49 research outputs found

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

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    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

    How inclusion became exclusion: Policy, teachers and inclusive education

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    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

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    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
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