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Journal articleParkes MA, Tompsett DA, d'Avezac M, et al., 2016,
The atomistic structure of yttria stabilised zirconia at 6.7 mol%: an ab initio study.
, Physical Chemistry Chemical Physics, Vol: 18, Pages: 31277-31285, ISSN: 1463-9084Yttria stabilized zirconia (YSZ) is an important oxide ion conductor used in solid oxide fuel cells, oxygen sensing devices, and for oxygen separation. Doping pure zirconia (ZrO2) with yttria (Y2O3) stabilizes the cubic structure against phonon induced distortions and this facilitates high oxide ion conductivity. The local atomic structure of the dopant is, however, not fully understood. X-ray and neutron diffraction experiments have established that, for dopant concentrations below 40 mol% Y2O3, no long range order is established. A variety of local structures have been suggested on the basis of theoretical and computational models of dopant energetics. These studies have been restricted by the difficulty of establishing force field models with predictive accuracy or exploring the large space of dopant configurations with first principles theory. In the current study a comprehensive search for all symmetry independent configurations (2857 candidates) is performed for 6.7 mol% YSZ modelled in a 2 × 2 × 2 periodic supercell using gradient corrected density functional theory. The lowest energy dopant structures are found to have oxygen vacancy pairs preferentially aligned along the ⟨210⟩ crystallographic direction in contrast to previous results which have suggested that orientation along the ⟨111⟩ orientation is favourable. Analysis of the defect structures suggests that the Y(3+)-Ovac interatomic separation is an important parameter for determining the relative configurational energies. Current force field models are found to be poor predictors of the lowest energy structures. It is suggested that the energies from a simple point charge model evaluated at unrelaxed geometries is actually a better descriptor of the energy ordering of dopant structures. Using these observations a pragmatic procedure for identifying low energy structures in more complicated material models is suggested. Calculation of the oxygen vacancy migration activat
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Journal articlePetrov PK, Zou B, Walker C, et al., 2016,
Growth of Epitaxial Oxide Thin Films on Graphene
, Scientific Reports, Vol: 6, ISSN: 2045-2322The transfer process of graphene onto the surface of oxide substrates is well known.However, for many devices, we require high quality oxide thin films on the surface ofgraphene. This step is not understood. It is not clear why the oxide should adopt theepitaxy of the underlying oxide layer when it is deposited on graphene where there isno lattice match. To date there has been no explanation or suggestion of mechanismswhich clarify this step. Here we show a mechanism, supported by first principlessimulation and structural characterisation results, for the growth of oxide thin films ongraphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a grapheneand show that local defects in the graphene layer (e.g. grain boundaries) act as bridgepillarspots that enable the epitaxial growth of STO thin films on the surface of thegraphene layer. This study, and in particular the suggestion of a mechanism forepitaxial growth of oxides on graphene, offers new directions to exploit thedevelopment of oxide/graphene multilayer structures and devices.
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Journal articleCarta D, Salaoru I, Khiat A, et al., 2016,
Investigation of the Switching Mechanism in TiO2-Based RRAM: A Two-Dimensional EDX Approach
, ACS Applied Materials & Interfaces, Vol: 8, Pages: 19605-19611, ISSN: 1944-8244The next generation of nonvolatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multistate memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and lifetime of TiO2-based resistive random access memory (RRAM).
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Journal articleKorotana RK, Mallia G, Fortunato NM, et al., 2016,
A combined thermodynamics and first principles study of the electronic, lattice and magnetic contributions to the magnetocaloric effect in La0.75Ca0.25MnO3
, Journal of Physics D: Applied Physics, Vol: 49, ISSN: 0022-3727Manganites with the formula La1−x Ca x MnO3 for 0.2 < x < 0.5 undergo a magnetic field driven transition from a paramagnetic to ferromagnetic state, which is accompanied by changes in the lattice and electronic structure. An isotropic expansion of the La0.75Ca0.25MnO3 cell at the phase transition has been observed experimentally. It is expected that there will be a large entropy change at the transition due to its first order nature. Doped lanthanum manganite (LMO) is therefore of interest as the active component in a magnetocaloric cooling device. However, the maximum obtained value for the entropy change in Ca-doped manganites merely reaches a moderate value in the field of a permanent magnet. The present theoretical work aims to shed light on this discrepancy. A combination of finite temperature statistical mechanics and first principles theory is applied to determine individual contributions to the total entropy change of the system by treating the electronic, lattice and magnetic components independently. Hybrid-exchange density functional (B3LYP) calculations and Monte Carlo simulations are performed for La0.75Ca0.25MnO3. Through the analysis of individual entropy contributions, it is found that the electronic and lattice entropy changes oppose the magnetic entropy change. The results highlighted in the present work demonstrate how the electronic and vibrational entropy contributions can have a deleterious effect on the total entropy change and thus the potential cooling power of doped LMO in a magnetocaloric device.
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Journal articleCamino B, Noakes TCQ, Surman M, et al., 2016,
Photoemission simulation for photocathode design: theory and application to copper and silver surfaces
, Computational Materials Science, Vol: 122, Pages: 331-340, ISSN: 0927-0256Obtaining a bright and low emittance electron beam directly from the photocathode is mandatory in order to design high performance free electron lasers (FELs). To achieve this goal a clear understanding of how the emission process is influenced by structure, morphology and composition of the photocathode surface is needed. This is difficult from an experimental point of view because often the atomic scale details of the surface whose emission has been measured are unknown. A predictive theoretical approach capable of determining the effects of surface structure on emission is therefore or great interest. A model to extend the well known three step model (as proposed by Berglund and Spicer) to surface calculations is discussed in this paper. It is based on a layer-by-layer decomposition of the surface electronic structure that can be calculated through reliable and efficient DFT calculations. The advantage of this approach with respect to other existing photoemission calculations is being able to correlate directly the photoemission to the electronic, atomic and chemical structure of the surface. The proposed approach retains, therefore, the simple chemical intuition in the study of surface modifications and their effect on the photoemission. The approach is validated in calculations of the emission from clean copper and silver surfaces. The ability of the model to simulate the change in photoemission in response to adsorbates is tested by simulating monolayers of oxygen, hydrogen and lithium on the copper (1 1 1) surface.
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Journal articleBusayaporn W, Duncan DA, Allegretti F, et al., 2016,
Structure of a Model Dye/Titania Interface: Geometry of Benzoate on Rutile-TiO2 (110)(1 × 1)
, Journal of Physical Chemistry C, Vol: 120, Pages: 14690-14698, ISSN: 1932-7455 -
Journal articleMartÃnez-Casado R, Chen VH, Mallia G, et al., 2016,
A hybrid-exchange density functional study of the bonding and electronic structure in bulk CuFeS2
, Journal of Chemical Physics, Vol: 144, ISSN: 1089-7690The geometric, electronic, and magnetic properties of bulk chalcopyrite CuFeS2 have been investigated using hybrid-exchange density functional theory calculations. The results are compared with available theoretical and experimental data. The theoretical description of the bonding and electronic structure in CuFeS2 is analyzed in detail and compared to those computed for chalcocite (CuS2) and greigite (Fe3S4). In particular, the behavior of the 3d electrons of Fe3+ is discussed in terms of the Hubbard-Anderson model in the strongly correlated regime and found to be similarly described in both materials by an on-site Coulomb repulsion (U) of ∼8.9 eV and a transfer integral (t) of ∼0.3 eV.
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Journal articleInkpen MS, Du S, Hildebrand M, et al., 2015,
The unusual redox properties of fluoroferrocenes revealed through a comprehensive study of the haloferrocenes
, Organometallics, Vol: 34, Pages: 5461-5469, ISSN: 1520-6041We report the synthesis and full characterization of the entire haloferrocene (FcX) and 1,1′-dihaloferrocene (fcX2) series (X = I, Br, Cl, F; Fc = ferrocenyl, fc = ferrocene-1,1′-diyl). Finalization of this simple, yet intriguing set of compounds has been delayed by synthetic challenges associated with the incorporation of fluorine substituents. Successful preparation of fluoroferrocene (FcF) and 1,1′-difluoroferrocene (fcF2) were ultimately achieved using reactions between the appropriate lithiated ferrocene species and N-fluorobenzenesulfonimide (NFSI). The crude reaction products, in addition to those resulting from analogous preparations of chloroferrocene (FcCl) and 1,1′-dichloroferrocene (fcCl2), were utilized as model systems to probe the limits of a previously reported “oxidative purification” methodology. From this investigation and careful solution voltammetry studies, we find that the fluorinated derivatives exhibit the lowest redox potentials of each of the FcX and fcX2 series. This counterintuitive result is discussed with reference to the spectroscopic, structural, and first-principles calculations of these and related materials.
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Journal articleChen VH-Y, Mallia G, Martinez-Casado R, et al., 2015,
Surface morphology of CuFeS2: The stability of the polar (112)/((112)over-bar) surface pair
, Physical Review B, Vol: 92, ISSN: 1550-235XThe reconstruction and energetics for a range of chalcopyrite (CuFeS2) surfaces have been investigated using hybrid-exchange density functional theory. The stable nonpolar surfaces in increasing order of surface energy are (110), (102), and (114). In addition, the polar (112)/(112¯¯¯¯¯) surface pair was found to be remarkably stable with a surface formation energy that is only slightly higher than that of the (110) surface. The stability of (112)/(112¯¯¯¯¯) can be attributed to a combination of geometric and electronic mechanisms that result in the suppression of the electrostatic dipole perpendicular to the surface. Defect formation is a third mechanism that can further stabilize the (112)/(112¯¯¯¯¯) surface pair to an extent that it is thermodynamically preferred over the (110) surface. The stability of (112)/(112¯¯¯¯¯) means that regardless of the growth conditions, (112) and (112¯¯¯¯¯) facets are expected to have a significant presence in the surface morphology of CuFeS2.
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Journal articleAhmad EA, Tileli V, Kramer D, et al., 2015,
Optimizing Oxygen Reduction Catalyst Morphologies from First Principles
, Journal of Physical Chemistry C, Vol: 119, Pages: 16804-16810, ISSN: 1932-7455Catalytic activity of perovskites for oxygen reduction (ORR) wasrecently correlated with bulk d-electron occupancy of the transition metal. Weexpand on the resultant model, which successfully reproduces the high activity ofLaMnO3 relative to other perovskites, by addressing catalyst surface morphology asan important aspect of the optimal ORR catalyst. The nature of reaction sites onlow index surfaces of orthorhombic (Pnma) LaMnO3 is established from FirstPrinciples. The adsorption of O2 is markedly influenced by local geometry andstrong electron correlation. Only one of the six reactions sites that result from experimentally confirmed symmetry-breakingJahn−Teller distortions is found to bind O2 with an intermediate binding energy while facilitating the formation of superoxide, animportant ORR intermediate in alkaline media. As demonstrated here for LaMnO3, rational design of the catalyst morphology topromote specific active sites is a highly effective optimization strategy for advanced functional ORR catalysts.
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Journal articleWebster R, Bernasconi L, Harrison NM, 2015,
Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations.
, Journal of Chemical Physics, Vol: 142, Pages: 214705-214705, ISSN: 1089-7690We present a study of the electronic and optical properties of a series of alkali halide crystals AX, with A = Li, Na, K, Rb and X = F, Cl, Br based on a recent implementation of hybrid-exchange time-dependent density functional theory (TD-DFT) (TD-B3LYP) in the all-electron Gaussian basis set code CRYSTAL. We examine, in particular, the impact of basis set size and quality on the prediction of the optical gap and exciton binding energy. The formation of bound excitons by photoexcitation is observed in all the studied systems and this is shown to be correlated to specific features of the Hartree-Fock exchange component of the TD-DFT response kernel. All computed optical gaps and exciton binding energies are however markedly below estimated experimental and, where available, 2-particle Green's function (GW-Bethe-Salpeter equation, GW-BSE) values. We attribute this reduced exciton binding to the incorrect asymptotics of the B3LYP exchange correlation ground state functional and of the TD-B3LYP response kernel, which lead to a large underestimation of the Coulomb interaction between the excited electron and hole wavefunctions. Considering LiF as an example, we correlate the asymptotic behaviour of the TD-B3LYP kernel to the fraction of Fock exchange admixed in the ground state functional cHF and show that there exists one value of cHF (∼0.32) that reproduces at least semi-quantitatively the optical gap of this material.
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Journal articleParkes MA, Refson K, d'Avezac M, et al., 2015,
Chemical descriptors of yttria-stabilized zirconia at low defect concentration: an ab initio study.
, Journal of Physical Chemistry A, Vol: 119, Pages: 6412-6420, ISSN: 1520-5215Yttria-stabilized zirconia (YSZ) is an important oxide ion conductor with applications in solid oxide fuel cells (SOFCs) and oxygen sensing devices. Doping the cubic phase of zirconia (c-ZrO2) with yttria (Y2O3) is isoelectronic, as two Zr(4+) ions are replaced by two Y(3+) ions, plus a charge compensating oxygen vacancy (Ovac). Typical doping concentrations include 3, 8, 10, and 12 mol %. For these concentrations, and all below 40 mol %, no phase with long-range order has been observed in either X-ray or neutron diffraction experiments. The prediction of local defect structure and the interaction between defects is therefore of great interest. This has not been possible to date as the number of possible defect topologies is very large and to perform reliable total energy calculations for all of them would be prohibitively expensive. Previous theoretical studies have only considered a selection of representative structures. In this study, a comprehensive search for low-energy defect structures using a combined classical modeling and density functional theory approach is used to identify the low-energy isolated defect structures at the dilute limit, 3.2 mol %. Through analysis of energetics computed using the best available Born-Mayer-Huggins empirical potential model, a point charge model, DFT, and a local strain energy estimated in the harmonic approximation, the main chemical and physical descriptors that correlate to the low-energy DFT structures are discussed. It is found that the empirical potential model reproduces a general trend of increasing DFT energetics across a series of locally strain relaxed structures but is unreliable both in predicting some incorrect low-energy structures and in finding some metastable structures to be unstable. A better predictor of low-energy defect structures is found to be the total electrostatic energy of a simple point charge model calculated at the unrelaxed geometries of the defects. In addition, the strain relaxation energ
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Journal articleKucernak ARJ, 2015,
Electrochemical Characterization and Quantified Surface Termination Obtained by LEIS and XPS of Orthorhombic and Rhombohedral LaMnO<sub>3</sub> Powders
, Journal of Physical Chemistry C, Vol: 119, Pages: 12209-12217, ISSN: 1932-7455LaMnO3 powder synthesized by glycine combustion synthesis with the rhombohedral and orthorhombic structures has been characterized by the combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), while the electrocatalytic activity for the oxygen reduction reaction is measured with the rotating disk electrode (RDE) method. Quantification of the surface terminations obtained by LEIS suggests that the orthorhombic LaMnO3 crystallites are near thermodynamic equilibrium as surface atomic ratios compare well with those of equilibrium morphologies computed by a Wulff construction based on computed surface energies. Both rhombohedral and orthorhombic structures present the same La/Mn atomic ratio on the surface. Electrochemical activity of the two structures is found to be the same within the error bar of our measurements. This result is in disagreement with results previously reported on the activity of the two structures obtained by the coprecipitation method [Suntivich et al. Nat. Chem. 2011, 3 (7), 546], and it indicates that the preparation method and the resulting surface termination might play a crucial role for the activity of perovskite catalysts.
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Journal articleTseng H-H, Serri M, Harrison NM, et al., 2015,
Thin film properties of tetracyanoquinodimethane (TCNQ) with novel templating effects
, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 8694-8699, ISSN: 2050-7526- Author Web Link
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- Citations: 8
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Journal articlePatel M, Sanches FF, Mallia G, et al., 2014,
A quantum mechanical study of water adsorption on the (110) surfaces of rutile SnO<sub>2</sub> and TiO<sub>2</sub>: investigating the effects of intermolecular interactions using hybrid-exchange density functional theory
, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 16, Pages: 21002-21015, ISSN: 1463-9076- Author Web Link
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- Citations: 20
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Journal articleMartinez-Casado R, Usvyat D, Mallia G, et al., 2014,
Diffraction of helium on MgO(100) surface calculated from first-principles
, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 16, Pages: 21106-21113, ISSN: 1463-9076- Author Web Link
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- Citations: 10
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Journal articleTomic S, Bernasconi L, Searle BG, et al., 2014,
Electronic and Optical Structure of Wurtzite CuInS<sub>2</sub>
, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 118, Pages: 14478-14484, ISSN: 1932-7447- Author Web Link
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- Citations: 45
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Journal articleSanches FF, Mallia G, Liborio L, et al., 2014,
Hybrid exchange density functional study of vicinal anatase TiO<sub>2</sub> surfaces
, PHYSICAL REVIEW B, Vol: 89, ISSN: 2469-9950- Author Web Link
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- Citations: 14
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Journal articleMartinez-Casado R, Usvyat D, Maschio L, et al., 2014,
Approaching an exact treatment of electronic correlations at solid surfaces: The binding energy of the lowest bound state of helium adsorbed on MgO(100)
, PHYSICAL REVIEW B, Vol: 89, ISSN: 2469-9950- Author Web Link
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- Citations: 15
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Journal articleKorotana R, Mallia G, Gercsi Z, et al., 2014,
Hybrid density functional study of structural, bonding, and electronic properties of the manganite series La<sub>1-x</sub>Ca<sub>x</sub>MnO<sub>3</sub> (x=0, 1/4, 1)
, PHYSICAL REVIEW B, Vol: 89, ISSN: 2469-9950- Author Web Link
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- Citations: 10
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Conference paperHarrison N, 2014,
Computational characterisation of catalysts in reactive environments: Phase stability, surface compostion, structure, and reaction sites
, 247th National Spring Meeting of the American-Chemical-Society (ACS), Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727 -
Journal articleSerri M, Wu W, Fleet LR, et al., 2014,
High-temperature antiferromagnetism in molecular semiconductor thin films and nanostructures
, Nature Communications, Vol: 5, Pages: 1-9, ISSN: 2041-1723The viability of dilute magnetic semiconductors in applications is linked to the strength of the magnetic couplings, and room temperature operation is still elusive in standard inorganic systems. Molecular semiconductors are emerging as an alternative due to their long spin-relaxation times and ease of processing, but, with the notable exception of vanadium-tetracyanoethylene, magnetic transition temperatures remain well below the boiling point of liquid nitrogen. Here we show that thin films and powders of the molecular semiconductor cobalt phthalocyanine exhibit strong antiferromagnetic coupling, with an exchange energy reaching 100 K. This interaction is up to two orders of magnitude larger than in related phthalocyanines and can be obtained on flexible plastic substrates, under conditions compatible with routine organic electronic device fabrication. Ab initio calculations show that coupling is achieved via superexchange between the singly occupied a1g () orbitals. By reaching the key milestone of magnetic coupling above 77 K, these results establish quantum spin chains as a potentially useable feature of molecular films.
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Journal articleWu W, Harrison NM, Fisher AJ, 2013,
Suitability of chromium phthalocyanines to test Haldane's conjecture: First-principles calculations
, PHYSICAL REVIEW B, Vol: 88, ISSN: 2469-9950- Author Web Link
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- Citations: 6
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Journal articleSanches FF, Mallia G, Harrison NM, 2013,
Simulating constant current STM images of the rutile TiO<inf>2</inf> (110) surface for applications in solar water splitting
, Materials Research Society Symposium Proceedings, Vol: 1494, Pages: 339-344, ISSN: 0272-9172Solar water splitting has shown promise as a source of environmentally friendly hydrogen fuel. Understanding the interactions between semiconductor surfaces and water is essential to improve conversion efficiencies of water splitting systems. TiO2 has been widely adopted as a reference material and rutile surfaces have been studied experimentally and theoretically. Scanning Tunneling Microscopy (STM) is commonly used to study surfaces, as it probes the atomic and electronic structure of the surface layer. A systematic and transferable method to simulate constant current STM images using local atomic basis set methods is reported. This consists of adding more diffuse p and d functions to the basis sets of surface O and Ti atoms, in order to describe the long range tails of the conduction and valence bands (and, thus, the vacuum above the surface). The rutile TiO2 (HO) surface is considered as a case study. © 2013 Materials Research Society.
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Journal articleWu W, Harrison NM, Fisher AJ, 2013,
<i>p</i>-orbital nanomagnetism in an organic chain magnet
, PHYSICAL REVIEW B, Vol: 88, ISSN: 1098-0121- Author Web Link
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- Citations: 9
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Journal articleRefson K, Montanari B, Mitev PD, et al., 2013,
Comment on "First-principles study of the influence of (110)-oriented strain on the ferroelectric properties of rutile TiO<sub>2</sub>"
, PHYSICAL REVIEW B, Vol: 88, ISSN: 2469-9950- Author Web Link
- Open Access Link
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- Citations: 9
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Journal articleZuehlsdorff TJ, Hine NDM, Spencer JS, et al., 2013,
Linear-scaling time-dependent density-functional theory in the linear response formalism
, JOURNAL OF CHEMICAL PHYSICS, Vol: 139, ISSN: 0021-9606- Author Web Link
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- Citations: 56
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Journal articleWu W, Harrison NM, Fisher AJ, 2013,
Electronic structure and exchange interactions in cobalt-phthalocyanine chains
, PHYSICAL REVIEW B, Vol: 88, ISSN: 2469-9950- Author Web Link
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- Citations: 29
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Conference paperPatel M, Mallia G, Harrison NM, 2013,
The structure of water on rutile TiO2(110) for applications in solar hydrogen production: towards a predictive model using hybrid-exchange density functional theory
, MRS Spring Meeting 2013 -
Conference paperKorotana R, Mallia G, Gercsi Z, et al., 2013,
A hybrid-exchange density functional study of Ca-doped LaMnO<sub>3</sub>
, 12th Joint MMM-Intermag Conference, Publisher: AMER INST PHYSICS, ISSN: 0021-8979- Author Web Link
- Open Access Link
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- Citations: 7
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