Td Sites Research Articles

Unveiling the Physicochemical Features of CoFe2O4 Nanoparticles Synthesized via a Variant Hydrothermal Method: NMR Relaxometric Properties

A series of CoFe2O4 nanoparticles were formed through a variant hydrothermal synthesis based on a self-assembled oil–water system in autoclaves at 200 °C in the presence of octadecylamine and the trivalent iron and cobalt acetylacetonates. The variation of the water content, the different valence of the cobalt precursors (Co(II) and Co(III)) as well as Fe:Co precursor ratios (2:1 and 1:1) were studied. CoFe2O4 nanoparticles with a size range of 9–16 nm of high crystallinity and enhanced saturation magnetization (∼89 emu g–1) have been isolated and characterized. Raman spectroscopy provided information concerning the lattice strain, while incorporation of Co2+ at Td sites of the spinel indicated a different inversion degree (0.67–0.60) among the samples. EPR studies showed that EPR signal and spin relaxation process were size dependent and influenced by aggregation effects. CoFe2O4 nanoparticles were converted to dual agents via a reaction between the free amine groups of the organic coating and the sulfon...

Determination of the cation site distribution of the spinel in multiferroic CoFe2O4/BaTiO3 layers by X-ray photoelectron spectroscopy

The properties of CoFe2O4/BaTiO3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe2O4 layers grown on BaTiO3, with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor.

Cation distribution and photoluminescence properties of Mn-doped ZnGa2O4 nanoparticles

Mn-doped ZnGa2O4 nanoparticles with the composition of MnxZn1−xGa2O4 (0≤x≤1) were prepared by the citrate sol–gel method. The cation distribution in the nanoparticles was studied by using X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results show that the nanoparticles with the size of 20–48nm were obtained when the precursor was annealed at 600°C or above, and the size increases with increasing annealing temperature. The cations (Ga3+, Zn2+ and Mn2+) occupy the tetrahedral (Td) sites as well as the octahedral (Oh) sites of spinel structure in the nanoparticles. Ga3+ ions are located in a large proportion at the octahedral sites and in a small proportion at the tetrahedral sites. The inversion parameter (two times the fraction of Ga3+ in the tetrahedral sites) increases with decreasing annealing temperature and Mn-enrichment. The luminescent properties were studied by using fluorescence spectrophotometer. The photoluminescence (PL) spectra exhibit green and red emissions with excitation wavelength of 304nm, which is attributed to 4T1(4G)→6A1(6S) transition of Mn2+ ions in the tetrahedral and octahedral sites of MnxZn1−xGa2O4 nanoparticles, respectively.

Origin of surface canting within Fe3O4 nanoparticles.

The nature of near-surface spin canting within Fe3O4 nanoparticles is highly debated. Here we develop a neutron scattering asymmetry analysis which quantifies the canting angle to between 23° and 42° at 1.2 T. Simultaneously, an energy-balance model is presented which reproduces the experimentally observed evolution of shell thickness and canting angle between 10 and 300 K. The model is based on the concept of Td site reorientation and indicates that surface canting involves competition between magnetocrystalline, dipolar, exchange, and Zeeman energies.

Rectilinear evolution in arvicoline rodents and numerical dating of Iberian Early Pleistocene sites

Lozano-Fernández et al. (2013a) have recently published a method intended for numerical dating of Early Pleistocene sites, which is based on the assumption of uniform, constant rate increase through time of mean lower molar tooth length of water voles (Mimomys savini) in a number of levels sampled in the stratigraphic sequence of Atapuerca TD site. They suggest that the regression equation obtained in this local section for site chronology on tooth size could be useful for estimating the numerical age of other localities from southwestern Europe. However, in our opinion this biostratigraphic approach has severe conceptual and methodological problems, which discourage its use as a chronometric tool. These problems include that: (1) the logic behind their approach represents a ‘fallacy of hasty generalization’, because the results obtained for a local section are generalized to all possible stratigraphic sequences; (2) the study is based on tooth measurements from a limited set of samples taken in a single locality, which are represented by small numbers of specimens, cover a short time span and have a high level of age uncertainty; (3) the samples analyzed show small, statistically non-significant differences between their mean tooth length values; (4) the approach assumes a linear-straight, orthoevolutionary model of change at constant, monotonic rate for the apparent trend to increasing tooth size in the M. savini lineage; (5) these changes are better described in the Atapuerca TD section by a random walk, or even by a series of independent events, than by a model of rectilinear change; and (6) the application of this methodology to other localities such as the Orce sites, which preserve the oldest evidence of human presence in Europe, would mean that an equation adjusted within a restricted chronological range (1.01–0.8 Ma) is used for extrapolating the ages of sites that are clearly older (∼1.4 Ma).

Multi-scale simulation of lithium diffusion in the presence of a 30° partial dislocation and stacking fault in Si

The multi-scale simulation method is employed to investigate how defects affect the performances of Li-ion batteries (LIBs). The stable positions, binding energies and dynamics properties of Li impurity in Si with a 30° partial dislocation and stacking fault (SF) have been studied in comparison with the ideal crystal. It is found that the most table position is the tetrahedral (Td) site and the diffusion barrier is 0.63 eV in bulk Si. In the 30° partial dislocation core and SF region, the most stable positions are at the centers of the octagons (Oct-A and Oct-B) and pentahedron (site S), respectively. In addition, Li dopant may tend to congregate in these defects. The motion of Li along the dislocation core are carried out by the transport among the Oct-A (Oct-B) sites with the barrier of 1.93 eV (1.12 eV). In the SF region, the diffusion barrier of Li is 0.91 eV. These two types of defects may retard the fast migration of Li dopant that is finally trapped by them. Thus, the presence of the 30° partial dislocation and SF may deactivate the Li impurity and lead to low rate capability of LIB.

Insight into lithium distribution in lithium-stuffed garnet oxides through neutron diffraction and atomistic simulation: Li 7-x La 3 Zr 2-x Ta x O 12 (x = 0–2) series

Abstract Lithium-stuffed garnet oxides are promising candidates of lithium ion electrolyte materials due to their high ionic conductivities and good chemical stability. In this study, time-of-flight (TOF) neutron diffraction experiments and the Rietveld refinement were performed on four garnet compounds in the Li 7-x La 3 Zr 2-x Ta x O 12 (x = 0–2) series to study their average structures. Different structural models (split-site and anisotropic displacement) for lithium disorder at tetrahedral (Td) and octahedral (Oh) sites were compared and results suggested possible lithium disorder around both ideal Oh and ideal Td sites. To study the local lithium distribution, atomistic simulations based on static energy minimization with interatomic potentials were carried out on a large number of configurations of eight compositions. We demonstrated that energy probability distribution plots were helpful in understanding the lithium disorder/order in these materials. Nuclear density maps provided a visual presentation of structural disorder at both Td and Oh lithium sites. Our simulation results suggested that Td lithium occupancy is generally lower than that obtained from the average structure (Rietveld refinement) and the exclusion principle (no nearest-neighbor Td–Oh–Td lithium triplets). In addition, the nearest-neighbor Td–Oh lithium pairs and (Oh,□)–Td–(Oh,□) lithium clusters appear to be the characteristic local features contributing to the structure and conduction of these lithium-stuffed garnet oxides.

Electronic and crystal structure analysis of the FeCrO3 oxide

The magnetic and electronic behaviors of FeCrO3 crystal were investigated by X-ray absorption near edge (XANES) and X-ray magnetic linear dichroism (XMLD) techniques. The measured data were compared with the parent oxides Cr2O3 and α-Fe2O3 of the compound to identify the structural and electronic change during the process of FeCrO3 structure. The electronic properties of the sample were investigated via Fe, Cr L3,2 and O K-edges. The XMCD measurements to probe magnetic properties were performed by an external magnetic field of 0.4T. In the FeCrO3 structure, traces of the Oh and Td site symmetric local Fe3+ formations were observed. The Fe3+ ions with Oh symmetry as in the parent oxide α-Fe2O3 (hematite) were determined to have antiferromagnetic order in the structure. However, domains who have Td site symmetry with ferrimagnetic order due to the γ-Fe2O3 (maghemite) formation were determined.

Guest-Triggered ZnII Translocation and Supramolecular Nuclearity Control in Calix[6]arene-Based Complexes

Two new polytopic ligands based on a calix[6]arene scaffold were synthesized. The truncated cone-shaped calixarene was functionalized at its small rim by a tris-imidazole site, aimed at generating a tetrahedral Zn(II) complex, where a fourth labile site inside the cavity is accessible through the funnel provided by its large rim. Tridentate aza ligands (either two or three) were then grafted at this large rim (the entrance of the cavity). Zn(II) coordination studies, monitored by (1)H NMR spectroscopy, showed unprecedented behavior in this family of heteropolytopic ligands. Indeed, it gives access to complexes of various nuclearities in acetonitrile, where zinc binding is under the supramolecular control of the guest. It is first shown that, in the absence of a good guest donor (a primary amine), Zn(II) binding is favored at the large rim where two tridentate nitrogenous groups can form an octahedral complex. The addition of a long guest such as heptylamine induces the quantitative translocation of the Zn(II) ion from the large rim octahedral (O(h)) site to the small rim tetrahedral (T(d)) site provided by the trisimidazole core and the guest ligand. With 2 equiv of Zn(II), well-defined dinuclear complexes were obtained and isolated, with one Zn(II) ion bound at each rim. Interestingly, it is shown that the binding mode at the large rim is under the supramolecular control of the guest bound at the small rim (with short guests, the O(h) environment is obtained at the large rim, whereas long guests disrupt this core through an induced-fit process); the partially included and dangling alkyl chain opens the large rim (entrance of the cavity) and pushes apart the tridentate moieties. As a result, a guest-induced switch of Zn(II) binding mode occurs and frees one of the tridentate groups from coordination, allowing further extension of the complex nuclearity.

Electronic structure of Co-doped ZnO nanorods

The optical transmission spectra, the photoluminescence (PL), and the photoluminescence excitation (PLE) spectra of the cobalt doped zinc oxide nanorods Zn1−xCoxO (x = 0.01, 0.10) were measured by Loan et al. [J. Phys. D: Appl. Phys. 42, 065412 (2009)] in the region 1.5-4 eV. These spectra exhibit a group of ultraviolet narrow lines in the region of 3.0-3.4 eV related to the near-band-edge emission of the host ZnO materials and a group of emission lines in the red region of 1.8–1.9 eV assigned to the radiative transitions within the tetrahedral Co2+ ions in the ZnO host crystal. The group of lines in the visible region provides important information about the electronic structure of the cobalt doped zinc oxide nanorods. This work investigates a theoretical crystal-field analysis of the visible lines associated to the Co2+ ion transition occupying a Td site symmetry in ZnO host crystal. A satisfactory correlations were obtained between experimental and calculated energy levels. The electronic structure was compared with the reported for cobalt transition ion doped in ZnO nanoparticles and bulk crystals [Volbers et al., Appl. Phys. A 88, 153 (2007) and H. J. Schulz and M. Thiede, Phys. Rev. B 35, 18 (1987)]. In order to explain the existence of excitation peaks observed near the band edge of the ZnO host, an energy transfer mechanism is proposed.

Crystal Chemistry and Electronic Structure of the Photovoltaic Buffer Layer, (In1-xAlx)2S3

We have investigated the effects of the aluminum substitution in In2S3 using an approach that combines X-ray diffraction, nuclear magnetic resonance (NMR) experiments, and density functional theory (DFT) calculations. For the very first time, an understanding of the evolution of the local structure in this family with Al-content is reached based on the synergy of these three techniques. In particular, the comparison of the measured and simulated 27Al NMR quadrupolar constants allows an unambiguous attribution of the Al-sites in the investigated (In1-xAlx)2S3 compounds (0 ≤ x ≤ 0.185). It shows that for high Al-contents a new tetrahedral site (Td(4a)) is occupied, leading to a tetragonal to cubic modification. The present paper provides a definite explanation of the band gap reduction upon the aluminum substitution in In2S3, which is directly related to the occupation of the Td(4a) site. The present results could help to improve the performances of solar cells using In2S3-based compounds as alternative buffer layers.

First principles study of lithium insertion in bulk silicon

Si is an important anode material for the next generation of Li ion batteries. Here the energeticsand dynamics of Li atoms in bulk Si have been studied at different Li concentrations on thebasis of first principles calculations. It is found that Li prefers to occupy an interstitial site as ashallow donor rather than a substitutional site. The most stable position is the tetrahedral(Td) site. The diffusion of a Li atom in the Si lattice is through aTd–Hex–Td trajectory, where the Hex site is the hexagonal transition site with an energy barrier of0.58 eV. We have also systematically studied the local structural transition of aLixSi alloywith x varying from 0 to 0.25. At low doping concentration (x = 0–0.125),Li atoms prefer to be separated from each other, resulting in a homogeneous doping distribution. Startingfrom x = 0.125, Li atoms tend to form clusters induced by a lattice distortionwith frequent breaking and reforming of Si–Si bonds. Whenx ≥ 0.1875, Li atoms will break some Si–Si bonds permanently, which results in dangling bonds.These dangling bonds create negatively charged zones, which is the main driving force forLi atom clustering at high doping concentration.

Electronic Structure of Vertically Aligned Mn-Doped CoFe2O4 Nanowires and Their Application as Humidity Sensors and Photodetectors

Vertically aligned CoFe2O4 and 10% Mn-doped CoFe2O4 nanowires were synthesized by the gas-phase reduction/substitution reaction of pregrown α-Fe2O3 nanowires. They consisted of perfect single-crystalline cubic structures grown along the [110] direction. The increase of the lattice constant provides evidence for the effective Mn substitution. We investigated their electronic structures using X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The results suggest that as the Mn substitutes, Mn2+/Mn3+ ions would occupy the octahedral (Oh) sites, Mn4+ ions form at the tetrahedral (Td) sites, and a fraction of the Co2+ ions migrate from the Oh sites to the Td sites. The effective Mn substitution can increase the hole concentration, which makes it possible to develop humidity sensors operating at room temperature. Furthermore, it induces high photocurrents upon irradiation of visible light.

Probing the site occupancies of Co-, Ni-, and Mn-substituted biogenic magnetite using XAS and XMCD

Ferrimagnetic nanoparticles have many uses in industry including in magnetic recording media and transformers, however these particles are often expensive to synthesize. In this study, the Fe 3+ - reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula M x Fe 3-x O 4 , where M is either Co, Ni, Mn, Zn, or a combination of Mn and Zn. This was done at ambient temperatures through the dissimilatory reduction of Fe 3+ -oxyhydroxides containing the appropriate substitutional cations. A combination of L-edge and K-edge X-ray absorption spectroscopy (XAS) and L-edge X-ray magnetic circular dichroism (XMCD) was used to determine the site occupancies, valence, and local structure of the Fe and substitutional cations within the spinels. The Ni and Co ferrites produced using each bacterium were very similar and therefore this study concludes that, despite the difference in reduction mechanism of the bacteria used, the end-product is remarkably unaltered. Nickel ferrites contained only Ni 2+ , with at least 80% in Oh coordination. Cobalt ferrites contained only Co 2+ but with a significant proportion (up to 45%) in Td coordination, showing a slight preference for Td sites. The Mn-ferrites contained Mn 2+ only on the Oh sites but a mixture of Mn 2+ and Mn 3+ on Td sites when the amount of Mn exceeded 3% (compared to the amount of Fe) or some Zn was also present. This study successfully produced a range of nanoparticulate ferrites that could be produced industrially using relatively environmentally benign methodologies.

Investigation of the valence states of Fe and Co inFe1−xCoxOy (0

The unusual magnetic properties of thin films ofFe1−xCoxOy {0≤x≤1} ferrite solid solutions have been explained through an investigation of their electronicstructure. The x-ray absorption near-edge structure (XANES) spectra of the FeL3-edge and K-edgesshow that Co replaces Fe3+ in the tetrahedral (Td) sites up to x = 0.34 and octahedral (Oh) sites from 0.39<x<0.89. The Co L3-edge and O K-edge results indicate mixed valence states of Co as it replaces Fe at dissimilarsites. From the results, it is concluded that the magnetic properties of the thin films of the spinelFe1−xCoxOy arise fromthe Fe3+ at theTd sites. Thereplacement of Fe3+ and Fe2+ byCo at the Oh sites results in a crossover to the rocksalt structure and a loss of the magnetic properties.

Electronic structure of B-doped diamond: A first-principles study

Electronic structure of B-doped diamond is studied based on first-principles calculations with supercell models for substitutional and interstitial doping at 1.5–3.1 at.% B concentrations. Substitutional doping induces holes around the valence-band maximum in a rigid-band fashion. The nearest neighbor C site to B shows a large energy shift of 1score state, which may explain reasonably experimental features in recent photoemission and X-ray absorption spectra. Doping at interstitial Td site is found to be unstable compared with that at the substitutional site.

Correlation between the crystalline environment and optical property of Mn2+ ions in ZnGa2O4: Mn2+ phosphor

Sol-gel-driven ZnGa2O4:Mn2+ phosphors are annealed at various temperatures. As an increase of the annealing temperature of as-grown ZnGa2O4:Mn2+ phosphors, their lattice constants and crystallinities are increased. The resonance signal intensity of Mn2+ ions on Td sites is drastically increased whereas that of Mn2+ ions on Oh sites is decreased as the annealing temperature is increased. The decay curves are well fitted with two simple exponential decay functions indicating the existence of two origins. Liquid nitrogen-temperature emission spectra show two emission peaks suggesting that they originate from Mn2+ ions on Td and on Oh sites. The emission intensity of Mn2+ ions on Td sites is drastically increased whereas that of Mn2+ ions on Oh sites is decreased. The variation of the optical characteristics agrees with the concentration of Mn2+ ions at Td site and its site environment.

Direct observation of the Fe substitution effect on the MCD spectra of the dysprosium iron garnet family

Soft x-ray magnetic circular dichroism (MCD) and absorption spectra (XAS) were measured at the Fe L2,3-edges and the Dy M4,5-edges for a family of dysprosium iron garnets. The Fe ions sit in either Oh symmetry sites or Td symmetry sites in Dy3Fe5O12. Only Fe ions in the Oh sites are substituted with In ions in Dy3Fe4.25In0.75O12 and only Fe ions in the Td sites are substituted with Ga ions in Dy3Fe4.25Ga0.75O12. The XAS spectrum profiles of the three-dysprosium iron garnets are very similar. The MCD signals of both the Fe L2,3-edges and the Dy M4,5-edges of Dy3Fe4.25Ga0.75O12 are similar to that of Dy3Fe5O12, but Dy3Fe4.25In0.75O12 is remarkably different. The results directly show that the spin moment of Fe in the Td sites play a major role in determining the total magnetization of Dy3Fe5O12.

Stabilization of mixed valencies in Cu, Zn-based oxides

ABSTRACTCu and Zn-based oxides have been developed for the last decades because of their remarkable electronic properties and their relevant UV-Visible-NIR absorption properties. Cu in oxides can adopt various oxidation states (+I, +II, +III) stabilized in various environments (h, Td, D4h, C4v, D2h, Oh). Divalent copper cations can occupy Td or Oh sites in zinc aluminate spinel network. Solid state routes lead to homogeneous phases with Zn1−xCuxAl2O4 compositions. Depending on the inversion rate in the spinel matrix, various absorption bands have been identified in the UV-Visible-CNIR spectrum. The synthesis of the zinc-aluminate spinel solid solution Zn1−xCuxAl2O4 by an esterification route led to monocrystalline nanosized oxides. Two intense absorption bands at 300 and 500 nm can be attributed to charge transfer phenomena between oxygen and Cu2+ cations in tetrahedral and octahedral coordinations. Two other less intense absorption bands centred at 800 nm and 1500 nm are also appearing when the copper rate in the spinel increases but their relative intensity are not in good agreement with those observed in the case of the solid-state synthesis. In the case of the esterification route, the absorption band at 800 nm is much more intense than in the case of the solid-state synthesis. It can only be explained either by a deviation to the centrosymmetric character of octahedral sites or by the occurence of a non negligible amount of monovalent copper cations which give electronic transitions in this energy range. A magnetic study correlated to EPR measurements confirms the occurence of a mixed valence state for copper cations in the solid solution Zn1−xCuxAl2O4. EPR spectra at T=4K show for the small concentration of Cu2+ (x&lt;0.10) a strong anisotropic signal due to the presence of Cu2+ ions in a distorted octahedral symmetry (gx=2.07, gy= 2.15 gz=2.23). Moreover the hyperfine structure, identified on EPR spectra tend to disappear as the compounds are annealed under air because the content of paramagnetic centers Cu2+ (3d9) as well as their interactions become significant. Furthermore, the color changes drastically with the Cu+ (3d10) content. Finally the structural features and UV-Visible-NIR absorption properties of copper-zinc aluminates will be discussed and compared to Cu-doped ZnO.

EPR Spectra of Mn2+-Doped ZnS Quantum Dots

Powder Mn2+ EPR spectra of 5-nm ZnS:Mn quantum dots have been analyzed in detail to account for all the observed fine spectral features. The same spectral features are observed in samples with low doping of Mn2+ (0.003−0.008%), showing 10 weak forbidden transitions between the six-line pattern of the MS = −1/2 to MS = 1/2 transition for the six MI values. Spectral simulations have been performed with the Simfonia program with parameters gxx = 2.0064, gyy = 2.0064, gzz = 2.0066, D = 37.4 × 10-4 cm-1, E = 12.5 × 10-4 cm-1, Axx = 63.9 × 10-4 cm-1, Ayy = 64.0 × 10-4 cm-1, and Azz = 64.4 × 10-4 cm-1, suggesting a lower than Td site symmetry for the doped Mn2+ in the cubic ZnS structure. At higher doping concentration (15.9%), a single broad line is observed with isotropic g and A.