Effect Of Chemical Order Research Articles

Electrochemical Reactions, Chemical Ordering Effects, and Calculated Electronic Structure, for Pt100-xMx (M = V, Zr) Thin-Film Surfaces in Acid Electrolytes

ABSTRACTMicrostructural, chemical, and electrochemical property measurements results, for (111) crystallographically oriented Pt100-xMx (M = V, Zr; 3 < xv < 14; and 4 < xZr < 35, At.%) sputtered thin films are presented, with electronic structure calculations. These Pt-based alloys were prepared to investigate early transition metal (ETM), late-transition-metal (LTM) alloys as potential electrode materials in hydrogen-air polymer-electrolyte-membrane fuel cells (PEMFCs). The Pt100-xMx oxygen-reduction-reaction (ORR) currents peak for 8 < x < 10 atomic percent, so local chemical-short-range-order, may exist; as the peak in ORR activity is commensurate with the strong ordering in Pt8M (M = Ti, V, Zr). The hydrogen under potential deposition (Hupd) at Pt active area, and ORR reaction kinetics, on the alloyed surfaces are composition dependent, suggesting three possible effects: 1) charge transfer from V-(3d)3 [or Zr- (4d)2] states, to the hole in the top of the Pt-(5d)9 band alters the electronic structure at the Fermi energy; 2) alloying Pt with the ETM elements introduces a bi-functional character to the electrode surface, and 3) or the presence of short range chemical order induces a Fermi energy shift. To confirm the 1st and 3rd hypotheses, the electronic structure of Pt8Ti, Pt8V, and Pt8Zr, were calculated using the WIEN2k program package. The electronic structure calculations for ordered Pt8M give strong confirmation of the hypotheses, as they reveal that the Pt8M Fermi energy lies within the Pt-5d anti-bonding band, and also falls into a pseudogap in between the Md bonding and anti-bonding bands. In addition, the Pt8M DOS calculations confirm the presence of a deep pseudogap formed across the Fermi energy for both the Pt-sp and M-sp electrons. These experimental and theoretical results motivate additional studies of the novel Pt8M phases.

The effect of chemical ordering and lattice mismatch on structural transitions in phase segregating nanoalloys.

We elucidate the effect of lattice mismatch and chemical ordering on structural transitions in bimetallic nanoalloys of ∼1.5 nm. We show that collective screw dislocation motions happen in small mismatch shell@core systems while strongly mismatched ones favour incomplete outer shell rearrangements. Cooperative transitions can also become hindered when the chemical ordering breaks the geometrical symmetry. Escaping from an unfavourable morphological basin occurs first via re-arrangements of the geometry and then changes towards a better chemical pattern. We observe that the chemical re-ordering mechanisms are independent of system composition and stoichiometry but hinge on the initial and final chemical arrangements.

Phase competition and effect of chemical ordering in ferroelectric relaxor PbSc0.5Nb0.5O3 from first principles

ABSTRACTWe present results of first principles calculations for ferroelectric relaxor PbSc0.5Nb0.5O3. An ordered supercell with Sc and Nb atoms alternating in all directions (NaCl-like superstructure) has a ferroelectric ground state with R3 spacegroup. It is characterized by a tilt system a−a−a− as well as different sizes of adjacent oxygen octahedra. Analysis of phonon instabilities in cubic reveals similarity with an antiferroelectric PbZrO3. This is further confirmed by the fact that possibly antiferroelectric P21/b structure of PbSc0.5Nb0.5O3 (symmetry lowered from Pbam due to Sc/Nb superstructure) is energetically very close to the ground state. The ferroelectric order is shown to be less sensitive to deviations in the B-site distribution than the antiferroelectric one.

Effect of chemical ordering annealing on martensitic transformation and superelasticity in polycrystalline Ni–Mn–Ga microwires

Polycrystalline Ni–Mn–Ga microwires of diameter 30–80μm were prepared by melt-extraction technique on a large scale. The rapidly solidified microwires exhibit a fairly high ductility and excellent shape memory property. Here, with the aim to reduce the defect density, internal stress and compositional inhomogeneity in the as-extracted microwires, a stepwise chemical ordering annealing heat treatment was carried out and the effect of annealing on martensitic transformation, magnetic properties and superelastic behavior were investigated. The results indicate that annealing increase the transformation temperature and decrease the transformation hysteresis. These are related to composition homogenization, increase of atomic ordering and decrease in internal stress and defects. During mechanical tests, the stress-induced martensite (SIM) formation took place at a much lower stress after annealing treatment. The annealed microwires also demonstrate a lower superelastic hysteresis and a higher recovery rate compared to the as-extracted microwires. The temperature dependence of SIM stress is weaker after annealing, which is related to the enthalpy change (ΔH) and phase transformation temperature change according to the Clausius–Clapeyron relation.

Effect of chemical order on the magnetic and electronic properties of epitaxial off-stoichiometryFexSi1−xthin films

Off-stoichiometry, epitaxial $\mathrm{F}{\mathrm{e}}_{x}\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}$ thin films $(0.5lxl1.0)$ exhibit $D{0}_{3}$ or $B2$ chemical order, even far from stoichiometry. Theoretical calculations show the magnetic moment is strongly enhanced in the fully chemically disordered $A2$ phase, while both theoretical and experimental results show that the magnetization is nearly the same in the $B2$ and $D{0}_{3}$ phases, meaning partial chemical disorder does not influence the magnetism. The dependencies of the magnetic moments are directly and nonlinearly linked to the number of Si atoms, primarily nearest neighbor but also to a lesser extent (up to 10%) next nearest neighbor, surrounding Fe, explaining the similarities between $B2$ and $D{0}_{3}$ and the strong enhancement for the $A2$ structure. The calculated electronic density of states shows many similarities in both structure and spin polarization between the $D{0}_{3}$ and $B2$ structures, while the $A2$ structure exhibits disorder broadening and a reduced spin polarization.

Effects of chemical ordering and composition on the magnetic properties of disordered FeAl alloys

Recent studies have shown that FeAl alloys are promising candidate for magnetically patterned media owing to their disorder induced magnetism property at room temperature. In this paper, we have studied the effect of chemical ordering on the magnetic properties of FeAl alloys. At first we have calculated the magnetic exchange interactions in disordered FexAl1-x (50⩽x⩽65) alloys from first-principles theories. With these exchange interaction parameters, we carried out Monte-Carlo simulations with the resulting Ising Hamiltonian, onto which the alloy system was mapped and obtained the x-T phase diagram of the system. We have studied the effect of chemical ordering from a body-centered cubic (A2 phase) to an ordered cubic B2 phase in FeAl on the paramagnetic to ferromagnetic transition as a function of alloy composition and ordering fraction. We have also studied the behavior of hysteresis loop and coercive field as a function of alloy composition, temperature and ordering fraction.

Intrinsic magnetic properties of bimetallic nanoparticles elaborated by cluster beam deposition.

In this paper, we present some specific chemical and magnetic order obtained very recently on characteristic bimetallic nanoalloys prepared by mass-selected Low Energy Cluster Beam Deposition (LECBD). We study how the competition between d-atom hybridization, complex structure, morphology and chemical affinity affects their intrinsic magnetic properties at the nanoscale. The structural and magnetic properties of these nanoalloys were investigated using various experimental techniques that include High Resolution Transmission Electron Microscopy (HRTEM), Superconducting Quantum Interference Device (SQUID) magnetometry, as well as synchrotron techniques such as Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Magnetic Circular Dichroism (XMCD). Depending on the chemical nature of the nanoalloys we observe different magnetic responses compared to their bulk counterparts. In particular, we show how specific relaxation in nanoalloys impacts their magnetic anisotropy; and how finite size effects (size reduction) inversely enhance their magnetic moment.

Order Parameter in L10 FePt Nanocrystals

FePt nanocrystals with L10 chemical order have high magnetic anisotropy. FePt nanocrystals with fcc structure need to be heated to 600 °C to form the hard magnetic L10 phase. The order parameter (S) in nanocrystals changes during annealing. The order parameter in L10 FePt nanocrystals determines the relation between chemical ordering in this structure and the position and intensity of XRD peaks. To evaluate the effect of chemical ordering in the L10 phase in a quantitative way, some equations were used to calculate the S. In this work these suggested equations are compared and the results are reported for synthesized samples. The most suitable equation is suggested by physical discussion.

EXAFS Study of the Local Order in Ge-As-Se Glasses

The nearest-neighbour environment of the Ge, As and Se atoms in ternary GexAsySe1-x-y systems with same mean coordination number (MCN = 2.5) have been investigated by the Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). For all the glass compositions, Ge, As and Se are always 4, 3 and 2 coordinated, respectively. Ge atoms are bonded to Se atoms exclusively in the compositional range studied. Homopolar bonds are formed between some As or Se atoms in non-stoichiometric glasses. The results suggest a combined effect of chemical ordering and topological in this system.

Chemical ordering dependence of interlayer exchange coupling in Co-Mn-Si/Cr/Co-Mn-Si trilayer structures

The effect of chemical ordering of a half-metallic full-Heusler alloy ${\text{Co}}_{2}\text{MnSi}$ (CMS) on interlayer exchange coupling (IEC) behavior was investigated in Co-Mn-Si(20 nm)/Cr(1.2 nm)/Co-Mn-Si(7 nm) trilayers. Two types of experimental investigations were employed, namely, the variations in IEC with changing the annealing temperature of the bottom CMS from room temperature to $500\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and with changing the Co concentration in top and bottom Co-Mn-Si layers. The $90\ifmmode^\circ\else\textdegree\fi{}$ coupling parameter ${J}_{2}$ and the bilinear coupling parameter ${J}_{1}$ were estimated from the numerical simulations of magnetization curves. Only the strong $90\ifmmode^\circ\else\textdegree\fi{}$ coupling with almost the same strength was observed in both the completely $B2$ ordered and in a mixture of ordered $L{2}_{1}$ and $B2$ structures. The degree of $B2$ ordering as well as the strength of $90\ifmmode^\circ\else\textdegree\fi{}$ coupling was observed to decrease simultaneously with the Co concentration in Co-Mn-Si and $180\ifmmode^\circ\else\textdegree\fi{}$ coupling appeared in highly Co antisite defective $(A2)$ structures, i.e., for the Co concentration over $73\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. This observation showed a clear relationship between the degree of $B2$ ordering and the IEC behavior in full-Heusler Co-Mn-Si-based structures.

Effects of chemical order and atomic relaxation on the electronic and magnetic properties ofLa2/3Sr1/3MnO3

We investigate the effects of Sr/La cation ordering/disordering andatomic relaxation on the electronic and magnetic properties ofLa2/3Sr1/3MnO3 (LSMO) by means of ab initio pseudopotential calculations. We consider a cation-orderedlayered structure and a more homogeneously Sr-bulk-doped structure. Cation disorderingand atomic relaxation both tend to push the LSMO system towards half-metallicity,increasing the minority-spin and spin-flip gaps. Lattice relaxation has a significant effect onthe electronic density of states (DOS) of the layered LSMO and drastically reduces theinitial differences found comparing the electronic properties of the perovskitestructures with different dopant configurations. The trend with structural relaxation isunderstood in terms of an effective screening, due to the displacement of the O anionsand La cations, of the local electric field produced by the ordered Sr dopants.

Atomic ordering and magnetic properties in the Ni45Co5Mn36.7In13.3 metamagnetic shape memory alloy

The effects of chemical order on the phase stability and magnetic properties of the metamagnetic shape memory alloy Ni45Co5Mn36.7In13.3 were investigated. Alloys quenched from the B2 and L21 phase regions were found to transform to the L10 and 6M martensite phases, respectively. For alloys quenched from the B2 region the martensitic transformation starting temperature is about 80 K higher than that for alloys quenched from the L21 region. The Curie temperature of the parent phase and the magnetization of the martensite phase were both lower for the alloy quenched from the B2 region than those for the alloy quenched from L21 region.

Magnetic moment of Fe in oxide-free FePt nanoparticles

We present results of x-ray absorption spectroscopy and x-ray magnetic circular dichroism investigations at the ${L}_{3,2}$ edges of Fe in FePt nanoparticles. Plasma treated, oxide-free FePt nanoparticles with a high degree of $L{1}_{0}$ ordering and a mean diameter of about $6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were produced by gas phase condensation followed by in situ flight annealing. The presence of the $L{1}_{0}$ phase is observed by high resolution transmission electron microscopy. Compared to chemically disordered A1 FePt particles of similar size and composition, we find a fourfold enhanced orbital magnetic moment ${\ensuremath{\mu}}_{L}=0.19$ $(\ifmmode\pm\else\textpm\fi{}0.04){\ensuremath{\mu}}_{B}$, a nearly unchanged effective spin magnetic moment ${\ensuremath{\mu}}_{S}^{\mathrm{eff}}=2.21$ $(\ifmmode\pm\else\textpm\fi{}0.24){\ensuremath{\mu}}_{B}$ and a 2% increase in the white-line ratio of the ${L}_{3}$ and ${L}_{2}$ Fe edges. Nevertheless, the room temperature coercive field amounts to ${\ensuremath{\mu}}_{0}{H}_{C}=38(\ifmmode\pm\else\textpm\fi{}7)\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ only. These results are discussed in relation to possible effects of chemical order and surface anisotropy.

Size‐Dependent Chemical and Magnetic Ordering in L10‐FePt Nanoparticles

FePt nanoparticles have great application potential in advanced magnetic materials such as ultrahigh-density recording media and high-performance permanent magnets. The key for applications is the very high uniaxial magnetocrystalline anisotropy of the L10-FePt phase, which is based on crystalline ordering of the face-centered tetragonal (fct) structure, described by the chemical-ordering parameter S. Higher chemical ordering results in higher magnetocrystalline anisotropy. Unfortunately, as-synthesized FePt nanoparticles take a disordered face-centered cubic (fcc) structure that has low magnetocrystalline anisotropy. Heat-treatment is necessary to convert the fcc structure to the ordered fct structure. Several previous theoretical and experimental investigations have been reported on the size-dependent chemical ordering of FePt nanoparticles. It has been observed that the degree of ordering decreases with decreasing particle size of the sputtered FePt nanoparticles. Theoretical simulation predicted that the ordering would not take place when the particle size is below a critical value. However, there have not been systematic experimental studies on quantitative size dependence of chemical ordering of FePt nanoparticles due to the lack of monodisperse L10-FePt nanoparticles with controllable sizes. There are also few studies reported to date on the quantitative particle size dependence of magnetic properties, including the Curie temperature, coercivity, and magnetization of the L10-FePt phase, although it has been well accepted that there is a size effect on the ferromagnetism of any low-dimensional magnets. Additionally, the magnetic properties of FePt ferromagnets, as observed in thin-film samples, are affected by the degree of chemical ordering, which is in turn size dependent. It is therefore highly desirable to understand the size and chemical-ordering effects, and their influence on the magnetic properties of the nanoparticles. A major hurdle in obtaining the particle size dependence of structural and magnetic properties of the L10 phase is particle sintering during heat-treatments that convert the fcc phase to the fct phase. This long-pending problem has been solved recently by adopting the salt-matrix annealing technique. With this technique, particle aggregation during the phase transformation has been avoided so that the true size-dependent properties of the fct phase can be measured. In this paper, we report results on quantitative particle size dependence of the chemical-ordering parameter S and selected magnetic properties, including the Curie temperature, Tc, magnetization, Ms, and coercivity, Hc, with the particle size varying from 2 to 15 nm. Figure 1 shows the transmission electron microscopy (TEM) images of the FePt nanoparticles with different sizes before and after annealing in a salt matrix at 973 K for 4 h. The images, from left to right, show nanoparticles with nominal diameters of 2, 4, 6, 8, and 15 nm, respectively. The upper and lower rows are images of as-synthesized and salt-matrixannealed nanoparticles, respectively. As shown in Figure 1, the particle size is retained well upon annealing. Both the assynthesized and annealed nanoparticles are monodisperse with a standard deviation of 5–10 % in diameter. TEM observations also revealed that when the particle size is smaller than or equal to 8 nm, the fct nanoparticles are monocrystalline, whereas the 15 nm fct particles are polycrystalline. It is interesting to see that the L10 nanoparticles, tiny ferromagnets at room temperature, are dispersed very well without agglomeration despite the dipolar interaction between the particles, if a solvent with high viscosity is chosen and if the solution is diluted. Extensive TEM and X-ray diffraction (XRD) analyses have proved that the technique of salt-matrix annealing can be applied to heat-treatments of the FePt nanoparticles without leading to particle agglomeration and sintering, if a suitable salt-to-particle ratio and proper annealing conditions are chosen. Figure 2 shows the XRD patterns of the 4 nm, as-synthesized, fcc-structured nanoparticles and the particles annealed in a salt matrix at 873 K for 2 h, 973 K for 2 h, and 973 K for 4 h (from bottom to top), respectively. As shown in the figure, the positions of the (111) peaks shift in the higher-anC O M M U N IC A TI O N

The structural properties of Gex Sb40?x Se60 system

High purity glasses with an average coordination number N c in the Ge x Sb 40-x Se 60 ternary system between 2.4 and 2.7, were prepared under vacuum with x = 0,10, 20 and 30. X-ray diffraction (XRD) as, well as the Differential Scanning Calorimetry (DSC) studies were carried out to identify the amorphous and polycrystalline structure for the Ge-Sb-Se system. The dependence of properties such as the mean atomic volume V a , the glass transition temperature Tg and the number of topological constraints N con were studied as a function of N c , in the light of recent model proposed for covalent random network. The occurrence of this dependence in addition to those due to topological effects leads to the conclusion that the effects of chemical ordering and the effect, which have topological origin, coexist in this system. To understand the effect of N c on the glass-forming capability, the number of lone-pair electrons, (N cs - N c )/2, and the efficiency of the structural modification ΔΦwere calculated and discussed in relation to topological consideration. This then renders support to Phillips' theoretical model.

The structural properties of Gex Sb40?x Se60 system

High purity glasses with an average coordination number Nc in the GexSb40−xSe60 ternary system between 2.4 and 2.7, were prepared under vacuum with x = 0, 10, 20 and 30. X-ray diffraction (XRD) as, well as the Differential Scanning Calorimetry (DSC) studies were carried out to identify the amorphous and polycrystalline structure for the Ge-Sb-Se system. The dependence of properties such as the mean atomic volume Va, the glass transition temperature Tg and the number of topological constraints Ncon were studied as a function of Nc, in the light of recent model proposed for covalent random network. The occurrence of this dependence in addition to those due to topological effects leads to the conclusion that the effects of chemical ordering and the effect, which have topological origin, coexist in this system. To understand the effect of Nc on the glass-forming capability, the number of lone-pair electrons, (Ncs−Nc)/2, and the efficiency of the structural modification Δ Φ were calculated and discussed in relation to topological consideration. This then renders support to Phillips’theoretical model.

Long-range chemical order effects upon the magnetic anisotropy of FePt alloys from an abinitio electronic structure theory

The magnetocrystalline anisotropy (MCA) of bulk and thick films of FePt is calculated froma ‘first-principles’ theory. The starting point is a description from electronic densityfunctional theory for systems of interacting electrons moving in lattices of ions. Relativisticeffects such as spin–orbit coupling are included. FePt readily transforms into a CuAu-type(L 10) ordered phase and this coincides with the material’s high anisotropy.Here we describe how to calculate the MCA of a partially ordered alloyand to extract its dependence on the long range chemical order parameterη. We present calculations of the MCA of FePt as a function ofη and find excellent agreement with the experimental data of Okamoto et al (2002 Phys. Rev. B 66 024413) and others with respect to the magneticeasy axis, the magnitude of the MCA and its trend with decreasingη. We also study the paramagnetic phase of the ordered alloy using the ‘disordered localmoment’ picture of metallic magnetism at finite temperatures. We calculate a Curietemperature of 935 K in reasonable agreement with experiment (710 K) and find the easyaxis for the onset of ferromagnetic order to coincide with the magnetic easy axis found atlow temperatures.

Ferroelectric Phase Transitions in Nano-Scale Chemically Ordered PbSc 0.5 Nb 0.5 O 3 Using a First-Principles Model Hamiltonian

Effects of chemical order, disorder, short range order, and anti-phase boundaries on phase transitions and dielectric properties of PbSc 1/2 Nb 1/2 O 3 are studied through molecular dynamics simulations of a FP model. Simulations of large systems are required to capture these effects, and we present an efficient reciprocal space method, based on fast Fourier transforms, for calculating long-range interactions and inhomogeneous strain. Calculations for random or partially disordered systems yield significant increases in T = 0 K dielectric response, and broadening of the ferroelectric phase transition. Coupling between random fields caused by chemical disorder and the inhomogeneous strain (acoustic modes) affects the dynamics of soft modes in chemically nano-structured configurations.

Chemical ordering in lanthanum-doped lead magnesium niobate relaxor ferroelectrics probed by A1g Raman mode

Chemical ordering of the B-site cations in undoped, 5, 10, 20, and 30 mol % La3+-doped Pb(Mg1/3Nb2/3)O3 (PMN) ceramics with perovskite structure, has been examined by Raman scattering. Two B-site chemical ordering models, the space-charge model, and the random layer model assume a space group of Fm3̄m for the ordered nanoregions. It is found that A1g Raman mode around 780 cm−1 from the B-site ordered nanoregions with a space group Fm3̄m, is very sensitive to the delicate changes of the chemical order at B site. Frequency changes of this A1g characteristic mode with La3+ doping, are analyzed according to the lattice parameters and chemical order effects. The present results provide stronge evidence supporting the random layer model in La-doped PMN.

Light ion irradiation of Co/Pt systems: Structural origin of the decrease in magnetic anisotropy

We study the structural properties of Pt/Co/Pt systems submitted to ${\mathrm{He}}^{+}$ ion irradiation, in order to understand why the magnetic anisotropy can be decreased in a controlled way. It is shown by grazing x-ray reflectometry that the irradiation-induced Pt and Co atom displacements can be largely accounted for by a simple ballistic recoil mechanism model. Our results indicate that even in these nm-thick films, irradiation may affect the upper and lower interfaces differently. Specifically, the upper Co interface undergoes short-range mixing, resulting in roughness, whereas the lower Co interface mostly evolves by longer-range mixing, leading to alloy formation. Irradiation also releases strain in these Co-Pt systems, but has no chemical ordering effect. Together with slow asymmetric interface roughening, the cobalt tensile strain relaxation at low fluences accounts for the magnetic anisotropy decrease. The type of analysis we propose could be useful to understand why other magnetic properties, such as interlayer exchange coupling, can be controlled by light ion irradiation.