Ln(SePh) 3 (Ln ) Ce, Pr, Nd) reacts with elemental Se in the presence of Na ions to give (py) 16 Ln 17 NaSe 18 (SePh) 16 , a spherical cluster with a 1 nm diameter. All three rare-earth metals form isostructural products. The molecular structure contains a central Ln ion surrounded by eight five-coordinate Se 2that are then surrounded by a group of 16 Ln that define the cluster surface, with additional µ 3 and µ 5 Se 2-, µ 3 and µ 4 SePh -, and pyridine donors saturating the vacant coordination sites of the surface Ln, and a Na ion coordinating to selenolates, a selenido, and pyridine ligands. NIR emission studies of the Nd compound reveal that this material has a 35% quantum efficiency, with four transitions from the excited state 4 F 3/2 ion to 4 I 9/2 , 4 I 11/2 , 4 I 13/2 , and 4 I 15/2 states clearly evident. The presence of Na + is key to the formation of these larger clusters, where reactions using identical concentrations of Nd(SePh) 3 and Se with either Li or K led only to the isolation of (py) 8 Nd 8 Se 6 (SePh) 12 .

Ln(SePh)(3) (Ln = Ce, Pr, Nd), prepared by reduction of PhSeSePh with elemental Ln and Hg catalyst, reacts with excess elemental Se to give (py)(11)Ln(7)Se(21)HgSePh, an ellipsoidal polyselenide cluster. The molecular structure contains two square arrays of eight- or nine-coordinate Ln fused at one edge to form a V shape that is also capped on the concave side by a centrally located nine-coordinate (Se(3))pyLn(Se(3)) and on the convex side by a 2-fold disordered SeHgSePh. The central Ln coordinates to selenido, triselenido, and pyridine ligands, while all other Ln coordinate to selenido, diselenido, triselenido, and pyridine ligands. Thermal treatment of the Pr compound at 650 °C gave Pr(2)Se(3) and Pr(3)Se(4). NIR emission studies of the Nd compound show four transitions from the excited-state (4)F(3/2) ion to (4)I(9/2), (4)I(11/2), (4)I(13/2), and (4)I(15/2) states. The (4)F(3/2) ion to (4)I(11/2) transition (1075 nm emission) exhibited 43% quantum efficiency. This is the highest quantum efficiency reported for a 'molecular' Nd compound and leads a group of selenide-based clusters that has shown extraordinary quantum efficiency. In terms of efficiency and concentration, these compounds compare favorably with solid-state materials.

The room temperature Ca(C 2 H 3 O 2 ) 2 -K 3 PO 4 -H 2 O equilibrium system was examined for the preparation of hydroxyapatite nanopowder with sizes less than 10 nm. The reaction products were characterized with X-ray diffraction, transmission electron microscopy (TEM), nitrogen-adsorption surface area, helium pycnometry, thermogravimetric analysis, and Karl Fisher titration methods. TEM revealed that ∼5 nm nanopowders could be successfully prepared with this synthesis approach. However, the vast instability of these powders brought upon by the method of sample separation or the characterization method itself made it impossible to use other conventional methods of characterization to validate TEM data. This study has identified key processing steps that control the order and disorder of these nanomaterials, as well as the conditions that lead to surface area reduction. The most unique phenomenon from this work is the observed crystalline to amorphous phase transformation when washed or unwashed nanopowders are aged for 5 months in 30% relative humidity. This transformation, the first of its kind to be reported in the literature, is accompanied by a surface area loss by a factor of 3 or greater. The uphill phase transformation from the nanocrystalline to amorphous state appears to be driven by the reduction of the large positive surface energy inherent in the as-crystallized ∼5 nm nanopowder.

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