MgO-Al,O,-SiO, Glasses Containing ZrO,, TiO,, and CeO, 163 were obtained in the region described by these workers, X-ray examination indicated that both quenched and slowly cooled samples were crystalline. The diffuse diffraction lines observed by Ruff et al. may have led to the assumption that a partly glassy material had been obtained. Also, the hope of using the suggested compositions as translucent refractories is unlikely to be realized, since the metastable high-temperature phase decomposes on annealing. The limits of the cubic solid solution in the binary system ZrO,-CaO were taken from G a r~i e ,~ who viewed the cubic field as extending from = 14 to 20 mol% CaO at 1600°C. The boundaries reported for the 2-phase region containing tetragonal ZrO,(ss) and CaZr,O,(ss) also vary considerably."-" Since the exact value is of little interest for the present investigation, a n arbitrary range was assumed. The mutual solubility of ZrO, and Tho, is Disagreement on the extent of CaO solubility in Tho, but, since Ref. 13 presents electron micrographs of second-phase inclusions at 2 mol% CaO, this maximum concentration was assumed for the present work. The existence at 1600°C of the compound CaThOB mentioned by Naray-Szaboi5 could not be confirmed, in agreement with the results of Moebius et a/. ', The ternary compound 2Ca0. 2Th0,. 5Zr0, is mentioned neither in the study of Ruff et a / . ' nor elsewhere. This compound seems to be stable up to a temperature near its melting point, since some superstructure lines which can be indexed by doubling the fluorite lattice constant were seen clearly in X-ray diffraction patterns of samples cooled slowly from the melt. An exact structural analysis will be given subsequently. In particular, the question of deviation from true pyrochlore stoichiometry , A,B,O,, should be answered.
Acknowledgments:The writers thank P. Eckerlin for many helpful discussions and M. Uitterhoeve, H. Bausen, and H. Liebner for performing the X-ray work.