Vidvuds Ozoliņš

Vacancy-mediated dehydrogenation of sodium alanate

Clarification of the mechanisms of hydrogen release and uptake in transition-metal-doped sodium alanate, NaAlH4, a prototypical high-density complex hydride, has fundamental importance for the development of improved hydrogen-storage materials. In this and most other modern hydrogen-storage materials, H2 release and uptake are accompanied by long-range diffusion of metal species. Using first-principles density-functional theory calculations, we have determined that the activation energy for Al mass transport via AlH3 vacancies is Q = 85 kJ/mol·H2, which is in excellent agreement with experimentally measured activation energies in Ti-catalyzed NaAlH4. The activation energy for an alternate decomposition mechanism via NaH vacancies is found to be significantly higher: Q = 112 kJ/mol·H2. Our results suggest that bulk diffusion of Al species is the rate-limiting step in the dehydrogenation of Ti-doped samples of NaAlH4 and that the much higher activation energies measured for uncatalyzed samples are controlled by other processes, such as breaking up of AlH4− complexes, formation/dissociation of H2 molecules, and/or nucleation of the product phases.

Compressed modes for variational problems in mathematics and physics

Significance Intuition suggests that many interesting phenomena in physics, chemistry, and materials science are “short-sighted”—that is, perturbation in a small spatial region only affects its immediate surroundings. In mathematical terms, near-sightedness is described by functions of finite range. As an example, the so-called Wannier functions in quantum mechanics are localized functions, which contain all the information about the properties of the system, including its spectral properties. This work’s main research objective is to develop theory and numerical methods that can systematically derive functions that span the energy spectrum of a given quantum-mechanical system and are nonzero only in a finite spatial region. These ideas hold the key for developing efficient methods for solving partial differential equations of mathematical physics. This article describes a general formalism for obtaining spatially localized (“sparse”) solutions to a class of problems in mathematical physics, which can be recast as variational optimization problems, such as the important case of Schrödinger’s equation in quantum mechanics. Sparsity is achieved by adding an regularization term to the variational principle, which is shown to yield solutions with compact support (“compressed modes”). Linear combinations of these modes approximate the eigenvalue spectrum and eigenfunctions in a systematically improvable manner, and the localization properties of compressed modes make them an attractive choice for use with efficient numerical algorithms that scale linearly with the problem size.

Compressive sensing as a paradigm for building physics models

The widely-accepted intuition that the important properties of solids are determined by a few key variables underpins many methods in physics. Though this reductionist paradigm is applicable in many physical problems, its utility can be limited because the intuition for identifying the key variables often does not exist or is difficult to develop. Machine learning algorithms (genetic programming, neural networks, Bayesian methods, etc.) attempt to eliminate the a priori need for such intuition but often do so with increased computational burden and human time. A recently-developed technique in the field of signal processing, compressive sensing (CS), provides a simple, general, and efficient way of finding the key descriptive variables. CS is a new paradigm for model building-we show that its models are just as robust as those built by current state-of-the-art approaches, but can be constructed at a fraction of the computational cost and user effort.

Reaction energetics and crystal structure of Li4 BN3 H10 from first principles

Using density functional theory we examine the crystal structure and the finite-temperature thermodynamics of formation and dehydrogenation for the quaternary hydride

Lattice Anharmonicity and Thermal Conductivity from Compressive Sensing of First-Principles Calculations

{\mathrm{Li}}_{4}{\mathrm{BN}}_{3}{\mathrm{H}}_{10}

Nanotwin formation in copper thin films by stress/strain relaxation in pulse electrodeposition

. Two recent studies based on x-ray and neutron diffraction have reported three bcc crystal structures for this phase. While these structures possess identical space groups and similar lattice constants, internal coordinate differences result in bond length discrepancies as large as

Effects of vibrational entropy on the Al-Si phase diagram

0.2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}

Compressed plane waves yield a compactly supported multiresolution basis for the Laplace operator

. Geometry optimization calculations on the experimental structures reveal that the apparent discrepancies are an artifact of x-ray interactions with strong bond polarization; the relaxed structures are essentially identical. Regarding reaction energetics, the present calculations predict that the formation reaction

Crystal field and magnetic structure of UO 2

3{\mathrm{LiNH}}_{2}+{\mathrm{LiBH}}_{4}\ensuremath{\rightarrow}{\mathrm{Li}}_{4}{\mathrm{BN}}_{3}{\mathrm{H}}_{10}

Non-Grotthuss proton diffusion mechanism in tungsten oxide dihydrate from first-principles calculations

is exothermic with enthalpy