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Alkaline‐Earth Tungstates: Equilibrium and Stability in the M‐W‐O Systems
Author(s) -
CHANG LUKE L. Y.,
SCROGER MARGARET G.,
PHILLIPS BERT
Publication year - 1966
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1966.tb13291.x
Subject(s) - eutectic system , tungstate , alkaline earth metal , wolframite , tungsten , materials science , mole fraction , scheelite , phase (matter) , crystallography , metal , analytical chemistry (journal) , chemistry , inorganic chemistry , metallurgy , microstructure , chromatography , organic chemistry
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‐WO 2 , no intermediate compounds were formed. Binary mixtures resulted in a eutectic at 1185°C and 37 mole % BeO. In the system MgO‐WO 3 , 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‐WO 3 , SrO‐WO 3 , and BaO‐WO 3 , 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 WO 3 at 1135°, 1073°, and 935°C, all with a eutectic composition near 75 mole % WO 3 . The 3:1 compounds having the distorted (NH,) 3 FeF 6 ‐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 (NH 4 ) 3 FeF 6 ‐type structure in Sr 3 WO 6 and Ba 3 WO 6 , 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, W 18 O 49 , the 3:1 ternary oxides, and with combinations thereof.