Density Functional Theory Code CASTEP Research Articles

First principles validation of energy barriers in Ni75Al25

Precipitates in nickel-based superalloys form during heat treatment on a time scale inaccessible to direct molecular dynamics simulation, but can be studied using kinetic Monte Carlo (KMC) modelling. This requires reliable values for the barrier energies separating distinct configurations over the trajectory of the system. In this study, we validate vacancy migration barriers found with the Activation-Relaxation Technique nouveau (ARTn) method in partially ordered Ni75Al25 with a monovacancy using published potentials for the atomic interactions against first-principles methods. In a first step, we confirm that the ARTn barrier energies agree with those determined with the nudged elastic band (NEB) method. As the number of atoms used in those calculations is too great for direct ab initio calculations, we cut the cell size to 255 atoms, thus controlling finite size effects. We then use the plane-wave density functional theory code CASTEP and its inbuilt NEB method in the smaller cells. This provides us with a continuous validation chain from first principles to KMC simulations with interatomic potentials (IPs). We evaluate the barrier energies of five further IPs with NEB, demonstrating that none yields values with sufficient reliability for KMC simulations, with some of them failing completely. This is a first step towards quantifying the errors incurred in KMC simulations of precipitate formation and evolution.

Mobile metal adatoms on single layer, bilayer, and trilayer graphene: Anab initioDFT study with van der Waals corrections correlated with electron microscopy data

The plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene. All edge site binding energies were found to be substantially higher than the adsorption energies for all metals. The adatom migration activation barriers for the lowest energy migration paths on pristine monolayer, bilayer, and trilayer graphene were then calculated and found to be smaller than or within an order of magnitude of ${k}_{B}T$ at room temperature, implying very high mobility for all adatoms studied. This suggests that metal atoms evaporated onto graphene samples quickly migrate across the lattice and bind to the energetically favorable edge sites before being characterized in the microscope. We then prove this notion for Al and Au on graphene with scanning transmission electron microscopy (STEM) images showing that these atoms are observed exclusively at edge sites, and also hydrocarbon-contaminated regions, where the pristine regions of the lattice are completely devoid of adatoms. Additionally, we review the issue of fixing selected atomic positions during geometry optimization calculations for graphene/adatom systems and suggest a guiding principle for future studies.

Ab-initio modelling, polarity and energetics of clean rutile surfaces in vacuum and comparison with water environment

All terminations of the (1x1) rutile (110), (101), (001), (100) and (111) surfaces are classified according to their electrostatic polarity. Six are found to be non-polar. The plane-wave density functional theory code CASTEP is used with a GGA-PBE exchange-correlation functional and a vacuum/material slab supercell method to calculate the surface energy density of symmetric thin rutile films with the six non-polar terminations in vacuum. The ratio of the surface energy densities of a rutile crystal with {111} and {110} facets in water is deduced using Lagrange multipliers and found to be consistent with the DFT vacuum results.

Synthesis and optoelectrical properties of ABO2 (A = Li, Na; B = Y, Yb)

The transparent electrically conductive composite materials ABO2 (A = Li, Na; B = Y, Yb) were synthesized under high temperature solid-state reactions from A2O and B2O3. The synthesized compounds have been investigated by Rietveld analysis, giving the crystal (monoclinic, c12/c1, a = 611.6 pm) for LiYO2, (monoclinic, p121/c1, a = 999 pm) for NaYO2, (trigonal, R3̄m, a = 335.2 pm) for NaYbO2, (tetragonal, I41/amd, a = 438.6 pm) for LiYbO2. Optical properties of the four compounds were studied by UV-vis spectral measurements, results show each of the compounds has an optical band gap more than 3.3 eV which is the standard for transparent property. Electrical conductivities of the four compounds have been studied by two probe direct current electrical conductivity measurements. The results show with increasing heating temperature, an obvious increase in electrical conductivity was observed for each compound. Also, the crystal energy band structure of each compound has been studied by using density functional theory code CASTEP. The results show the solid-state compounds of NaYO2, NaYbO2 and LiYbO2 are semiconductors with direct band gaps, LiYO2 is a semiconductor with an indirect band gap.

First-principles study of LiBaF 3 crystals containing interstitial fluoride

The positions of the interstitial fluoride atoms in LiBaF 3 crystal are simulated by GULP. It is found that the formation energy is low when interstitial fluoride atoms at the center of the edge of the cube formed by Ba ions. The electronic structures and absorption bands for the perfect LiBaF 3 crystal and the LiBaF 3 crystal containing interstitial fluoride have been calculated using density functional theory code CASTEP. It is predicted that the interstitial fluoride atom combine with a formal lattice fluoride ion forming H center causes the 320 nm absorption band.

First principle studies on the electronic structures and absorption spectra in KMgF3 crystal with fluorine vacancy

The experiments indicate that the perfect KMgF3 crystal has no absorption in the visible range, however the electron irradiation induces a complex absorption spectrum. The absorption spectra can be decomposed by five Gaussian bands peaking at 2.5 eV (488 nm), 3.4 eV (359 nm), 4.2 eV (295 nm), 4.6 eV (270 nm) and 5.2 eV (239 nm), respectively. The purpose of this paper is to seek the origins of the absorption bands. The electronic structures and absorption spectra either for the perfect KMgF3 or for KMgF 3: VF+ with electrical neutrality have been studied by using density functional theory code CASTEP with the lattice structure optimized. The calculation results predicate that KMgF3: VF+ also exhibits five absorption bands caused by the existence of the fluorine ion vacancy VF+ and the five absorption bands well coincide with the experimental results. It is believable that the five absorption bands are related to VF+ in KMgF3 crystal produced by the electron irradiation.

Analysis of computational EELS modelling results for MgO-based systems

The ab-initio density functional theory (DFT) code CASTEP was used to model oxygen K edges in various magnesium oxide systems. Firstly, for the bulk material the process of geometry optimisation was carried out. Predicted oxygen K edges were found for a single cell with experimental lattice parameters, and parameters obtained after geometry optimisation, both with single electron core-holes in place. After geometry optimisation, a different predicted result was obtained, although it was qualitatively similar to the result for experimental lattice parameters in some respects. For example, approximately the same sets of peaks are observed, though in different energy positions, and with different relative peak intensities within those sets. Ultimately for the single cell results the experimental lattice parameters generated the predicted result that was in the closest agreement with experiment. It was further observed that a large supercell result (based on the experimental lattice parameters, utilising a core-hole) led to a slightly improved comparison with experiment as compared to the corresponding single cell result, although the latter result, and indeed a ground state calculation also give reasonable agreement with experiment. To rationalise these observations it was necessary to investigate the density of states (DOS) for the MgO cell and its constituent atoms, and it was observed that the conduction bands were of predominantly magnesium character. Furthermore, the core-hole’s introduction had relatively little overall effect on the p DOS prediction for oxygen, though there is a significant localised change close to the Fermi level. This work also considers interface and surface results. The principal aim of the study was to explore the interface of Fe (0 0 1)/MgO (0 0 1), crucial in certain classes of magnetic tunnel junctions (MTJs), which have significant technological applications. An initial step was to consider a MgO (0 0 1) surface. It was verified that a surface could be constructed such that within that surface a theoretical result could be found that matched the bulk result. It was then valid to use this surface as part of an interface with iron. Theoretical results obtained at that interface compare well with experimental results from an epitaxially grown MTJ, and various conclusions are drawn with regard to the nature of the interface.

Color center of YAlO 3 with cation vacancies

High quality pure YAlO 3 crystal with dimension of Φ 30 × 50 mm 2 was grown by Czochralski technique. UV irradiation and air annealing bring additional absorption in the region 200–800 nm. The absorption spectra of perfect YAlO 3 and YAlO 3 containing cation vacancy (aluminium vacancy and yttrium vacancy) were calculated using density functional theory code CASTEP. Comparison of the simulated absorption spectra with the experimental absorption spectra of YAlO 3 after UV irradiation and air annealing treatments shows that cation vacancies are responsible for part of the coloration on YAlO 3 crystal.

Study on the doping mechanism and electronic structure for Nb 5+ doping PbWO 4 crystals

The existing forms of the impurity Nb 5+ in the Nb 5+:PbWO 4 (PWO) crystals are simulated by computer technology. The possible various kinds of defects in the Nb 5+:PWO crystals are also simulated. By analyzing the calculation results of defect formation energies and binding energies, the optimal substitution positions of the Nb 5+ ions and the charge compensating mechanism [ Nb w - + V O 2 + + Nb w - ] in the Nb 5+:PWO crystals were obtained. The electronic structures for Nb 5+:PWO were calculated with density functional theory code CASTEP. It shows that the doping of Nb 5+ ions could suppress the 350 nm absorption band.

First-principles calculations of electronic structures and absorption spectra of YAlO3 crystals with F center

The electronic structures and absorption spectra of perfect YAlO3 crystal and YAlO3 crystal containing F center (oxygen vacancy VO2+ catching two electrons) with lattice structure optimized were calculated using density functional theory code CASTEP. The calculated electronic structures of YAlO3 crystal containing F center appear new density of states in forbidden band compared with that of perfect YAlO3 crystal and the calculated absorption spectra of the YAlO3 containing F center along three lattice parameter directions exhibit absorption bands in the range from 200 to 300 nm which are in agreement with experimental values. These new absorption bands are caused by separated F center in YAlO3 crystal.

First-principles study on electronic structures and absorption spectra for BaWO 4 crystal containing barium vacancy

The electronic structures, dielectric function and absorption spectra for the perfect BaWO 4 (BWO) crystal and the BWO crystal containing barium vacancy ( V Ba 2 - ) have been studied using density functional theory code CASTEP with the lattice structure optimized. The results indicate that the optical properties of the BWO crystal exhibit anisotropy and its optical symmetry coincide with lattice structure geometry of the BWO crystal. For the BWO crystal containing V Ba 2 - , there exhibit four absorption bands peaking at 0.71 eV (1751 nm), 1.85 eV (672 nm), 3.43 eV (362 nm) and 3.85 eV (322 nm), respectively. The origins of the 370 nm absorption band should be related to the V Ba 2 - .

First-principles study on electronic structures and color centers in BaWO4 crystal with barium vacancy

Abstract The electronic structures and absorption spectra for the perfect BaWO4 (BWO) crystal and the BWO crystal containing barium vacancy ( V Ba 2 - ) have been studied by using density functional theory code CASTEP with the lattice structure optimized. For the BWO crystal containing V Ba 2 - , there exhibit four absorption bands peaking at 0.71 eV (1751 nm), 1.85 eV (672 nm), 3.43 eV (362 nm) and 3.85 eV (322 nm), respectively. The origins of the absorption bands have been discussed and corresponding color centers also have been established.

First‐principles study of the electronic structures and absorption spectra for the PbMoO4 crystal with lead vacancy

AbstractThe electronic structures and absorption spectra for the perfect PbMoO4 crystal and the crystal containing lead vacancy $ {\rm V}^{2-}_{\rm Pb} $ with lattice structure optimized are calculated using density functional theory code CASTEP. The calculated absorption spectra of the PbMoO4 crystal containing $ {\rm V}^{2-}_{\rm Pb} $ exhibit three absorption bands peaking at 2.0 eV (620 nm), 3.0 eV (413 nm) and 3.3 eV (375 nm), which are in good agreement with experimental values. The theory predicts that the 390 nm, 430 nm and 580 nm absorption bands are related to the existence of $ {\rm V}^{2-}_{\rm Pb} $ in the PbMoO4 crystal. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principles study on electronic structure and absorption spectra for the CaWO4 crystal with oxygen vacancy

The electronic structure and absorption spectra for the perfect CaWO4 and the CaWO4 containing oxygen vacancy VO2+ have been calculated using density functional theory code CASTEP with the lattice structure being optimized. The results indicate that the CaWO4 crystal containing VO2+, an additional absorption spectrum in the region of visible light. And the absorption spectrum can be fit into two Gaussian-shape absorption spectra with peaks at 360 and 415nm. These peaks are located at the experimentally observed position. It is predicted that the 340 and the 420nm absorption spectra are related to the existence of VO2+ in the CaWO4 crystal.

First-Principles Studies on Electronic Structures and AbsorptionSpectra of PbWO4 Crystals with Defect [VPb2−–VO2+–VPb2−]2−

Electronic structures and absorption spectra for perfect PbWO4(PWO) crystals and the crystal containing aggregated defect[VPb2−–VO2+–VPb2−]2−have been calculated using density functional theory code CASTEP withthe lattice structure optimized. The calculated absorption spectra ofthe PWO crystal containing the aggregated defect[VPb2−–VO2+–VPb2−]2−exhibit two absorption bands peaking at 1.90 eV (650 nm) and3.02 eV (410 nm). It is predicted that the 420 and 680 nm absorptionbands are related to the existence of the aggregated defect[VPb2−–VO2+–VPb2−]2− in the PWOcrystal.

First-principles study on the effect of the interstitial oxygen ion on the spectral properties of PWO crystal and the origin of the green luminescence band

The electronic structures and absorption spectra of PWO crystals containing interstitial oxygen ions have been calculated using density functional theory code CASTEP with lattice structure optimized. It is shown by calculation that: (1) the interstitial oxygen ion in the perfect PWO crystal doesn’t bring any obvious absorption in the visible region; (2) the green emission of lead tungstate origin is closely related to the interstitial oxygen ion, and probably originates from the center of “ WO 4+O i ”.

First-principles study on the electronic structures and absorption spectra for Th 4+: PbWO 4 crystal

The electronic structures and absorption spectra for Th 4+: PbWO 4 have been calculated using density functional theory code CASTEP with the lattice structure optimized. It is shown by calculation that the visible range absorptions of the lead vacancy contained PWO crystal can be completely quenched by the doping of Th 4+. The quenching of the 330, 360 and 420 nm absorption bands are discussed.

Study on the electronic structures and absorption spectra for the PbWO 4 crystal with the defect [V 2+O–V 2−Pb–V 2+O] 2+

The electronic structures and absorption spectra for the perfect PbWO 4 (PWO) crystal and the crystal containing aggregated defect [V 2+ O–V 2− Pb–V 2+ O] 2+ have been calculated using density functional theory code CASTEP with the lattice structure optimized. The calculated absorption spectra of the PWO crystal containing the aggregated defect [V 2+ O–V 2− Pb–V 2+ O] 2+ exhibit three absorption bands peaking at 1.68 eV (740 nm), 2.25 eV (560 nm), 2.50 eV (470 nm). It predicts that the 450–750 nm absorption bands are related to the existence of the aggregated defect [V 2+ O–V 2− Pb–V 2+ O] 2+ in the PWO crystal.

First-principles study on the electronic structures and absorption spectra for the PbWO 4 crystal with lead vacancy

The electronic structures and absorption spectra for the perfect PbWO 4 (PWO) crystal and the crystal containing lead vacancy V Pb 2 − have been calculated using density functional theory code CASTEP with the lattice structure optimized. The calculated absorption spectra of the PWO crystal containing V Pb 2 − exhibit seven absorption bands peaking at 1.72 eV (720 nm), 2.16 eV (570 nm), 2.81 eV (440 nm), 3.01 eV (410 nm), 3.36 eV (365 nm), 3.70 eV (335 nm) and 4.0 eV (310 nm), which are very close to the experimental values. It predicts that the 330, 360, 420, 500–750 nm absorption bands are related to the existence of V Pb 2 − in the PWO crystal.

Structure dependence of optical spectra of ferromagnetic Heusler alloy Ni–Mn–Ga

The influence of structural transformation on the optical properties of Heusler alloy Ni–Mn–Ga has been investigated by using Density Functional Theory code CASTEP. The calculated optical refractive and absorption spectra, the off-diagonal components of the dielectric function and the optical conductivity for L2 1 structure and the tetragonal phase show a noticeable structural dependence which were confirmed by the experiments (Y.V. Kudryavtsev et al., Phys. Rev. B 66 (2002) 115114).