Local Structural Properties Research Articles

First-principles studies of the local structure and relaxor behavior of Pb(Mg1/3Nb2/3)O3−PbTiO3 -derived ferroelectric perovskite solid solutions

We have investigated the effect of transition-metal dopants on the local structure of the prototypical $0.75\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}0.25{\mathrm{PbTiO}}_{3}$ relaxor ferroelectric. We find that these dopants give rise to very different local structure and other physical properties. For example, when Mg is partially substituted by Cu or Zn, the displacement of Cu or Zn is much larger than that of Mg and is even comparable to that of Nb. The polarization of these systems is also increased, especially for the Cu-doped solution, due to the large polarizability of Cu and Zn. As a result, the predicted maximum dielectric constant temperatures ${T}_{\mathrm{m}}$ are increased. On the other hand, the replacement of a Ti atom with a Mo or Tc atom dramatically decreases the displacements of the cations and the polarization, and thus, the ${T}_{\mathrm{m}}$ values are also substantially decreased. The higher ${T}_{\mathrm{m}}$ cannot be explained by the conventional argument based on the ionic radii of the cations. Furthermore, we find that Cu, Mo, or Tc doping increases the cation displacement disorder. The effect of the dopants on the temperature dispersion $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{m}}$, which is the change in ${T}_{\mathrm{m}}$ for different frequencies, is also discussed. Our findings lay the foundation for further investigations of unexplored dopants.

Implications of linear correlation between hyperfine parameters in iron oxide nanoparticles

Controlled annealing induced evolution of local structure and magnetic properties on oleic acid coated and bare nanoparticles composed of iron oxy hydroxide and cubic iron oxides were studied using Mössbauer spectroscopy. Mutually opposite variation of quadrupole splitting and magnetic hyperfine field with respect to that of isomer shift has been observed in both the cases of bare and oleic acid coated nanoparticles subjected to annealing treatments. This manifests in an interesting linear correlation between quadrupole splitting and magnetic hyperfine field. Physics of the observed linear correlation between hyperfine parameters has been elucidated based on the direct proportionality of superexchange interaction on the hyperfine contact interaction and a strong dependence of the strength of superexchange interaction on the structural distortion and point defects mainly due to anionic vacancies. Thus the result of this study plausibly points to a strong coupling between local structure in terms of structural distortions and point defects with magnetic properties while the nanoparticles of iron oxides are subjected to coarsening and reduction.

Efficient weighting strategy for enhancing synchronizability of complex networks

Networks with high propensity to synchronization are desired in many applications ranging from biology to engineering. In general, there are two ways to enhance the synchronizability of a network: link rewiring and/or link weighting. In this paper, we propose a new link weighting strategy based on the concept of the neighborhood subgroup. The neighborhood subgroup of a node i through node j in a network, i.e. [Formula: see text], means that node u belongs to [Formula: see text] if node u belongs to the first-order neighbors of j (not include i). Our proposed weighting schema used the local and global structural properties of the networks such as the node degree, betweenness centrality and closeness centrality measures. We applied the method on scale-free and Watts–Strogatz networks of different structural properties and show the good performance of the proposed weighting scheme. Furthermore, as model networks cannot capture all essential features of real-world complex networks, we considered a number of undirected and unweighted real-world networks. To the best of our knowledge, the proposed weighting strategy outperformed the previously published weighting methods by enhancing the synchronizability of these real-world networks.

Light Element Doping Induced Phase Change of Strongly Correlated Semiconductor SmNiO3

Introduction Correlated oxide semiconductors offer a unique relation between local structure and electrical properties because small amount of impurity atoms allow for significant changes in the band structure1,2. Electron doping of strongly correlated nickelates such as SmNiO3 (SNO) by small ion incorporation has been observed to change resistivity up to ten orders of magnitude – manifesting a quantum mechanical driven phase transition via ~3eV band gap opening3. During this process, cations with small atomic radii intercalate, reducing the Ni valence in of the SNO. Nevertheless, it is difficult to directly characterize the small amounts of such light elements within the structure. This study focuses on the band gap opening of the correlated oxide SmNiO3 upon charge doping as well as structural changes introduced by the dopants during this phase transition. The correlation among doping level – structural changes – electrical properties of SNO will be established. Experimental SNO thin films are grown by physical vapor deposition (PVD) from Sm and Ni targets in an Ar/O2 atmosphere and subsequent high pressure O2 annealing. LaAlO3 and Si/SiO2 substrates were used to create epitaxial and polycrystalline samples respectively. Films were then subjected to electrochemical treatment in a liquid environment to dope light elements into the lattice. Characterization of the changes in film resistance was used in conjunction to determine dopant distribution overall effect on SNO thin film properties. Synchrotron x-ray and neutron scattering will be used to investigate the structural evolution with doping of SNO. Results and Discussion Figure 1 shows resistivity changes as a function of temperature as the sample is doped with lightweight elements. It can be readily observed that this non-volatile doping process significantly changes not only the resistivity of the film, but also the characteristic metal-insulator transition observed in a number of pristine nickelates. This doping process is non-volatile due to resistivity remaining stable at ambient conditions. Notably, there is no change to the film resistance without further bias application. To better understand the structural implications of the doping process, X-ray diffraction was used to explore the effects on the SNO lattice with the incorporation of these impurity ions. Epitaxial SNO on LAO substrates before and after light element doping show a clear shift of the diffraction peaks of SNO to lower angle. This observation is consistent in that the lattice expands to accommodate the dopant ions. Additional investigations on the electrochemical doping process are also explored through cyclic voltammetry and will be discussed during the presentation. Summary SNO thin films show a large change in resistivity when doped with light elements via charge transfer to the Ni orbitals. To better understand the phase transition as it relates to this doping process, we highlight the association between the changing structure and properties as the light elements are intercalated into the lattice. A better understanding of the carrier density modulation and distribution are correlated with structural changes imparted with the electrochemical treatment. Ultimately, new insights into this highly sensitive wide gap semiconductor may be useful for future sensing applications. References Rondinelli, J.M., May, S.J. and Freeland, J.W., 2012. Control of octahedral connectivity in perovskite oxide heterostructures: An emerging route to multifunctional materials discovery.MRS bulletin, 37(03), pp.261-270.Middey, S., Chakhalian, J., Mahadevan, P., Freeland, J.W., Millis, A.J. and Sarma, D.D., 2016. Physics of Ultrathin Films and Heterostructures of Rare-Earth Nickelates.Annual Review of Materials Research, 46, pp.305-334.Shi, J., Zhou, Y. and Ramanathan, S., 2014. Colossal resistance switching and band gap modulation in a perovskite nickelate by electron doping. Nature communications, 5, 4860. Figure 1. Doping of SmNiO3 changing the resistance as a function of temperature and derivative. Figure 1

Understanding the Role of Li Inventory on Capacity Fade in Silicon Containing Anodes

Silicon containing anodes are an attractive alternative to graphite since the inclusion of intrinsically high capacity Si dramatically increases the energy density in a full Li-ion battery. However, problems persist with Si related to SEI formation/exfoliation, pore filling, and particle agglomeration due to >300% volume changes. This leads to severely limited Li diffusion kinetics and hence capacity fade. In this work we assess the impact of Li inventory on the capacity fade of Si containing electrodes by pairing them with a LiFePO4 reference/counter electrode. Using a capacity limited cycling protocol we assess the role of Li inventory in prolonging the lifetime of Si. We also explore the synergistic effects that arise from blending graphite with Si in composite electrodes. Overall, we relate the observed electrochemical behavior to the evolution of the local structure, electrode morphology and mechanical properties.

The Coupling between Mechanics and Intercalation Chemistry in Layered Battery Materials

In addition to a change in lattice parameters, many materials also experience complex structural transformations upon intercalation. For example, many layered lithium and sodium oxides transform between the O3/O1/P3 stacking sequences, and Ni and Mn oxides often undergo transformations associated with collective Jahn-Teller distortions. Many questions remain about how these structural transformations affect battery performance, and in particular, how they affect cycle life limitations arising from fracture and mechanical damage. Here we present new models to capture the coupling between intercalation chemistry and mechanics arising from stacking-sequence changes and Jahn-Teller distortions. This includes first-principles simulations and mesoscale models to elucidate the thermodynamics and kinetics of stacking sequence changes, as well as analyses of how these transformations might result in irreversible changes to microstructure. Regarding the mechanics of Jahn-Teller distortions, we have developed anharmonic vibrational models to explore the transition from a collective distortion to a dynamic one in layered materials such as lithium nickel and manganese oxides (as well as their sodium analogues). Numerical simulations reveal how the interplay between the shape of the Jahn-Teller energy landscape and rigidity of the crystal impact the local structure and thermodynamic properties of the dynamically distorted phase.

Electronic structure and dielectric properties of ZrO2-CeO2 mixed oxides

ZrO2–CeO2 mixed ceramics were studied for their local electronic structure and dielectric properties. X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray absorption fine structure (XAFS) and frequency dependence dielectric constant measurements were performed on the samples. Rietveld refinements and quantitative phase analysis of ZrO2-CeO2 ceramics showed an increased concentration of tetragonal ZrO2 and cubic Zr0.5Ce0.5O2 phases at the expense of parent monoclinic ZrO2. The morphology of the ZrO2-CeO2 ceramics remained invariant for all of the fractional percentages of CeO2 loading. O K-edge and Ce M5,4-edge NEXAFS showed the oxygen vacancies and Ce3+-Ce4+ redox pair formation in ZrO2CeO2 ceramics, which is further corroborated by Ce L3-edge XAFS. The higher CeO2 molar ratios containing ZrO2–CeO2 ceramics showed higher ε′, ε″ and σac and lower tan δ.

Complex network view of evolving manifolds.

We study complex networks formed by triangulations and higher-dimensional simplicial complexes representing closed evolving manifolds. In particular, for triangulations, the set of possible transformations of these networks is restricted by the condition that at each step, all the faces must be triangles. Stochastic application of these operations leads to random networks with different architectures. We perform extensive numerical simulations and explore the geometries of growing and equilibrium complex networks generated by these transformations and their local structural properties. This characterization includes the Hausdorff and spectral dimensions of the resulting networks, their degree distributions, and various structural correlations. Our results reveal a rich zoo of architectures and geometries of these networks, some of which appear to be small worlds while others are finite dimensional with Hausdorff dimension equal or higher than the original dimensionality of their simplices. The range of spectral dimensions of the evolving triangulations turns out to be from about 1.4 to infinity. Our models include simplicial complexes representing manifolds with evolving topologies, for example, an h-holed torus with a progressively growing number of holes. This evolving graph demonstrates features of a small-world network and has a particularly heavy-tailed degree distribution.

Interplay of local structure, charge, and spin in bilayered manganese perovskites

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. Here, we show results of high-energy resolution x-ray absorption and emission spectroscopies on a ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ family of bilayered manganites in a broad doping range $(0.5\ensuremath{\le}x\ensuremath{\le}1)$. We established a relation between local Mn charge and Mn-O distances as a function of doping. Based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.

Moiré structure of MoS2 on Au(111): Local structural and electronic properties

Monolayer islands of molybdenum disulfide (MoS2) on Au(111) form a characteristic moiré structure, leading to locally different stacking sequences at the S-Mo-S-Au interface. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we find that the moiré islands exhibit a unique orientation with respect to the Au crystal structure. This indicates a clear preference of MoS2 growth in a regular stacking fashion. We further probe the influence of the local atomic structure on the electronic properties. Differential conductance spectra show pronounced features of the valence band and conduction band, some of which undergo significant shifts depending on the local atomic structure. We also determine the tunneling decay constant as a function of the bias voltage by a height-modulated spectroscopy method. This allows for an increased sensitivity of states with non-negligible parallel momentum k∥ and the identification of the origin of the states from different areas in the Brillouin zone.

Effects of rapid thermal annealing on the local environment, electronic structure and magnetic properties of Mn doped TiO2 thin films

In this report, nanostructure thin films of Ti0.97 Mn0.03 O2−δ were deposited on Si (001) substrate by pulsed laser deposition (PLD) technique, followed by rapid thermal annealing (RTA) in different ambient gases; O2, N2 and Ar. The RTA treatment dramatically affected the surface defects, local environment, electronic structure and magnetic properties of the films, where, annealing of the film in O2 gas recovered the dislocated atoms of oxygen vacancy (Vo) at the surfaces and induced a diamagnetic phase, whereas the annealing of film in N2 gas introduced Vo, reduced the valence state of Ti4+ (TiO2) into Ti3+ (Ti2O3) and induced ferromagnetic (FM) signal. Therefore, the change in the local atomic defects of Vo at the surface texture is likely to be responsible for the magnetic response. The presence of oxygen vacancy has been traced by means of Raman scattering and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. The NEXAFS spectra were carried out in synchrotron facility at Ti/Mn L3,2 and O-K edges. The spectral feature at Ti L3,2 edges revealed the rutile phase of TiO2. The asymmetry of t2g and eg bands at the O-K edge confirmed the formation of Vo and reflected the modification in the O co-ordination around Ti4+cations. The Mn L3,2 edges revealed the incorporation of Mn ions in the TiO2 network with Mn2+ and Mn3+ mixed valence states. This different valance states (Mn2+ and Mn3+) is the basis for the ferromagnetism induced by Stoner spin-splitting of the local density of defects state at Fermi level (EF). The change in the magnetic moment of the films after RTA process is related to the change in the local density of defects band. Crystal field and charge transfer parameters were extracted roughly by using atomic multiplet calculations (theory).

An overview on the research of Sr2IrO4-based system probed by X-ray absorption spectroscopy

Investigations of materials with 5d transition metal ions have opened up new paradigms in condensed-matter physics because of their large spin-orbit coupling (SOC) interactions. The typical compound is Sr2IrO4, which attracted much attention due to its similarities to the parent compound of high-[Formula: see text] cuprate superconductor La2CuO4. Theoretical calculations predicted that the unconventional superconductivity can occur in carrier doped-Sr2IrO4 system. Until now, hundreds of experimental methods were devoted to investigate the carrier doping effect on Sr2IrO4. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) made great contributions to the local lattice and electronic structure, and also the intimate relationship between the local structure and physical properties induced by carrier doping. The aim of this review is a short introduction to the progress of research on Sr2IrO4-based system probed by the unique technique — XAS, including the strength of the SOC, valence changes upon doping and even local lattice structure with atomic level for this Sr2IrO4-based family.

An X-ray absorption spectroscopy study of La-Fe-Si-(H) magnetocaloric alloys

In order to unveil the lattice structure of La-Fe-Si-(H) alloys from local atomic perspective and uncover its relation with macro-properties, local structural environments of La and Fe in stoichiometric and Fe-rich La-Fe-Si-(H) have been investigated by X-ray absorption fine structure (XAFS) technique. According to the curve-fitting and real space analysis of extended X-ray absorption fine structure (EXAFS) spectra at La L3-edge and Fe K-edge, we found that the interstitial hydrogen atoms preferentially occupy 24d site, which is not affected by the existence of second phase. X-ray absorption near edge structure (XANES) spectra of La atom give a clear evidence of valence electron transfer from hydrogen to Fe, indicating a close relation between electronic structure and capability of hydrogen absorption. The hybridization effect between Fe and Si is proved to play dominant role on Curie temperature (Tc) of La-Fe-Si alloys, but have limited influence on Tc for La-Fe-Si-(H) alloys. The study establishes strong correlation between the local structure and above properties in La-Fe-Si-(H) alloys.

Local Structure and Magnetic Properties of Fe50Cr40Si10 Nanocrystalline Alloys

The structural and the magnetic properties of nanocrystalline Fe50Cr40Si10 alloys were prepared via mechanical alloying by using Fe, Cr, and Si elements with 1- to 24 hrs milling times. Structural analysis based on X-ray diffraction and Extended X-ray absorption fine structure spectroscopy. Concerning the magnetic behavior, the data obtained from a vibrating sample magnetometer at 300 K shows that both the magnetic saturation and the coercivity are dependent strongly on the milling time and the crystallite size. By adjusting the milling time, both appropriate structural transformation and magnetization values are obtained.

Effect of Nitrogenation and Hydrogenation on the Magnetic Properties and Structure of the Sm2Fe17 Alloy: Analysis of XMCD Data

Changes in the local magnetic and structural properties of Sm2Fe17 alloys at nitrogenation and hydrogenation of samples have been studied by the X-ray magnetic circular dichroism (XMCD) technique at the Fe K absorption edge and Sm L3 absorption edge using synchrotron radiation. The results have been discussed in comparison with X-ray diffraction data and macroscopic vibration magnetometry measurements. The observed changes in XMCD spectra indicate a noticeable effect of nitrogenation on the local magnetic properties of sublattices of both iron and samarium, whereas hydrogenation of samples makes a small effect. The mentioned effects have been analyzed and discussed in terms of the effect of nitrogen (N) and hydrogen (H) interstitial atoms on Sm 5d and Fe 4p electronic states. The effect of nitrogenation is larger than the effect of hydrogenation because the volume expansion of the crystal lattice of initial Sm2Fe17 in the case of nitrogenation is larger than that in the case of hydrogenation. The studied local magnetization curves for samarium and iron sublattices in magnetic fields up to 17 T also indicate a strong increase in the magnetocrystalline anisotropy at nitrogenation.

SAR Target Recognition via Local Sparse Representation of Multi-Manifold Regularized Low-Rank Approximation

The extraction of a valuable set of features and the design of a discriminative classifier are crucial for target recognition in SAR image. Although various features and classifiers have been proposed over the years, target recognition under extended operating conditions (EOCs) is still a challenging problem, e.g., target with configuration variation, different capture orientations, and articulation. To address these problems, this paper presents a new strategy for target recognition. We first propose a low-dimensional representation model via incorporating multi-manifold regularization term into the low-rank matrix factorization framework. Two rules, pairwise similarity and local linearity, are employed for constructing multiple manifold regularization. By alternately optimizing the matrix factorization and manifold selection, the feature representation model can not only acquire the optimal low-rank approximation of original samples, but also capture the intrinsic manifold structure information. Then, to take full advantage of the local structure property of features and further improve the discriminative ability, local sparse representation is proposed for classification. Finally, extensive experiments on moving and stationary target acquisition and recognition (MSTAR) database demonstrate the effectiveness of the proposed strategy, including target recognition under EOCs, as well as the capability of small training size.

Synchrotron radiation microbeam X-ray diffraction for nondestructive assessments of local structural properties of faceted InGaN/GaN quantum wells

Synchrotron radiation (SR) X-ray diffraction with a sub-µm spatial resolution is used to nondestructively evaluate the local thickness and alloy composition of three-dimensionally faceted InGaN/GaN quantum wells (QWs). The (0001) facet QW on a trapezoidal structure composed of (0001), , and facets is nonuniform, most likely owing to the migration of adatoms between facets. The thickness and composition markedly vary within a short distance for the facet QW of another pyramidal structure. The QW parameters acquired by SR microbeam X-ray diffraction reproduce the local emission property assessed by cathodoluminescence, thereby indicating the high reliability of this method.

Manipulation of domain-wall solitons in bi- and trilayer graphene.

Topological dislocations and stacking faults greatly affect the performance of functional crystalline materials1-3. Layer-stacking domain walls (DWs) in graphene alter its electronic properties and give rise to fascinating new physics such as quantum valley Hall edge states4-10. Extensive efforts have been dedicated to the engineering of dislocations to obtain materials with advanced properties. However, the manipulation of individual dislocations to precisely control the local structure and local properties of bulk material remains an outstanding challenge. Here we report the manipulation of individual layer-stacking DWs in bi- and trilayer graphene by means of a local mechanical force exerted by an atomic force microscope tip. We demonstrate experimentally the capability to move, erase and split individual DWs as well as annihilate or create closed-loop DWs. We further show that the DW motion is highly anisotropic, offering a simple approach to create solitons with designed atomic structures. Most artificially created DW structures are found to be stable at room temperature.

The influence of NaCl concentration on the (LiCl-KCl) eutectic system and temperature dependence of the ternary system

To explore the complex correlation between local structure and properties in the alkali chlorides ternary system, simulations were systematically conducted for 11 different (Li, Na, K) Cl compositions from 900 K to 1200 K as the fundamental study of magnesium chloride electrolysis. For the ternary system, the density was calculated across the full composition and database was established. Furthermore, the structure simulation results revealed that the coordination of Li-Cl pair tended to be a distorted tetrahedron and the Na-Cl and K-Cl pairs tended to be an octahedron. The prediction of the properties was compared with available experiments and extrapolated analytical data, the deviations are acceptable. The temperature had no significant effects on the local structure and thermal conductivity. The results also showed that the Tosi−Fumi potential predicted positive temperature dependence of self-diffusion coefficients and ionic conductivity, and negative temperature dependence of the viscosity. In addition, the correlation between the local structure and transport properties was explored. The hydrodynamic radius of Li+ and K+ is similar in the ternary system, it can be found that whilst the K+ ion diffuses as an atomic species, the Li+ ion carries at least some of its coordinating Cl− ions in the diffusion process, giving the diffusing species a larger effective size than the bare ion.

Modelling of novel polymer materials through atomistic molecular dynamics simulations

Molecular simulation methodologies are nowadays very important tools for the prediction of structure-properties relations of molecular systems, and, more general, for the computational design of novel materials. Here, we demonstrate that the atomistic molecular dynamic simulations can be successfully used to design well-defined polymer systems as well as to examine their structural and dynamical properties. More specifically, we study the behaviour of graphene-based nanocomposites and nanostructured polymer materials. We provide a detailed analysis adapted to every system with a special focus on detecting the local heterogeneities in the material. Our carefully developed method allows us to shed light on the relation between the local structure and final properties of the desired material. The obtained information can be applied in the development of novel complex multi-phase polymeric materials with a broad range of possible applications, as for instance in energy storage and electronics.