The promoting effect of Li2O on Al-2(MoO4)(3) formation at 500 degrees C was attributed to dissolution of a small portion of Li2O in the MoO3 lattice, with subsequent increase in the mobility of Mo6+ ions. The possible increase in the mobility of these ions might also account for the observed decrease in the solubility of MoO3 in Al2O3.