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Reverse microemulsions were used to synthesize barium fluoride doped with 0−65 mol % neodymium. Although the products were polydisperse, average particle sizes below 100 nm were achieved. XRD analysis showed that powders with 0−10 mol % Nd were single phase, while samples with dopant levels of 10−50 mol % contained two phases. Products with more than 50 mol % Nd were amorphous by XRD. Fluorescence of Nd:BaF2 showed an unusually high threshold for concentration quenching as well as very short lifetimes compared to those of bulk samples. The use of a cosurfactant and variation in reaction conditions provided control over particle size; smaller particles resulted by limiting the aqueous volume while simultaneously increasing the amount of cosurfactant for a given concentration of reactants.

The d-d band is only weakly structured in these complexes, but in some cases three components are distinguishable corresponding to transitions from the manifold of filled d orbitals of Figure 6 to the u* orbital of xy symmetry. The u interaction between the xy and zz d orbitals with the ligand u orbitals could be the same in the green and the orange complexes. Then the difference in the d-d absorption maximum for the two groups of complexes would be determined by 'a different amount of interaction between the yz, z x d orbitals and the ligand a orbitals. The most important interaction will be with the lowest lying empty a* orbital which stabilizes the d, orbitals. The stabilization must be most pronounced in the complexes having an even number of ligand ?r orbitals only half filled, Le., in the green complexes. We would therefore expect the d-d transitions in the green complexes to occur at a higher energy than in the orange complexes. The opposite is observed, and we interpret this as an indication of significantly different u interactions in the two groups of complexes. This effect is not evident from the variation of bond lengths.Acknowledgment. The authors are grateful to Dr. K. Schaumburg for measuring 270-MHz spectra and for taking part in discussions of the results and to Mr. F. Hansen for excellent technical assistance.

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A H202-assisted sol-gel method for the preparation of precursors for formation of thin films of MgO is described. Sols were prepared from the addition of Mg(OCH3)2 to methanolic H2O2. Two such sols, with Mg to H202 ratios of 1:1 and 1:10, were used to prepare, respectively, random or (111) oriented MgO thin films on Si(100). The xerogel from a sol with a 1:10 Mg:H202 ratio contained nanocrystalline Mg02-nH20, whereas the xerogel from a comparable sol with a 1:1 Mg:H202 ratio contained nanocrystals of Mg02,nH20, and Mg-(OH)(OCH3). The latter compound was the hydrolysis product of methanolic Mg(OCH3)2 even when treated with up to 4 equiv of water. The thermal decomposition properties of this compound are reported. Using the published method of treating Mg(N03)2 with excess 30% H202, Mg02-nH20, with a Mg2+:022-ratio of 1:0.74, was prepared. Based on IR, TGA, and DTA results, it is proposed that the material also contained oxide, and possibly hydroxide ions for charge balance. These ions were present either in an amorphous phase, or possibly substituted for 022-in the Mg02 crystal structure.