Energetics of brannerite-type solid solutions: The systems ZnV₂O₆LiMoVO₆ and MgV₂O₆LiMoVO₆

Enthalpies of mixing in brannerite-type solid solutions ZnV₂O₆LiMoVO₆ and MgV₂O₆LiMoVO₆ were determined by transposed temperature-drop solution calorimetry into a molten sodium molybdate solvent at 702°C. The ZnV₂O₆LiMoVO₆ system shows zero heats of mixing (to ± 2 kJ/mole) and symmetrical negative excess volumes of mixing. The MgV₂O₆LiMoVO₆ system shows more complex behavior, with positive enthalpies of mixing at MgV₂O₆-rich end and negative heats of mixing at LiMoVO₆-rich compositions. Volumes of mixing are negative at MgV₂O₆-rich compositions and essentially zero on the LiMoVO₆-rich side. This complex behavior suggests that several effects compete to result in the observed energetic and structural behavior. These factors may be (a) strain energy associated with size-mismatch, (b) the relaxation of rather long MgO bonds in MgV₂O₆ upon solid solution formation, and (c) the effect of simultaneous coupled substitution of Li on Mg sites and of Mo on V sites, possibly with some short-range order.