First-principles study of Ti-doped sodium alanate surfaces

We have performed first-principles calculations of thick slabs of Ti-doped sodium alanate (NaAlH_4), which allows to study the system energetics as the dopant progresses from the surface to the bulk. Our calculations predict that Ti stays on the surface, substitutes for Na, and attracts a large number of H atoms to its vicinity. Molecular dynamics simulations suggest that the most likely product of the Ti-doping is the formation of H-rich TiAl_n (n>1) compounds on the surface, and hint at the mechanism by which Ti enhances the reaction kinetics of NaAlH_4.Comment: 3 pages with 3 postscript figures embedded. Uses REVTEX4 and graphicx macros. More information at http://www.ncnr.nist.gov/staff/taner/alanates

Effective-Hamiltonian modeling of external pressures in ferroelectric perovskites

The phase-transition sequence of a ferroelectric perovskite such as BaTiO_3 can be simulated by computing the statistical mechanics of a first-principles derived effective Hamiltonian [Zhong, Vanderbilt and Rabe, Phys. Rev. Lett. 73, 1861 (1994)]. Within this method, the effect of an external pressure (in general, of any external field) can be studied by considering the appropriate "enthalpy" instead of the effective Hamiltonian itself. The legitimacy of this approach relies on two critical assumptions that, to the best of our knowledge, have not been adequately discussed in the literature to date: (i) that the zero-pressure relevant degrees of freedom are still the only relevant degrees of freedom at finite pressures, and (ii) that the truncation of the Taylor expansion of the energy considered in the effective Hamiltonian remains a good approximation at finite pressures. Here we address these issues in detail and present illustrative first-principles results for BaTiO_3. We also discuss how to construct effective Hamiltonians in cases in which these assumptions do not hold.Comment: 5 pages, with 2 postscript figures embedded. Proceedings of "Fundamental Physics of Ferroelectrics, 2002", R. Cohen and T. Egami, eds. (AIP, Melville, New York, 2002). Also available at http://www.physics.rutgers.edu/~dhv/preprints/ji_effp/index.htm

Structurally Triggered Metal-Insulator Transition in Rare-Earth Nickelates

Rare-earth nickelates form an intriguing series of correlated perovskite oxides. Apart from LaNiO3, they exhibit on cooling a sharp metal-insulator electronic phase transition, a concurrent structural phase transition and a magnetic phase transition toward an unusual antiferromagnetic spin order. Appealing for various applications, full exploitation of these compounds is still hampered by the lack of global understanding of the interplay between their electronic, structural and magnetic properties. Here, we show from first-principles calculations that the metal-insulator transition of nickelates arises from the softening of an oxygen breathing distortion, structurally triggered by oxygen-octahedra rotation motions. The origin of such a rare triggered mechanism is traced back in their electronic and magnetic properties, providing a united picture. We further develop a Landau model accounting for the evolution of the metal-insulator transition in terms of the $R cations and rationalising how to tune this transition by acting on oxygen rotation motions.Comment: Submitted in Nature Communicatio

Helping School Administrators Involve Hispanic Parents in Their Children\u27s Education

The purpose of this project was to help school administrators more effectively involve Hispanic parents in the education of their children both at school and in the home. To accomplish this task, principals and parents from four heavily impacted Hispanic school districts were interviewed. Research was undertaken of pertinent literature and a study of parent involvement programs was also included. Based upon analysis of interviews and research a conceptual framework on how to involve Hispanic parents in the education of their children was developed along with examples of specific programs and activities

Small Ruminant Breeding Plans for Smallholder Systems

The transfer of animal genetic improvement strategies in the environments of resource-poor smallholder producers in developing countries has not been successful. Therefore, the access by farmers to sources of animals with high genetic potential is difficult or just not possible. As producers improve or intensify their systems to capture market opportunities, this access need is accentuated. This paper reviews identified constraints to the transfer of breeding plans, the need for a new approach, and the lessons learned in applying a better-targeted approach. The issues and thoughts were derived from the experience of a working group involving the International Center for Agricultural Research in the Dry Areas; Austrian University of Natural Resources and Life Sciences; Argentinean National Institute of Agriculture Technology; Brazilian Agricultural Research Corporation (Embrapa Goats and Sheep); and National Research Institute for Forestry, Agriculture and Animal Production-Mexico. The transfer of breeding strategies in smallholder systems is not a straightforward task. Contrasting a successful application in commercial livestock production environments, the achievement of the primary objectives in smallholder systems requires a holistic approach beyond pure technical matters. Of all identified aspects with direct incidence in the implementation of breeding plans, the lack of specific national policies coupled with long-term funding to support the improvement of smallholder production systems, and the lack of involvement and participation of the communities emerge as issues that gravitate more to achieve sustainability and meet the objectives of a given breeding plan