The systems Y3-xCexA15012, Ys-~CexA15-zGazO12, and Y3-x-zGdzCexA15012 were studied. It was found that the solid solubility of Ce 3 § in Y3A15012 varies with the atmosphere during firing. Only 2 mole per cent (m/o) of Ce ~ § can be dissolved in the yttrium aluminum garnet in air while almost 6% Ce 3 § enters the garnet lattice when fired at 1450~ under a hydrogen atmosphere. Complete solid solutions were observed in the systems Y3A15Ol~-Gd~A15012 and Y3A15012-Y3GasO12. The cathodoluminescence emission-peak position of the Ce 3 § ion in Y~A15012 host shifted to a longer wavelength while the latttice parameter of the solid solution expanded when Gd 3. ions were substituted for y3 § On the other hand, the emission-peak position shifted to a shorter wavelength when the garnet host contained Ga s § ions in the A13 § position. The lattice parameter of the solid solution increased with Ga concentration. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.230.243.252 Downloaded on 2015-03-24 to IP Vol. 120, No. 2 Ce s § ACTIVATED Y3A15Ol~ 281

Solid solutions in the system YPO4–YVO4, with and without 2 mol% of Tb3, were studied to determine the reason for the total inactivity of Tb in YVO4. The comparison of the diffuse reflectivity of Tb-containing solutions with that of Tb-free ones shows a uv absorption at energies below the absorption edge of YVO4 in materials containing both terbium and vanadium. Selective excitation directly into Tb3+ in these wavelength regions produced reduced Tb3+ emission when there was vanadium present. The band emission of the VO43+ groups, and the Tb3+ emission under electron beam excitation, were strongly quenched whenever terbium and vanadium were both present. Emission of Dy3+, present as a 10-ppm impurity in all samples, was also quenched by the presence of both terbium and vanadium, but not by either alone. All these observations imply energy loss by Tb–V interaction. A particular loss mechanism—the well-known intervalence transfer of charge from Tb to V resulting in an abrupt change of bond strength leading to a transfer of electronic excitation into lattice vibrations—is supported by measurements of Tb activity as a function of V concentration. The intensity of Tb emission is proportionate to the probability that none of four metal ion sites about a Tb is occupied by a V. This suggests the intervalence transfer absorption mechanism because only four of the nearest six metal ion sites have Tb–O–M bond angles (about 150°) consistent with good charge transfer from Tb to V; the remaining two sites have a Tb–O–M near 90° and should be relatively inactive in charge transfer.

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The compounds MA12Si2Os, where M----Ca, Sr, or Ba, were synthesized. Compositions in the three binary systems were also prepared. Cathodoluminescence of these compositions activated with Ce 3+ and Eu 2+ was studied. It was found that the Ca and Sr feldspars formed complete solid solutions. In the systems Ba-Ca feldspars and Ba-Sr feldspars, only partial solid solutions are formed. The solid solubility limits are SrA12Si2Os to Sr0.6Bao.4A12Si2Os and BaA12Si._,Os to Ba0.sSro.~A12Si2Os CaA12Si208 to Ca0.gBa0.1A12Si2Os and BaA12Si2Os to Ba0.6Ca0.4A12Si2Os tem to study the effect of crystal chemical variation ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.173.72.87 Downloaded on 2014-12-26 to IP Vol. 118, No. 6 ALKALINE EARTH FELDSPARS 919 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.173.72.87 Downloaded on 2014-12-26 to IP

June 1 !1B3Journal of 7'he American Ceramic (3) At GOO'C, tlic large specimens showed a relatively large increase in density (1%) for runs up to 20 minutes. Devitrificatioii occurred along the edges of the specimens when rum were made for 45 minutes. X-ray investigation indicated the formation of coesite. As the specimens were not sealed in the platinum, some water vapor may have catalyzed the reaction. No bircfringencc mcasuremcnt could be made, as the cracks were too numerous; the specimens were definitely distorted.When small specimens were used, the following results were observed:(1) At 400"C, thc curve showing the increase in density versus time reached an asymptotic value much lower than in the previous experiments at the Same temperature. A slight birefringence was noted.(2) At 6OO0C, the trend of the curve was similar to that obtained previously a t the same temperature. The rate of coesite formation is small.A uniform pressure applied to a glass specimen should not introduce any double refraction in the material. Waxler and Ada1ns7 reported that no birefringence results when glasses and polymers are subjected to hydrostatic pressures. The birefringence found when silica is compressed a t temperatures shows that the pressure is not isostatic. The slight change of shape of the specimen shows that flow takes place. The shearing stresses at each temperature are not of the same magnitude (a large scatter of density data is found a t high temperatures; there is more uniformity a t low temperatures) and depend also on time. It sceiiis that the magnitude of the shearing stresses differs according to the high-pressure device and to the experimental cell design; when powders are used, shearing, effects a t the point of contact of particles should be considered. This explains the large discrepancy between the results obtained by various workers. The results reported here agree with Bridgman,s who in a later experiment concluded that plastic shearing deformation plays an essential role in the increase in density; a similar conclusion was reached by Mackenzie and L a f~r c e .~ The changes obtained in this study at low temperature may have been so small because a better approximation of isostatic pressure can be achieved in the tetrahedral anvil apparatus at low temperature. It seems to the writer that fused silica cannot be compacted irreversibly by hydrostatic means. Neutron-irradiated silica reverts to ordinary fused silica during annealing. In their study of the effect of pressure (up to 120 kbars) a t room temperature on optical absorption spectra of transition metal ions in glass, Drickamer and Tischerlo show that the effects are reversible on the release of pressure except for phosphate glasses, where irreversible behavior occurs only above 50 kbars. Weir et al." also reported that following the release of pressure the refractive index of glass recovers its initial value after a few hours.7R. M. Waxler and L. H. Adams, "Effect of Hydrostatic large positive resistivity-temperature anomaly (PTC) which occurs a...