Alkaline-Earth Tungstates: Equilibrium and Stability in the M-W-O Systems

Phase relations in the systems alkaline earth oxide‐tungsten trioxide and their stability with metallic tungsten were investigated by the quenching technique using sealed capsules. In the system BeO‐WO2, no intermediate compounds were formed. Binary mixtures resulted in a eutectic at 1185°C and 37 mole % BeO. In the system MgO‐WO3, the 1:1 compound was stable and melted congruently at 1358°C. There are two crystalline modifications of this compound. The well‐known wolframite‐type form is stable below 1165°C. Two eutectics were found: 1120°C and 28.5 mole % MgO and 1318°C and 55.0 mole % MgO. In the systems CaO‐WO3, SrO‐WO3, and BaO‐WO3, two binary compounds are stable. The 1:1 compounds with the scheelite‐type structure melt, respectively, at 1580°, 1535°, and 1475°C, and form eutectics with WO3 at 1135°, 1073°, and 935°C, all with a eutectic composition near 75 mole % WO3. The 3:1 compounds having the distorted (NH,)3FeF6‐type structure melt at ∼2250°, 2225°, and 1795°C. The eutectics between these two compounds are at 1490°C and 56.5 mole % CaO, 1410°C and 57.0 mole % SrO, and 1320°C and 58.2 mole % BaO. Phase transformations to an ideal (NH4)3FeF6‐type structure in Sr3WO6 and Ba3WO6, were observed at 1100° and 805° C, respectively, by application of both high‐temperature X‐ray diffraction and DTA. At 1700°C, metallic tungsten exists in equilibrium with liquid, the alkaline earth oxides, W18O49, the 3:1 ternary oxides, and with combinations thereof.