Regions of phase crystallization and new double tungstates in the system K 2O–Al 2O 3–WO 3

Abstract

The concentration and temperature regions of formation of different crystal phases of the ternary system K 2O–Al 2O 3–WO 3 were determined using the results on spontaneous crystallization in various high-temperature solutions of this system where the components varied within the limits of 10–50 mol.% for K 2O, 2–17 mol.% for Al 2O 3 and 50–85 mol.% for WO 3. Homogeneous solutions of compositions in these limits were prepared at temperatures below 1150 °C at which no measurable evaporation of WO 3 was observed. Crystallization regions of nine different compounds were established. Four of them belonged to known binary and ternary oxides such as WO 3, K 2WO 4, K 2W 3O 10 and Al 2(WO 4) 3. Crystals of double tungstates of potassium and aluminum were formed in the other five regions. The K 2O:Al 2O 3:WO 3 molar ratios in them were 3:1:10, 3:1:16, 3:1:22, 8:1:15 and 5:1:12, respectively. The latter three compounds were obtained for the first time. The crystals grown in all nine regions were characterized by X-ray diffraction. With the double tungstates, composition determinations were made by atomic absorption, X-ray fluorescence and microprobe analyses. The behavior of these compounds during heating was established by thermal analysis. All double tungstates were found to melt incongruently and the temperature of peritectic decomposition of each of them was determined. A regularity of the changes in composition of the potassium aluminum tungstates crystallizing in neighbouring regions of the ternary system was observed. Two of the sections in the system, which contained the double tungstates, were discussed. It was shown that these sections could be used successfully for choosing the optimum growth conditions of single crystals of these compounds.