Showing papers on "Lattice constant published in 2013"

X-Ray Diffraction: A Practical Approach

Abstract: Basics: X-Rays and Diffraction Lattices and Crystal Structures Practical Aspects of X-Ray Diffraction Experimental Modules: Crystal Structure Determination, I: Cubic Structures Crystal Structure Determination, II: Hexagonal Structures Precise Lattice Parameter Measurements Phase Diagram Determination Detection of Long-Range Ordering Determination of Crystallite Size and Lattice Strain Quantitative Analysis of Powder Mixtures Identification of an Unknown Specimen: Appendices: Plane-Spacing Equations and Unit Cell Volumes Quadratic Forms of Miller Indices for the Cubic System Atomic and Ionic Scattering Factors of Some Selected Elements Summary of Structure Factor Calculations Mass Absorption Coefficients mu/rho (cm2/g) and Densities rho (g/cm3) of Some Selected Elements Multiplicity Factors Lorentz-Polarization Factor Physical Constants and Conversion Factors JCPDS-ICDD Card Numbers for Some Common Materials Index

Phase stability, electrochemical stability and ionic conductivity of the Li10±1MP2X12 (M = Ge, Si, Sn, Al or P, and X = O, S or Se) family of superionic conductors

Abstract: We present an investigation of the phase stability, electrochemical stability and Li+ conductivity of the Li10±1MP2X12 (M = Ge, Si, Sn, Al or P, and X = O, S or Se) family of superionic conductors using first principles calculations. The Li10GeP2S12 (LGPS) superionic conductor has the highest Li+ conductivity reported to date, with excellent electrochemical performance demonstrated in a Li-ion rechargeable battery. Our results show that isovalent cation substitutions of Ge4+ have a small effect on the relevant intrinsic properties, with Li10SiP2S12 and Li10SnP2S12 having similar phase stability, electrochemical stability and Li+ conductivity as LGPS. Aliovalent cation substitutions (M = Al or P) with compensating changes in the Li+ concentration also have a small effect on the Li+ conductivity in this structure. Anion substitutions, however, have a much larger effect on these properties. The oxygen-substituted Li10MP2O12 compounds are predicted not to be stable (with equilibrium decomposition energies >90 meV per atom) and have much lower Li+ conductivities than their sulfide counterparts. The selenium-substituted Li10MP2Se12 compounds, on the other hand, show a marginal improvement in conductivity, but at the expense of reduced electrochemical stability. We also studied the effect of lattice parameter changes on the Li+ conductivity and found the same asymmetry in behavior between increases and decreases in the lattice parameters, i.e., decreases in the lattice parameters lower the Li+ conductivity significantly, while increases in the lattice parameters increase the Li+ conductivity only marginally. Based on these results, we conclude that the size of the S2− is near optimal for Li+ conduction in this structural framework.

Strain-controlled oxygen vacancy formation and ordering in CaMnO 3

Abstract: We use first-principles calculations to investigate the stability of biaxially strained Pnma perovskite CaMnO${}_{3}$ towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain-relaxation mechanism. Our first-principles calculations of oxygen vacancy defect formation energy indeed show a strong strain dependence: We find that tensile strain lowers the formation energy, consistent with the established chemical expansion concept that oxygen deficiency increases the molar volume in oxides. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.

Towards control of the size and helicity of skyrmions in helimagnetic alloys by spin-orbit coupling

Abstract: Chirality--that is, left- or right-handedness--is an important concept in a broad range of scientific areas. In condensed matter, chirality is found not only in molecular or crystal forms, but also in magnetic structures. A magnetic skyrmion is a topologically stable spin vortex structure, as observed in chiral-lattice helimagnets, and is one example of such a structure. The spin swirling direction (skyrmion helicity) should be closely related to the underlying lattice chirality via the relativistic spin-orbit coupling. Here, we report on the correlation between skyrmion helicity and crystal chirality in alloys of helimagnets Mn(1-x)Fe(x)Ge with varying compositions by Lorentz transmission electron microscopy and convergent-beam electron diffraction over a broad range of compositions (x = 0.3-1.0). The skyrmion lattice constant shows non-monotonous variation with composition x, with a divergent behaviour around x = 0.8, where the correlation between magnetic helicity and crystal chirality changes sign. This originates from continuous variation of the spin-orbit coupling strength and its sign reversal in the metallic alloys as a function of x. Controllable spin-orbit coupling may offer a promising way to tune skyrmion size and helicity.

Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystal Ag(111)

Abstract: Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN.

Monolayer semiconducting transition metal dichalcogenide alloys: Stability and band bowing

Abstract: expansion method and the special quasi-random structure approach. It is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3, which can be explained by the small lattice mismatch between the constituents and a large additional charge exchange, while no ordered configuration exists for (Se, Te) and (S, Te) alloys at 0K. The calculated phase diagrams indicate that complete miscibility in the alloys can be achieved at moderate temperatures. The bowing in lattice constant for the alloys is quite small, while the bowing in band gap, and more so in band edge positions, is much more significant. By decomposing the formation of alloy into multiple steps, it is found that the band bowing is the joint effect of volume deformation, chemical difference, and a low-dimensionality enhanced structure relaxation. The direct band gaps in these alloys continuously tunable from 1.8eV to 1.0eV, along with the moderate miscibility temperatures, make them good candidates for two-dimensional optoelectronics. V C 2013 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4799126]

Effect of Rb and Ta Doping on the Ionic Conductivity and Stability of the Garnet Li7+2x–y(La3–xRbx)(Zr2–yTay)O12 (0 ≤ x ≤ 0.375, 0 ≤ y ≤ 1) Superionic Conductor: A First Principles Investigation

Abstract: In this work, we investigated the effect of Rb and Ta doping on the ionic conductivity and stability of the garnet Li7+2x–y(La3–xRbx)(Zr2–yTay)O12 (0 ≤ x ≤ 0.375, 0 ≤ y ≤ 1) superionic conductor using first principles calculations. Our results indicate that doping does not greatly alter the topology of the migration pathway, but instead acts primarily to change the lithium concentration. The structure with the lowest activation energy and highest room temperature conductivity is Li6.75La3Zr1.75Ta0.25O12 (Ea = 19 meV, σ300K = 1 × 10–2 S cm–1). All Ta-doped structures have significantly higher ionic conductivity than the undoped cubic Li7La3Zr2O12 (c-LLZO, Ea = 24 meV, σ300K = 2 × 10–3 S cm–1). The Rb-doped structure with composition Li7.25La2.875Rb0.125Zr2O12 has a lower activation energy than c-LLZO, but further Rb doping leads to a dramatic decrease in performance. We also examined the effect of changing the lattice parameter at fixed lithium concentration and found that a decrease in the lattice paramet...

Superconducting phases in potassium-intercalated iron selenides.

Abstract: The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in AxFe2–ySe2 (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, KxFe2Se2(NH3)y (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K0.3Fe2Se2(NH3)0.47 corresponds to the 44 K phase with lattice constant c = 15.56(1) A and K0.6Fe2Se2(NH3)0.37 to the 30 K phase with c = 14.84(1) A. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH3 has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K0.3Fe2Se2 and K0.6Fe2Se2 SC phases. K0.3Fe2Se2(NH3)0.47 and K0.6Fe2Se2(NH3)0.37 stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence t...

Correlation Effects on Lattice Relaxation and Electronic Structure of ZnO within the GGA+U Formalism

Abstract: The electronic structures of ZnO were calculated using density functional theory, in which the electronic interactions are described within the GGA+U (GGA = generalized gradient approximation) formalism, where on-site Coulomb corrections are applied on the Zn 3d orbitals (Ud) and O 2p orbitals (Up). The relaxed GGA+U calculation can correct completely the band gap, the position of Zn 3d states, the transition levels of O vacancy in band gap, and so on, which is different from other GGA+U (equivalent LDA+U) calculations partially correcting the energy band structure for fixed lattice constants. By comparing with experimental data, the pair of Ud = 10 and Up = 7 eV was identified as an optimum choice for the energy band structure of W-ZnO. Then, the proper pair of Ud and Up parameters was taken to predict the energy band structure of ZB- and RS- ZnO, of which the former is in good agreement with experimental values, and the latter is in dispute, relating to the decrease of the octahedral symmetry. Subsequen...

Signatures of a pressure-induced topological quantum phase transition in BiTeI.

Abstract: We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-pressure structure up to 8 GPa. The lattice parameter ratio c/a shows a minimum between 2.0-2.9 GPa, indicating an enhanced c-axis bonding through p(z) band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition and are consistent with a recent theoretical proposal.

Tunable optoelectronic and ferroelectric properties in Sc-based III-nitrides

Abstract: Sc-based III-nitride alloys were studied using density functional theory with special quasi-random structure methodology. ScxAl1−xN and ScxGa1−xN alloys are found to be stable in hexagonal phases up to x ≈ 0.56 and x ≈ 0.66, respectively, above which rock-salt structures are more stable. Epitaxial strain stabilization can prevent spinodal decomposition up to x ≈ 0.4 (ScxAl1−xN on AlN or GaN) and x = 0.27 (ScxGa1−xN on GaN). The increase in Sc content expands the in-plane lattice parameter of ScxAl1−xN and ScxGa1−xN alloys, leads to composition- and strain-tunable band gaps and polarization, and ultimately introduces ferroelectric functionality in ScxGa1−xN at x ≈ 0.625. A modified Becke-Johnson exchange-correlation potential was applied to study the electronic structures, which yielded band gaps comparable to those from hybrid functional calculations, yet in a much shorter computational time. The alloys were found to retain wide band gaps, which stay direct up to x = 0.25 (ScxAl1−xN) and x = 0.5 (ScxGa1−xN). The band gaps decrease with increasing x for ScxAl1−xN, in which the Sc-3d states dominate at the conduction band minimum and lead to flat electron dispersion at the Γ point. Conversely, the band gaps increase with increasing x for ScxGa1−xN (up to x = 0.5), in which Sc-3d states do not contribute to the conduction band minimum at the Γ point.

Transient phase change in two phase reaction between LiFePO4 and FePO4 under battery operation

Abstract: Transient states of phase transition in LiFePO4/FePO4 for lithium ion battery positive electrodes are investigated by time-resolved measurements. To directly detect changes in electronic and crystal structures under battery operation, in situ time-resolved X-ray absorption and diffraction measurements are performed, respectively. The phase fraction change estimated by the iron valence change is similar to the electrochemically expected change. The transient change of lattice constant during two phase reaction is clearly observed by the time-resolved X-ray diffraction measurement. The nonequilibrium lithium extraction behavior deviates from the thermodynamic diagram of the two phase system, resulting in continuous phase transition during electrochemical reactions.

Octahedral and truncated high-voltage spinel cathodes: the role of morphology and surface planes in electrochemical properties

Abstract: High-voltage spinel cathodes LiMn1.5Ni0.5O4 are promising candidates for large-scale energy-storage applications such as electric vehicles. However, the widespread adoption of this high-voltage spinel cathode is hampered by severe capacity fade, particularly at elevated temperatures, resulting from aggressive formation of a thick solid-electrolyte interphase (SEI) layer through side reactions with the electrolyte at the high operating voltage, cationic ordering between Mn4+ and Ni2+ ions in the crystal lattice, and formation of a rock salt LixNi1−xO impurity phase. While these issues have been explored, the wide variation in physical and electrochemical properties with different synthesis methods is not fully understood. In this investigation, we present how the synthesis conditions of the co-precipitation method influence the microstructure and morphology through nucleation and growth of crystals in solution. The samples were prepared by two similar wet-chemical routes and were characterized by microscopy and electrochemical methods to determine the role of microstructure and morphology in the electrochemical performance. Various factors such as the degree of cation ordering between Mn4+ and Ni2+, Mn3+ content, Ni–Mn ratio in the sample, change in lattice parameter with the state of charge, and surface crystal planes were examined to develop a better understanding of the factors influencing the electrochemical performance. It is found that the surface crystal planes, the arrangement of lithium ions near the surface, and the lithium diffusion mechanism have a dominant effect on the capacity retention and rate performance.

Phonon Scattering through a Local Anisotropic Structural Disorder in the Thermoelectric Solid Solution Cu2Zn1–xFexGeSe4

Abstract: Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu2Zn1–xFexGeSe4. Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the c/a lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce the lattice thermal conductivity in this class of materials.

Lattice constants and cohesive energies of alkali, alkaline-earth, and transition metals: Random phase approximation and density functional theory results

Abstract: We present lattice constants and cohesive energies of alkali, alkaline earth, and transition metals using the correlation energy evaluated within the adiabatic-connection fluctuation-dissipation (ACFD) framework in the random phase approximation (RPA) and compare our findings to results obtained with the meta-GGA functional revTPSS and the gradient corrected PBE (Perdew-Burke-Ernzerhof) functional and the PBEsol functional (PBE reparametrized for solids), as well as a van der Waals (vdW) corrected functional optB88-vdW. Generally, the RPA reduces the mean absolute error in the lattice constants by about a factor 2 compared to the other functionals. Atomization energies are also on par with the PBE functional, and about a factor 2 better than with the other functionals. The study confirms that the RPA describes all bonding situations equally well including van der Waals, covalent, and metallic bonding.

Mechanical and thermodynamic properties of Al3Sc and Al3Li precipitates in Al-Li-Sc alloys from first-principles calculations

Abstract: The mechanical, electronic and thermodynamic properties of L12-type Al3Sc and Al3Li precipitates have been investigated from first-principles method The calculated equilibrium parameters such as lattice constants are in good agreement with the available experimental results and other theoretical reports It is found that the Al3Sc exhibits a higher structural stability and stronger alloying ability than that of Al3Li since the Al3Sc possesses the lower cohesive energy and formation enthalpy Mechanical parameters, such as the bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio ν and universal anisotropic index AU are determined by the Voigt–Reuss–Hill approximation In addition, the anisotropic sound velocity, Debye temperature, density of states and charge density distribution of the two precipitates are studied The calculations associated with phonon properties confirm the dynamical stability of the L12-type structures studied Finally, the temperature-dependent behaviors of thermodynamic properties of the two precipitates are determined within the quasi-harmonic approximation

Lattice constant and substitutional composition of GeSn alloys grown by molecular beam epitaxy

Abstract: Single crystal epitaxial Ge1−xSnx alloys with atomic fractions of tin up to x = 0.145 were grown by solid source molecular beam epitaxy on Ge (001) substrates. The Ge1−xSnx alloys formed high quality, coherent, strained layers at growth temperatures below 250 °C, as shown by high resolution X-ray diffraction. The amount of Sn that was on lattice sites, as determined by Rutherford backscattering spectrometry channeling, was found to be above 90% substitutional in all alloys. The degree of strain and the dependence of the effective unstrained bulk lattice constant of Ge1−xSnx alloys versus the composition of Sn have been determined.