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Phosphor particles (ca. 60 nm in size) of yttrium oxide co-doped with Er 3+ and Yb 3+ ions were prepared by a precipitation method in the presence of EDTA. Their physical properties were compared to much larger particles (ca. 600-800 nm in size) prepared in the absence of EDTA. All of the particles were shown to have crystallized in the cubic phase, and all exhibited blue, blue-green, and green-yellow upconversion emission when excited with laser light of wavelength equal to 632.8 nm. These upconversion emissions were shown to be excited by a two-photon process. The most intense yellow-green upconversion emission occurs when the crystallite size is between 75 and 200 nm and the particle size is 600 to 800 nm. Cross-relaxation processes between Er 3+ ions are suggested to be responsible for the more efficient upconversion in the larger particles. There is evidence from spectra taken in the temperature range 30 to -190°C that there are two different hot bands in the given region of the spectrum. These two emission manifolds are explained as arising from the two different Er 3+ lattice sites in the cubic Y 2 O 3 :Er 3+ structure. When using red excitation (rather than infrared excitation), the presence of Yb 3+ was found to be detrimental, as it diminished the upconversion intensity.

Lead tin yellow type I (Pb,SnO,) and type II (PbSn,-,Si,O,) have each been prepared in a hightemperature furnace, and the preparative conditions defined. The crystal structure of type II has been refined from powder X-ray diffraction data and indicates that the Sn and Si atoms are randomly distributed over the same type of site with Sn/Si ratio z 3/1 (i.e., x z ; ) . The Raman spectra of each form and also that of lead antimonate yellow (Pb,Sb,O,) have been obtained. Raman microscopy is shown to be an effective technique whereby these inorganic yellow pigments as minute ( z l pm) grains may be identified on medieval manuscripts and paintings.

The Raman and IR spectra of monoclinic naturally occurring raspite, a-PbWO 4 (space group P2 1 /a) were measured; it is irreversibly transformed to the tetragonal stolzite, b-PbWO 4 , by heating to 450 • C, and this process was followed by Raman spectroscopy. Assignments of fundamental modes are proposed. New data are also presented on single crystals of stolzite (space group I4 1 /a), and it is shown that raspite and stolzite can easily be distinguished by their Raman spectra. We also present Raman and IR spectroscopic data on natural and synthetic studtite, UO 4 ·4H 2 O, and its synthetic dehydration product metastudtite, UO 4 ·2H 2 O, the only two naturally occurring peroxide-containing minerals, and on partially deuterated synthetic forms of these materials. Assignments of fundamental modes are proposed and both TGA and DSC data are presented for studtite.

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