28 research outputs found

    Density Functional Theory

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    Density Functional Theory (DFT) is a powerful technique for calculating and comprehending the molecular and electrical structure of atoms, molecules, clusters, and solids. Its use is based not only on the capacity to calculate the molecular characteristics of the species of interest but also on the provision of interesting concepts that aid in a better understanding of the chemical reactivity of the systems under study. This book presents examples of recent advances, new perspectives, and applications of DFT for the understanding of chemical reactivity through descriptors forming the basis of Conceptual DFT as well as the application of the theory and its related computational procedures in the determination of the molecular properties of different systems of academic, social, and industrial interest

    The 1st International Electronic Conference on Chemical Sensors and Analytical Chemistry

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    The 1st International Electronic Conference on Chemical Sensors and Analytical Chemistry was held on 1–15 July 2021. The scope of this online conference was to gather experts that are well-known worldwide who are currently working in chemical sensor technologies and to provide an online forum for the presention and discussion of new results. Throughout this event, topics of interest included, but were not limited to, the following: electrochemical devices and sensors; optical chemical sensors; mass-sensitive sensors; materials for chemical sensing; nano- and micro-technologies for sensing; chemical assays and validation; chemical sensor applications; analytical methods; gas sensors and apparatuses; electronic noses; electronic tongues; microfluidic devices; lab-on-a-chip; single-molecule sensing; nanosensors; and medico-diagnostic testing

    Commemorative Issue in Honor of Professor Karlheinz Schwarz on the Occasion of His 80th Birthday

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    A collection of 18 scientific papers written in honor of Professor Karlheinz Schwarz's 80th birthday. The main topics include spectroscopy, excited states, DFT developments, results analysis, solid states, and surfaces

    Coupling of crystal, electronic, and magnetic structures in quantum materials

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    The focus of this thesis is to understand a variety of emergent materials properties that are driven by quantum phenomena. A range of materials are investigated where the crystal, electronic and magnetic structures are delicately coupled to give rise to intriguing macroscopic properties. Computational methods are the primary tool for tackling these problems. This is largely centred around quantum mechanical, ab initio calculations using density functional theory (DFT), but also includes mean-field analyses and stochastic simulations of a model Hamiltonian. Tin telluride, SnTe, is a crystalline topological insulator with potential applications in a new generation of spintronics devices. In practice, SnTe shows a low temperature ferroelectric distortion and contains a large number of bulk carriers from Sn vacancies, so stands as a rare example of coexistence between metallicity and ferroelectricity. The implications of these effects for topological and transport properties require accurate modelling of the electronic structure. Here, in close collaboration with experiment, the evolution of the Fermi surface across this structural transition is probed by calculating quantum oscillation frequencies. The image analysis tools of Mathematica were exploited to develop a code for computing these frequencies from DFT electronic structure calculations. Agreement between experiment and theory is crucially dependent on the crystal structure. Calcium ruthenate, Ca2RuO4, is a layered perovskite compound with a rich phase diagram, which can be traced back to its strongly correlated electronic structure. Hydrostatic pressure can be used as a means to manipulate its crystal structure, which encourages several interesting effects. In particular, Ca2RuO4 undergoes anomalous expansion of the c axis, and a first-order structural transition coupled with a Mott insulator-metal transition. Here, this pressure response is investigated with DFT+U calculations, which account for the importance of electron correlation by adding an on-site Hubbard-like repulsion term. This work presents the first fully self-consistent electronic structure for Ca2RuO4, obtained from optimised crystal structures along a sequence of pressures. This appreciation of the coupling between lattice and electronic degrees of freedom sheds some light on its unusual phase diagram. The insulator-metal transition is reproduced and naturally coincides with a structural transition and associated orbital order. For the metallic phase, a complex energetic landscape with several competing phases emerges. Uranium gold, UAu2, is a heavy fermion metal with a complex spin-density-wave (SDW) phase at low temperature. This phase consists of frustrated, incommensurate ordering that is very robust in external fields, but suppressed by pressure to reveal unconventional superconductivity. The origin of this exotic magnetic ordering is of interest here, which is explored by several different approaches with DFT+U. Both itinerant and local-moment pictures of the magnetism are entertained. Fermi surface nesting is shown to be ineffective, but mapping the system to a Heisenberg model and computing effective exchange interactions identifies an instability towards modulated order. In addition to these material-specific investigations, the treatment of longitudinal magnetic fluctuations in computational methods is studied. Magnetic fluctuations are important for understanding materials on the border between itinerant and local-moment magnetism. The continuous-spin Ising (CSI) model is investigated here as a phenomenological model of these fluctuations. Using a bespoke simulation technique this model is extensively explored, firstly on a cubic ferromagnet and secondly on a highly frustrated, stacked triangular lattice with a variety of interaction topologies. The introduction of fluctuations is shown to alter the ground state of the prototypical frustrated triangular lattice, and to enhance the transition temperature of more severely frustrated systems. In all of these cases, different degrees of freedom in the system couple to one another, either to make an accurate theoretical description difficult, or to give rise to unexpected emergent properties. Both SnTe and Ca2RuO4 represent examples of delicately coupled crystal and electronic structures. In SnTe, the tuning of the crystal structure is vital to correctly describe the Fermi surface, while in Ca2RuO4 the Ru orbital structure very directly determines the structural properties, and drives the unusual pressure response. UAu2 introduces a non-trivial magnetic structure, which requires careful treatment of electron-electron interactions to locate. In the study of the CSI model, altering the interaction topology, which acts as a proxy for manipulating the underlying electronic structure, has drastic implications for the role of magnetic fluctuations in the system

    Structures, Structural Transfromations and Properties of Selected Elemental and Extended Solids

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    The current boom in computer power has created avenue to study materials’ properties under extreme thermodynamic conditions where experimental characterization is very challenging. This thesis is an aggregation of several objectives ranging from the study of elemental as well as extended materials for technological, high energy density (HED), and geophysical applications; all at high pressure. The density functional theory (DFT), ab initio metadynamics and ab initio molecular dynamics (AIMD) methods have been employed to analyze structural phase transitions, electronic, vibrational, and mechanical properties of selected materials at high pressure. Where available, high-pressure-high-temperature (HPHT) experiments were combined with the various theoretical methods for complete elucidation of the system. The first set of projects in this thesis involve study of structural phase transition in two elements: carbon (C) and nitrogen (N). The first part presents the results of structural phase transition in a two-dimensional polymeric C60 after being subjected to uniaxial compression at high temperature in a metadynamics simulation. The new structure exhibits a mixed sp2/sp3 hybridization. The structure is stable at ambient condition and exhibits superior mechanical performance than most of widely used hard ceramics. The second part presents theoretical results on the identification, and characterization of single bonded nitrogen in crystal structure isostructural to black phosphorus (BP-N) at 146 GPa and 2200 K. The crystal structure exhibits a unique puckered two-dimensional layer exhibiting exciting physical and chemical phenomena including prospect for high energy density (HED) applications. Synchrotron x-ray diffraction and Raman spectroscopy were used for experimental characterization of the BP-N. First-principles methods were employed in the theoretical characterization. The second set of projects involve the theoretical studies of transition metal (TM) -TM alloys/compounds. The first part of the chapter investigates structural phase transition leading to shape memory loss in the shape memory alloy NiTi. The second part investigates the formation of Au-Fe compounds at high pressure. A detailed analysis of the transition kinetics and dynamical pathway in NiTi using the metadynamics method reveals the possibility of the B19′ phase of NiTi losing its shape memory when subjected to high stress conditions and heated above a critical temperature (Tc) of 700 K. Using the particle swarm-intelligence optimization algorithm interfaced with first principles methods, we predicted the formation of bulk intermetallic compounds of two bulk-immiscible components, Fe and Au. the systems are stabilized by pressure and notable electron transfer. Next, the results of theoretical studies of the formation of noble gas element - TM compound were presented. The identification of a thermodynamically stable compound of Argon (Ar) and nickel (Ni) under thermodynamic conditions representative of the Earth’s core using density functional calculations were presented. The study present evidence of the reactability of Ar with one of the Earth’s core’s main constituents, Ni. The compound of Ar and Ni was identified as ArNi with a L11 Laves structure. It was found that ArNi compound is stabilized by notable electron transfer from Ni to Ar. The final project is an extensive theoretical study of the formation of alkali metal-transition metal intermetallic compounds at high pressure and temperature relevant to the upper mantle and the core of the Earth. These studies were carried out using particle swarm-intelligence optimization and genetic algorithms interfaced with first principles methods. The first part investigates the formation of K-Fe compounds at thermodynamics conditions relevant to the Earth’s interior while the second part investigates the formation of K-Ni compounds in the Earth’s interior. It was found that K and Fe can form intermetallic compounds that are stabilized by high pressure and energy reordering of atomic orbital. Phase transitions were also reported and the instabilities that induce them were also investigated. Furthermore, the study on K-Ni systems identify the crystal structure for the long-sought structure of the only experimentally known K-Ni compound to date. The identified K2Ni exhibits a semiconducting ground state with an indirect bandgap. The results of both studies indicate that the chemical properties of elements can change dramatically under extreme conditions and could have significant implications for understanding the Earth’s interior

    The LAPW method with eigendecomposition based on the Hari--Zimmermann generalized hyperbolic SVD

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    In this paper we propose an accurate, highly parallel algorithm for the generalized eigendecomposition of a matrix pair (H,S)(H, S), given in a factored form (F∗JF,G∗G)(F^{\ast} J F, G^{\ast} G). Matrices HH and SS are generally complex and Hermitian, and SS is positive definite. This type of matrices emerges from the representation of the Hamiltonian of a quantum mechanical system in terms of an overcomplete set of basis functions. This expansion is part of a class of models within the broad field of Density Functional Theory, which is considered the golden standard in condensed matter physics. The overall algorithm consists of four phases, the second and the fourth being optional, where the two last phases are computation of the generalized hyperbolic SVD of a complex matrix pair (F,G)(F,G), according to a given matrix JJ defining the hyperbolic scalar product. If J=IJ = I, then these two phases compute the GSVD in parallel very accurately and efficiently.Comment: The supplementary material is available at https://web.math.pmf.unizg.hr/mfbda/papers/sm-SISC.pdf due to its size. This revised manuscript is currently being considered for publicatio

    ATOMIC SCALE SIMULATION OF ACCIDENT TOLERANT FUEL MATERIALS FOR FUTURE NUCLEAR REACTORS

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    The 2011 accident at the Fukushima-Daiichi power station following the earthquake and tsunami in Japan put renewed emphasis on increasing the accident tolerance of nuclear fuels. Although the main concern in this incident was the loss of coolant and the Zr cladding reacting with water to form hydrogen, the fuel element is an integral part of any accident tolerant fuel (ATF) concept. Therefore, to license a new commercial nuclear fuel, the prediction of fuel behavior during operation becomes a necessity. This requires knowledge of its properties as a function of temperature, pressure, initial fuel microstructure and irradiation history, or more precisely the changes in microstructure due to irradiation and/or oxidation. Amongst other nuclear fuels, uranium diboride (UB2) and uranium silicide (U3Si2) are considered as potential fuels for the next generation of nuclear reactors due to their high uranium density and high thermal conductivity compared to uranium dioxide (UO2). However, the thermophysical properties and behavior of these fuels under extreme conditions are not well known, neither are they readily available in the literature. Therefore, in this thesis, density functional theory (DFT) and classical molecular dynamic (MD) simulations were used to investigate the thermophysical properties, radiation tolerance and oxidation behavior of UB2 and U3Si2 as potential fuels or burnable absorbers for the next generation of nuclear reactors. UB2 was studied in order to understand its thermophysical properties as a function of temperature. The phonon-assisted thermal conductivity (kph) exhibits large directional anisotropy with larger thermal conductivity parallel to the crystal direction. This has implications for the even dissipation of heat. The increase in thermal conductivity with temperature is justified by the electronic contribution to the thermal transport, especially at high temperatures. This shows that UB2 is a potential ATF candidate. In terms of radiation tolerance, Zr is more soluble in UB2 than Xe, while uranium vacancy is the most stable solution site. Furthermore, as the concentration of Zr fission product (FP) increases, there is a contraction in the volume of UB2, while an increase in Xe results in swelling of the fuel matrix. In terms of diffusion, the presence of an FP in the neighboring U site increases the migration of U in UB2, making U migrate more readily than B as observed in the ideal system. The thermophysical properties of U3Si2 as a possible ATF were studied and discussed considering the neutronic penalty of using a SiC cladding in a reactor. The calculated molar heat capacity and experimental data are in reasonable agreement. Due to the anisotropy in lattice expansion, a directional dependence in the linear thermal expansion coefficient was noticed, which has also been experimentally observed. The thermal conductivity of U3Si2 increases with temperature due to the electronic contribution while the phonon contribution decreases with increasing temperature. A comparison of the thermal conductivity in two different crystallographic directions sheds light on the spatial anisotropy in U3Si2 fuel material. The inherent anisotropic thermophysical properties can be used to parametrize phase field models by incorporating anisotropic thermal conductivity and thermal expansion. This allows for a more accurate description of microstructural changes under variable temperature and irradiation conditions. Due to the metallic nature of U3Si2, the oxidation mechanism is of special interest and has to be investigated. Oxidation in O2 and H2O was investigated using experimental and theoretical methods. The presence of oxide signatures was established from X-ray diffraction (XRD) and Raman spectroscopy after oxidation of the solid U3Si2 sample in oxygen. Surface oxidation of U3Si2 can be linked to the significant charge transfer from surface uranium ions to water and/or oxygen molecules. Detailed charge transfer and bond length analysis revealed the preferential formation of mixed oxides of U-O and Si-O on the U3Si2 (001) surface as well as UO2 alone on the U3Si2 (110) and (111) surfaces. Formation of elongated O−O bonds (peroxo) confirmed the dissociation of molecular oxygen before U3Si2 oxidation. Experimental analysis by Raman spectroscopy and XRD of the oxidized U3Si2 samples has revealed the formation of higher uranium oxides such as UO3 and U3O8. Overall, this work serves as a step towards understanding the complex anisotropic behavior of the thermophysical properties of metallic UB2 and U3Si2 considered as potential accident tolerant nuclear fuel. The calculated anisotropy of thermophysical properties can be used to parametrize phase field model and to incorporate in it anisotropic thermal conductivity and thermal expansion

    Synthesis of new pyrazolium based tunable aryl alkyl ionic liquids and their use in removal of methylene blue from aqueous solution

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    In this study, two new pyrazolium based tunable aryl alkyl ionic liquids, 2-ethyl-1-(4-methylphenyl)-3,5- dimethylpyrazolium tetrafluoroborate (3a) and 1-(4-methylphenyl)-2-pentyl-3,5-dimethylpyrazolium tetrafluoroborate (3b), were synthesized via three-step reaction and characterized. The removal of methylene blue (MB) from aqueous solution has been investigated using the synthesized salts as an extractant and methylene chloride as a solvent. The obtained results show that MB was extracted from aqueous solution with high extraction efficiency up to 87 % at room temperature at the natural pH of MB solution. The influence of the alkyl chain length on the properties of the salts and their extraction efficiency of MB was investigated

    Ab-Initio Studies into Intrinsic Piezoelectric Properties

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    We formulate a method of understanding the intrinsic piezoelectric properties of a perovskite system on an atomistic level using a density functional theory (DFT) methodology implemented into the electron density code CASTEP. First we consider the basic unary perovskites; barium titanate, lead titanate and potassium niobate. Geometry optimisation, elastic compliance, linear response, and simulated strain calculations are performed and the polarisation and piezoelectric coefficients are calculated. We then define the partial piezoelectric coefficient and demonstrate a way to generate the electron density shift, two novel methods in the understanding of intrinsic piezoelectric properties. We then study the binary piezoelectric lead titanate, performing the same calculations and analysing the electron density shift and partial piezoelectric coefficient in order to identify new features of binary systems, the equalisation between basic perovskite units and the "sawtooth" bonding asymmetry between the different B-site ions. Then the feasibility of these calculations is evaluated for bismuth ferrite, a material showing multiferroic properties. The conversion of non-orthonormal lattice axes to a cartesian coordinate system is addressed. We discuss the phonon and electric field calculations, and evaluate what is and is not possible. We find that structural and electron calculations are possible and report the optimised geometry, the elastic compliance, total electron density and spin maps, and the electron density shifts. We identify the symmetry, non-locality, and rotational modes in rhombohedral bismuth ferrite and suggest future research based on these properties
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