Energetics of brannerite-type solid solutions: The systems ZnV 2O 6LiMoVO 6 and MgV 2O 6LiMoVO 6

Abstract

Enthalpies of mixing in brannerite-type solid solutions ZnV 2O 6LiMoVO 6 and MgV 2O 6LiMoVO 6 were determined by transposed temperature-drop solution calorimetry into a molten sodium molybdate solvent at 702°C. The ZnV 2O 6LiMoVO 6 system shows zero heats of mixing (to ± 2 kJ/mole) and symmetrical negative excess volumes of mixing. The MgV 2O 6LiMoVO 6 system shows more complex behavior, with positive enthalpies of mixing at MgV 2O 6-rich end and negative heats of mixing at LiMoVO 6-rich compositions. Volumes of mixing are negative at MgV 2O 6-rich compositions and essentially zero on the LiMoVO 6-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 MgO bonds in MgV 2O 6 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.