Bulk Vacancies Research Articles

Vacancy ordering and phonon spectrum of the iron-based superconductor K0.8Fe1.6Se2

We have performed Raman-scattering measurements on a high-quality single crystal of the recently discovered Fe-based superconductor K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ (${T}_{c}$ $=$ 32 K). At least 13 phonon modes were observed in the wave number range 10--300 cm${}^{\ensuremath{-}1}$. The spectra possess a fourfold symmetry indicative of bulk vacancy order in the Fe-deficient planes. We perform a vibration analysis based on first-principles calculations, which both confirms the ordered structure and allows a complete mode assignment. We observe an anomaly at ${T}_{c}$ in the 180 cm${}^{\ensuremath{-}1}$ ${A}_{g}$ mode, which indicates a rather specific type of electron-phonon coupling.

Strain effect on diffusion properties of oxygen vacancies in bulk and subsurface of rutile TiO2

The influences of external strain on diffusion properties of the bulk and subsurface oxygen vacancy (OV) in rutile TiO2 are systematically studied using first-principle calculations. For OVs in bulk, we find that tensile (compressive) strain applied in the [001] direction or isotropically applied in the equivalent [110] and [11¯0] directions reduces (increases) the energy barriers of diffusion. Anisotropic strain applied in [110] and [11¯0] increases the energy barriers of diffusion in the two directions. Meanwhile it results in anisotropic diffusion behaviors. Between [110] and [11¯0], the bulk OV prefers to diffuse along the one in which more compressive or less tensile strain is applied. From subsurface to surface, the most energetically favorable OV pathway is along the [110] rows terminated with the surface bridging oxygen atoms. The diffusion barrier of the OV in the first trilayer is much lower than that of a bulk OV. External in-plane tensile strain can further reduce the energy barrier of the subsurface OV diffusion, and thus help to improve the diffusion of OVs from bulk to surface.

Charge distribution and transport properties in reduced ceria phases: A review

The question of the charge distribution in reduced ceria phases (CeO 2− x ) is important for understanding the microscopic physics of oxygen storage capacity, and the electronic and ionic conductivities in these materials. All these are key properties in the application of these materials in catalysis and electrochemical devices. Several approaches have been applied to study this problem, including ab initio methods. Recently [1], we applied the bond valence model (BVM) to discuss the charge distribution in several different crystallographic phases of reduced ceria. Here, we compare the BVM results to those from atomistic simulations to determine if there is consistency in the predictions of the two approaches. Our analysis shows that the two methods give a consistent picture of the charge distribution around oxygen vacancies in bulk reduced ceria phases. We then review the transport theory applicable to reduced ceria phases, providing useful relationships which enable comparison of experimental results obtained by different techniques. In particular, we compare transport parameters obtained from the observed optical absorption spectrum, α ( ω ) , dc electrical conductivity with those predicted by small polaron theory and the Harrison method. The small polaron energy is comparable to that estimated from α ( ω ) . However, we found a discrepancy between the value of the electron hopping matrix element, t, estimated from the Marcus–Hush formula and that obtained by the Harrison method. Part of this discrepancy could be attributed to the system lying in the crossover region between adiabatic and nonadiabatic whereas our calculations assumed the system to be nonadiabatic. Finally, by considering the relationship between the charge distribution and electronic conductivity, we suggest the possibility of low temperature metallic conductivity for intermediate phases, i.e., x ∼ 0.3 . This has not yet been experimentally observed.

Dislocation dynamics simulations with climb: kinetics of dislocation loop coarsening controlled by bulk diffusion

Dislocation climb mobilities, assuming vacancy bulk diffusion, are derived and implemented in dislocation dynamics simulations to study the coarsening of vacancy prismatic loops in fcc metals. When loops cannot glide, comparison of the simulations with a coarsening model based on the line tension approximation shows good agreement. Dislocation dynamics simulations with both glide and climb are then performed. Allowing for glide of the loops along their prismatic cylinders leads to faster coarsening kinetics, as direct coalescence of the loops is now possible.

Control of Thermal and Electronic Transport in Defect-Engineered Graphene Nanoribbons

The influence of the structural detail and defects on the thermal and electronic transport properties of graphene nanoribbons (GNRs) is explored by molecular dynamics and non-equilibrium Green's function methods. A variety of randomly oriented and distributed defects, single and double vacancies, Stone-Wales defects, as well as two types of edge form (armchair and zigzag) and different edge roughnesses are studied for model systems similar in sizes to experiments (>100 nm long and >15 nm wide). We observe substantial reduction in thermal conductivity due to all forms of defects, whereas electrical conductance reveals a peculiar defect-type-dependent response. We find that a 0.1% single vacancy concentration and a 0.23% double vacancy or Stone-Wales concentration lead to a drastic reduction in thermal conductivity of GNRs, namely, an 80% reduction from the pristine one of the same width. Edge roughness with an rms value of 7.28 Å leads to a similar reduction in thermal conductivity. Randomly distributed bulk vacancies are also found to strongly suppress the ballistic nature of electrons and reduce the conductance by 2 orders of magnitude. However, we have identified that defects close to the edges and relatively small values of edge roughness preserve the quasi-ballistic nature of electronic transport. This presents a route of independently controlling electrical and thermal transport by judicious engineering of the defect distribution; we discuss the implications of this for thermoelectric performance.

Diffusion-Assisted Formation Mechanism of Molecular Break Junctions: A First-Principles Study of Benzenethiolate on Au(111)

Density-functional theory (DFT) calculations have been used to obtain the binding energies of benzenethiolate on Au(111) under applied stress. The binding energetics to ideal Au(111), as well as to Au adatoms and small 2−3 adatom islands, have been calculated as functions of the normal displacement applied to the free end of the molecule. We find that binding to adatoms and adatom islands is energetically preferred over breaking the Au−S bond for sufficiently large displacements. On the basis of these results, we propose a diffusion-assisted mechanism of pyramidal break junction formation on Au(111) that involves creation and capture of surface adatoms, as well as surface or bulk vacancies. Quantitative estimates of stretching rates that are compatible with the required diffusion times are given on the basis of random walk theory.

Study of the structural and atomic diffusive properties of the ordered Cu3Au (110) surface

AbstractThe structural properties, the formation and migration energies of a single vacancy migrating intralayer and interlayer in the CuAu‐terminated (110) surface of Cu3Au ordered alloy have been calculated and discussed by using the modified analytical embedded‐atom method (MAEAM) and molecular dynamics (MD) methods. The surface layer exhibits rippling that the Au atoms are raised above Cu atoms about 0.117 Å in the topmost layer. The displacements of the topmost two layers are comparatively larger, while the third layer relaxes slightly and there are no changes in the nether layers. From energy minimization, the vacancy is most likely to be formed in the first layer (1L), especially on the Au site. The surface vacancy shows the smallest formation energy compared to the interlayer and bulk vacancies, while the corresponding value converges after the fifth layer (5L). For Cu vacancy originally sited in the second layer (2L) and migrated intralayer and interlayer, the diffusion without causing the local disorder is the most favorable, and the vacancy tends to migrate to the topmost layer. In the topmost layer of the CuAu‐terminated (110) surface, the circularity path is preferred over the beeline path. Copyright © 2011 John Wiley & Sons, Ltd.

Ferromagnetism Induced by Vacancies in Bulk and the (1010) Surfaces of ZnO: Density Functional Theory Calculations

Ferromagnetism Induced by Vacancies in Bulk and the (1010) Surfaces of ZnO: Density Functional Theory Calculations

Ferromagnetism Induced by Vacancies in Bulk and the (1010) Surfaces of ZnO: Density Functional Theory Calculations

We studied the vacancy-induced ferromagnetism in bulk and at the (1010) surface of wurtzite ZnO using density functional theory calculations. It is found out that surface vacancies are energetically favored to bulk vacancies and that the ferromagnetism is induced by Zn vacancies but not by O vacancies. Moreover, local magnetic moments induced at O atoms adjacent to the Zn vacancy are calculated to be larger on the surface than in bulk. These findings indicate that Zn vacancies at surfaces are responsible for the experimentally observed room-temperature ferromagnetism in undoped ZnO thin films and nanoparticles.

Oxygen permeation of Ba xSr 1 − xCo 0.8Fe 0.2O 3 − δ perovskite-type membrane: Experimental and modeling

Ba x Sr 1 − x Co 0 .8 Fe 0 .2 O 0.3 − δ ( x = 0.2, 0.5, 0.8) dense membranes were prepared by a combined EDTA and citrate complexing method. In our previous works, effects of sintering temperature, sintering dwell time and pressing pressure on microstructure and theoretical densities of the membranes were examined and finally the best corresponding values were reported as 1100 °C, 8–9 h and 200–250 MPa, respectively. In the present work, effects of temperature (650–950 °C), feed flow rate (100–200 cm³/min), sweep gas flow rate (40–80 cm³/min) and membrane thickness (4–5 mm) on oxygen permeation behavior of the Ba x Sr 1 − x Co 0 .8 Fe 0 .2 O 0.3 − δ membranes were investigated. Also, a mathematical model based on Nernst–Planck equation was developed to predict oxygen permeation through the perovskite-type membranes. Both bulk diffusion and surface reactions were incorporated into the model. It was observed that surface reactions are not elementary and a correction term should be introduced into the model to compensate this effect. Also, using a dimensionless Reynolds number, effect of feed flow rate on oxygen flux was taken into account. With aids of these modifications, it was realized that, there is a reasonable agreement between predicted results and experimental data with correlation coefficient (R) of higher than 0.960 and mean squared relative error (MSRE) of lower than 0.022 for all the membranes. Oxygen vacancy bulk diffusion coefficient (D v), surface exchange rate constants (k f and k r), contribution of each resistance to oxygen permeation and characteristic thickness (L c) of the BSCF membranes were also estimated.

Nitrogen vacancies at InN (11¯00) surfaces: A theoretical study

We present an ab initio atomistic study of the effects of nitrogen vacancies both in InN bulk and at (11¯00) nonpolar surface and confirm that these defects act as n-type impurities. Based on the dependence of the vacancy formation energy on the distance from the surface, we predict that at thermodynamic equilibrium these defects tend to segregate in the outermost surface layers reaching a concentration which is few orders of magnitude higher than in the bulk phase. Considering out-of-equilibrium growth and in view of the large calculated energy barrier that characterize vacancy diffusion, we also predict that the N enriched layer will extend hundreds of angstroms from the surface for typical growth processes. This vacancy accumulation at surface is consistent with the observed high electron accumulation layer.

Periodic step arrays on the aperiodici-Al-Pd-Mn quasicrystal surface at high temperature

We have observed the configuration and motion of surface steps on the aperiodic icosahedral ($i$-) Al-Pd-Mn quasicrystal using low-energy electron microscopy and scanning tunneling microscopy. As the quasicrystal is cooled from high temperature, bulk vacancies migrate to the surface causing the surface to be etched. Surprisingly, this etching occurs by two types of steps with different heights moving in different directions with different velocities. The steady-state surface morphology is a uniformly spaced rhomboidal step network. This network requires that the layer stacking near the surface deviates from the bulk quasicrystal stacking.

Hydrogen in tungsten: Absorption, diffusion, vacancy trapping, and decohesion

Understanding the interaction between atomic hydrogen and solid tungsten is important for the development of fusion reactors in which proposed tungsten walls would be bombarded with high energy particles including hydrogen isotopes. Here, we report results from periodic density-functional theory calculations for three crucial aspects of this interaction: surface-to-subsurface diffusion of H into W, trapping of H at vacancies, and H-enhanced decohesion, with a view to assess the likely extent of hydrogen isotope incorporation into tungsten reactor walls. We find energy barriers of (at least) 2.08 eV and 1.77 eV for H uptake (inward diffusion) into W(001) and W(110) surfaces, respectively, along with very small barriers for the reverse process (outward diffusion). Although H dissolution in defect-free bulk W is predicted to be endothermic, vacancies in bulk W are predicted to exothermically trap multiple H atoms. Furthermore, adsorbed hydrogen is predicted to greatly stabilize W surfaces such that decohesion (fracture) may result from high local H concentrations.

Conductance quantization in strongly disordered graphene ribbons

We present numerical studies of conduction in graphene nanoribbons with different types of disorder. We find that even when defect scattering depresses the conductance to values two orders of magnitude lower than $2{e}^{2}/h$, equally spaced conductance plateaus occur at moderately low temperatures due to enhanced electron backscattering near subband edge energies if bulk vacancies are present in the ribbon. This work accounts quantitatively for the surprising conductance quantization observed by Lin et al. [Phys. Rev. B 78, 161409(R) (2008)] in ribbons with such low conductances.

First-principles study of bulk and surface oxygen vacancies in SrTiO3 crystal

The structural and electronic properties of the neutral and positively charged oxygen vacancies (F and F + centres) in the bulk and on the (001) surfaces of SrTiO3 crystal are examined within the hybrid Hartree-Fock and density functional theory (HF-DFT) method based upon the linear combination of atomic orbital (LCAO) approach. A comparison of the formation energy for surface and bulk defects indicates a perceptible propensity for the segregation of neutral and charged vacancies to both SrO and TiO2 surface terminations with a preference in the latter case which is important for interpretation of space charge effects at ceramic interfaces. It is found that the vacancies reveal more shallow energy levels in the band gap on surfaces rather than in the bulk, in particular, on the TiO2 surface. The charged F + centre has significantly deeper energy levels both in bulk and on the surfaces, as compared with the neutral F centre.

Annealing of dislocation loops in dislocation dynamics simulations

We report of 3-dimensional discrete dislocation dynamics (DDD) simulations of dislocation loops coarsening by vacancy bulk diffusion. The calculation is based upon a model which couples the diffusion theory of vacancies to the DDD in order to obtain the climb rate of the dislocation segments. Calculation of isolated loops agrees with experimental observations, i.e. loops shrink or expand, depending on their type and vacancy supersaturation. When an array of dislocation loops of various sizes is considered, and the total number of vacancies in the simulation is maintained constant, the largest dislocations are found to increase in size at the expense of small ones, which disappear in a process known as Ostwald ripening.

A comparative ab initio study of bulk and surface oxygen vacancies in PbTiO 3, PbZrO 3 and SrTiO 3 perovskites

Using the DFT-HF hybrid LCAO approach as implemented in the CRYSTAL computer code, we have performed large supercell comparative calculations of neutral O vacancies ( F centers) in the bulk and on the (001) surface of three cubic perovskite crystals (SrTiO 3, PbTiO 3, and PbZrO 3). The local lattice relaxation, charge redistribution, and positions of defect energy levels within the band gap are compared. It is shown that the difference in the chemical composition of host materials leads to quite different defect properties.

Electronic structure of bulk and defect α- and γ-Al 2O 3

Electronic structure of bulk and oxygen vacancies of high-k dielectric Al 2O 3 (α- and γ- phases) was calculated from the first principles. It was found, that oxygen vacancy can be both electron and hole trap in α- and γ-Al 2O 3. High leakage current trough thin films of Al 2O 3 can be caused by oxygen vacancies.

Measurement and Modeling of the Impedance Characteristics of Porous La1 − xSrxCoO3 − δ Electrodes

Porous (LSC) electrodes having Sr composition (LSC-64) and (LSC-82) were fabricated on Sm-doped ceria electrolytes, and studied using electrochemical impedance spectroscopy between and . The faradaic portion of the impedance was found to exhibit Gerischer or Gerischer-like characteristics, indicating colimitation by kinetics and transport. The characteristic resistance and frequency response was analyzed using a transport and reaction model that considers parallel bulk and surface diffusion of oxygen, as well as three-dimensional transport effects near the electrode electrolyte interface. In the case of LSC-64, measured characteristics appear to be largely consistent with a bulk transport path, based on independent measurements of the thermodynamic, kinetic, and transport properties of LSC-64. In the case of LSC-82, which has a much lower bulk vacancy concentration under the same conditions, results were inconsistent with an entirely bulk transport path. Results for LSC-82 could be rationalized assuming a parallel surface transport path, where surface mobility is governed by some kind of interstitial or adatom diffusion mechanism.

Ion mass dependence of irradiation-induced local creation of ferromagnetism inFe60Al40alloys

Ion irradiation of ${\text{Fe}}_{60}{\text{Al}}_{40}$ alloys results in the phase transformation from the paramagnetic, chemically ordered B2 phase to the ferromagnetic, chemically disordered A2 phase. The magnetic phase transformation is related to the number of displacements per atom (dpa) during the irradiation. For heavy ions (${\text{Ar}}^{+}$, ${\text{Kr}}^{+}$, and ${\text{Xe}}^{+}$), a universal curve is observed with a steep increase in the fraction of the ferromagnetic phase that reaches saturation, i.e., a complete phase transformation, at about 0.5 dpa. This proves the purely ballistic nature of the disordering process. If light ions are used (${\text{He}}^{+}$ and ${\text{Ne}}^{+}$), a pronounced deviation from the universal curve is observed. This is attributed to bulk vacancy diffusion from the dilute collision cascades, which leads to a partial recovery of the thermodynamically favored B2 phase. Comparing different noble gas ion irradiation experiments allows us to assess the corresponding counteracting contributions. In addition, the potential to create local ferromagnetic areas embedded in a paramagnetic matrix is demonstrated.