Enhanced magnetoresistance and surface state of CrO2 particles improved by chemical process

The paper presents an approach to enhance a magnetoresistance (MR) effect in CrO₂ powder compact by an oxidization reaction process. An aqueous potassium permanganate (KMnO₄) was used to react with the CrO₂ particles coated naturally with Cr₂O₃ layer. The experiment indicates that the strong oxidant can effectively adjust thickness of the natural Cr₂O₃ layer, and thereby change the surface state of the CrO₂ particles. Structural and magnetic properties for the improved CrO₂ particles have been characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and SQUID magnetometer. The results exhibit that the magnetotransport behavior of CrO₂ particles depends sensitively on the chemical reaction time. An optimal reaction process yields an obvious increase up to −33% in magnetoresistance at a temperature of 5 K for the chemical treated CrO₂ powder, compared to MR=-27% for the original CrO₂ powder. The mechanism of magnetotransport is assumed to originate from the spin-dependent tunneling in the granular system, which is consistent with our experimental results. The simple chemical approach has a potential to achieve an enhanced magnetoresistance in a metallic particle system by adjusting the surface state of the magnetic nanoparticle.     

An XPS study of CrOx on a thin alumina film and in alumina supported catalysts

We have investigated chromium layers evaporated onto a thin alumina film at room temperature. The oxidation and reduction behavior of this model catalyst was compared to atomic layer deposition (ALD) and impregnated alumina supported catalysts using X-ray photoelectron spectroscopy (XPS) with a detailed analysis method utilizing asymmetric peak shapes to represent both metallic and oxidic states. The ALD and impregnated catalysts were measured after calcination in air and after reduction with several gases at 850 K. Both catalysts show Cr³⁺ and Cr⁶⁺ species after calcination and mostly Cr³⁺ after reduction. The chromium layers deposited in vacuum show initially small partial oxidation due to the interaction with the oxygen terminated alumina film. These model catalysts can be oxidized in vacuum to Cr³⁺ species but not to higher oxidation states. The model catalysts were also subjected to calcination and reduction treatments after deposition in vacuum. Under these conditions the model systems exhibit similar oxidation/reduction behavior as the supported catalysts. Photoreduction of Cr⁶⁺ during the measurements was also studied and found to be very slow having a negligible effect on the results.     

Characterization of water exposed plasma sprayed oxide coating materials using XPS

The surface compositions and oxidation states of non-exposed and water exposed plasma sprayed oxide coatings were studied using X-ray photoelectron spectroscopy (XPS). Coating materials were TiO₂, Al₂O₃ and Cr₂O₃ and their mixtures. Water exposures were performed for free standing coating disks at mild electrolyte (1 mmol NaCl solution) at pH 4, 7 and 9. The exposure time was two weeks.It was observed that pure plasma sprayed TiO₂ material was chemically stable over whole experiment pH range and only slight surface hydroxylation was observed for this material.In case of plasma sprayed Al₂O₃ materials the surface O/Al ratio increased considerably during water exposure especially at exposure pH 7. This was probably result of surface conversion to hydrous form. No surface oxidation state changes were observed for this material.The non-exposed Cr₂O₃ materials contained both Cr(III) and Cr(VI) oxides. The water exposures increased the surface oxygen and Cr(VI) contents at the expense of Cr(III). The most probable reason for that was the dissolution of surface Cr(VI) oxide phase during water exposures and the (re)adsorption of dissolved Cr(VI) species back to the surface.     

Crystal field analysis of energy level structure of the Cr2O3 antiferromagnet

A detailed analysis of the energy level structure of the six-fold coordinated Cr³⁺ ion in the chromium oxide Cr₂O₃ is performed using the exchange charge model of the crystal field theory. Parameters of the crystal field acting on the Cr³⁺ optical electrons are calculated from the crystal structure data for the [CrO₆]⁹⁻ impurity center. The energy levels obtained are compared with the experimental absorption spectra for the considered crystal; a good agreement with experimental data is demonstrated. One possible explanation for the ultraviolet p₁ absorption band is proposed based on the results of crystal field calculations.     

Magnetic ordering in La0.75Sr0.25CrO3: A neutron diffraction study

Low-temperature neutron diffraction measurements were carried out on a powder sample of the compound La_0.75Sr_0.25CrO₃ in order to elucidate its magnetic structure. Rietveld analysis of the neutron diffraction data, as a function of temperature, showed that it possesses a G-type antiferromagnetic alignment of Cr spins at all temperatures below 300 K. Down to the lowest achievable temperature, viz. 17 K, the Cr site moments were found to be the weighted average of the 75% Cr³⁺ and 25% Cr⁴⁺ spin-only ionic moments. At 17 K, the Cr site moment was 2.71(5) μ_B/Cr ion. There is no observable change in the Cr–O bond lengths as a function of temperature. The tilt angles of the CrO₆ octahedra marginally increase with decreasing temperature.     

Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy

Divalent and trivalent chromium ions Cr²⁺ and Cr³⁺ replacing magnesium ions at octahedral positions in Mg₂SiO₄: Cr and Mg₂SiO₄: Cr: Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65–230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed-valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is \(P_{O_2 } \) = 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr²⁺ ions are distributed over the M1 and M2 positions in a ratio of approximately 2: 1. A change in the oxygen partial pressure \(P_{O_2 } \) and the chromium concentration, as well as an additional doping with lithium, does not lead to substantial changes in the distribution of divalent chromium ions over the positions. It is shown that an increase in the oxygen partial pressure \(P_{O_2 } \) from 0.01 to 2.00 kPa results in a decrease in the coefficient of divalent chromium distribution between the crystal and the melt. Doping with lithium also decreases the concentration of Cr²⁺ centers. In crystals grown without lithium, approximately half of the trivalent chromium ions are associated with magnesium vacancies. The addition of lithium leads to the destruction of these associates, an increase in the concentration of individual Cr³⁺ centers, and the formation of lithium associates with trivalent chromium ions. The conditions for the formation of associates of trivalent chromium ions with lithium ions are optimum when the crystal contains approximately identical amounts of Cr³⁺ and Li⁺ ions. Doping with lithium increases the concentration of Cr³⁺ ions and, thus, decreases the fraction of Cr²⁺ and Cr⁴⁺ ions in the total content of chromium centers.     

Luminescence of Cr ions associated with surpassing the green-emissive defect centers in β-Ga2O3

Photoluminescence of undoped and Cr³⁺-doped β-Ga₂O₃ was investigated. The transparent, undoped β-Ga₂O₃ film was successfully prepared by thermal conversion from GaOOH. The film exhibited predominant green luminescence in response to ultraviolet light excitation at 250 nm. This luminescence behavior, which was proposed to result from the oxygen defect centers, was used in examining excitation and emission mechanisms for Cr³⁺ ions doped in β-Ga₂O₃. It was found that red luminescence of Cr³⁺ surpasses green luminescence of the host lattice, as evidenced by the dependence of the spectral structure on the Cr³⁺ concentration. The excitation of Cr³⁺ was then suggested to be caused by the energy transfer from Ga³⁺O₆ octahedra present in the monoclinic β-Ga₂O₃ lattice.     

Energy transfer among three luminescent centers in full-color emitting ZnGa2O4:Mn, Cr phosphors

The ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors show three colors; the blue band of 380 nm from the charge transfer between Ga-O, the green band of 505 nm from Mn²⁺ and the red band of 705 nm from Cr³⁺. As a variation of Mn²⁺ or Cr³⁺ concentrations in ZnGa₂O₄:Mn²⁺, Cr³⁺, the relative emission intensity can be tuned. This phenomenon is explained in terms of the energy transfer based on four factors: the spectral overlap between the energy donors (Ga-O) and the energy accepters of Mn²⁺ or Cr³⁺, the absorption cross section of the energy accepters, the distance between them, and the decay time of the energy donors. ZnGa₂O₄:0.0025Mn²⁺, 0.010Cr³⁺ shows the CIE coordinates of x=0.4014, y=0.3368, which is a pure white light. The single-phased full-color emitting ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors can be applied to illumination devices.     

Investigation of thin TiO2 films cosputtered with Si species

Titanium dioxide (TiO₂) films were fabricated by cosputtering titanium (Ti) target and SiO₂ or Si slice with ion-beam-sputtering deposition (IBSD) technique and were postannealed at 450 °C for 6 h. The variations of oxygen bonding, which included high-binding-energy oxygen (HBO), bridging oxygen (BO), low-binding-energy oxygen (LBO), and three chemical states of titanium (Ti⁴⁺, Ti³⁺ and Ti²⁺) were analyzed by X-ray photoelectron spectroscopy (XPS). The enhancement of HBO and reduction of BO in O 1s spectra as functions of SiO₂ or Si amount in cosputtered film imply the formation of Si-O-Ti linkage. Corresponding increase of Ti³⁺ in Ti 2p spectra further confirmed the property modification of the cosputtered film resulting from the variation of the chemical bonding. An observed correlation between the chemical structure and optical properties, refractive index and extinction coefficient, of the SiO₂ or Si cosputtered films demonstrated that the change of chemical bonding in the film results in the modification of optical properties. Furthermore, it was found that the optical properties of the cosputtered films were strongly depended on the cosputtering targets. In case of the Si cosputtered films both the refractive indices and extinction coefficients were reduced after postannealing, however, the opposite trend was observed in SiO₂ cosputtered films.     

Mössbauer study of the iron atom state in modified chromium dioxide

Powders of modified chromium dioxide produced by the hydrothermal method were studied using ⁵⁷Fe Mössbauer spectroscopy at a temperature of 298 K. The content of the modifier, i.e., ⁵⁷Fe compound, was varied from 2 to 10 mmol/mol Cr at a Sb content of 2.2 and 10 mmol/mol Cr. It was shown that, independently of concentrations, Fe³⁺ ions are distributed between three magnetic solid solutions (sextets): based on CrO₂ (bulk material and iron-enriched surface layer), based on Cr₂O₃, and surface β-CrOOH (doublet). In this case, chromium atoms were not substituted in the CrSbO₄ nucleation (12 nm in size) phase with an accuracy up to the Mössbauer factor. It was assumed that the powder coercivity, in addition to the size factor, is controlled by the iron concentration in the CrO₂ surface layer.     

Spectroscopic characteristics of LiGaSiO4:Cr nano-glass-ceramics and glassy precursors

We have performed a complex of spectroscopic investigations (extinction, luminescence, and luminescence excitation spectra and luminescence decay kinetics) of LiGaSiO₄:Cr nano-glass-ceramics and Cr-Li-Ga-Si-O glassy precursors. It has been shown that 94–96% of chromium in precursors synthesized in air are in the trivalent form, while remaining chromium is in the form of Cr⁶⁺ and Cr⁴⁺. The luminescence of the latter form in precursors at 300 K is strongly quenched. In precursors synthesized in an inert atmosphere, only trivalent chromium occurs. We have calculated the absorption cross sections of Cr³⁺ and Cr⁶⁺ ions in Cr-Li-Ga-Si-O glassy precursors. It has been shown that, in the case of synthesis of these precursors, a considerable amount of chromium is lost as a result of evaporation and fails to enter the specimens. Upon partial controlled crystallization of the precursors (formation of LiGaSiO₄:Cr nano-glass-ceramics), the oxidation state of chromium that passed to crystallites from the glass phase becomes tetravalent. In this case, two types of luminescence centers arise, which correspond to Cr⁴⁺ ions localized in two different polymorphic modifications of LiGaSiO₄. Spectral characteristics of these two centers (bandshape, peak position, and spectral range) are almost identical, whereas luminescence lifetimes are radically different.     

Spectroscopic investigations of Cr, CrN and TiCr anti-multipactor coatings grown by cathodic-arc reactive evaporation

Cr, CrN, TiCr coatings have been investigated as potential anti-multipactor coatings. The coatings were synthesized by cathodic-arc reactive evaporation in Ar-N₂ atmosphere where the ion energy is controlled by substrate biasing. Chemical state analysis and surface composition were studied by X-ray photoemission spectroscopy (XPS), whereas bulk composition and depth profile were studied by glow discharge optical emission spectroscopy (GDOES). The surface morphology was studied by optical profilometry (OP) and scanning electron microscopy (SEM). The compositions of the coatings were CrN and Ti₄₀Cr₆₀ and they were homogeneous in depth. Surface oxidation was higher in Ti₄₀Cr₆₀ than in CrN. Coatings deposited at high negative bias show lower deposition rate and had lower surface roughness than those obtained at low bias. Secondary electron emission yield (SEY) was higher for CrN than for Ti₄₀Cr₆₀, both before and after low-energy Ar⁺ ion bombardment. The SEY of Ti₄₀Cr₆₀ (1.17 maximum) was clearly smaller than the others. The maximum yield, σ_m, and the first crossover electron energy, E₁, are the most important parameters, and (E₁/σ_m)^1/2 is a good figure of merit. This quantity was approximately 3 eV^1/2 for Cr and CrN and 4 eV^1/2 for Ti₄₀Cr. After Ar⁺ ion bombardment, the average value improved significantly to 8.9 eV^1/2 for Cr and CrN and 10.2 eV^1/2 for Ti₄₀Cr₆₀. The radio-frequency multipactor performance of these materials was simulated using the experimentally determined SEY parameters.     

Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers

The objective of this study was to identify a material suitable to absorb radiation at the wavelength of neodymium-doped Yttrium Aluminum Garnet (Y₃Al₅O₁₂:YAG), 1064 nm. M-(M= Sm³⁺, Co²⁺, Co³⁺, Cr³⁺, and Cr⁴⁺) doped highly transparent YAG ceramics were fabricated, and their absorption spectra were measured. Unlike Co²⁺ and Cr³⁺-doped ceramic samples, Co³⁺ and Cr⁴⁺ and Sm³⁺-doped:YAG ceramics were found to have significant absorption at 1064 nm. However, the Sm³⁺-doped YAG clearly emerged as the best candidate because it is also transparent at 808 nm, the pumping wavelength laser diode (LD), and also at most absorption bands used for flash-lamp pumping.     

Spectrométrie électronique (ESCA) appliquée à la caractérisation de dérivés oxygénés de chrome et de sodium,NaxCryOz.

A series of compounds of chromium and sodium (Na₂CrO₄, Na₃CrO₄, Na₄CrO₄, NaCrO₂) and for comparison Cr₂O₃ and chromium metal were examined by ESCA. The binding energy values for Cr 2p, Cr 3s, Cr 3p levels increase with the oxidation number of chromium for the Na_xCr_yO_z derivatives. The separation, Δ, between Cr 2p $\text{1}\text{2}$ and Cr 2p $\text{3}\text{2}$ increases with the number of unpaired 3d electrons. The splitting of the Cr 3s peak is observed for NaCrO₂ and Cr₂O₃. For the O 1s, Na 2s and Na 2p levels, the binding energies vary little in the series. Chromium is in a tetrahedral environment of oxygens in all the compounds Na_xCrO₄ (x = 2, 3, 4), as shown by IR absorption spectroscopy.     

Spectroscopic characteristic and energy levels of Cr in Cr:KAl(MoO4)2 crystal

This paper reports polarized spectral properties and energy levels of Cr³⁺ in KAl(MoO₄)₂ crystal. The absorption and emission cross sections are estimated as 3.72×10^-20 cm² at 669 nm and 2.74×10^-20 cm^-2 at 823 nm for σ-polarization, respectively. The energy levels of Cr³⁺ ion in KAl(MoO₄)₂ crystal were calculated based on the Tanabe-Sugano theory. It is suggested that Cr³⁺ ions occupy at an intermediate crystal field site in Cr³⁺:KAl(MoO₄)₂.     

六配位中铬离子不同价态的MS-Xα计算

本文采用自旋极化的MS-X_α方法,以铬离子与氧六配位形成的(CrO₆)铬离子为模型,计算铬离子三种价态(Cr²⁺,Cr³⁺,Cr⁴⁺的电子结构,在D_4k群下给出单电子轨道本征值和本征函数、能级分布和光学跃迁,对计算结果作了详细的讨论。 关键词：     

Characterization of Pd-CeOx interaction on α-Al2O3 support

The Pd-Ce interaction was studied over CeO₂ (0.3-2.5 wt.%)-Pd (1 wt.%)/α-Al₂O₃ catalysts used in the reforming reaction of CH₄ with CO₂. The samples were characterized by using high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The activity and selectivity behavior was in good agreement with that of other supported metal catalysts (Ni and Pd) modified with different promoters. The preliminary results of HRTEM would indicate that the CeO_x forms small crystallites around the Pd particle. The XPS analysis for the regions of Ce 3d and Pd 3d, gives an account of Ce being present mostly as Ce³⁺ and a high binding energy for Pd 3d_5/2 (335.3 eV), an evidence of Pd-Ce chemical interaction. The Pd/Al XPS intensity ratios vs. the Pd average particle size, determined by TEM, show an excellent correlation for fresh and used catalyst. These results indicate that the diminution of the Pd/Al ratios was due to Pd sintering. Consequently, the small amounts of CeO_x species do not cover the Pd particle, in agreement with the HRTEM results. The overall results stand for the promoter action mechanism of the CeO_x for the reforming reaction with CO₂.     

Optical spectrum, local lattice structure and EPR g factors for Cr impurity ions in MgTiO3 and LiTaO3

On the basis of the 120×120 complete energy matrices for a d³ configuration ion in a trigonal ligand field, for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃, the local structures and EPR g factors of the octahedral (CrO₆)⁹⁻ clusters have been studied, respectively. By simulating the calculated optical spectra and the EPR spectra data to the experimental results, local structure parameters are obtained. The calculated results show that although the local lattice structures around the M (M=Mg²⁺, Ta⁵⁺) ions are obviously different, after Cr³⁺ replacing the M, the local lattice structures around the Cr³⁺ ions are quite similar and close to those of the Cr₂O₃. This may be ascribed to the fact that the octahedral Cr³⁺ center in MgTiO₃:Cr³⁺ and LiTaO₃:Cr³⁺ systems and that in Cr₂O₃ exhibit similar octahedral (CrO₆)⁹⁻ clusters. Moreover, the corresponding theoretical values of the optical spectra have been reported. It is also found that the orbital reduction factor k is very important to understand the EPR g factors for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃.     

In situ Raman spectroscopy studies on chromium oxide catalyst in an anhydrous hydrogen fluoride atmosphere

Changes of different chromium oxide species [Cr(VI), Cr(V) and Cr(III)] were investigated using in situ Raman spectroscopy in an anhydrous hydrogen fluoride (AHF) atmosphere. It was found that the Cr(VI) (CrO₃) and Cr(V) (YCrO₄) species were easy to fluorinate, and generated CrO₂F₂ or/and CrOF₃, while the Cr(III) (Cr₂O₃) species was difficult to fluorinate. Moreover, the CrO₂F₂ and CrOF₃ species were stable in the AHF atmosphere, but they easily transform into CrO₃ in N₂ atmosphere and further decomposed to Cr₂O₃ at 400 °C. Moreover, Cl/F exchange reaction of CF₃CH₂Cl to CF₃CH₂F over these Cr species indicated that the high‐oxidation‐state CrO_xF_y (CrO₂F₂ and CrOF₃) species exhibited much higher reactivities than Cr₂O₃, suggesting that these species (CrO₂F₂ and CrOF₃) may be the active phases of the reaction. Copyright © 2010 John Wiley & Sons, Ltd.     

In situ sol-gel composition of multicomponent hybrid precursors to luminescent novel unexpected microrod of Y2SiO5:Eu employing different silicate sources

Y₂SiO₅ doped with Eu³⁺ were in situ synthesized by a hybrid precursor assembly sol-gel technology employing four different silicate sources, 3-aminopropyl-trimethoxysilane (APMS), 3-aminopropyl-triethoxysilane (APES), 3-aminopropyl-methyl-diethoxysilane (APMES) and tetraethoxysilane (TEOS), respectively. The SEM result shows that there exist some novel unexpected morphological microrod structures owing to using the crosslinking reagents other than TEOS as silicate source. The photoluminescent properties of Y₂SiO₅:Eu³⁺ have been studied as a function of Eu³⁺ doping concentration. A cross-relaxation process between identical Eu³⁺ ions results in the quenching of the ⁵D₁ emission for high concentration sample.