Energetics of brannerite-type solid solutions: The system MnV 2O 6LiMoVO 6Mo 2O 6

Abstract

Brannerite-type solid solutions in the system MnV 2O 6LiMoVO 6Mo 2O 6 (MnLiф) were studied by high-temperature drop-solution calorimetry and X-ray diffraction. Enthalpies of mixing, referred to brannerite standard states, are ΔH mix = y(1 − y)(24.8 − 51.7 y) kJ/mole for (1 − y)MnV 2O 6 - yLiMnVO 6 (MnLi) and ΔH mix = 23.4 x(1 − x) for (1 − x) MnV 2O 6 - x Mo 2O 6 ( Mnф) . A fit to all the data simultaneously (the two joins above, three ternary points, and one point along Liф) gives values similar to those above for ΔH mix(MnLi) and ΔH mix ( Mnф) while suggesting a negative heat of mixing along the Liф binary and negligible ternary excess terms. Parameters for the transformation of Mo 2O 6 from the MoO 3 structure to the hypothetical brannerite structure are estimated to be: ΔH = 10.0 kJ/mole Mo 2O 6, ΔV° = 7.65 cm 3/mole. Both the MnV 2O 6LiMoVO 6 system studied here and the MgV 2O 6LiMoVO 6 system studied previously show positive heats of mixing at MnV 2O 6- or MgV 2O 6-rich compositions and negative heats of mixing at LiMoVO 6-rich compositions. In each case, a linear correlation of the enthalpy interaction parameter, λ H , and the volume interaction parameter, λ V , is observed. Both systems show that positive heats of mixing correlate with negative volumes of mixing and negative heats of mixing correlate with positive volumes of mixing, but the MgLi correlation has a steeper slope than that for MnLi. This rather unusual but systematic behavior thus persists in several systems with brannerite structure.