High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, sigma, in prototypical transition metal oxide ABO_3 perovskite crystals. Our principal findings are 1) a large anisotropy of sigma between deshielded sigma_x ~ sigma_y and shielded sigma_z components (z along the Ti-O bond); 2) a nearly linear variation, across all the systems studied, of the isotropic sigma_iso and uniaxial sigma_ax components, as a function of the B-O-B bond asymmetry. We show that the anisotropy and linear variation arise from large paramagnetic contributions to sigma_x and sigma_y due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic delta_iso and uniaxial delta_ax chemical shifts are in good agreement with recent BaTiO_3 and SrTiO_3 single crystal 17O NMR measurements. In PbTiO_3 and PbZrO_3, calculated delta_iso are also in good agreement with NMR powder spectrum measurements. In PbZrO_3, delta_iso calculations of the five chemically distinct sites indicate a correction of the experimental assignments. The strong dependence of sigma on covalent O(2p)-B(nd) interactions seen in our calculations indicates that 17O NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to study the local structure in complex perovskite alloys.Comment: 12 pages, 3 figures, and 3 Table

Investigation of pin-post monochromators for a wiggler beamline

Three water-cooled pin-post monochromators, to be used on a wiggler beamline at the Advanced Photon Source (APS), were built with the heat exchanger engineered to provide very high heat transfer. The geometry of the heat exchanger as well as calculated data on the heat transfer will be presented. Before using the monochromators on the beamline, they were checked by x-ray diffraction topography. Reflections (333) and (220) in Bragg case were utilized. In all crystals, similar patterns of strain in the diffracting silicon layers were revealed, which can be attributed to the geometry of the heat exchangers, the bonding technology, and the thickness of the top layer. Conclusions about construction of future pin-post monochromators have been drawn

Laue diffraction protein crystallography at the National Synchrotron Light Source

A new facility for the study of protein crystal structure using Laue diffraction has been established at the X26 beam line of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The characteristics of the beam line and diffraction apparatus are described. Selected results of some of the initial experiments are discussed briefly by beam line users to illustrate the scope of the experimental program. Because the Laue method permits the recording of large data sets in a single shot, one goal in establishing this facility has been to develop the means to study time-resolved structures within protein crystals. Systems being studied include: the reactions catalyzed by trypsin; photolysis of carbonmonoxy myoglobin; and the photocycle of photoactive yellow protein

How inclusion became exclusion: Policy, teachers and inclusive education

It is almost two decades since a concept of inclusion as selective segregation was proposed as an alternative to the concept of full inclusion and inclusive education was reconfigured as providing children with varied educational settings in order to meet their needs. A version of this model of inclusive education subsequently gained political traction in England where the issue of segregated or mainstream provision is now constructed as a matter of parental choice and child voice. Meanwhile, the implications of this latest model of inclusive education for teachers and schools in a rapidly changing wider educational landscape have largely been ignored or reduced to a question of training. This paper explores how the inclusive education landscape has changed in England in recent years, charting recent key developments in areas such as policy, statutory guidance and teacher training, with particular reference to teacher workload and the positioning of teachers within political and polemical educational discourse

Cognitive development in Dutch primary education, the impact of individual background and classroom composition

A sample of 815 Dutch pupils from 49 classes was followed from age 6 (Dutch Grade 3) through age 11 (Grade 7) to estimate growth trajectories for pupils with different socio-ethnic backgrounds. The results indicate that the disadvantage for spelling already present in Grade 3 increases more strongly for Dutch low-socioeconomic status (SES) pupils than for low-SES minority pupils. With regard to mathematics, the initial disadvantage of Dutch low-SES pupils hardly changes, whereas the gap is clearly reduced for low-SES minorities. Effects of classroom composition appear to be quite modest, but classes with very high proportions of low-SES minority pupils show a clear disadvantage for both spelling and mathematics at age 6. In subsequent grades, the spelling gap between advantaged and disadvantaged classroom populations disappears completely. For mathematics, the gap is initially reduced to some extent, but at age 11 the disadvantage increases again