The influence of oxygen vacancies on the magnetic state of La<Subscript>0.50</Subscript>D<Subscript>0.50</Subscript>MnO<Subscript>3−γ</Subscript> (D=Da,Sr) manganites

The crystal structure and magnetic and electric transport properties of polycrystalline La_0.50D_0.50MnO_3?γmanganites (D=Ca, Sr) were studied experimentally depending on the concentration of oxygen vacancies. The La_0.50Sr_0.50MnO_3?γ system of anion-deficient compositions was found to be stable and possess a perovskite structure only up to the γ=0.25 concentration of oxygen vacancies, whereas, for the La_0.50Ca_0.50MnO_3?γ system, we were able to obtain samples with the concentrations of oxygen vacancies up to γ=0.50. The stoichiometric La_0.50D_0.50MnO₃ (D=Ca, Sr) compositions had O-orthorhombic (Ca) and tetragonal (Sr) unit cells. The unit cell of the anion-deficient La_0.50Sr_0.50MnO_3?γ manganites also became O-orthorhombic when the concentration of oxygen vacancies increased γτ;0.16). Oxygen deficiency in La_0.50Sr_0.50MnO_3?γ first caused the transition from the antiferromagnetic to the ferromagnetic state γ～0.06) and then to the spin glass state γ～0.16). Supposedly, the oxygen vacancies in the reduced La_0.50Sr_0.50MnO_{3? γ} samples with γ≥0.16 were disordered. The special feature of the La_0.50Ca_0.50MnO_3?γ manganites was a nonuniform distribution of oxygen vacancies in the La_0.50Ca_0.50MnO_2.75 phase. In the La_0.50Ca_0.50MnO_2.50 phase, the type of oxygen vacancy ordering corresponded to that in Sr₂Fe₂O₅, which led to antiferromagnetic ordering. The specific electric resistance of the La_0.50D_0.50MnO_3?γ anion-deficient samples increased with increasing oxygen deficiency. The magnetoresistance of all samples gradually increased as a result of the transition to the magnetically ordered state. Supposedly, the La_0.50Ca_0.50MnO_3?γ manganites in the range of oxygen vacancy concentrations 0.09≤γ≤0.50 had a mixed state and contained microdomains with different types of magnetic ordering. The experimentally observed properties can be interpreted based on the model of phase layering and the model of superexchange magnetic ordering.     

Conductivity and water uptake of Sr4(Sr2Nb2)O11·nH2O and Sr4(Sr2Ta2)O11·nH2O

The hydrated oxygen deficient complex perovskite-related materials Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 °C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 °C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr₄(Sr₂Nb₂)O₁₁·nH₂O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)₂. The stability of Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and SrO/Sr(OH)₂ secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials.     

Electronic structure and stability of nonstoichiometric titanium monoxide TiO y with structural vacancies in one of the sublattices

The electronic structure of nonstoichiometric titanium monoxide TiO _y with different compositions y, which contains structural vacancies either in the metallic sublattice or in the nonmetallic sublattice, has been investigated using the supercell method within the DFT-GGA approximation with pseudopotentials. The cases of ordered and disordered arrangements of vacancies have been considered. It has been found that the complete removal of vacancies from the sublattice is energetically unfavorable, and the ordering of oxygen vacancies according to the type of the Ti₆O₅□₁ superstructure, as well as titanium vacancies according to the Ti₅?₁O₆ type, does not lead to the stabilization of the B1 basic structure of titanium monoxide.     

Effects of O2/Ar ratio and annealing temperature on electrical properties of Ta2O5 film prepared by magnetron sputtering

The effects of the oxygen-argon ratio on electric properties of Ta2O5 film prepared by radio-frequency magnetron sputtering were investigated.The Ta2O5 partially transforms from the amorphous phase into the crystal phase when annealing temperatures are 800℃ or higher.The lattice constant of Ta2O5 decreases with the increase of the O2/Ar ratio,which indicates that oxygen gas in the working gas mixture contributes to reducing the density of oxygen vacancies during the deposition process.For the films deposited in working gas mixtures with different O2/Ar ratios and subsequently annealed at 700℃,the effective dielectric constant is increased from 14.7 to 18.4 with the increase of the O2/Ar ratio from 0 to 1.Considering the presence of an SiO2 layer between the film and the silicon substrate,the optimal dielectric constant of Ta2O5 film was estimated to be 31.Oxygen gas in the working gas mixture contributes to reducing the density of oxygen vacancies,and the oxygen vacancy density and leakage current of Ta2O5 film both decrease with the increase of the O2/Ar ratio.The leakage current decreases after annealing treatment and it is minimized at 700℃.However,when the annealing temperature is 800℃ or higher,it increases slightly,which results from the partially crystallized Ta2O5 layer containing defects such as grain boundaries and vacancies.     

Oxide ion conduction in Nd9.33(SiO4)6O2 and Sr2Nd8(SiO4)6O2 single crystals grown by floating zone method

Single crystals of Nd_9.33(SiO₄)₆O₂ and Sr₂Nd₈(SiO₄)₆O₂ oxide ion conductors with the oxyapatite structure were grown without any macroscopic defect by the floating zone method. The oxide ion conductivity of the Sr₂Nd₈(SiO₄)₆O₂ single crystal varied with the distance from its seed crystal along the growth direction, because of its changing deficiency in Sr content. Its stoichiometric portion had a lower electrical conductivity by about five orders of magnitude at 600 °C than the value for Nd_9.33(SiO₄)₆O₂ single crystal. Structural refinement of neutron diffraction data for powdered Nd_9.33(SiO₄)₆O₂ single crystal showed that the previous structure analysis was misleading. Cation vacancies were present only at 4f site and its channel oxygen site was fully occupied in space group of P6₃/m. The oxygen had an anisotropic displacement along the channel in the refined oxyapatite structure.     

Neutron diffraction analysis of a defect vanadium monoxide close to the equiatomic vanadium monoxide

Neutron and X-ray diffraction analyses are applied to studying the defect structure of synthesis-temperature quenched and low-temperature annealed vanadium monoxides VO _y (0.90 ≤ y ≤ 0.97) close to the equiatomic monoxide VO_1.0. It is found that the monoxides VO_0.90 and VO_0.97 contain structural vacancies not only in the oxygen sublattice, but also in the metal sublattice. In addition to the cubic disordered phase VO _y with the structure B1, the monoclinic superstructure V₁₄O₆ with space group C2/m is present in the synthesized VO_0.90 sample and in the annealed VO_0.90 and VO_0.97 samples. The formation of the V₁₄O₆ superstructure is attributed to the ordering of oxygen atoms and nonmetal vacancies in the lattice of the tetragonal solid solution of oxygen in vanadium. No simultaneous ordering of metal and oxygen vacancies in two sublattices of the cubic vanadium is observed.     

Oriented Growth of Nanocrystalline Gamma Ferric Oxide in Electrophoretically Deposited Films

Films of nanocrystalline γ-Fe₂O₃ were deposited on silicon substrates by using the technique of electrophoretic deposition. The precursor powder was nanocrystalline γ-Fe₂O₃, which was synthesized, using DC arc plasma in the oxygen ambient by vapour–vapour interaction in gas phase condensation; at a stabilized arc current of 40 A. This powder was characterized by X-ray diffraction, Transmission Electron Microscopy, Vibrating Sample Magnetometer and Mössbauer Spectroscopy. An increase in directional coercivity was observed in case of films deposited on silicon substrates, which is dramatically significant. Preferred orientation of almost similar sized nanocrystalline magnetic domains in deposited films is evident from the results of X-ray diffraction and Transmission Electron Microscopy results. The preferred alignment of the nanocrystallites seems to be responsible for the significant changes observed in magnetic properties of films.     

Synthesis of boron nitride nanostructures from catalyst of iron compounds via thermal chemical vapor deposition technique

For the first time, patterned growth of boron nitride nanostructures (BNNs) is achieved by thermal chemical vapor deposition (TCVD) technique at 1150 °C using a mixture of FeS/Fe₂O₃ catalyst supported in alumina nanostructured, boron amorphous and ammonia (NH₃) as reagent gas. This innovative catalyst was synthesized in our laboratory and systematically characterized. The materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The X-ray diffraction profile of the synthesized catalyst indicates the coexistence of three different crystal structures showing the presence of a cubic structure of iron oxide and iron sulfide besides the gamma alumina (γ) phase. The results show that boron nitride bamboo-like nanotubes (BNNTs) and hexagonal boron nitride (h-BN) nanosheets were successfully synthesized. Furthermore, the important contribution of this work is the manufacture of BNNs from FeS/Fe₂O₃ mixture.     

Influence of nickel on the electronic structure and magnetic properties in Gd7Pd3-xNix

With increasing amount of nickel in the Gd₇Pd_3-xNi_x series, the unit cell volume decreases due to difference between atomic radius of Pd and Ni but the type of crystal structure is preserved. The paper presents X-ray diffraction, Scanning Electron Microscopy with Energy Dispersive X-ray Spectrometer, Transmission Electron Microscopy, AC and DC magnetic susceptibility and magnetocaloric examinations of the series. The Ni substitution into Pd position causes the decrease in the Curie temperature. Moreover, the dispersion observed in the AC-susceptibility at different frequencies vanishes at the Curie point. Just below the ordering temperature, a Hopkinson peak is observed. Complex processes depending on frequency and magnetic field occur in the transition region. The magnetization phenomena are very sensitive to the applied DC magnetic field and even very weak fields producing thermomagnetic hysteresis. A reorientation process around 150 K exists in all compounds and it is connected with increasing interactions at low temperatures. After nickel substitution, the relative cooling power (RCP) is greater in comparison to the parent compound.     

Reversible resistance switching properties in Ti-doped polycrystalline Ta2O5 thin films

Unipolar reversible resistance switching effects were found in 5 at% Ti-doped polycrystalline Ta₂O₅ films with the device structure of Pt/Ti–Ta₂O₅/Pt. Results suggest that the recovery/rupture of the conductive filaments which are involved in the participation of oxygen vacancies and electrons leads to the resistance switching process. Ti-doped Ta₂O₅ thin films possess higher resistance whether in low-resistance state or high-resistance state and higher resistance switching ratio than Ta₂O₅ thin films, where Ti addition plays an important role in the resistance switching process by suppressing the migration of oxygen vacancies via forming an electrically inactive Ti/O–vacancy complex. Excellent retention properties of the high and low resistances under constant stress of applied voltage were obtained.     

Microstructure and polarity of epitaxial ZnO films grown on LSAT(111) substrate studied by transmission electron microscopy

Transmission electron microscopy is used to investigate the structural characteristics of epitaxial ZnO thin films grown on (LaAlO₃)_0.3(Sr_0.5Ta_0.5O₃)_0.7(111) (LSAT) by rf plasma-assisted molecular beam epitaxy. It is found that the growth temperature plays a key role in the formation of microstructures in ZnO film. Growth temperature dependence of rotation domain, interface and dislocation structures is studied, and the mechanism for polarity selection is discussed.     

Effect of Magnetic Pulse Processing on the Structure and Magnetic Properties of Ferrites

The effect a pulsed magnetic field has on the crystal structure and macroscopic magnetic parameters of hexagonal ferrites BaFe₁₂O₁₉ and SrFe₁₂O₁₉ are studied. It is shown that changes in the physical properties of ferrites are due to the ordering of cation vacancies on the boundaries of hexagonal and spinel blocks that minimize local distortion of the oxygen polyhedrons. Violation of the collinear ordering of the magnetic moments of iron ions in the nonequivalent positions of SrFe₁₂O₁₉ ferrite is observed, due to the selective localization of such vacancies (and thus violations of the magnetic relationships in Fe–O–Fe).     

Magnetic ordering on Yb3+ in YbBa2Cu3O7−x

The high critical temperature superconductor YbBa₂Cu₃O_7−x has been studied over the range 0.05 to 95K using ¹⁷⁰Yb Mössbauer absorption spectroscopy. Magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K. The saturated magnetic moments are 1.7μ_B. At all temperatures the hyperfine parameters show the presence of distributions attributed to some disordering of the oxygen vacancies.     

Comparative study of Pt, Au and Ag growth on Fe3O4(0 0 1) surface

The epitaxial growth of Pt, Au and Ag layers on Fe₃O₄(0 0 1) as a function of temperature and thickness have been studied. The layers were deposited by sputtering in an ultra high vacuum chamber and the structural properties were investigated by Reflection High Energy Electron Diffraction, X-ray reflectivity and diffraction, High Resolution Transmission Electron Microscopy and Atomic Force Microscopy. Our studies give evidence for three different growth behaviours depending both on the nature of the metals and the temperature. Comparison between the growth modes of the three metals will be discussed in relation with surface and interfaces energies.     

Ordering of vacancies in LiAl

The intermetallic compound β-LiAl contains relatively high concentrations of constitutional vacancies on the Li sublattice for Li deficient compositions. Neutron diffraction measurements on a Li_0.486Al_0.514 single crystal show that these vacancies order on every tenth (840) plane at 97K. This vacancy ordering is responsible for the “100K” anomaly observed in several physical properties.     

Application of diffraction methods to the study of volume variations in the composition of a langatate single crystal

A langatate single crystal with the initial composition La₃Ga_5.5Ta_0.5O₁₄, which is grown by means of Czochralski pulling along the 〈0001〉 direction in a weakly oxidizing medium, is investigated via X-ray and neutron diffraction techniques. The crystal is inhomogeneous in color and composition. The middle part of the single crystal is revealed to have a more homogeneous composition than its upper part. It is demonstrated that the color change depends on the amount of oxygen vacancies in different parts of the single-crystal boule.     

Facile synthesis of multifunctional Ag/Fe3O4-CS nanocomposites for antibacterial and hyperthermic applications

Nanocomposites containing two or more functional constituents are attractive candidates for advanced nanomaterials. In this study, multifunctional Ag/Fe₃O₄-CS nanocomposites were successfully prepared, using chitosan as a stabilizing and cross-linking agent. The as-synthesized nanocomposites were characterized by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), UV–visible spectrophotometer (UV–Vis) and vibrating sample magnetometer (VSM). The results demonstrated that Ag/Fe₃O₄-CS composite nanoparticles (NPs) were composed of parent components, Fe₃O₄ and Ag NPs, which were uniformly dispersed in the chitosan matrix. The hybrid NPs exhibited strong antibacterial property against Pseudomonas aeruginosa. With high magnetization value (67 emu/g), the synthesized Ag/Fe₃O₄-CS composite can be easily separated or recycled in potential biomedical applications. Furthermore, the results showed that the multicomponent hybrid nanostructures appeared to be the promising material for local hyperthermia, which can be used as thermoseeds for localized hyperthermia treatment of cancers.     

Ab initio simulation of the electronic structure of δ-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with oxygen vacancy and comparison with experiment

The electronic structure of a Ta₂O₅ insulator with oxygen vacancies is studied theoretically and experimentally. The ab initio calculations of δ-Ta₂O₅ are performed in terms of density functional theory using the generalized gradient (GGA) and GGA + U approximations. The electronic structure of Ta₂O₅ is experimentally studied by X-ray photoelectron spectroscopy (XPS). To study oxygen vacancies, an amorphous Ta₂O₅ film is irradiated by argon ions. The calculated XPS spectra of the valence band of δ-Ta₂O₅ agree satisfactorily with the corresponding experimental spectra of the amorphous films. The oxygen vacancy in δ-Ta₂O₅ is found to be a trap for holes and electrons. The minimum and maximum effective masses of electrons and holes in δ-Ta₂O₅ are calculated.     

Advantages and limitations of OM, SEM, TEM and AFM in the study of ancient decorated pottery

This paper presents results from the study of two fragments of pre-Hispanic pottery, decorated with red pigment, using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM) and Magnetic Force Microscopy (MFM). Capabilities and limitations of these techniques in the analysis of archaeological material are highlighted with special emphasis on TEM, AFM and MFM due to their contribution in the study of the pigment layer at micro and nano scale. The analyzed samples come from the archaeological sites of El Tajin and Xochicalco, both in Mexico. Results of conventional TEM and HRTEM analysis of the red pigment showed nanometric Fe₂O₃ particles in both samples but different particle shape and size distributions: specimen from El Tajin presented irregular particles between 50–100 nm while that from Xochicalco exhibited semispherical shapes in the 3–25 nm range. AFM images showed the topography of the pigments, which are related to the texture of their surface and thus to the production process. Finally, MFM showed different contrast regions suggesting the presence of ferromagnetic elements forming clusters and domain orientations on the color layer.