Showing papers on "Lattice constant published in 2015"

Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides

Abstract: We present a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition-metal dichalcogenides and -oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin–orbit coupling are calculated in the G0W0 approximation, and comparison is made with different density functional theory descriptions. Pitfalls related to the convergence of GW calculations for two-dimensional (2D) materials are discussed together with possible solutions. The monolayer band edge positions relative to vacuum are used to estimate the band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analyzed and rationalized in terms of the electronic structure. Finally, the q-dependent dielectric functions and effective electron and hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to estimate exciton binding energies. Throughout the paper we focus on...

On the structural origins of ferroelectricity in HfO2 thin films

Abstract: Here, we present a structural study on the origin of ferroelectricity in Gd doped HfO2 thin films. We apply aberration corrected high-angle annular dark-field scanning transmission electron microscopy to directly determine the underlying lattice type using projected atom positions and measured lattice parameters. Furthermore, we apply nanoscale electron diffraction methods to visualize the crystal symmetry elements. Combined, the experimental results provide unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization, resolving the origin of ferroelectricity in HfO2 thin films.

Band Gap Tuning of CH3NH3Pb(Br1–xClx)3 Hybrid Perovskite for Blue Electroluminescence

Abstract: We report on the structural, morphological and optical properties of AB(Br1–xClx)3 (where, A = CH3NH3+, B = Pb2+ and x = 0 to 1) perovskite semiconductor and their successful demonstration in green and blue emissive perovskite light emitting diodes at room temperature. The bandgap of perovskite thin film is tuned from 2.42 to 3.16 eV. The onset of optical absorption is dominated by excitonic effects. The coulomb field of the exciton influences the absorption at the band edge. Hence, it is necessary to explicitly account for the enhancement of the absorption through the Sommerfield factor. This enables us to correctly extract the exciton binding energy and the electronic bandgap. We also show that the lattice constant varies linearly with the fractional chlorine content satisfying Vegards law.

Nucleon isovector couplings from Nf = 2 lattice QCD

Abstract: We compute the axial, scalar, tensor and pseudoscalar isovector couplings of the nucleon as well as the induced tensor and pseudoscalar charges in lattice simulations with $N_f=2$ mass-degenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions. The simulations are carried out down to a pion mass of 150 MeV and linear spatial lattice extents of up to 4.6 fm at three different lattice spacings ranging from approximately 0.08 fm to 0.06 fm. Possible excited state contamination is carefully investigated and finite volume effects are studied. The couplings, determined at these lattice spacings, are extrapolated to the physical pion mass. In this limit we find agreement with experimental results, where these exist, with the exception of the magnetic moment. A proper continuum limit could not be performed, due to our limited range of lattice constants, but no significant lattice spacing dependence is detected. Upper limits on discretization effects are estimated and these dominate the error budget.

New Insights on the Burstein-Moss Shift and Band Gap Narrowing in Indium-Doped Zinc Oxide Thin Films

Abstract: The Burstein-Moss shift and band gap narrowing of sputtered indium-doped zinc oxide (IZO) thin films are investigated as a function of carrier concentrations. The optical band gap shifts below the carrier concentration of 5.61 × 1019 cm-3 are well-described by the Burstein-Moss model. For carrier concentrations higher than 8.71 × 1019 cm-3 the shift decreases, indicating that band gap narrowing mechanisms are increasingly significant and are competing with the Burstein-Moss effect. The incorporation of In causes the resistivity to decrease three orders of magnitude. As the mean-free path of carriers is less than the crystallite size, the resistivity is probably affected by ionized impurities as well as defect scattering mechanisms, but not grain boundary scattering. The c lattice constant as well as film stress is observed to increase in stages with increasing carrier concentration. The asymmetric XPS Zn 2p3/2 peak in the film with the highest carrier concentration of 7.02 × 1020 cm-3 suggests the presence of stacking defects in the ZnO lattice. The Raman peak at 274 cm-1 is attributed to lattice defects introduced by In dopants.

Phase stability and transformations in the halide perovskite CsSnI3

Abstract: We employ the quasiharmonic approximation to study the temperature-dependent lattice dynamics of the four different phases of cesium tin iodide $({\mathrm{CsSnI}}_{3})$. Within this framework, we obtain the temperature dependence of a number of structural properties, including the cell volume, bulk modulus, and Gr\"uneisen parameter. The Gibbs free energy of each phase is compared against the temperature-dependent Helmholtz energy obtained from the equilibrium structure within the harmonic approximation. We find that the black tetragonal perovskite phase is not dynamically stable up to at least 500 K, with the phonon dispersion displaying negative optic modes, which pass through all of the high-symmetry wave vectors in the Brillouin zone. The main contributions to the negative modes are found to be motions of the Cs atom inside the perovskite cage. The black cubic perovskite structure shows a zone-boundary instability, indicated by soft modes at the special $\mathbf{q}$ points $M$ and $R$. These modes are present in calculations at the equilibrium (0 K) lattice constant, while at finite temperature additional negative modes develop at the zone center, indicating a ferroelectric instability. The yellow crystal, composed of one-dimensional $({\mathrm{SnI}}_{6}){}_{n}$ double chains, has the same heat of formation as the orthorhombic perovskite phase at 0 K, but becomes less energetically favorable at higher temperatures, due to its higher free energy.

A Novel Synthesis of Zn2+-Doped CoFe2O4 Spinel Nanoparticles: Structural, Morphological, Opto-magnetic and Catalytic Properties

Abstract: Zn2+-doped CoFe2O4 (Zn x Co1−x Fe2O4: where x= 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) spinel nanoparticles (NPs) were synthesized by microwave combustion method using nitrates of cobalt, zinc, and iron as the starting materials and urea used as the fuel. Powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), energy dispersive X-ray (EDX), and selected area electron diffraction (SAED) pattern analyses showed that all composition was found to have pure cubic spinel structure with well crystalline nature. The average crystallite size of the samples was found to be in the range of 25.43 and 29.46 nm. The lattice parameter increased from 8.432 to 8.441 A with increasing the Zn2+ content, due to the smaller ionic radius of Co2+ substituted by larger ionic radius of Zn2+, which was determined by Rietveld analysis. High-resolution scanning electron microscopy (HR-SEM) and transmission electron microscopy (HR-TEM) analyses were used to study the morphological variation, and the results showed a nano-sized spherical-shaped particle-like morphology. The band gap (E g) of the undoped CoFe2O4 was estimated to be 2.69 eV from UV-Vis diffuse reflectance spectroscopy (DRS). With the increase of Zn2+ dopant, the E g value decreased from 2.61 to 2.01 eV, due to the difference of particle size of the samples. The magnetic hysteresis (M−H) loop confirmed the ferromagnetic nature of undoped CoFe2O4 with magnetization (M s) of 64.85 emu/g, and it is decreased with increasing the Zn2+ content in CoFe2O4 spinel, which was confirmed by a vibrating sample magnetometer (VSM). All composition of spinel Zn x Co1−x Fe2O4 samples were successfully tested as catalyst for the conversion of benzyl alcohol, which has resulted 91.73 and 95.82 % conversion efficiency of CoFe2O4 and Zn0.4Co0.6Fe2O4 nano-catalysts, respectively.

Crystal Structure, Transformation Strain, and Superelastic Property of Ti–Nb–Zr and Ti–Nb–Ta Alloys

Abstract: The composition dependences of transformation strain and shape memory, and superelastic properties were extensively investigated in Ti–Nb–Zr and Ti–Nb–Ta alloys in order to establish the guidelines for alloy design of biomedical superelastic alloys. The effects of composition on the crystal structure of the parent (β) phase and the martensite (α″) phase were also investigated. Results showed that not only transformation temperature but also transformation strain is tunable by alloy design, i.e., adjusting contents of Nb, Zr, and Ta. The lattice constant of the β phase increased linearly with increasing Zr content, while it was insensitive to Nb and Ta contents. On the other hand, the lattice constants of the α″ phase are mainly affected by Nb and Ta contents. The increase of Zr content exhibited a weaker impact on the transformation strain compared with Nb and Ta. The addition of Zr as a substitute of Nb with keeping superelasticity at room temperature significantly increased the transformation strain. On the other hand, the addition of Ta decreased the transformation strain at the compositions showing superelasticity. This study confirmed that the crystallography of martensitic transformation can be the main principal to guide the alloy design of biomedical superelastic alloys.

In Situ X-Ray Diffraction Studies on Structural Changes of a P2 Layered Material during Electrochemical Desodiation/ Sodiation

Abstract: Sodium layered oxides with mixed transition metals have received signifi cant attention as positive electrode candidates for sodium-ion batteries because of their high reversible capacity. The phase transformations of layered compounds during electrochemical reactions are a pivotal feature for understanding the relationship between layered structures and electrochemical properties. A combination of in situ diffraction and ex situ X-ray absorption spectroscopy reveals the phase transition mechanism for the ternary transition metal system (Fe‐Mn‐Co) with P2 stacking. In situ synchrotron X-ray diffraction using a capillary-based microbattery cell shows a structural change from P2 to O2 in P2‐Na 0.7 Fe 0.4 Mn 0.4 Co 0.2 O 2 at the voltage plateau above 4.1 V on desodiation. The P2 structure is restored upon subsequent sodiation. The lattice parameter c in the O2 structure decreases signifi cantly, resulting in a volumetric contraction of the lattice toward a fully charged state. Observations on the redox behavior of each transition metal in P2‐Na 0.7 Fe 0.4 Mn 0.4 Co 0.2 O 2 using X-ray absorption spectroscopy indicate that all transition metals are involved in the reduction/oxidation process.

Li+‐Ion Extraction/Insertion of Ni‐Rich Li1+x(NiyCozMnz)wO2 (0.005<x<0.03; y:z=8:1, w≈1) Electrodes: In Situ XRD and Raman Spectroscopy Study

Abstract: We present the results of our in situ X-ray diffraction (XRD) and Raman spectroscopy measurements for the first Li-ion extraction/insertion (charge/discharge) processes of nickel-rich Li1+x(NiyCozMnz)wO2 (0.005

Strain-Induced Modification of Optical Selection Rules in Lanthanide-Based Upconverting Nanoparticles

Abstract: NaYF4:Yb3+,Er3+ nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. Although hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly 2-fold increase in efficiency. In-situ X-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.

Nature of the band gap of halide perovskites ABX 3 (A = CH 3 NH 3 , Cs; B = Sn, Pb; X = Cl, Br, I): First-principles calculations

Abstract: The electronic structures of cubic structure of ABX3(A=CH3NH3, Cs; B=Sn, Pb; X=Cl, Br, I) are analyzed by density functional theory using the Perdew–Burke–Ernzerhof exchange–correlation functional and using the Heyd–Scuseria–Ernzerhof hybrid functional. The valence band maximum (VBM) is found to be made up by an antibonding hybridization of B s and X p states, whereas bands made up by the π antibonding of B p and X p states dominates the conduction band minimum (CBM). The changes of VBM, CBM, and band gap with ion B and X are then systematically summarized. The natural band offsets of ABX3 are partly given. We also found for all the ABX3 perovskite materials in this study, the bandgap increases with an increasing lattice parameter. This phenomenon has good consistency with the experimental results.

Electric Field and Strain Effect on Graphene-MoS2 Hybrid Structure: Ab Initio Calculations

Abstract: The electronic structures of graphene-MoS2 heterojunction under tension and external electric field were examined on the basis of density-functional theory. The tension of MoS2 changes the hybrid structure from semiconductor to metal. The transition is from the sensitive dependence of the bandgap of MoS2 on the lattice constant. The vertical electric field has little influence on the bandgap of MoS2, while it can also adjust the charge transfer between monolayer MoS2 and graphene. In addition, the Schottky barrier is linearly dependent on the electric field intensity with an effective vacuum spacing of 1.3 A. It is also discussed in detail that the bandgap of MoS2 dependence on the lattice constant and the S–S spacing.

Pt–Au–Co Alloy Electrocatalysts Demonstrating Enhanced Activity and Durability toward the Oxygen Reduction Reaction

Abstract: Here we investigate the oxygen reduction reaction electrocatalytic activity and the corrosion stability of several ternary Pt–Au–Co and Pt–Ir–Co alloys, with Pt–Au–Co having never been previously studied for ORR. The addition of Au fine tunes the lattice parameter and the surface electronic structure to enable activity and cycling stability that is unachievable in Pt–25 atom % Co (state-of-the-art binary baseline). The ternary alloys exhibit a volcano-type dependence of catalytic efficacy on the content of Au or Ir. Pt–2.5 atom % Au–25 atom % Co alloy shows a specific activity of 1.41 mA cm–2 at 0.95 V, which is 16% and 404% higher than those of identically synthesized Pt–Co and pure Pt, respectively. This enhancement is promising in comparison to a range of previously published Pt “skeleton” and Pt “skin” alloys and is in fact the most optimum reported for a skeleton-type system. The catalysts exhibit dramatically improved corrosion stability with increasing levels of Au or Ir substitution, with the spec...

Crystallographic and optical properties of (Cu, Ag)2ZnSnS4 and (Cu, Ag)2ZnSnSe4 solid solutions

Abstract: (Cu1-xAgx)2ZnSnS4 (CAZTS) and (Cu1-xAgx)2ZnSnSe4 (CAZTSe) solid solutions with 0 ≤ x ≤ 1.0 were synthesized. Their crystal structures were analyzed by Rietveld refinement of X-ray diffraction data. The refined lattice constants a of the kesterite-type CAZTS and CAZTSe increase with Ag content (x) increasing, while their lattice constants c slightly decrease. Therefore, by increasing x, the c /a ratio decreases from 2 for Cu-based compounds to a value lower than 1.9 for Ag-based compounds. The band-gap energies (Eg) of CAZTS and CAZTSe solid solutions were determined by their diffuse reflectance spectra. The reflectance edges of the CAZTS and CAZTSe shifted to the longer wavelength side with Ag content increasing at 0.0 ≤ x ≤ 0.2, while they shifted to the shorter wavelength side at 0.2 ≤ x ≤ 1.0. By increasing x, the Eg of CAZTS decreases from 1.49 eV (x=0.0) to 1.47 (x=0.2) and then increases to 2.01 eV (x=1.0); and the Eg of CAZTSe decreases from 0.98 eV (x=0.0) to 0.95 (x=0.2) and then increases to 1.34 eV (x=1.0). There is small bowing in the band gaps of the CAZTS and CAZTSe systems. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Understanding chemical expansion in perovskite-structured oxides.

Abstract: In this work, chemical expansion in perovskite oxides was characterized in detail, motivated, inter alia, by a desire to understand the lower chemical expansion coefficients observed for perovskites in comparison to fluorite-structured oxides. Changes in lattice parameter and in local atomic arrangements taking place during compositional changes of perovskites, i.e., stoichiometric expansion, were investigated by developing an empirical model and through molecular dynamics and density functional theory atomistic simulations. An accurate empirical expression for predicting lattice constants of perovskites was developed, using a similar approach to previous reports. From this equation, analytical expressions relating chemical expansion coefficients to separate contributions from the cation and anion sublattices, assuming Shannon ionic radii, were developed and used to isolate the effective radius of an oxygen vacancy, rV. Using both experimental and simulated chemical expansion coefficient data, rV for a variety of perovskite compositions was estimated, and trends in rV were studied. In most cases, rV was slightly smaller than or similar to the radius of an oxide ion, but larger than in the fluorite structured materials. This result was in good agreement with the atomistic simulations, showing contractive relaxations of the closest oxide ions towards the oxygen vacancy. The results indicate that the smaller chemical expansion coefficients of perovskites vs. fluorites are largely due to the smaller change in cation radii in perovskites, given that the contraction around the oxygen vacancy appears to be less in this structure. Limitations of applicability for the model are discussed.

(Ba,Ca)(Zr,Ti)O3 lead-free piezoelectric ceramics—The critical role of processing on properties

Abstract: Lead-free piezoelectric compositions based on (Ba,Ca)(Zr,Ti)O3 have been reported to exhibit many piezoelectric properties similar to the conventionally used Pb(Zr,Ti)O3 materials, and have thus been attracting much attention as potential replacements for lead-based piezoceramics. However, there appears quite a wide variation in the reported piezoelectric properties of the BCZT ceramics, indicating that such properties may be sensitive to fabrication and processing methods. This paper reports an investigation of a wide range of processing factors, including composition (e.g. ratio of Ba(Zr,Ti)O3 to (Ba,Ca)TiO3), sintering conditions (temperature and cooling rate), particle size of the calcined ceramic powder, structure and microstructure (e.g. phase, lattice parameters, density and grain size), and their effect on the piezoelectric properties. For individual compositions, lattice constants and grain size, which are themselves dependent on the ceramic powder particle size and sintering conditions, have been shown to be very important in terms of optimising piezoelectric properties in these materials.

Microstructure and magnetic properties of MFe2O4 (M = Co, Ni, and Mn) ferrite nanocrystals prepared using colloid mill and hydrothermal method

Abstract: Three kinds of spinel ferrite nanocrystals, MFe2O4 (M = Co, Ni, and Mn), are synthesized using colloid mill and hydrothermal method. During the synthesis process, a rapid mixing and reduction of cations with sodium borohydride (NaBH4) take place in a colloid mill then through a hydrothermal reaction, a slow oxidation and structural transformation of the spinel ferrite nanocrystals occur. The phase purity and crystal lattice parameters are estimated by X-ray diffraction studies. Scanning electron microscopy and transmission electron microscopy images show the morphology and particle size of the as-synthesized ferrite nanocrystals. Raman spectrum reveals active phonon modes at room temperature, and a shifting of the modes implies cation redistribution in the tetrahedral and octahedral sites. Magnetic measurements show that all the obtained samples exhibit higher saturation magnetization (Ms). Meanwhile, experiments demonstrate that the hydrothermal reaction time has significant effects on microstructure, mo...

Structural band-gap tuning in g-C3N4

Abstract: g-C3N4 is a promising material for hydrogen production from water via photo-catalysis, if we can tune its band gap to desirable levels. Using a combined experimental and ab initio approach, we uncover an almost perfectly linear relationship between the band gap and structural aspects of g-C3N4, which we show to originate in a changing overlap of wave functions associated with the lattice constants. This changing overlap, in turn, causes the unoccupied pz states to experience a significantly larger energy shift than any other occupied state (s, px, or py), resulting in this peculiar relationship. Our results explain and demonstrate the possibility to tune the band gap by structural means, and thus the frequency at which g-C3N4 absorbs light.

Synthesis, Crystal Chemistry, and Electrochemical Properties of Li7–2xLa3Zr2–xMoxO12 (x = 0.1–0.4): Stabilization of the Cubic Garnet Polymorph via Substitution of Zr4+ by Mo6+

Abstract: Cubic Li7La3Zr2O12 (LLZO) garnets are exceptionally well suited to be used as solid electrolytes or protecting layers in "Beyond Li-ion Battery" concepts. Unfortunately, cubic LLZO is not stable at room temperature (RT) and has to be stabilized by supervalent dopants. In this study we demonstrate a new possibility to stabilize the cubic phase at RT via substitution of Zr(4+) by Mo(6+). A Mo(6+) content of 0.25 per formula unit (pfu) stabilizes the cubic LLZO phase, and the solubility limit is about 0.3 Mo(6+) pfu. Based on the results of neutron powder diffraction and Raman spectroscopy, Mo(6+) is located at the octahedrally coordinated 16a site of the cubic garnet structure (space group Ia-3d). Since Mo(6+) has a smaller ionic radius compared to Zr(4+) the lattice parameter a0 decreases almost linearly as a function of the Mo(6+) content. The highest bulk Li-ion conductivity is found for the 0.25 pfu composition, with a typical RT value of 3.4 × 10(-4) S cm(-1). An additional significant resistive contribution originating from the sample interior (most probably from grain boundaries) could be identified in impedance spectra. The latter strongly depends on the prehistory and increases significantly after annealing at 700 °C in ambient air. Cyclic voltammetry experiments on cells containing Mo(6+) substituted LLZO indicate that the material is stable up to 6 V.