No abstract

A study of the system A1203-Ga203-H20 has resulted in the determination of equilibrium diagrams for the systems Al2O3-Ga2O3 and AlZO3.-Hz0-GazO3.HZ0. Extensive solid solution characterizes the cr-Al~O~ and @-Ga208 structures at high temperatures, but it is shown that below 810°C. a compound, GaA103, and a new series of (Al,Ga)203 structures are stable. Among the hydrates, a complete series of diaspore solid solutions extends from A1203.Hz0 to Gaz03 .HzO. Boehmite solid solutions extend to approximately the composition '7OAl2o3 .HzO, 30Gaz03 .HzO.

It is demonstrated experimentally that even in the purest laboratory silicas available the temperature of the a-0 inversion in cristobalite is variable and depends on the structure of the starting material and on the temperature and length of heat-treatment. It is shown that this variability is a n index of the order achieved in the cristobalite structure quite independent of impurities. There is no specific order characteristic of a particular temperature ; the completely ordered 3C stacking of cristobalite is the most stable cristobalite throughout the temperature range and has an a-6 inversion temperature of 267' =k 2OC. All disordered cristobalites will tend toward the 3C 267" cristobalite with time. The conversion of cristobalite to tridymite involving the 3 C d 2 H stacking change does not alter the cristobalite inversion temperature, nor have any regular polytypes other than those recognized as tridymite been encountered.

RESEARCH prograiii to grow K T N (KTao.csNbo.ss03) single A crystals hydrothernially revealed a lack of inforniation on the subsolidus cquilibriurii relations in the system KTa03-KNbOt. The present study was initiated to determine why thc crystals grown were eithcr Taor Nb-rich. Further, the cornposition of the crystals appeared to be related to the growth temperature rather than to the composition of the nutrient.Single crystals of potassium tantalate niobate in the cubic paraelcctric state are of interest because the material has very useful electrooptical properties just above the Curie temperature.' KTN also has excclleut acoustical parametric amplification characteristics. K T K single crystals which display tlie electrooptical properties zit room temperature were grown by Bonner et al.,2 Wilcox and F~l l r n e r ,~ and Gentile and Andres4 using flux methods, and by Marshall and Laudisc6 using the hydrothermal tnethod. Tlie major difiiculty encountered was compositional zouing. This zoning has been attributed to the wide loop between the solidus and liquidus curves of the system KNb03-KTa03.G-s Howcver, this mechanism does not explain why the crystals from a single hydrothermal run have two widely different compositions since the crystals grcw below the solidus.The chemicals used were TaeOa, Nb20s, and KrC03 (mannfacturer's stated purity 99.9';/,). Stoichiornetric mixtures of KTaOa, KNbOr, arid compositions betwcen these, including KT;LO.,XWJ~.~~O:~, wcrc prepared from the rcquircd weights of K2COs and oxides previously dried at 400°C. These were homogenizcd by grinding in aii agate mortar using acetone as the suspciiding agent. After drying, they were heated to 600°C for 48 t o 72 hr and then a t 800°C for 1 lir to produce a very finegrained KTN. Portions of some mixtures were crystallized after heating to 1300°C in sealcd crucibles to prevent loss of alkali.Cold-seal microreactors (0.25 and 0.5 in. I D ) fabricated of RenC or Stellite alloys, were used for the hydrotherttial work. The charges were scaled inside gold capsules and reacted hydrothernidly a t terripcraturcs up to 1000°C and pressures up to 4 kbars. A t the end of the runs the microreactors were removed