Site Occupancy Of Atoms Research Articles

Site occupation of Nb atoms in ternary Ni–Ti–Nb shape memory alloys

Site occupation of Nb atoms in ternary Ni–Ti–Nb shape memory alloys

Inelastic Neutron Scattering and Density Functional Theory–Molecular Dynamics Study of Si Dynamics inTi3SiC2

Observed differences between measured and calculated elastic constants forTi3SiC2are investigated using Density Functional Theory and Inelastic Neutron Scattering. The agreement between the calculated lattice dynamics and the dynamics measured by inelastic neutron scattering is considered good except at energies below ~20 meV where discrepancies suggest anharmonic potentials. This suggestion is confirmed by Density Functional Theory—Molecular Dynamics simulation which shows multiple site occupancy of the Si atoms within the basal plane at finite temperature and produces a calculated inelastic spectrum in better agreement with the measured spectrum in the low‐energy region. The highly anharmonic potential energy surface of theSiatoms offers an explanation for the failure of elastic constants, calculated based on the harmonic approximation, to agree with initial experimental measurements.

In Search of the True Structure of the Sodium Chromium Alum II: Crystal Growth and Structure of the Double Salt NaCr(SO4)2(H2O)6·(CH3OH)0.6(2)· (H2O)1.5(5)

Attempts to crystallize the sodium chromium alum, NaCr(SO4)2(H2O)12, from water=methanol solutions at temperatures around 281 K yielded crystals of the title compound instead. Their monoclinic structure can be described as a centered rectangular array of linear {1 ∞}[Na(SO4)2]3- rods aligned parallel to [001] with vicinal rods held together by columns of isolated [Cr(H2O)6]3+ complexes. The structure forms channels parallel to [001] which are occupied by methanol and water molecules with atomic site occupancy factors of 1/5 and less. Various attempts to crystallize the compound or the sodium chromium alum from purely aqueous solutions remained unsuccessful

β-Mn-Type Co8+xZn12–x as a Defect Cubic Laves Phase: Site Preferences, Magnetism, and Electronic Structure

The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co-Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co-Zn phases crystallize in the space group P4(1)32 [Pearson symbol cP20; a = 6.3555(7)-6.3220(7)], and their stoichiometry may be expressed as Co(8+x)Zn(12-x) [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co(8+x)Zn(12-x) reveals that the β-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed β-Mn structure type, a result that offers greater insight into the β-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).

Site-specific order and magnetism in tetragonal Mn3Ga thin films

Mn${}_{3}$Ga bulk material and thin films deposited on several different substrates have been investigated using x-ray and neutron diffraction, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and electronic structure calculations using density-functional theory with the aim of determining the atomic site occupancy, magnetic moments, and magnetic structure of this tetragonal D0${}_{22}$-structure compound. The Mn${}_{3}$Ga has close to ideal site occupancy, with Ga on $2a$ sites and Mn on $2b$ and $4d$ sites. The magnetic structure is basically ferrimagnetic, with the larger Mn moment of about 3 ${\ensuremath{\mu}}_{B}$ on the $2b$ site, which is coordinated by 8 Mn $4d$ and 4 Ga, and the smaller one on the $4d$ site, which is coordinated by 4 Mn $2b$, 4 Ga, and 4 Mn $4d$. The Mn $d$-band occupancy is close to 5 on both sites, and the orbital moments are small, $l$$0.2$ ${\ensuremath{\mu}}_{B}$. The material nevertheless exhibits substantial uniaxial anisotropy, ${K}_{u}=1.0\phantom{\rule{0.28em}{0ex}}{\mathrm{MJ}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}3}$, which originates from the $4d$ site. The $2b$ site has hard axis anisotropy, which together with an oscillatory exchange coupling from the first and second nearest neighbors, leads to a soft component of the magnetization in the $c$ plane, coexisting with $c$-axis hysteresis loops exhibiting coercivity of up to 1.2 T, and magnetization in the range 110--220 kA m${}^{\ensuremath{-}1}$ at room temperature, depending on preparation conditions. Tetragonal Mn${}_{2}$Ga films behave similarly. Manganese is lost from both sites, but the films have substantially larger magnetization (480 kA m${}^{\ensuremath{-}1}$) and anisotropy constant (2.35 MJ m${}^{\ensuremath{-}3}$) than Mn${}_{3}$Ga.

Structural and relaxor characteristics of Ca0.18Sr0.226Ba0.594Nb2O6

Abstract Polycrystalline Ca0.18Sr0.226Ba0.594Nb2O6 (CSBN18) was synthesized via the solid-state reaction route. X-ray structural studies confirmed it belonged to the tetragonal tungsten bronze family. Rietveld refinement of the X-ray data has been carried out for CSBN18 where the atomic positions and site occupancy factors for A-sites have been determined. The dielectric properties of CSBN18 ceramic were studied as a function of temperature in the 100 Hz–1 MHz frequency range. The dielectric relaxation followed the Vogel–Fulcher relation wherein E a = 37.4 meV; T f = 131.5 °C and ω 0 = 4.31 × 109 rad s−1. A high pyroelectric coefficient of ∼ 250 μC m−2.K was obtained around the transition temperature (∼150 °C). This is significantly higher than that reported for polycrystalline Sr x Ba1-xNb2O6 (SBN). However, the piezoelectric coefficient (d 33) of the title composition was as low as 6 pC N−1.

Synthesis of Binary Magnesium–Transition Metal Oxides via Inverse Coprecipitation

Synthesis of binary magnesium–transition metal oxides, MgM2O4 (M: Cr, Mn, Fe, Co) and MgNiO2, was performed by calcination at relatively low temperatures of 500 and 750 °C for 24 h through inverse coprecipitation of carbonate hydroxide precursors. The important roles of the precipitation agent, sodium carbonate, were clarified by considering equilibria in an aqueous solution. The structure parameters of the obtained binary magnesium–transition metal oxide powders, specifically the occupancy of atomic sites, were evaluated from synchrotron X-ray diffraction (XRD) profiles by Rietveld refinement in addition to the magnetic properties at room temperature. The present work provides general guidelines for low-cost and high-volume synthesis of complex oxides, which are easily decomposed at high temperatures.

How to monitor guest exchange in host–guest systems

The process of guest-exchange in host–guest compounds is reviewed, with a brief discussion of the mechanism and followed by an outline of various analytical techniques, which are commonly employed to follow the progress of the exchange reaction. These include thermal gravimetry (TG), differential scanning calorimetry (DSC), gas–liquid chromatography, nuclear magnetic resonance (NMR), colour change and other more specialised methods. In-situ methods are emphasized and structural changes as monitored by powder X-ray diffraction or single-crystal to single-crystal transformation are considered. An example is given of a guest exchange in an organic system comprising a xanthenol host that initially contains chlorobenzene as guest, which is exchanged with toluene. The procedure is carried out on a single crystal and the structure is monitored as a function of time. The refinement of the structures, which exhibit disordered mixed guests, poses special problems, and a good starting model for the site occupancies of the guest atoms is important. Many such reactions may be carried out using a simple apparatus.

X-ray Rietveld and 57Fe M^|^ouml;ssbauer study of babingtonite from Kouragahana, Shimane Peninsula, Japan

Babingtonite from Kouragahana, Shimane Peninsula, Japan, was investigated using electron microprobe, X-ray Rietveld, and 57Fe Mössbauer spectral analyses to characterize its chemical compositions, crystal structure, oxidation state of Fe, and distribution of Fe between two crystallographically independent octahedral Fe1 and Fe2 sites. The Kouragahana babingtonite occurs as single parallelohedrons with {100}, {001}, {001}, {111}, {110}, and {101} and sometimes shows penetration twinning. Both normal and sector-zoned crystals occur. Babingtonite crystals with sector zoning consist of sectors relatively enriched in Fe and of sectors enriched in Mg, Mn, and Al. Babingtonite also shows compositional zoning with higher Fe2+ and Al core and higher Fe3+ and Mn rim. The average Fe content of the babingtonite without sector zoning is similar to the Fe2+-rich sector of the sector-zoned babingtonite. The chemical formula based on the average composition of all analytical data (n = 193) is [Na0.01(2)Ca2.01(2)] [Mg0.11(4)Mn0.09(3)Fe2+0.76(7)Fe3+0.93(5)Ti0.01(1)Al0.06(5)]Si5.01(4)O14(OH). X-ray Rietveld refinement was carried out using a model of space group P1. The result of the refinement is characterized by R-weighted pattern = 9.91, R-expected pattern = 6.37, and goodness-of-fit = 1.56. The unit cell parameters are a = 7.4667(3), b = 11.6253 (6), c = 6.6820(2) Å, α = 91.533(4), β = 93.886(3), γ = 104.203(4)°, and V = 560.43(4) Å3. The refined site occupancies of atoms in the Fe1 and Fe2 sites are [Fe0.91(2)Mg0.09] and [Fe0.91(2)Al0.09], respectively, if Mg is assumed to distribute in the Fe1 site and Al in the Fe2 site. By allocating Mn, 0.09 atoms per formula unit (a.p.f.u.), to the Fe1 site, the site populations in the Fe1 and Fe2 sites are determined as [Fe0.82(2)Mn0.09Mg0.09] and [Fe0.91(2)Al0.09] a.p.f.u., respectively. The 57Fe Mössbauer spectrum taken at room temperature consists of three peaks, which were resolved into two doublets assigned to Fe2+ and Fe3+ at the two octahedral sites. The Fe2+:Fe3+ ratio was determined as Fe2+:Fe3+ = 43.3(3):56.7(4) and 47.1(4):52.9(3) by applying two fitting models, and the average Fe2+:Fe3+ ratio was 45.2(4):54.8(4). The results of X-ray Rietveld analysis and Mössbauer spectroscopy indicate that Fe2+ and Fe3+ are ordered at the Fe1 and Fe2 sites, respectively, in the Kouragahana babingtonite.

Phase relations and crystal structures in the system Ce–Ni–Zn at 800 °C

Phase relations have been established for the system Ce–Ni–Zn in the isothermal section at 800°C using electron microprobe analysis and X-ray powder diffraction. Phase equilibria at 800°C are characterized by a large region for the liquid phase covering most of the Ce-rich part of the diagram, whereas a Zn-rich liquid is confined to a small region near the Zn-corner of the Gibbs triangle. Whereas solubility of Ce in the binary Ni-Zn phases is negligible, mutual solubilities of Ni and Zn at a constant Ce content are large at 800°C for most Ce–Zn and Ce–Ni compounds. The solid solution Ce(Ni1−xZnx)5 with the CaCu5-type is continuous throughout the entire section and for the full temperature region from 400 to 800°C. Substitution of Zn by Ni is found to stabilize the structure of CeZn11 to higher temperatures. At 800°C Ce(NixZn1−x)11 (0.03≤x≤0.22) appears as a ternary solution phase. Similarly, a rather extended solution forms for Ce2(NixZn1−x)17 (0≤x≤0.53). Detailed data on atom site occupation and atom parameters were derived from X-ray structure analyses for single crystals of Ce2+y(NixZn1−x)17, y=0.02, x=0.49 (a=0.87541(3), c=1.25410(4)nm; Th2Zn17 type with space group R3¯m,RF2=0.018) and Ce(Ni0.18Zn0.82)11 (a=1.04302(2), c=0.67624(3)nm, BaCd11 type with space group I41/amd, RF2=0.049).

Structural modulations and magnetic properties of off-stoichiometric Ni-Mn-Ga magnetic shape memory alloys

A comparative description of the crystallographic structural details and the magnetic properties of Ni-Mn-Ga magnetic shape memory alloys are necessary to understand the physical origins of their magnetoelastic properties. The crystallographic structural details of an off-stoichiometric Ni${}_{2}$Mn${}_{1.14}$Ga${}_{0.86}$ alloy have been obtained from the refinement of high-resolution single-crystal neutron diffraction data, following a (3 $+$ 1)-dimensional [(3 $+$ 1)-D] superspace formalism. In particular, the structure adopts a $P$2/$m$($\ensuremath{\alpha}$0$\ensuremath{\gamma}$)00 superspace group symmetry with the following fundamental lattice parameters: $a$ $=$ 4.255(4) $\AA{}$, $b$ $=$ 5.613(4) $\AA{}$, $c$ $=$ 4.216(3) $\AA{}$, a commensurate periodicity of 5M, and a modulation wave vector of $\stackrel{P\vec}{\mathbf{q}}=2/5\stackrel{P\vec}{\mathbf{c}}$. The magnetic properties of the crystal are characterized from magnetic susceptibility measurements, as well as from neutron diffraction intensities. The overall magnetic moment per unit cell is 2.7 $\ensuremath{\mu}$${}_{\mathrm{B}}$ per Ni${}_{2}$Mn${}_{1.14}$Ga${}_{0.86}$ formula unit and the magnetic moments are aligned along the $b$ axis. The modulations of the atomic site displacements, site occupancies, and magnetic moments are elucidated from a (3 $+$ 1)-D refinement of the neutron diffraction intensities. In addition to atomic displacements corresponding to shear waves along $\ensuremath{\langle}110\ensuremath{\rangle}$, distortions of the Ni-centric tetrahedra are also evident. Physical interpretations for the different structural distortions and their relationship to magnetic properties are discussed.

Homogeneity range and crystal structure of the Ca2Mg5Zn13 compound

The homogeneity range and crystal structure of the Ca2Mg5Zn13 ternary solid solution were determined using SEM, EPMA, EBSD, TEM and X-ray diffraction. This compound has the CaxMgyZnz (8.2≤x≤9.1; 27.1≤y≤31.0; 60.8≤z≤64.7) composition range at 335°C. The refinement of the XRD patterns was carried out by Rietveld analysis. XRD data showed that this solid solution crystallizes in a hexagonal structure having P63/mmc (194) space group and Sm3Mg13Zn30 prototype. The well indexed SAED patterns and Kikuchi diffraction pattern obtained from TEM and EBSD confirmed the crystallographic information obtained by XRD. The atomic coordination spheres and site occupancy were determined. On the basis of the atomic occupancy results and the crystallographic details, a three-sublattice model is proposed for this compound.

A Brief Introduction to Alloy Phase Chemistry Determination under EPMA/SEM-EDS Conditions and its Applications

A so called multiphase separation method (MPSM) is proposed to quantitatively separate precipitated phases from their surrounding matrix phase in chemistry for bulk alloy/steel samples under EPMA/SEM-EDS measurement conditions. Applied examples to comparisons of the results through the MPSM with the values either cited or obtained via other analytical means relevant are indicative of the feasibility, accuracy as well as applicability of the MPSM, which deal with chemistry, amount, lattice parameter, elemental partitioning, atomic-site occupancy and stability of precipitated phases of either superalloy or heat-resistant steel samples analyzed. Successful applications of the MPSM not only show a significant improvement for difficulties in accurate quantification in phase chemistry under the EPMA/SEM-EDS measurement conditions but also provide with a useful and helpful tool to determine some other important physical quantities in alloys and steels, which make it possible to quantitatively and more widely evaluate structure-property relationships of the materials investigated through analyzing their bulk samples under EPMA/SEM-EDS measurement conditions.

Atomic Anti-Site Evolution during L1&lt;sub&gt;2&lt;/sub&gt; Precipitation Using Microscopic Phase-Field Simulation

The influence of temperature on atomic site occupation probability (SOP) in L12--Ni3(Al1-xFex) has been studied using microscopic phase-field simulation for Ni3Al-Fe alloy with the lower Fe content. Microscopic phase-field kinetics model was further extended to study the temporal evolution of atomic site occupation and atomic anti-sites behavior, which was revealed in detail at the different temperatures. The results show that the size, number and the precipitation mechanism of L12phases also present obvious changes with the increase of temperature. The SOP of each atom on its right-sites decreases slowly; while the SOP of Fe atoms are far lower than those of Al atoms. On the other hand, the SOP of each atom on its anti-sites increases gradually. The formation of NiAl(Ni atoms form anti-sites on Al sites) is more easily and the formation of NiAlis more sensitive to temperature than that of FeNi(Fe atoms form anti-sites on Ni sites). Atomic site occupation will not evolve during the early stage of precipitation, which becomes more and more obvious.

The Atomic Site Occupancies in the Fe-Cr σ-Phase

The atomic site distribution of the complex σ-phase structure (P42/mnm) has been studied using density functional theory (within the EMTO and WIEN2k codes) applying the cluster expansion method in a mean field approximation at finite temperatures. We found that at low temperatures Fe atoms predominantly occupy the icosahedrally coordinated (A,D) sites, Cr atoms prefer the (B,E) sites with the high coordination numbers, while the C site remains mixed. However, at higher temperature close to 1000 K all occupations become more and more mixed and reproduce well the available experimental data.

Design of a technique to measure the density of ultracold atoms in a short-period optical lattice in three dimensions with single-atom sensitivity

A measurement technique is described that has the potential to map the atomic site occupancies of ultracold atoms in a short-period three-dimensional optical lattice. The method uses accordion and pinning lattices, together with polarization gradient cooling and fluorescence detection, to measure the positions of individual atoms within the sample in three dimensions at a resolution of around half the atomic resonant wavelength. The method measures the site occupancy, rather than the parity of the site occupancy, of atoms in the lattice. It is expected that such measurements hold significant potential for the study of ultracold quantum dynamics.

Formation ofα-MnandRstructures from theβ-Mnstructure in Mn-Si alloys

The Mn-Si alloy system undergoes the $(\ensuremath{\beta}\text{-Mn}\ensuremath{\rightarrow}\ensuremath{\alpha}\text{-Mn}+R)$ eutectoid reaction at 908 K around $8\text{ }\text{at}\text{.}\text{ }\mathrm{%}$ Si. The crystal structures of the $\ensuremath{\beta}\text{-Mn}$, $\ensuremath{\alpha}\text{-Mn}$, and $R$ phases were reported to consist of complex coordination polyhedra such as a polyhedron with coordination number (CN) 12: that is, an icosahedral atomic cluster. The notable features of these structures are that both the $\ensuremath{\alpha}\text{-Mn}$ and $R$ structures are characterized by arrays of penetrated CN16 pairs while penetrated CN14 pairs are present in the $\ensuremath{\beta}\text{-Mn}$ structure. In a penetrated CN16 pair consisting of two CN16 polyhedra, for instance, a center atom of one CN16 polyhedron is one of 16 atoms forming a shell of the other. To understand the crucial factors controlling a change between two coordination-polyhedra structures, in this study, the crystallographic features of the $(\ensuremath{\beta}\text{-Mn}\ensuremath{\rightarrow}\ensuremath{\alpha}\text{-Mn})$ and $(\ensuremath{\beta}\text{-Mn}\ensuremath{\rightarrow}R)$ structural changes related to the eutectoid reaction have been investigated by transmission electron microscopy. It was found that the former $(\ensuremath{\beta}\text{-Mn}\ensuremath{\rightarrow}\ensuremath{\alpha}\text{-Mn})$ change started with the conversion of a penetrated CN14 pair into a penetrated CN16 pair by successive and simple atomic shifts. Among three types of orientation relations between the $\ensuremath{\beta}\text{-Mn}$ and $R$ structures in the latter change, the direct conversion of a penetrated CN16 pair from a penetrated CN14 pair should first occur for the major relation. As for two other minor relations found mainly for higher Si contents, in the starting changes, a penetrated pair consisting of CN14 and CN12 polyhedra, and two neighboring CN14 and CN12 polyhedra in the $\ensuremath{\beta}\text{-Mn}$ structure must be converted to a penetrated CN16 pair, and two neighboring CN16 and CN12 polyhedra in the $R$ structure, respectively. The directional relation in the orientation relationship for these two structural changes was, as a result, determined by orientations of the penetrated CN14 pairs or two neighboring polyhedra in the $\ensuremath{\beta}\text{-Mn}$ structure. It is thus understood that the crucial factor controlling the $(\ensuremath{\beta}\text{-Mn}\ensuremath{\rightarrow}\ensuremath{\alpha}\text{-Mn})$ and $(\ensuremath{\beta}\text{-Mn}\ensuremath{\rightarrow}R)$ structural changes should be the conversion between the structural units consisting of only two neighboring polyhedra, which are involved in these complex structures. Furthermore, both site occupancies of atoms and atomic-bond lengths in the penetrated CN16 pairs for the $\ensuremath{\alpha}\text{-Mn}$ and $R$ structures were evaluated to understand the physical origin of interplay between the appearance of a magnetic moment in the former structure and the formation of covalentlike atomic bonding in the latter.

High-Temperature Stability of Lead Zinc Niobate: In Situ X-Ray Diffraction

The thermal stability of the relaxor ferroelectric Pb(Zn 1/3 Nb 2/3 ) 0 3 (PZN) upon heating was studied using high-temperature X-ray diffraction (XRD) and scanning electron microscopy. It was found that single-phase PZN is stable below 700°C, whereupon the first decomposition phase, the pyrochlore Pb 1.83 Nb 1 . 71 Zn 0 . 29 0 6 . 39 was observed. With the increasing temperature, the amount of pyrochlore increased until at 1100°C there is no PZN remaining. Pyrochlore formation was accompanied by the precipitation of ZnO above 800°C and after holding at 1100°C, the formation of Nb 2 Z H O 6 . Rietveld refinements based on the XRD patterns have allowed the relative phase proportions as well as the atomic site occupancies of the metal ions in PZN and the pyrochlore to be estimated. The Zn site occupancy within the perovskite PZN decreases upon heating in parallel with the pyrochlore formation. An elemental mass balance based on the XRD results shows that PZN decomposition is well under way before any Pb is lost from the sample. This indicates that Zn egress rather than Pb volatility is the determining factor in the instability of PZN, contrary to prior published work.

Effect of Fe substitution on the phase stability and magnetic properties of Mn-rich Ni–Mn–Ga ferromagnetic shape memory alloys

The influence of Fe additions on the martensitic transformation and magnetic properties of Mn-rich Ni–Mn–Ga alloys was investigated by substituting either 1 at% Fe for each atomic species or by substituting Ni with varying amounts of Fe. The magnetic structure of the alloys was studied using 57Fe Mössbauer spectroscopy. Mössbauer spectra revealed typical paramagnetic features in Mn-rich Ni–Mn–Ga–Fe alloys owing to the preferential site occupancy of Fe atoms at Ni sites. The evolution of the magnetic properties and phase stability has been correlated with the chemical and atomic ordering in these alloys.

A new phase in the system lithium–aluminum: Characterization of orthorhombic Li 2Al

Investigation of the Li rich part of the binary Li–Al system revealed the existence of a new phase, orthorhombic Li 2Al, which is isostructural to Li 2Ga and Li 2In. The crystal structure was determined from single crystal X-ray diffraction data ( Cmcm, a=4.658(2) Å, b=9.767(4) Å, c=4.490(2) Å, Z=4). Refinement of atomic position site occupancies yielded a composition Li 1.92Al 1.08 (64 at% Li) indicating a small homogeneity range, Li 2− x Al 1+ x . Li 2Al is the peritectic decomposition product of the stoichiometric compound Li 9Al 4, which is stable below 270±2 °C. Li 2Al itself decomposes peritectically to Li 3Al 2 and Li rich melt at 335±2 °C. The discovery of Li 2Al (Li 2− x Al 1+ x ) settles a long standing inconsistency in the Li–Al phase diagram which was based on the assumption that Li 9Al 4 possesses a high temperature modification.