Analysis of solid solutions stability in scheelite-type molybdates and tungstates

Mutual solubility of bivalent metal molybdates and tungstates with scheelite structure was theoretically estimated by calculating formation enthalpies and the maximal decomposition temperatures of solid solutions at different temperatures. The theoretical stability of continuous solid solutions in binary systems of bivalent metal molybdates and tungstates was found to be higher than reported literature data. After cooling down continuous substitution solid solution should remain in following systems: CaMoО4–CdMoО4, SrMoО4–MMoО4 (M=Ba, Pb), BaMoО4–PbMoО4, SrWO4–MWO4 (M=Ca, Pb), and BaWO4–PbWO4. There is a probability that at room temperature in systems CaMoО4–SrMoО4, CaWO4–PbWO4, and BaWO4–SrWO4 the single homogeneity region may decompose to limited solid solutions. It was shown experimentally that a continuous series of scheelite-structure solid solutions M1−xMIxТO4 can be formed via citrate synthesis at temperatures below 500°С.

Calculated boundaries of solid solutions in BaWO4–CaWO4 (1) and PbMoO4–CaMoO4 (2) systems. Figure optionsDownload as PowerPoint slideHighlights
► Stability of solid solutions molybdates and tungstates has been investigated.
► The ionic model for isovalent substitutions was used.
► In “polyhedral substitution model” enthalphies of mixing were calculated.
► Most of the examined series of solid solutions is stable at room temperatures.
► Solid solutions M1−xMIxMo(W)O4 (x=0–1) were formed via citrate synthesis below 500 С.