Upconversion luminescence of Er3+/Yb3+ co-doped nanolangasite synthesized by a modified Pechini route

Nanopowders of langasite (La₃Ga₅SiO₁₄) doped with 1 at.% Er³⁺ and 3 at.% Yb³⁺ were synthesized for the first time by a modified Pechini route and annealed in air at 700, 750, 800, 900, and 1,000?°C. The langasite powders were characterized by XRD, FTIR and luminescence techniques. Crystallization began at 750?°C and pure langasite phase was obtained for the samples annealed at 800 and 900?°C. Traces of LaGaO₃ and Ga₂O₃ were observed in the sample annealed at 1,000?°C. Bright green and red luminescence was observed for pumping at 973?nm whose intensity increased with annealing temperature due to the removal of the adsorbed impurities and the improvement of crystallinity.     

Laser-induced luminescence of model Fe/Al2O3 and Cr/Al2O3 catalysts

The laser-induced luminescence of Cr³⁺ impurity ions in model Fe/Al₂O₃ and Cr/Al₂O₃ catalysts with different calcination temperatures was studied. It was found that an additional luminescence band at 770 nm appeared in the luminescence spectra of low-temperature samples as a result of the interaction of octahedrally coordinated Cr³⁺ ions with Fe³⁺ impurity ions. In the θ-Al₂O₃ phase with a concentration of Cr³⁺ ions higher than 0.1 wt %, the interaction of the Cr³⁺-Cr³⁺ ion pairs in the immediate surroundings resulted in the appearance of N _θ lines due to the splitting of R _θ lines. The differences of these lines from the N _α lines of α-Al₂O₃ were related to the individuality of the crystal lattice of the θ phase and the coordination of Cr³⁺ impurity ions in the immediate surroundings, which is different from that in the α phase. Based on the laser-induced luminescence spectroscopic data, it was found that regions with a local Cr³⁺ concentration higher than the average Cr³⁺ concentration in the bulk of a catalyst by one order of magnitude were formed in the α-Al₂O₃-Fe₂O₃ system with the bulk Fe and Cr concentrations of 2.5 and 0.04 wt %, respectively, which was calcined at 1220°C, as a result of the diffusion of chromium and iron ions.     

Luminescence and long-lasting afterglow in Mn<Superscript>2+</Superscript> and Eu<Superscript>3+</Superscript> co-doped ZnO–GeO<Subscript>2</Subscript> glasses and glass ceramics prepared by sol–gel method

To develop new fluorescent and afterglow materials, Mn²⁺ and Eu³⁺ co-doped ZnO–GeO₂ glasses and glass ceramics were prepared by a sol–gel method and their optical properties were investigated by measuring luminescence, excitation and afterglow spectra, and luminescence quantum yield (QY). Under UV irradiation at 254 nm, some glasses and all of the glass ceramics showed green luminescence peaking at 534 nm due to the ⁴T₁ → ⁶A₁ transition of tetrahedrally coordinated Mn²⁺ ions. The strongest luminescence was observed in a glass ceramic of 0.1MnO–0.3Eu₂O₃–25ZnO–75GeO₂ heat treated at 900 °C, with QY of 49.8%. All of the green-luminescent glasses and glass ceramics showed green afterglow, and the afterglow lasting for more than 60 min was obtained in a glass ceramic heat treated at 900 °C. It is considered that the Eu³⁺ ions may behave as electron trapping centers to be associated with the occurrence of the green afterglow due to the Mn²⁺ ions in the co-doped system.     

The effect of active oxygen species in nano-ZnCr2O4 spinel oxides for methane catalytic combustion

The nano-ZnCr₂O₄ spinel oxides was synthesized by a ethylene glycol mediated solvothermal method. Catalytic combustion of methane test showed that an excellent activity over nano-ZnCr₂O₄ with T_10% = 300 °C and T_90% = 400 °C. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption-desorption measurements (BET) indicated that a uniform nano-ZnCr₂O₄ spinel oxides particles with the high surface area (96.2 m²g⁻¹) was successfully synthesized. Oxygen temperature programmed desorption (O₂-TPD) profile revealed there were two obvious desorption of oxygen species from nano-ZnCr₂O₄ in the range of 300–400 °C and 500–700 °C. It was clear that the desorption temperature range of the first oxygen species coincided with the methane catalytic combustion temperature. X-ray photoelectron spectroscopy (XPS) analysis exhibited that Cr⁶⁺ was present in the lattice of ZnCr₂O₄ apart from Cr³⁺. High valence cations of chromium in crystal lattice probable caused the presence of interstitial oxygen species in the structure to maintain the electroneutrality. Additionally, Raman spectra proved that there is the interstitial oxygen species in the crystal lattice of ZnCr₂O₄. Therefore, the excellent catalytic activity for methane combustion was contributed to the flexible interstitial oxygen in the ZnCr₂O₄.     

Les differentes formes cristallines de Y3ReO8: Relations avec la structure fluorine

The monoclinic Y₃ReO₈β phase is synthetized at 40 kbar and 900°C. The crystal structure is solved to an R value of 0.058 for 1341 observed reflections, by conventional methods using automated four-circle diffractometer data. This structure can be visualized as a fluorite-related superstructure, the large difference between the ionic radii of Re⁷⁺ and Y³⁺ being favorable to cation ordering. The heating to 500°C of this phase under atmospheric pressure leads to a disordered fluorite structure of the form Y₃ReO₈α. An intermediary metastable phase Y₃ReO₈α′ is obtained by heating a Y₃ReO₈β single crystal to 350°C. The structural relations between these three crystalline forms of Y₃ReO₈ and the fluorite structure are discussed.     

A novel spinel coating on cobalt‐based Superalloy GH605 via an in‐situ oxidation approach

Cr‐Mn‐O spinel coating was prepared on the surface of cobalt‐based superalloy GH605 via an in‐situ oxidation method in H₂O‐H₂ environment. The composition, morphology, and chemical value state of the oxide spinel coatings were investigated by SEM, EDS, XRD, Raman spectra, and XPS. It indicated that the morphology of coating varied with oxidation temperature, and granular surface appeared when oxidation temperature increased to 1100°C. The formed Cr‐Mn‐O spinel coating was composed of Cr₂O₃ and MnCr₂O₄, and the thickness increased significantly with oxidation temperature. In the coating, Cr element existed in the state of Cr³⁺ ions and Cr⁶⁺ ions, while Mn element only existed in the form of Mn²⁺ ions.     

Sr2(OLi2Sr4)[CrN4]2, ein Nitridochromat(VI)‐Oxid mit Sauerstoff in tetragonal‐bipyramidaler Koordination durch Lithium und Strontium

Sr₂(OLi₂Sr₄)[CrN₄]₂, a Nitridochromate(VI)‐Oxide with Oxygen in Tetragonal‐Bipyramidal Coordination by Lithium and Strontium Green gleaming crystals of Sr₂(OLi₂Sr₄)[CrN₄]₂ were prepared by reaction of Li, Sr and CrN/Cr₂N (approximate 1 : 1 mixture) with flowing nitrogen at 900 °C (molar overall composition Li : Sr : Cr = 6 : 1 : ∼3). The oxygen content results from a leak in the gas supply. The crystal structure was determined by single crystal methods (triclinic; P1; a = 615.87(9) pm, b = 682.50(10) pm, c = 754.30(8) pm, α = 82.302(14)°, β = 75.197(10)°, γ = 70.133(13)°; Z = 1) and contains distorted tetragonal bipyramids (OLi₂Sr₄)⁸⁺ and [Cr^VIN₄]^6–‐tetrahedra besides Sr²⁺.     

Beiträge zum thermischen Verhalten wasserfreier Phosphate. IX. Darstellung und Kristallstruktur von Cr6(P2O7)4. Ein gemischtvalentes Pyrophosphat mit zwei- und dreiwertigem Chrom

Contributions on the Thermal Behaviour of Anhydrous Phosphates. IX. Synthesis and Crystal Structure of Cr₆(P₂O₇)₄. A Pyrophosphate Containing Di- and Trivalent Chromium Cr₆(P₂O₇)₄ (Cr₂²⁺Cr₄³⁺(P₂O₇)₄) can be obtained reducing CrPO₄ by phosphorus (950°C, 48 h, 100 mg iodine as mineralizer). By means of chemical transport reactions (transport agent iodine; 1050 → 950°C) the compound has been separated from its neighbour phases (Cr₂P₂O₇, CrP₃O₉) and crystallized (greenish, transparent crystals; edge length up to 0.3 mm). The crystal structure of Cr₆(P₂O₇)₄ (Spcgrp.: P-1; z = 1; a = 4.7128(8) Å, b = 12.667(3) Å, c = 7.843(2) Å, α = 89.65(2)°, β = 92.02(2)°, γ = 90.37(2) has been solved and refined from single crystal data (2713 unique reflections, 194 parameter, R = 0.035). Cr²⁺ is surrounded by six oxygen atoms which occupy the corners of an elongated octahedron (4 × d^{Cr? O} ≈? 2.04 Å; 2 × d^{Cr? O} ≈? 2.62 Å). The Cr³⁺ ions are also coordinated octahedraly (1.930 Å ≤ d_{Cr? O} ≤ 2.061 Å). The crystallographically independent pyrophosphate groups show nearly eclipsed conformation. The bridging angles (P? O? P) are 136.5° and 138.9° respectively.     

Effect of annealing temperatures and of high content of the iron ion (Fe<Superscript>3+</Superscript>)-doping on transition anatase–rutile phase of nanocrystalline TiO<Subscript>2</Subscript> thin films prepared by sol–gel spin coating

Titanium dioxide doped with iron (III) was prepared by sol–gel Spin Coating method. The phase structures, morphologies, particle size of the doped TiO₂ have been characterized by X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM) and ultraviolet–visible (UV–Vis) spectrophotometer. The XRD and Raman results show that the 10% Fe³⁺-doped TiO₂ thin films crystallize in anatase phase between 600 and 800 °C, and into the anatase–rutile phase at 1,000 °C, and further into the rutile phase when the content of Fe³⁺ increases (20%). The grain size calculated from XRD patterns shows that the crystallinity of the obtained anatase particles increased from 39.4 to 43.4 nm as the temperature of annealing increase, whereas the size of rutile crystallites increases, with increasing Fe³⁺ concentrations from 36.9 to 38.1 nm. The AFM surface morphology results confirmed that the particle size increases by increasing the annealing temperature and also with an increasing of Fe³⁺ content. The optical band gap (E _g) of the films was determined by the UV–Vis spectrophotometer. We have found that the optical band gap decreased with an increasing of annealing temperatures and also with an increasing of Fe³⁺ content.     

Synthesis and properties of a solid solution formed in the CrVMoO7 - AlVMoO7 system

It has been shown by the methods of X-ray powder diffraction (XRD), differential thermal analysis (DTA) and infrared spectroscopy (IR) that solid solutions of a formula Cr_1−xAl_xVMoO₇, where xε (0−0.65), are formed in the system CrVMoO₇-AlVMoO₇. The obtained research results have proven that the ions Al3+ are incorporated into the crystal lattice of CrVMoO₇ instead of Cr³⁺, which causes a contraction of the lattice and a shift of IR absorption bands towards higher values of wavenumbers. The phases Cr_1−xAl_xVMoO₇ melt incongruently in the temperature range from 710°C (for x=0.65) to ∼820°C in the case of x close to zero This revised version was published online in July 2006 with corrections to the Cover Date.     

Homogeneity regions of neodymium,samarium, and europium manganites in air

XRD phase analysis of homogeneous phases and heterogeneous compositions of general formula Ln_2?x Mn_xO_3±δ (Ln = Nd, Sm, Eu; 0.90 ≤ x ≤ 1.20; Δx = 0.22) prepared by ceramic synthesis from oxides in air at 900–1400°C was used to determine the solubility boundaries for Ln₂O₃ oxides and maganese oxides in LnMnO_3±δ. The results were represented as fragments of the phase diagrams for the Ln-Mn-O systems in air. It was assumed that the solubility of Ln₂O₃ oxides in LnMnO_3±δ is determined by lattice defects, while that of manganese oxides, in addition to above mechanism, by the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺ followed by the partial substitution of divalent magnesium for Ln³⁺ at cuboctahedral positions of the perovskitelike crystal lattice.     

Chemical and structural transformations in manganese aluminum spinel of the composition Mn1.5Al1.5O4 during heating and cooling in air

Single phase cubic spinel of the composition Mn_1.5Al_1.5O₄ is synthesized. Its crystal structure refinement shows that 0.4Mn+0.6Al are in the octahedral sites and 0.7Mn+0.3Al are in the tetrahedral sites. High temperature X-ray diffraction is used to analyze Mn_1.5Al_1.5O₄ behavior during heating and cooling in air. In a temperature range of 600°C to 700°C, initial spinel splits into layers, and the sample represents a twophase system: cubic spinel Mn_0.4Al_2.4O₄ and a phase based on β-Mn₃O₄. Above 900°C the sample again turns into single phase cubic spinel. The role of oxidizing processes in the decomposition of Mn_1.5Al_1.5O₄ caused by oxygenation and partial oxidation of Mn²⁺ to Mn³⁺ is shown. A scheme of structural transformations of manganese aluminum spinel during heating from room temperature and cooling from 950°C is proposed.     

Effect of high-temperature treatment on the properties of an alumina-chromium catalyst for the dehydrogenation of lower paraffins

The crystal and pore structures of a microspherical alumina-chromium catalyst calcined at 800–1100°C were studied using a set of currently available physicochemical techniques (X-ray diffraction, lowtemperature nitrogen adsorption, diffuse reflectance UV-vis spectroscopy, Raman spectroscopy, and EPR spectroscopy); the state of its active component and the catalytic properties in isobutane dehydrogenation were examined. As the calcination temperature was increased from 800 to 900–1000°C, the properties of the catalyst were improved as a result of the formation of Cr₂O₃ clusters in an optimum amount and a decrease in the surface acidity of the catalyst due to the dehydroxylation and phase transformations of the aluminum oxide support. Calcination at 1100°C was accompanied by a decrease in the yield of isobutylene as a result of the formation of inactive macrocrystalline chromium (III) oxide and a chromium species inaccessible to reacting molecules; this chromium species was encapsulated in closed pores as the constituent of a solid solution of α-Al₂O₃-Cr₂O₃.     

Influence of annealing temperature on the electrochemical and surface properties of the 5-V spinel cathode material LiCr0.2Ni0.4Mn1.4O4 synthesized by a sol–gel technique

LiCr_0.2Ni_0.4Mn_1.4O₄ was synthesized by a sol–gel technique in which tartaric acid was used as oxide precursor. The synthesized powder was annealed at five different temperatures from 600 to 1,000 °C and tested as a 5-V cathode material in Li-ion batteries. The study shows that annealing at higher temperatures resulted in improved electrochemical performance, increased particle size, and a differentiated surface composition. Spinel powders synthesized at 900 °C had initial discharge capacities close to 130 mAh g^?1 at C and C/2 discharge rates. Powders synthesized at 1,000 °C showed capacity retention values higher than 85 % at C/2, C, and 2C rates at 25 °C after 50 cycles. Annealing at 600–800 °C resulted in formation of spinel particles smaller than 200 nm, while almost micron-sized particles were obtained at 900–1,000 °C. Chromium deficiency was detected at the surface of the active materials annealed at low temperatures. The XPS results indicate presence of Cr⁶⁺ impurity when the annealing temperature was not high enough. The study revealed that increased annealing temperature is beneficial for both improved electrochemical performance of LiCr_0.2Ni_0.4Mn_1.4O₄ and for avoiding formation of Cr⁶⁺ impurity on its surface.     

ChemInform Abstract: Temperature Induced Phase Transition of CaMn0.5Zr1.5(PO4)3 Phosphate.

In view of a known structural phase transition at 800—875 °C and the by 10 times increased luminescence of Mn²⁺ in the high‐temperature phase, low‐ (LT) and high‐temperature (HT) polymorphs of CaMn_0.5Zr_1.5(PO₄)₃ are prepared by sol—gel reaction of Mn(O‐Ac)₂, Ca(NO₃)₂, ZrOCl₂, and NH₄H₂PO₄ in ethylene glycol followed by a final annealing (700 or 900 °C, 20 h, resp.).     

Synthesis and Characterization of Nanostructured Fe3O4 Micron‐Spheres and Their Application in Removing Toxic Cr Ions from Polluted Water

We present a simple and effective method for the synthesis of nanostructured Fe₃O₄ micron‐spheres (NFMSs) by annealing hydrothermally formed FeCO₃ spheres in argon. The phase structure, particle size, and magnetic properties of the product have been characterized by X‐ray diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR), X‐ray photoelectron spectroscopy (XPS), field‐emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and by means of a superconducting quantum interference device (SQUID). The results have shown that the as‐obtained NFMSs have a diameter of about 5 μm and are composed of nanometer‐sized porous lamellae. The NFMSs have a large specific surface area (135.9 m² g^?1), reductive Fe²⁺ incorporated into their structure, and intense magnetic properties. These properties suggest that NFMSs have potential application in removing toxic Cr⁶⁺ ions from polluted water. At 25 °C, each gram of NFMSs product can remove 43.48 mg of Cr⁶⁺ ions, as compared to just 10.2 mg for nanometer‐sized Fe₃O₄ and 1.89 mg for micron‐sized Fe₃O₄. The enhanced removal performance can be ascribed to the structural features. Moreover, the Cr⁶⁺ ion removal capacity of the NFMSs can reach up to 71.2 mg g^?1 at 50 °C. The influencing parameters in the removal of Cr⁶⁺ ions, such as contact time, pH, and temperature, have been evaluated. The Cr⁶⁺‐removal mechanism has been investigated. We have found that the NFMSs product not only serves as an effective adsorbent to remove toxic Cr⁶⁺ ions from polluted water, but also as an effective reductant in reducing the adsorbed toxic Cr⁶⁺ ions to much less toxic Cr³⁺ through the Fe²⁺ incorporated into its structure.     

The obtaining of NiCr2O4 nanoparticles by unconventional synthesis methods

This paper presents a study regarding the obtaining of NiCr₂O₄ by two new unconventional synthesis methods: (i) the first method is based on the formation of Cr(III) and Ni(II) carboxylate-type precursors in the redox reaction between the nitrate ion and 1,3-propanediol. The thermal decomposition of these complex combinations, at ~300 °C, leads to an oxide mixture of Cr₂O_3+x and NiO, with advanced homogeneity, small particles and high reactivity. On heating this mixture at 500 °C, Cr₂O₃ reacts with NiO to form NiCr₂O₄, which was evidenced by FT-IR and X-ray diffractometry (XRD) analysis; (ii) the second method starts from a mechanical mixture of (NH₄)₂Cr₂O₇ and Ni(NO₃)₂·6H₂O. On heating this mixture, a violent decomposition at 240 °C with formation of an oxides mixture (Cr₂O₃ + CrO₃) and NiO takes place. On thermal treatment up to 500 °C, an intermediary phase NiCrO₄ is formed, which by decomposition at ~700 °C leads to NiCr₂O₄, evidenced by FT-IR and XRD analysis. NiCr₂O₄ is formed, in both cases, starting with a temperature higher than 400 °C, when the non-stoichiometric chromium oxide (Cr₂O_3+x ) loses the oxygen excess and turns to stoichiometric chromium oxide (Cr₂O₃), which further reacts with NiO.     

Studies regarding the formation from metal nitrates and diol of NiM 2 III O4 spinels, inside a silica matrix

The present study deals with preparation and characterization of spinel mixed oxide systems NiM ₂ ^III O₄, where M^III?=?Fe^III, Cr^III. In order to obtain 50% NiFe₂O₄/50% SiO₂ and 50% NiCr₂O₄/50% SiO₂ nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO₂ matrix, of some particular precursors, coordination compounds of the involved M^II and M^III cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140?°C of the gels (tetraethylorthosilicate?Cmetal nitrates?C1,3-propanediol?Cwater). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350?°C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe₂O₄ starting with 600?°C, and NiCr₂O₄ starting with 400?°C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300?°C, the spinelic phase ??-Fe₂O₃ is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr₂O_3+x ) is formed. In the range 380?C400?°C, Cr₂O_3+x turns into Cr₂O₃ which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.     

Darstellung,Kristallstrukturen und Eigenschaften der Chrom(II)-phosphat-halogenide Cr2(PO4)Br und Cr2(PO4)I

Synthesis, Crystal Structures, and Properties of the Chromium(II) Phosphate Halides Cr₂(PO₄)Br and Cr₂(PO₄)I The new compounds Cr₂(PO₄)Br and Cr₂(PO₄)I have been obtained by reaction of CrPO₄, Cr and Br₂ or I₂ in evacuated silica tubes at elevated temperatures (Cr₂(PO₄)Br: 900 °C, Cr₂(PO₄)I: 700 °C). Single crystals of deep blue Cr₂(PO₄)Br and turquoise Cr₂(PO₄)I with edge-lengths up to 2 mm and 0.3 mm, respectively, have been grown in experiments involving the gaseous phase. Single crystal data have been used for structure determination and refinement. Though being not isotypic, the two crystal structures are closely related. Two crystallographically independent Cr²⁺, in polyhedra [Cr1O₃X₃] and [Cr2O₅X], form dimers [Cr1₂O₂O_2/2X₄] and [Cr2₂O₈X₂]. Distances are 1.978 Å ≤ d(Cr–O) ≤ 2.096 Å (for the iodide: 1.959 Å ≤ d(Cr–O) ≤ 2.105 Å), 2.587 Å ≤ d(Cr–Br) ≤ 3.158 Å and 2.867 Å ≤ d(Cr–I) ≤ 3.327 Å. The structures of bromide and iodide can be distinguished by the different way of connection of the Cr1 containing dimers. The phosphate group shows slightly distorted tetrahedral geometry with 1.491 Å ≤ d(P–O) ≤ 1.559 Å (1.486 Å ≤ d(P–O) ≤ 1.567 Å) and angles of 106.48° ≤ ∠(O–P–O) ≤ 111.69° (106.57° ≤ ∠(O–P–O) ≤ 111.72°. IR-spectra of Cr₂(PO₄)Br and Cr₂(PO₄)I, the Raman-spectrum of Cr₂(PO₄)Br and electronic spectra of the two compounds in the UV/vis region at low temperature are reported and discussed.     

Octahedron-dependent near-infrared luminescence in Cr3+-activated phosphors

Searching for broadband near-infrared (NIR) materials with high efficiency and excellent thermal luminescence stability is of great significance because of their widespread spectroscopic applications. Different element substitution can modulate the structure and crystal field of host lattice so as to regulate the luminescent properties. Herein, we report the octahedron-dependent NIR luminescence in Cr³⁺-doped KMP₂O₇ (M = Ga, Sc, In, and Lu) phosphors and investigate the effect of octahedral environment on luminescent properties, aiming to provide guidance for host material selection. The decreased crystal field strength leads to the apparent spectral red shift from 815 to 900 nm for the samples of M = Ga to Lu. The small Stokes shift as well as weak electron–phonon coupling effect decreases the non-radiative transition probability and thus gives rise to the highest emission intensity and excellent thermal stability of Cr³⁺-doped KGaP₂O₇. The optimal sample, KGa_0.89P₂O₇:0.11Cr³⁺, possesses an internal/external quantum efficiency of 55.8%/36.6%, and its integrated emission intensity at 423 K can maintain 68% of that at room temperature. Finally, we investigate the potential applications in non-destructive examination field by manufacturing a NIR phosphor-conversion light-emitting diode device.