We present the results of a systematic study of the reconstruction of the Si(100) surface based upon total energies calculated within the framework of the local density approximation. We focus on the extent to which total energy differences may be calculated reliably by examining these differences for the ideal surface and four proposed reconstructions: p(2 x 1) symmetric, p(2 x 1) asymmetric, p(2 x 2), and c(4 x 2). The calculations were performed using norm-conserving pseudopotentials and a plane-wave basis. The convergence of the total energy differences was assessed by varying the energy cutoff used to truncate the plane-wave basis and the number of sampling points used to perform Brillouin zone (BZ) integrals over a large range. The effect of optimizing atomic geometries as a function of the energy cutoff and density of BZ sampling points was determined. With the exception of the p(2 x 2) and c(4 x 2) reconstructions, whose energies only difFer by 3 meV per dimer, we are able to unambiguously determine the energy ordering of the 6ve systems studied. Disagreements between previous calculations can be largely understood in terms of the different energy cutoffs and BZ samplings used. The electronic structures of the different reconstructions are calculated and compared.

Nanoparticulate ferrites (39-105 nm), including magnetite (Fe 3 O 4 ) and materials containing additional metals (cobalt, zinc, and nickel), have been synthesized continuously in near-critical and supercritical water. For comparison, a cobalt ferrite (particle size, 16 nm) was synthesized by a conventional hydrothermal procedure. The local metal environments of the iron atoms and the additional metal in the ferrites have been studied by X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) in order to determine the type of spinel structure. Previously reported results for the bulk-phase materials show that Fe 3 O 4 (magnetite), CoFe 2 O 4 , and NiFe 2 O 4 exhibit inverse spinel structure, while ZnFe 2 O 4 is a normal spinel. Our results show that the inverse spinel structures extend to the nanoregime. For the ZnFe 2 O 4 material, XANES and EXAFS show that the sample adopts the normal structure in the same size regime. All of the materials are very polydisperse. In the case of CoFe 2 O 4 synthesized under supercritical conditions the EXAFS clearly indicates that the particle sizes are weighted heavily toward 10 nm. To interpret the EXAFS, it was necessary to calculate actual or apparent multiplicities of the different interactions. These are based on 1 / 3 of the metal atoms occupying the tetrahedral positions and 2 / 3 the octahedral positions of the spinel structure.

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