Structural and luminescence characterization of synthetic Cr-doped Ni3(PO4)2

Ni_3–xCr_2x/3(PO₄)₂ (x=0 and 0.02) microcrystalline powders were obtained as single phases via a modified sol–gel Pechini-type in situ polymerizable complex method. The samples were characterized using scanning electron microscopy, X-ray diffraction, cathodoluminescence (CL), and thermoluminescence (TL) techniques. We found that Cr³⁺ doping modified the average particle and distribution. The mean particle size was 0.441 μm for Ni₃(PO₄)₂ and 0.267 μm for Ni_2.98Cr_0.013(PO₄)₂. The results also reveal that Cr³⁺ doping notably enhanced the CL and TL UV-blue emission.     

Synthesis,structure and magnetic properties of layered manganate Sr4Mn3−xCrxO10 (x=0, 0.2)

The manganates Sr₄Mn_3−xCr_xO₁₀ (x=0 and 0.2) have been synthesized by solid state reaction. Powder X-ray diffraction analysis shows orthorhombic symmetry with space group Cmca for both compounds. The magnetic susceptibility measurements show an antiferromagnetic transition at 192 and 176 K for x=0 and 0.2, respectively. The magnetic susceptibility data were estimated using a model based on spin exchange antiferromagnetic interactions in isolated (Mn⁴⁺) trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials.     

Site preference and magnetism of Fe3−xCrxAl0.5Si0.5

In order to gain better insight into the origin of the observed differences between Fe_3−xCr_xAl and Fe_3−xCr_xSi, alloys of Fe_3−xCr_xAl_0.5Si_0.5 (x=0, 0.125, 0.250, 0.375 and 0.5) were prepared and studied by means of X-ray and neutron diffraction as well as by magnetization measurements. Electronic structure calculations of these alloys have been performed by means of TB-LMTO-ASA method. It was expected, and experimentally verified, that the presence of silicon and aluminum atoms in 1:1 proportion will result in the independence of the lattice parameter on the iron/chromium concentration. All samples have been proved to be a single phase of the DO₃-type of structure. Theoretical and experimental results indicate that chromium atoms locate preferentially in B sublattice. Cr magnetic moments are oriented antiparallel to Fe magnetic moments. Neutron measurements show a linear dependence of the magnetic moments of Fe(A,C), Fe(B) and Cr(B) as a function of Cr concentration. However the calculated total magnetic moment decreases faster with chromium content than indicated by the experiment.     

Characterization of Cr/SiO2 catalysts and ethylene polymerization by XPS

Cr/SiO₂ catalysts with 1 or 3 wt.% Cr loadings and different chromium precursors were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A method to determine chromium species in the sample was developed through the decomposition of the Cr 2p XPS spectrum in Cr⁶⁺ and Cr³⁺ standard spectra. The results of the binding energy from the Cr 2p region and of the distribution of chromium species allowed to evaluate the dynamic photo-reduction of the surface chromium species during XPS analysis. Photo-reduction of surface Cr⁶⁺ to Cr³⁺ species was verified for all samples supported in silica, depending on the precursor and chromium content. Bulk CrO₃ and Cr₂O₃ standards did not reveal variation in the binding energy of Cr 2p_3/2, but a physical mixture of CrO₃ with SiO₂ presented photo-reduction. The behavior of this mixture resembled to the catalysts and suggests the participation of the surface hydroxyls of silica in the photo-reduction process. XPS intensity measurements for assessing dispersion of chromium oxide were used to compare the calcined and reduced catalysts to different chromium precursors. Polyethylene chains were detected by in situ XPS, while oligomerization products were not observed.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Structure and surface characterization of ZrO2-Y2O3-Cr2O3 system

3-mol% Y₂O₃ and 0.3 to 3-mol % Cr₂O₃ co-doped ZrO₂ nanopowders were synthesized using co-precipitation technique and investigated by terms of X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Structural analysis shows no significant impact of chromium on powders structure except of presence of small amount of m-phase. Surface analysis reveals segregation of yttrium and chromium atoms to the surface along with surface enrichment by oxygen that can be attributed to residual water. Chromium surface atoms present in three oxidation states with catalytically active Cr²⁺ sites possibly controlling m-phase appearance through lattice distortion.     

Site occupancy and magnetic study of Al and Cr co-substituted Y3Fe5O12

Single-phased polycrystalline Y₃Fe_5−2xAl_xCr_xO₁₂ garnet samples (x=0, 0.2, 0.4 and 0.6) have been prepared by the conventional ceramic technique. Rietveld refinement of X-ray diffraction patterns of the samples shows them to crystallize in the Ia3d space group and the corresponding lattice constant to decrease with increasing Al³⁺ and Cr³⁺ contents (x). Mössbauer results indicate that Cr³⁺ substitutes for Fe³⁺ at the octahedral sites whilst Al³⁺ essentially replaces Fe³⁺ at the tetrahedral sites. This result indicates that co-doping of Y₃Fe₅O₁₂ does not affect the preferential site occupancy for separate individual substitution of either Cr³⁺ or Al³⁺. The magnetization measurements reveal that the Curie temperature (T_c) monotonically decreases with increasing x while the magnetic moment per unit formula decreases up to x=0.4 and then slightly increases for x=0.6. This reflects a progressive weakening of the ferrimagnetic exchange interaction between the Fe³⁺ ions at octahedral and tetrahedral sites due to co-substitution. The magnetic moment was calculated using the cations distribution inferred from the Mössbauer data and the collinear ferrimagnetic model, and was found to agree reasonably with the experimentally measured value. The phenomenological amplitude crossover, characterized by the temperature T*, has also been observed in the doped YIG and briefly discussed.     

EPR investigation of the trivalent chromium complexes in SrTiO3

The trivalent chromium centers were investigated by means of electron paramagnetic resonance (EPR) in SrTiO₃ single crystals grown using the Verneuil technique. It was shown that the charge compensation of the Cr³⁺-V_O dominant centers in octahedral environment is due to the remote oxygen vacancy located on the axial axis of the center. In order to provide insight into spin-phonon relaxation processes the studies of axial distortion of Cr³⁺-V_O centers have been performed as function of temperature. The analysis of the trigonal Cr³⁺ centers found in SrTiO₃ indicates the presence of the nearest-neighbor strontium vacancy. The next-nearest-neighbor exchange-coupled pairs of Cr³⁺ in SrTiO₃ has been analyzed from the angular variation of the total electron spin of S=2 resonance lines.     

Phase relationships and magnetic phase diagram in the system Mn1−xCuxCr2S4

Ternary spinel compounds have been found in the system Mn_1?xCu_xCr₂S₄ for x < 0.4 and x ? 0.8. The unit cell parameter does not follow a Vegard law. Magnetization measurements up to 150 kOe for Mn_0.95Cu_0.05Cr₂S₄ show three magnetic structures: Néel ferrimagnet, Yafet-Kittel and tridimensional. The magnetic phase diagram has been calculated within the molecular field approximation. There is strong evidence for the non-existence of Mn³⁺. Our findings are in conformity with a model proposed by Goodenough.     

Magnetic and electrical properties of amorphous Ge1−xCrx thin films grown by low temperature vapor deposition

Amorphous Ge_1−xCr_x thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10⁻³–0.96×10² Ω cm at 300 K, and decrease with Cr concentration. The Ge_1−xCr_x thin films are p-type. The hole concentrations are 5×10¹⁶–7×10²¹ cm⁻³ at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm³ at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge_1−xCr_x thin films are paramagnetic.     

Antiferromagnetism and spin-glass transition in the FeInxCr2−xSe4 series of selenides

The magnetic behavior of the FeIn_xCr_2−xSe₄ system (with x=0.0, 0.2 and 0.4) has been investigated by magnetic and Mössbauer spectroscopy. Hyperfine parameters indicate that iron is in the Fe²⁺ oxidation state, with a minor (∼9%) Fe³⁺ fraction, located at different layers in the structure. Low-field magnetization curves as a function of temperature showed that the antiferromagnetic (AFM) order temperature is T_N=208(2) K for FeCr₂Se₄ and decreases to 174(3) K for FeIn_0.4Cr_1.6Se₄. The effective magnetic moment μ_eff decreases with increasing In contents, and shows agreement with the expected values from the contribution of Fe²⁺ (⁵D) and Cr³⁺ (⁴F) electronic states. A second, low-temperature transition is observed at T_G∼13 K, which has been assigned to the onset of a glassy state.     

A XPS study of Fe1-χCuχCr2S4

The X-ray photoelectron spectroscopy is applied for a chalcogen chromite system Fe_{1 ? x}Cu_xCr₂S₄ to obtain microscopic information about valence states of the constituting atoms. The features of the Cu 2p spectra show that copper is always in a monovalent state in this system. This supports a model Fe³⁺_{1 ? x}Cu¹⁺_xCr³⁺₂S^1-_{2x ? 1}S^2-_{5 ? 2x} for a compositional range 0.5 < x < 1.     

Cr-doping effect on the structural, magnetic, transport properties and Raman spectroscopy of La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 perovskites

A series of the double-doping samples La_(2+x)/3Sr_(1−4x)/3Mn_1−xCr_xO₃ (0?x?0.25) with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been fabricated. The structural, magnetic, transport properties and Raman spectroscopy have been investigated, and no apparent crystal structure change is introduced by Cr doping up to x=0.25. But the Curie temperature T_C and metal-insulator transition temperature T_MI are strongly affected by Cr substitution. The room temperature Raman spectra start exhibiting some new features following the increasing concentration of Cr substitutions. Moreover, it is worth noting that the frequency of the A_1g phonon mode can also be well correlated with the A-site mismatch effect (σ²), which is influenced mainly by the variety of the Sr content.     

Effects of Cr doping in electron-doped manganites La0.9Te0.1MnO3

The effects of Cr doping on Mn sites in the electron-doped manganites La_0.9Te_0.1MnO₃ have been studied by preparing the series La_0.9Te_0.1Mn_1−xCr_xO₃ (0.05≤x≤0.20). Upon Cr doping, both the Curie temperature T_C and magnetization M are suppressed. The resistivity measurements indicate that there exists a weak metal-insulator (M-I) transition for the sample with x=0.05, with an increase in the doping level, the M-I transition disappears and the resistivity increases. Thermopower S(T) exhibits a maximum near T_C for all samples. By fitting the S(T) and ρ(T) curves, it is found that the temperature dependences of both S(T) and ρ(T) in the high temperature paramagnetic (PM) region follow the small polaron conduction (SPC) mechanism for all samples. The fitting parameters obtained imply changes of both the average-hopping distance of the polarons and the polaron concentration with Cr doping in our studied samples. In the case of the thermal conductivity κ(T), the variation of κ(T) is analyzed based on the combined effects due to the suppression of the local Mn³⁺O₆ Jahn-Teller (JT) lattice distortion because of the substitution of Cr³⁺ for Mn³⁺ ions, which results in the increase in κ, and the introduction of the disorder due to Cr-doping, which contributes to the decrease in κ.     

Effects of Cr doping in bilayered manganite LaSr2Mn2O7: Resistivity, thermoelectric power, and thermal conductivity

Systematic studies of resistivity, thermoelectric power, and thermal conductivity have been performed on polycrystalline bilayered manganites LaSr₂Mn_2−xCr_xO₇ (0≤x≤0.2). It is found that the temperature dependence of both Seebeck coefficient S(T) and resistivity ρ(T) in the high temperature region follows the small polaron transport mechanism for all the samples. But in the low temperature region, variable-range-hopping (VRH) model matches the experimental data better. In addition, the maximum of absolute S(T) at low temperatures is gradually suppressed for the sample with Cr-doping level of x>0.04, implying that a new FM order probably arises. With decreasing the temperatures further, S(T) has a sign change and becomes positive for the sample with Cr-doping level of x>0.04, indicating that there may occur a variation of the type of charge carrier. As to thermal conduction κ(T), the low-temperature peak is suppressed due to Cr-doping. The variation of κ(T) is analyzed based on the combined effect due to the suppression of local Mn³⁺O₆ Jahn-Teller (JT) lattice distortion because of the substitution of Cr³⁺ ions for Mn³⁺ ions, which results in the increase in thermal conduction, and the introduction of the disorder due to Cr-doping, which contributes to the decrease in thermal conduction.     

La solution solide Cr1−xRhxO2

A new solid solution Cr_1?x,Rh_xO₂ (0?x?1) has been prepared using oxidizing acids or ammonium perchlorate under pressure. The magnetic study suggests that chromium (IV) and rhodium (IV) are present in the structure at x ? 0.2 and chromium (V) and rhodium (III) at 0.2 < x ? 0.5. Forx > 0.5 increasing rhodium (IV) content gives rise to properties similar to those of RhO₂.     

Synthesis and characterization of hexakis(acetonitrile)chromium(III) tetrafluoroborate, [Cr(NCMe)6][BF4]3. A nonaqueous Cr source

Oxidation of [Cr^II(NCMe)₄][BF₄]₂ with thianthrinium tetrafluoroborate forms [Cr^III(NCMe)₆][BF₄]₃ exhibiting two ν_CN absorptions at 2331 and 2301 cm⁻¹, and has been structurally characterized with an average Cr-N distance of 1.999 Å. From the electronic absorption spectra the ligand field splitting, Δ₀, is 20,160 cm⁻¹, which is slightly larger than [Cr^III(OH₂)₆]³⁺ in accord with the divalent chromium analogues. The 298 K ESR has a resonance at g=1.9884, and the magnetic susceptibility has a 300 K moment of 3.85μ_B characteristic of S=3/2 Cr(III). The field dependence of the magnetization can be fit to the Brillouin function also characteristic of S=3/2.     

Room temperature magnetoresistance in Ln2/3A1/3MnO3 manganites

The investigation of the manganites La_2/3−xPr_xSr_1/3MnO₃, La_2/3Sr_1/3−xCa_xMnO₃ and La_2/3+xCa_1/3−2xAg_xMnO₃, which all exhibit Mn³⁺:Mn⁴⁺=2, shows that it is possible to reach high magnetoresistance at room temperature, up to 21% under 1.2 T. These materials are compared to La_5/6Ag_1/6MnO₃ which corresponds to the same Mn³⁺:Mn⁴⁺ ratio and exhibits a magnetoresistance of 25% in this field. An interesting feature deals with the value of the insulator-metal transition temperature T_IM, often higher than T_C, especially for Ag-based compounds. It is suggested that the latter results either from a better oxygenation of the surface of the grains or from a migration of silver toward the surface.     

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.     

On the mixed valence of the chromium ions in the noncollinear ferrimagnetic spinels Mn1−xCuxCr2S4 (Where x = 0.85, 0.90, 0.95)

For the polycrystalline samples of Mn_1?xCu_xCr₂S₄ (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, T_C, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between T_C and about 600 K. The Curie temperatures were obtained using the kink point method.In the temperature range between 4.2 and 77 K the magnetization was measured in stationary magnetic fields up to 14 T. The data indicate a noncollinear ferrimagnetic structure. The compounds under investigation can be treated as CuCr₂S₄ slightly doped with Mn, with a valence distribution Mn²⁺_1?xCu¹⁺_xCr³⁺_2?xCr⁴⁺_xS^2?₄.