Regions of phase crystallization and new double tungstates in the system Na2O-Al2O3-WO3

The concentration and temperature regions of formation of crystals from different phases of the ternary system Na2O-Al2O3-WO3 were determined using the results on spontaneous crystallization in various high-temperature solutions of this system where the content of each of the three oxide components varied within the limits of 13-40 mol.% for Na2O, 2-15 mol.% for Al2O3 and 57.5-77.5 mol.% for WO3. Homogeneous solutions with compositions within these limits were prepared at temperatures below 1150degreesC at which no measurable evaporation of WO3 was observed. Crystals of seven different phases were found to grow in the respective regions. Four of them belonged to the known compounds Na2W2O7, Na2W4O13, WO3 and Al-2(WO4)(3). The double sodium aluminum tungstates, NaAl(WO4)(2) and Na3AlW5O18, and a sodium tungstate with not yet specified composition, Na2O.xWO(3) (x=16-18), crystallizing in the remaining three regions were obtained for the first time. The crystals isolated from the above seven regions were characterized by X-ray diffraction. With the new compounds, composition determinations were made by atomic absorption, X-ray fluorescence and microprobe analyses. Their behavior during heating was established by thermal analysis. The results obtained indicate the possibility of developing convenient methods for the growth of single crystals of the aluminum-containing phases, to be used, upon suitable doping with transition metal ions, as tunable solid state laser hosts. (C) 2002 Elsevier Science B.V. All rights reserved.