Development of electron holes across the temperature-induced semiconductor-metal transition in Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ)(x, y = 0.2-0.8): a soft x-ray absorption spectroscopy study.

The x-ray absorption spectra of Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) (BSCF) powders quenched in air from 623 and 1173 K were measured at the oxygen K and transition metal L(II,III) edges. All the samples show a predominantly Fe high spin ground state of 3d(5)L character, while the 3d(6)L Co ions are intermediate spin at 623 K and high spin at 1173 K. Further changes in the metal L(II,III) peaks caused by higher temperature quenching are attributed to changes in symmetry around the cations associated with oxygen loss. The oxygen K spectra show the development of unoccupied states just above the Fermi level for samples quenched from 1173 K. At 1173 K, Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) shows metallic conductivity, while at 623 K it is a semiconductor; the states developed at high temperature with strong oxygen character are pathways for hole conductivity. Splitting of the transition metal 3d energy levels by the ligand field was observed in the oxygen K spectra, and the range for 10Dq is 1.6-1.8 eV, while the 3d bandwidth is 1.1-1.4 eV in samples quenched from 623 K. On the basis of the soft x-ray absorption results, the classification of Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) as a material with a negative charge-transfer energy is proposed.

Surface diffusion and surface growth in nanofilms of mixed rocksalt oxides.

We have modelled the surface diffusion and growth of BaO and SrO both in the homoepitaxial and heteroepitaxial (BaO on SrO and SrO on BaO) cases. The diffusion proceeds most favourably by an exchange mechanism involving the surface layer. When impurities are adsorbed on the surface this can lead to intermixing between the layers. This strongly suggests that ionic materials may not be grown on a substrate with a similar structure without significant intermixing. Island growth begins with the formation of individual clusters which grow and merge together.

Iron wheels on silicon: wetting behavior and electronic structure of adsorbed organostannoxane clusters.

Atomic force microscopy and synchrotron radiation (SR) spectroscopy have been used to study the wetting behavior and electronic structure of thin films of a novel organometallic cluster--[BuSn(O)OC(O)Fc]6 ("Fc" = ferrocenyl)--on silicon substrates. This cluster comprises six ferrocene units connected to a stannoxane central core--"an iron wheel on a tin drum" (V. Chandrasekhar; et al. Angew. Chem., Int. Ed. 2000, 39, 1833). Thin films spin-cast onto native oxide-terminated silicon readily dewet the substrate. We have utilized advanced image analysis techniques based on Minkowski functionals to provide a detailed quantitative analysis of the morphology of the stannoxane overlayers. This analysis shows that the dewetting patterns are rather far removed from those expected to arise from a simple Poisson distribution of centers, and we discuss the implications of this finding in terms of nucleated and spinodal dewetting. Variations in both the surface roughness and the in-plane correlation length have been followed as a function of annealing time to probe the surface dewetting dynamics. SR valence band photoemission illustrates that the highest occupied molecular orbital (HOMO) of the cluster is found 2 eV below the Fermi level. Fe 2p --> 3d and Sn 3d --> 5p resonant photoemission spectroscopy have been used to enhance the cross sections of the partial density of states associated with the Fe and Sn atoms. Sn atoms make a large contribution to the HOMO of the cluster, whereas the Fe atoms are associated with an electronic environment seemingly very similar to that in the "parent" ferrocene molecule.

The 'zero charge' partitioning behaviour of noble gases during mantle melting.

Noble-gas geochemistry is an important tool for understanding planetary processes from accretion to mantle dynamics and atmospheric formation. Central to much of the modelling of such processes is the crystal-melt partitioning of noble gases during mantle melting, magma ascent and near-surface degassing. Geochemists have traditionally considered the 'inert' noble gases to be extremely incompatible elements, with almost 100 per cent extraction efficiency from the solid phase during melting processes. Previously published experimental data on partitioning between crystalline silicates and melts has, however, suggested that noble gases approach compatible behaviour, and a significant proportion should therefore remain in the mantle during melt extraction. Here we present experimental data to show that noble gases are more incompatible than previously demonstrated, but not necessarily to the extent assumed or required by geochemical models. Independent atomistic computer simulations indicate that noble gases can be considered as species of 'zero charge' incorporated at crystal lattice sites. Together with the lattice strain model, this provides a theoretical framework with which to model noble-gas geochemistry as a function of residual mantle mineralogy.

Dopant incorporation into garnet solid solutions--a breakdown of Goldschmidt's first rule.

Atomistic simulations suggest trace elements are more soluble in a 50:50 pyrope (Mg3Al2Si3O12)-grossular (Ca3Al2Si3O12) garnet mixture than in either end-member; consistent with partitioning experiments, and, contrary to Goldschmidt's first rule, large trace element cations may substitute for Mg2+, small trace elements for Ca2+.