Regular Sites Research Articles

Effect of Zn doping on the photoluminescence properties of LiNbO3 single crystals

An extraordinary violet luminescence at 418 nm together with the intrinsic blue band at 440 nm with excitation in the UV region (380 nm) was observed in a series of LiNbO3 single crystals doped with Zn. Structural and photoluminescence properties were studied by Raman, UV reflectance, and fluorospectrometry, respectively. The emission peaks in PL spectra varied in amplitude according to the level of concentration of Zn in LiNbO3 crystals. It was found, from Raman spectroscopy studies, a connection between the appearance of the new emission band centered at 418 nm and a diminishing in the Li concentration in crystals, which suggest the displacement of the Li ions from their regular sites by the Zn ions. The maximum peak intensity of each one of the three PL emission components found from a deconvolution process shows a linear dependence on the Zn concentration present in the crystals studied.

Chemical ordering beyond the superstructure in long-range ordered systems

To describe chemical ordering in solid solutions systems Warren–Cowley short-range parameters are ordinarily used. However, they are not directly suited for application to long-range ordered systems, as they do not converge to zero for large separations. In this paper, the theory is generalized to long-range ordered systems and chemical short-range order beyond the superstructure arrangements is discussed quantitatively on the example of a non-stoichiometric B2-ordered intermetallic alloy. Parameters of interatomic potentials are taken from an embedded atom method calculations and the degree of order is simulated by the Monte Carlo method. Both on-lattice and off-lattice methods, where the latter allows individual atoms to deviate from their regular lattice sites, were used, and the resulting effects are discussed.

High Temperature Oxide Ion Conduction Path for Pb1-XLaxWO4+x/2

Introduction Partially substituting lanthanum ions for the Scheelite-type structured PbWO4, oxide ion interstitials are formed as Pb1-xLaxWO4+x/2 and high oxide ion conduction appears at high temperatures [1]. While most of the oxide ion conductors including zirconias, cerias, or LSGM employ the oxide ion vacancy for the high-oxide ion conduction, the present system utilize the slight amount of oxide ion interstitials. Since there is no sufficient space for the oxide ion to migrate through only the interstitial sites, oxide ion diffusion should be made using regular and interstitial sites. However, unlike the vacancy diffusion mechanism, actual diffusion path is hardly estimated without direct experiment such as neutron diffraction. We have previously measured the neutron diffraction on Pb1-xLaxWO4+x/2(x = 0.1 and 0.2) at room temperature to evaluate the diffusion path from the interatomic distances and direction of anisotropic temperature factors of regular oxide ions [2]. Nevertheless, the estimated model only support the 2 D diffusion path, and oxide ion distribution at high temperature would be different from that at room temperature.We therefore carried out the neutron diffraction experiments on Pb0.8La0.2WO4.1in the temperature range from 200°C to 800°C and estimated the high temperature conduction path of oxide ions by means of maximum entropy method (MEM). Experimental Pb0.8La0.2WO4.1 sample has been prepared by solid state reaction method. Stoichiometric mixtures of PbO, H2WO4 and La2O3 were at first calcined at 800°C in air for 10 h, finely ground, pressed into pellets under a hydrostatic pressure of 100 MPa, and finally sintered at 900°C in air for 10 h. The base material of PbWO4 was also prepared for comparison. Crystalline phase of sintered sample was identified by X-ray diffraction measurement. About 10 g of the sample pellets were placed in vanadium holder through a thin quartz tube and the neutron powder diffraction experiments were performed in the temperature range between 200°C and 800°C by using SHRPD diffractometer in J-PARC and the Rietveld refinements were carried out by using Z-Rietveld code, and the oxide ion conduction path at high temperatures was estimated with maximum entropy method using Z-MEM. Results and Discussions Fig.1 represent the refined lattice parameters, a and c, of Pb1-xLaxWO4+x/2 (x = 0 and 0.2), both of which grow larger with increasing temperature. On the other hand, the interatomic distances between W and O, d W-O, gradually decreases with temperature in x = 0.2, while it grows larger for x = 0 as shown in Fig. 2. This indicates that the regular oxide ion is distorted by the diffusing oxide ions through the interstitial sites at high temperature. As for the oxide ion interstitials, two possible sites are deduced for the present high-temperature diffraction data in x = 0.2. The fractional coordinates of Oi(1) and Oi(2) are (0 0 0.208) and (0.667 0.333 0.238), respectively, which are represented in Fig. 3. The former corresponds to the previously reported one obtained from the room temperature neutron diffraction data. On the other hand, the latter was found by the present high-temperature diffraction experiments. Therefore, a new diffusion route through Oi(2) site is expected at high temperature in comparison with the previous route estimated at room-temperature. Fig.4 shows the nuclear density diagram of Pb0.8La0.2WO4.1 at 800°C. It is indicated that oxide ions migrate through Oi(2) as well as Oi(1) in Pb0.8La0.2WO4.1. It is indicated that actual diffusion path through Oi(1) (black arrow) is rather complex in comparison with that estimated from previous room temperature data (red arrow). In addition, additional new diffusion path is found through both Oi(1) and Oi(2) in the present high temperature experiment. Using these two typical conduction paths, oxide ion can migrate to achieve 3 D diffusion network. On the other hand, PbWO4doesn’t have such diffusion path even at high temperature.

Temperature dependence of small polaron population decays in iron-doped lithium niobate by Monte Carlo simulations

The population decay of light-induced small polarons in iron-doped lithium niobate is simulated by a Monte-Carlo method on the basis of Holstein's theory. The model considers random walks of both bound polarons (NbLi4+) and free polarons (NbNb4+) ending to deep traps (FeLi3+). The thermokinetic interplay between polaron species is introduced by trapping and de-trapping rates at niobium antisites (NbLi). The decay of the NbLi4+ population proceeds by three possible channels: direct trapping at FeLi3+ sites, hopping on niobium antisites and hopping on Nb regular sites after conversion to the free state. Up to three regimes, each one reflecting the predominance of one of these processes, appear with different activation energies in the Arrhenius plots of the decay time. The influence of FeLi and NbLi concentrations on the transition temperatures is evidenced. For both polaron species, the length of the final hop (trapping length) is found much larger than the usual hopping length and decreases at rising temperature. This trap size effect is a natural consequence of Holstein's theory and may explain some unclear features of polaron-related light-induced phenomena, such as the temperature-dependent stretching exponent of light-induced absorption decays and the anomalous increase of the photoconductivity at high doping levels.

Structural and optical properties of GaAs(100) with a thin surface layer doped with chromium

The aim of this study is to explore the structural and optical properties of single-crystal GaAs(100) doped with Cr atoms by burning them into the substrate at high temperatures. The diffusion of chromium into single-crystal GaAs(100) substrates brings about the formation of a thin (~20–40 μm) GaAs:Cr transition layer. In this case, chromium atoms are incorporated into the gallium-arsenide crystal lattice and occupy the regular atomic sites of the metal sublattice. As the chromium diffusion time is increased, such behavior of the dopant impurity yields changes in the energy structure of GaAs, a decrease in the absorption at free charge carriers, and a lowering of the surface recombination rate. As a result, the photoluminescence signal from the sample is significantly enhanced.

Diffusion Barriers Block Defect Occupation on Reduced CeO_{2}(111).

Surface defects are believed to govern the adsorption behavior of reducible oxides. We challenge this perception on the basis of a combined scanning-tunneling-microscopy and density-functional-theory study, addressing the Au adsorption on reduced CeO_{2-x}(111). Despite a clear thermodynamic preference for oxygen vacancies, individual Au atoms were found to bind mostly to regular surface sites. Even at an elevated temperature, aggregation at step edges and not decoration of defects turned out to be the main consequence of adatom diffusion. Our findings are explained with the polaronic nature of the Au-ceria system, which imprints a strong diabatic character onto the diffusive motion of adatoms. Diabatic barriers are generally higher than those in the adiabatic regime, especially if the hopping step couples to an electron transfer into the ad-gold. As the population of O vacancies always requires a charge exchange, defect decoration by Au atoms becomes kinetically hindered. Our study demonstrates that polaronic effects determine not only electron transport in reducible oxides but also the adsorption characteristics and therewith the surface chemistry.

The winter distribution of the Spoon-billed Sandpiper Calidris pygmaeus

SummaryDeclines in populations of the Critically Endangered Spoon-billed Sandpiper Calidris pygmaeus have been rapid, with the breeding population now perhaps numbering fewer than 120 pairs. The reasons for this decline remain unresolved. Whilst there is evidence that hunting in wintering areas is an important factor, loss of suitable habitat on passage and wintering areas is also of concern. While some key sites for the species are already documented, many of their wintering locations are described here for the first time. Their wintering range primarily stretches from Bangladesh to China. Comprehensive surveys of potential Spoon-billed Sandpiper wintering sites from 2005 to 2013 showed a wide distribution with three key concentrations in Myanmar and Bangladesh, but also regular sites in China, Vietnam and Thailand. The identification of all important non-breeding sites remains of high priority for the conservation of the species. Here, we present the results of field surveys of wintering Spoon-billed Sandpipers that took place in six countries between 2005 and 2013 and present species distribution models which map the potential wintering areas. These include known and currently unrecognised wintering locations. Our maximum entropy model did not identify any new extensive candidate areas within the winter distribution, suggesting that most key sites are already known, but it did identify small sites on the coast of eastern Bangladesh, western Myanmar, and the Guangxi and Guangdong regions of China that may merit further investigation. As no extensive areas of new potential habitat were identified, we suggest that the priorities for the conservation of this species are habitat protection in important wintering and passage areas and reducing hunting pressure on birds at these sites.

Synthesis of nitrogen and lanthanum codoped barium titanate with a novel thermal ammonolysis reactor

Lanthanum doped barium titanate was nitridated by thermal ammonolysis with a newly designed reactor, which performs extended efficiency to supply ammonia with less decomposition at elevated temperatures above 600°C. The kinetics of nitrogen-uptake was studied systematically at elevated temperatures. Nitrogen-content, thermal stability and crystallographic phase of nitridated lanthanum doped barium titanate were characterized by several techniques. The final nitrogen-content of nitridated lanthanum doped barium titanate was much higher than the content of lanthanum, resulting in the formation of oxygen vacancies compensating the additional negative electrical charge of nitrogen anions occupying regular oxygen sites in the lattice. Through thermal ammonolysis, strongly reducing conditions because of the decomposition of ammonia generating hydrogen gas causes the formation of barium orthotitanate as a secondary phase. Ammonolysis temperature below 800°C and suppression of thermal decomposition of ammonia gas are essential to realize a nitridated lanthanum doped barium titanate as a single phase.

First-principles study of the structural, electronic, and magnetic properties of double perovskite Sr2FeReO6 containing various imperfections* *Project supported by the National Natural Science Foundation of China (Grant No. 51501017).

The structural, electronic, and magnetic properties of double perovskite Sr2FeReO6 containing eight different imperfections of FeRe or ReFe antisites, Fe1–Re1 or Fe1–Re4 interchanges, VFe, VRe, VO or VSr vacancies have been studied by using the first-principles projector augmented wave (PAW) within generalized gradient approximation as well as taking into account the on-site Coulomb repulsive interaction (GGA+U). No obvious structural changes are observed for the imperfect Sr2FeReO6 containing FeRe or ReFe antisites, Fe1–Re1 or Fe1–Re4 interchanges, or VSr vacancy defects. However, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VRe (VO) vacancies. The half-metallic (HM) character is maintained for the imperfect Sr2FeReO6 containing FeRe or ReFe antisites, Fe1–Re4 interchange, VFe, VO or VSr vacancies, while it vanishes when the Fe1–Re1 interchange or VRe vacancy is presented. So the Fe1–Re1 interchange and the VRe vacancy defects should be avoided to preserve the HM character of Sr2FeReO6 and thus usage in spintronic devices. In the FeRe or ReFe antisites, Fe1–Re1 or Fe1–Re4 interchanges cases, the spin moments of the Fe (Re) cations situated on Re (Fe) antisites are in an antiferromagnetic coupling with those of the Fe (Re) cations on the regular sites. In the VFe, VRe, VO, or VSr vacancies cases, a ferromagnetic coupling is obtained within each cation sublattice, while the two cation sublattices are coupled antiferromagnetically. The total magnetic moments μ tot (μ B/f.u.) of the imperfect Sr2FeReO6 containing eight different defects decrease in the sequence of VSr vacancy (3.50), VRe vacancy (3.43), FeRe antisite (2.74), VO vacancy (2.64), VFe vacancy (2.51), ReFe antisite (2.29), Fe1–Re4 interchange (1.96), Fe1–Re1 interchange (1.87), and the mechanisms of the saturation magnetization reduction have been analyzed.

Study of divalent elements (Mg, Sr and Ba)-doped LaMnO3 nano-manganites

Doping of group-II divalent cations in LaMnO3 can produce advanced multi-functionalities in these nano-manganite materials according to recent theoretical approaches. An experimental investigation of Mg, Sr and Ba-doped LaMnO3 nano-structures has been made by synthesizing these compositions with fuel agent assistive sol–gel auto-combustion technique. Detailed structural studies were carried out for crystal structure, lattice parameters, crystallite size, density and strain in structure using the data obtained from X-ray diffraction (XRD). Structural parameters were largely affected and significantly varied in all compositions due to doping of group-II elements with different sized ionic radii. Morphological and compositional analyzes were carried out using scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Electrical and magnetic properties were explored using four-point probe and physical property measurement setups and observed a significant change in various compositions. The substitution of group-II atoms with different sized ionic radii on the regular sites in structure provides a distinctive behavior in various properties.

Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110)

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photo-activated dehydrogenation of ethanol on reduced and oxidized rutile TiO2(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO2.

Phase-shift-controlled logic gates in Y-shaped nonlinearly coupled chains.

We introduce a model system composed of two input discrete chains nonlinearly coupled to a single output chain which mimics the geometry of Y-shaped carbon nanotubes, photonic crystal wave guides, and DNA junctions. We explore the capability of the proposed system to perform logic gate operations based on the transmission of phase-shifted harmonic incoming waves. Within a tight-binding approach, we determine the exact transmission spectrum which exhibits a nonlinear induced bistability. Using a digitalization scheme of the output signal based on amplitude modulation, we show that AND, OR, and XOR logic operations can be achieved. Nonlinearity strongly favors the realization of logic operations in the regime of large wavelengths, while phase shifting is required for the OR logic gate to be realizable. A detailed analysis of the contrast ratio shows that optimal operation of the AND and OR logic gates takes place when the nonlinear response is the predominant physical property distinguishing the coupling and regular sites. These results point towards the possibility of Y-branched junctions to perform logic operations based on the transmission of traveling waves.

Land suitability for final waste disposal with emphasis on septic systems installation in southern Minas Gerais, Brazil

ABSTRACT Environmental pollution is a problem that has been noted due to changes in the environment, affecting natural resources. Regarding the soil, it may offer great potential for waste disposal. Thus, this study aims to propose criteria for evaluating local suitability for waste disposal, according to soil and terrain attributes for southern Minas Gerais State, and to apply those criteria to define the most appropriate locations for installation of septic systems in a pilot watershed. Literature and the authors' experience were used to propose the more important criteria regarding the suitability of sites for waste disposal. The set of attributes taken into account was grouped into four suitability classes: Adequate, Regular, Restricted and Inadequate. The defined criteria and considered limiting were: soil depth, texture, textural gradient, structure, natural drainage, water infiltration, type of surface horizon, water table depth, depth of perched water table, distance from water bodies, relief, stoniness, rockiness and risk of flooding. From these, soil depth, natural drainage, water table depth, relief and distance from water bodies were adopted for the installation of septic systems. From the total area of the watershed, 5.29% fit in the Adequate suitability class. The Regular, Restricted, and Inadequate sites accounted for, respectively, 19.72%, 41.99% and 33% of the wathershed. Factors such as soil and terrain attributes provide a basis for defining more appropriate places for waste disposal. Future work should involve the refinement of these propositions, since there are rare studies in this research line in Brazil.

Hydroxyl Defect Effect on Reoxidation of Sc‐Doped (Ba,Ca)(Ti,Zr)O3 Fired in Reducing Atmospheres

The behavior of grain and grain‐boundary conductivity of acceptor (Sc)‐doped (Ba,Ca)(Ti,Zr)O3 ceramics sintered in moist reducing atmosphere and subsequently reoxidized in dry and moist atmosphere was investigated by means of impedance spectroscopy (IS). In moist firing atmosphere, water vapor was found to react with oxygen vacancies, forming positively charged hydroxyl defects on regular oxygen sites in the crystal lattice. Proton hopping is considered to raise the ionic conductivity significantly. Therefore, hydroxyl defects in turn influence the grain conduction. Hydroxyl defects are also considered to be responsible for alternations of the dielectric maximum at the Curie point.

Surface induced vibrational modes in the fluorescence spectra of PTCDA adsorbed on the KCl(100) and NaCl(100) surfaces.

We report a combined experiment-theory study on low energy vibrational modes in fluorescence spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules. Using very low coverages, isolated molecules were adsorbed on terrace sites or at sites located at residual steps on (100) oriented alkali halide films (KCl and NaCl). The low energy modes couple to the optical transition only because the PTCDA molecule is geometrically distorted (C2v) upon adsorption on the surface; they would be absent for the parent planar (D2h) PTCDA molecule. The modes differ in number and energy for molecules adsorbed on regular terrace sites and molecules adsorbed at step edge sites. Modes appearing for step edge sites have the character of frustrated rotations. Their coupling to the optical transition is a consequence of the further reduced symmetry of the step edge sites. We find a larger number of vibrational modes on NaCl than on KCl. We explain this by the stronger electrostatic bonding of the PTCDA on NaCl compared to KCl. It causes the optical transition to induce stronger changes in the molecular coordinates, thus leading to larger Franck-Condon factors and thus stronger coupling. Our results demonstrate how optical spectroscopy can be used to gain information on adsorption sites of molecules at low surface concentrations.

Effect of Surface Site on the Spin State for the Interaction of NO with Pd<sub>2</sub>, Rh<sub>2</sub> and PdRh Nanoparticles Supported at Regular and Defective MgO(001) Surfaces

An attempt has been made to analyze the effect of surface site on the spin state for the interaction of NO with Pd2, Rh2 and PdRh nanoparticles that supported at regular and defective MgO(001) surfaces. The adsorption properties of NO on homonuclear, Pd2, Rh2, and heteronuclear transition metal dimers, PdRh, that deposited on MgO(001) surface have been studied by means of hybrid density functional theory calculations and embedded cluster model. The most stable NO chemisorption geometry is in a bridge position on Pd2 and a top configuration of Rh2 and PdRh with N-down oriented. NO prefers binding to Rh site when both Rh and Pd atoms co-exist in the PdRh. The natural bond orbital analysis (NBO) reveals that the electronic structure of the adsorbed metal represents a qualitative change with respect to that of the free metal. The adsorption properties of NO have been analyzed with reference to the NBO, charge transfer, band gaps, pairwise and non-pairwise additivity. The binding of NO precursor is dominated by the E(i)Mx-NO pairwise additive components and the role of the support was not restricted to supporting the metal. The adsorbed dimers on the MgO surface lose most of the metal-metal interaction due to the relatively strong bond with the substrate. Spin polarized calculations were performed and the results concern the systems in their more stable spin states. Spin quenching occurs for Rh atom, Pd2, Rh2 and PdRh complexes at the terrace and defective surfaces. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support and dimer-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals and dimers. Although the interaction of Pd, Rh, Pd2, Rh2 and PdRh particles with Fs sites is much stronger than the regular sites O2-, the adsorption of NO is stronger when the particular dimers are supported on an anionic site than on an Fs site of the MgO(001). The encountered variations in magnetic properties of the adsorbed species at MgO(001) surface are correlated with the energy gaps of the frontier orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and the role of precursor molecules on the magnetic and binding properties of complexes need to be explicitly taken into account.

Optical nonlinearities of small polarons in lithium niobate

An overview of optical nonlinearities of small bound polarons is given, which can occur in the congruently melting composition of LiNbO3. Such polarons decisively influence the linear and nonlinear optical performance of this material that is important for the field of optics and photonics. On the basis of an elementary phenomenological approach, the localization of carriers in a periodic lattice with intrinsic defects is introduced. It is applied to describe the binding energies of four electron and hole small polarons in LiNbO3: small free NbNb4+ polarons, small bound NbLi4+ polarons, small bound NbLi4+:NbNb4+ bipolarons, and small bound O− hole polarons. For the understanding of their linear interaction with light, an optically induced transfer between nearest-neighboring polaronic sites is assumed. It reveals spectrally well separated optical absorption features in the visible and near-infrared spectral range, their small polaron peak energies and lineshapes. Nonlinear interaction of light is assigned to the optical formation of short-lived small polarons as a result of carrier excitation by means of band-to-band transitions. It is accompanied by the appearance of a transient absorption being spectrally constituted by the individual fingerprints of the small polarons involved. The relaxation dynamics of the transients is thermally activated and characterized phenomenologically by a stretched exponential behavior, according to incoherent 3D small polaron hopping between regular and defect sites of the crystal lattice. It is shown that the analysis of the dynamics is a useful tool for revealing the recombination processes between small polarons of different charge. Nonlinear interaction of small polarons with light furthermore results in changes of the index of refraction. Besides its causal relation to the transients via Kramers-Kronig relation, pronounced index changes may occur due to optically generated electric fields modulating the index of refraction via the linear electro-optic effect, also. Based on a microscopic picture and by considering the local structural environment of bound polarons, the appearance of photovoltaic currents is explained straightforwardly as a result of the optically induced carrier transfer. Both transient absorption and index changes are spatially confined to the intensity profile of the interacting light allowing for the recording of efficient mixed absorption and phase volume holograms. By means of holographic spectroscopy, these small-polaron based optical nonlinearities are verified either without or with the action of the linear electro-optic effect; their prominent features are highlighted by appropriate experimental studies wherin the ultrafast response on the picosecond time scale is the most recognized one. Based on these findings, the consequences for applications of LiNbO3 in the field of nonlinear optics and photonics are presented. Besides visionary examples like real-time, 3D holographic displays, the impact of optical nonlinearities of small polarons for present applications are discussed with frequency conversion and respective limiting effects, such as green-induced infrared absorption and optical damage, as important example.

Interstitial lithium diffusion pathways in γ-LiAlO2: a computational study.

Although the Li diffusion in single crystalline γ-LiAlO2 was studied with temperature-dependent Li-7 NMR spectroscopy and conductivity measurements recently, the exact diffusion pathways are not yet clearly identified. Therefore, the present study aims at elucidating the diffusion pathways in γ-LiAlO2 theoretically from first principles. Competing pathways for Li diffusion are investigated using the climbing-image nudged-elastic-band approach with periodic quantum-chemical density functional theory (DFT) method. Li can migrate between two regular LiO4 tetrahedral sites via Li point defect (VLi) and via a Li Frenkel defect (VLi + Lii). On the basis of calculated activation energies for Li diffusion pathways, it is concluded that Li conductivity is strongly dependent on the distribution of Li vacancies and interstitial Li in the lattice. For Frenkel defects where Lii is far away from the migrating Li atom, the calculated activation energies for jumps to nearest-neighbor vacant sites agree with experimental values.

Comprehensive studies of structural, electronic and magnetic properties of Zn0.95Co0.05O nanopowders

X-ray absorption (XANES, EXAFS, XMCD) and photoelectron (XPS) spectroscopic techniques were employed to study local structural, electronic and magnetic properties of Zn0.95Co0.05O nanopowders. The substitutional Co2+ ions are incorporated in ZnO lattice at regular Zn sites and the sample is characterized by high structural order. There was no sign of ferromagnetic ordering of Co magnetic moments and the sample is in paramagnetic state at all temperatures down to 5K. The possible connection of the structural defects with the absence of ferromagnetism is discussed on the basis of theoretical calculations of the O K-edge absorption spectra.

Effects of the defects on the structural, electronic and magnetic properties of Sr2FeMoO6

From the point of view of the half-metallic character and magnetization reduction, the structural, electronic and magnetic properties of the imperfect Sr2FeMoO6 compound containing seven different defects of the FeMo or MoFe antisites, Fe–Mo interchange, VFe, VMo, VO or VSr vacancies have been studied by using first-principles projector augmented wave (PAW) within generalized gradient approximation (GGA) as well as taking into account on-site Coulomb repulsive interaction (GGA + U). No obvious structural changes are observed for the imperfect Sr2FeMoO6 compound containing FeMo or MoFe antisites, Fe–Mo interchange or VSr vacancy defects. However, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VMo (VO) vacancies. The half-metallic character is maintained for the imperfect Sr2FeMoO6 compound containing FeMo antisite, VFe, VO or VSr vacancies, while it vanishes when MoFe antisite, Fe–Mo interchange or VMo vacancy are presented. So the MoFe antisite, Fe–Mo interchange or VMo vacancy defects have to be avoided in order to preserve the half-metallic character of the Sr2FeMoO6 compound and thus usable in spintronics devices. In FeMo or MoFe antisites, Fe–Mo interchange cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. In VFe, VMo, VO or VSr vacancies cases, a ferromagnetic coupling is obtained within each cation sublattice, while two cation sublattices are coupled antiferromagnetically. The total magnetic moments μtot (μB/f.u.) of the imperfect Sr2FeMoO6 compound containing seven different defects decrease in the sequence of VSr vacancy (4.38), VMo vacancy (3.92), VO vacancy (3.51), VFe vacancy (3.40), FeMo antisite (3.40), MoFe antisite (3.33), Fe–Mo interchange (2.47) and the mechanisms of the saturation magnetization reduction have been discussed.