Structural, spectroscopic characterization and EPR studies of tetramethylammonium-hydrogen fumarate-fumaric acid complex

The tetramethylammonium-hydrogen fumarate-fumaric acid (hereafter, [TMAHF-FA]) complex was synthesized and characterized by spectroscopic (IR), structural (XRD) and electron paramagnetic resonance (EPR) techniques. In the crystal structure, tetramethylammonium cation lies on a mirror plane of the space group P2₁/m. Crystal structure analysis reveals that there is a large degree of proton sharing between the fumaric acid and hydrogen fumarate, with the H atom lying almost symmetrically between the donor and acceptor sites, as evidenced by the long O-H and short H?O distances [1.19(3) Å, 1.26(3) Å], respectively. γ-Irradiation damage centers in [TMAHF-FA] single crystal have been investigated by EPR at room temperature. The spectra indicated the existence of radical. The EPR spectra recorded in the three mutually perpendicular planes have shown two magnetically distinct paramagnetic sites in monoclinic lattice. The principal values of g and hyperfine constants for both sites were calculated. IR spectrum was resolved and transitions were assigned based on the molecular structure.     

Characterization of nanosized TiO2 synthesized inside a porous glass-ceramic monolith by metallo-organic decomposition process

This work reports the preparation of TiO₂ by decomposition of a metallo-organic precursor (MOD process) in the pores of an α-NbPO₅ glass-ceramic monolith (PGC-NbP) and the study of the TiO₂ anatase-rutile transition phase. The impregnation of titanium di-(propoxy)-di-(2-ethylhexanoate) in the PGC-NbP was confirmed by diffuse reflectance infrared spectroscopy. In the restrictive porous environment the decomposition of the metallo-organic compound exhibits a lower initial decomposition temperature but a higher final decomposition temperature, in comparison to the free precursor. The pure TiO₂ rutile phase is formed only above 700 °C when the titanium precursor is decomposed outside the pores. The TiO₂ anatase obtained inside the PGC-NbP was stabilized up to 750 °C and exhibits a smaller average crystallite size in comparison with the MOD process performed without PGC-NbP. Furthemore, the temperature of the TiO₂ anatase-rutile transformation depends on crystallite size, which was provided by XRD and Raman spectroscopy. The precursor impregnation-decomposition cycle revealed a linear mass increment inside PGC-NbP. Micro-Raman spectroscopy shows the presence of a gradient concentration of the TiO₂ inside the PGC-NbP. The use of the MOD process in the PGC-NbP pores has several advantages: control of the amount and the nature of the phase formed and preservation of the pore structure of PGC-NbP for subsequent treatments and reactions.     

Structural and vibrational studies of anilinium nitrate

The crystals of anilinium nitrate, , were obtained by slow evaporation of an aqueous solution. The crystals belong to the Pbca (no. 61) space group of orthorhombic system, Z=8, a=10.158(2), b=9.277(2), c=16.177(3) Å. Positively charged anilinium cations and anions are present in the structure. Powder FT IR and FT Raman spectra for normal and deuterated samples are discussed with respect to the crystal structure. DSC measurements do not indicate clearly on the occurrence of phase transition in the temperature region 113-293 K.     

Synthesis and structural characterization of a series of lanthanide stannate pyrochlores

A simple hydrothermal method has been employed to prepare a series of lanthanide stannate pyrochlores Ln₂Sn₂O₇ (Ln=Y, La, Pr-Yb) at a relatively low temperature of less than 200 °C successfully. On the basis of structural characterizations by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) absorption spectroscopy and Raman spectroscopy, it was found that the positions of bands in vibrational spectra are sensitive to the ionic radius of Ln³⁺, and the linear relationship can be seen between the frequency of Sn-O stretching mode and the lanthanide ionic radius in IR spectrum, as well as the frequency of O-Sn-O bending mode and the lanthanide ionic radius in Raman spectrum.     

Solvothermal synthesis and characterization of CdSe nanocrystals with controllable phase and morphology

Zinc blende (ZB) CdSe hollow nanospheres were solvothermally synthesized from the reaction of Cd(NO₃)₂·4H₂O with a homogeneously secondary Se source, which was first prepared by dissolving Se powder in the mixture of ethanol and oleic acid at 205 °C. As Se power directly reacted with Cd(NO₃)₂·4H₂O in the above mixed solvents, wurtzite (W) CdSe solid nanoparticles were produced. Time-dependent experiments suggested that the formation of CdSe hollow nanospheres was attributed to an inside-out Ostwald ripening process. The influences of reaction time, temperature and ethanol/oleic acid volume ratio on the morphology, phase and size of the hollow nanospheres were also studied. Infrared (IR) spectroscopy investigations revealed that oleic acid with long alkene chains behaved as a reducing agent to reduce Se powder to Se²⁻ in the synthesis. Photoluminescence (PL) measurements showed that the ZB CdSe hollow nanospheres presented an obvious blue-shifted emission by 42 nm, and the W CdSe solid nanoparticles exhibited a band gap emission of bulk counterpart.     

Effect of co-dopant on the formation and properties of anatase-type titania solid solutions doped with niobium

The effect of co-dopant M (M=gallium (Ga), aluminum (Al), and scandium (Sc)) on the formation, crystallite growth, optical band gap, photocatalytic activity, and phase stability of anatase-type titanium dioxide solid solutions (Ti_1−2XNb_XM_XO₂) containing the same amount of dopant niobium (Nb) that were directly formed as nanoparticles under mild hydrothermal conditions at 180 °C for 5 h was investigated. The composition range X of the anatase-type solid solutions (Ti_1−2XNb_XM_XO₂) depended on the co-dopant M, i.e., X=0.15-0.20 for M=Ga and Al, and X=0.33 for M=Sc. A remarkable increase in the lattice parameter c₀ was detected in the solid solutions co-doped with M=Sc. The increase in the amount of co-dopant M=Ga and Al enhanced the crystallite growth of the anatase-type solid solutions under the hydrothermal conditions. The photocatalytic activity of the solid solutions (Ti_0.80Nb_0.10M_0.10O₂) co-doped with M=Sc, Ga, and Al increased in that order. The co-dopant M=Ga promoted the anatase-to-rutile phase transformation of the solid solutions at lower temperature.     

The lithium- and sodium-enhanced transformation of Ba-exchanged zeolite LTA into celsian phase

The Li⁺- and Na⁺-doped hexacelsians (HC) synthesized from Ba-LTA synthetic zeolite as precursors were used for preparation of monoclinic celsians (MC). The doped pure MC and mixture HC/MC species were obtained by thermally induced polymorphous transformation at 1200 °C. Synthesized-doped MC have been characterized by X-ray powder diffraction and spectroscopic (infrared, Raman and ²⁹Si magic angle spinning nuclear magnetic resonance) methods. The obtained results suggest that in all investigated samples Na⁺ or Li⁺ dopants were incorporated in MC crystal structures during thermal transformation of a zeolite. It has been shown that HC→MC transformation depends on molar fractions of alkali cations. These findings combined with used temperature/time conditions could help in optimization of MC synthesis route.     

Influence of pH on microstructural and optical properties of electrosynthesized CdSe thin films

Cadmium selenide (CdSe) thin films were electrosynthesized onto well cleaned stainless steel and fluorine-doped tin oxide (FTO) coated glass (10-15 Ω/cm²) substrates at different pH of the electrolytic solution. X-ray diffraction study reveals a cubic structure with preferential orientation along the (1 1 1) direction. The structural parameters such as grain size (D), lattice constant (a), strain (ε), dislocation density (δ), average internal stress (S) and degree of preferred orientation (I₁₁₀/I₂₂₀) in the film are calculated and they are found to be dependent on the pH of the depositing bath. EDAX analysis confirms nearly stoichiometric composition of the film deposited at pH 2.70. AFM analysis shows uniform deposition of the film over the entire substrate surface. In optical studies, the transition of the deposited film is found to be a direct allowed transition. The optical energy gaps are found to be in the range from 1.87 to 2.04 eV depending on the pH of the depositing bath. Photoluminescence (PL) spectrum shows blue shift in PL peak position and reduction in luminescence intensity for the film deposited at pH other than 2.70.     

Production of Sr-deficient bismuth tantalates from microwave-hydrothermal derived precursors: Structural and dielectric properties

Strontium bismuth tantalates were produced for the first time from microwave-hydrothermal precursors at , for 2 h. Structural and dielectric properties were investigated by X-ray diffraction and complex impedance spectroscopy. A high ferroelectric-paraelectric transition temperature of was observed, together with two different dielectric regimes for the ac electrical conductivity below T_c. The activation energies were calculated as 0.155 and 0.531 eV, and are related to conduction by oxygen vacancies. It was concluded that the low activation energies showed by these materials could contribute to their fatigue-free nature.     

Structural studies of Cr-doped mullite derived from single-phase precursors

Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr₂O₃ using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr₂O₃). A second phase, namely θ-(Al,Cr)₂O₃, was revealed by XRD in the sample containing 9.60 wt% Cr₂O₃. The structure of mullites was refined by the Rietveld method, location of Cr³⁺ was performed by the EPR spectroscopy. At low chromium doping level (Cr₂O₃ content less than ∼5 wt%) Cr³⁺ ions were substituted for Al³⁺ in the AlO₆ octahedra of the mullite structure (M1 site). For higher doping level, Cr³⁺ ions were additionally substituted for Al³⁺ in the AlO₆ octahedra of the second phase [θ-(Al,Cr)₂O₃ at 1400 °C, or α-(Al,Cr)₂O₃ at 1600 °C] which segregated in the system. Substitution of Cr³⁺ for Al³⁺ on M1 site in the mullite structure resulted in increase of average distances in (M1)O₆ octahedron and decrease of average distances in T*O₄ tetrahedron, while average distances in TO₄ tetrahedron stayed almost constant.     

Structural and morphological characterization of chemically deposited silver films

Silver thin films were deposited on glass slide substrates at room temperature by the chemical bath deposition (CBD) technique, using silver nitrate (AgNO₃) as Ag⁺¹ source and triethanolamine [(N(CH₂CH₂OH)₃)] as the complex reductor agent. We determined the conditions of the CBD process to obtain homogeneous, opaque silver films with good adhesion to the substrate and white coloration. The silver films were studied by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The results show that the films are composed of several layers with different morphology depending on the deposition time. In all the cases, the crystalline structure of the films was the face cubic centered phase with a moderate [111] texture. Strains and stresses were calculated by the Vook-Witt grain interaction model.     

Synthesis, growth and characterization of a new derivative 4-ethoxy-N-methyl-4-stilbazolium besylate monohydrate single crystal

A new organic material 4-ethoxy-N-methyl-4-stilbazolium besylate monohydrate, a derivative in the stilbazolium family, known for efficient nonlinear optical materials, has been synthesized for the first time. Employing the slow evaporation technique, optically transparent good quality single crystals of size 15 mm×10 mm×5 mm were grown from methanol. The cell dimensions obtained by single crystal X-ray diffraction studies reveal that the crystal belongs to the monoclinic system. Functional groups of the grown crystal were identified from FTIR and NMR spectral analysis. UV−vis−NIR studies show that the crystal is transparent in the wavelength range 460-1100 nm. The thermal stability of the compound was determined by thermal analyses of the specimen.     

The effect of zinc concentration on vacancies jump rate, diffusion coefficient and jump length in Cu-Zn ferrite

Samples with the chemical formula Cu_1−xZn_xFe₂O₄ (x=0.2, 0.4, 0.6, 0.8 and 1) were prepared by the standard ceramic method. The dielectric constant and dielectric loss tangent were studied as a function of vacancy jump rate. The results show that the dielectric constant and dielectric loss tangent decrease with increasing vacancy jump rate. In addition, the electron jump length in the octahedral sites was studied as a function of zinc concentration. The increase in jump length with Zn concentration has been attributed to the substitution of Fe⁺³ for Zn²⁺ at the A-sites, which increases the B-B interaction. The increase of diffusion coefficient with increasing Zn concentration was reinforced by the increase of jump rate.     

Structural, optical and scintillation properties of cerium cyclotriphosphates and polyphosphates

The cerium cyclotriphosphate CeP₃O₉·3H₂O and polyphosphate Ce(PO₃)₃ have been optically investigated for the first time. In both materials, excitation and emission spectra under UV and X-ray excitations as well as emission decays of Ce³⁺ were measured at room temperature. The spectroscopic results of anhydrous Ce(PO₃)₃, prepared by progressively heating the corresponding CeP₃O₉. 3H₂O, are discussed and correlated with the structural data.For the Ce(PO₃)₃ polyphosphate material, the Stokes shift of the d-f emission is very small (760 cm⁻¹), inducing an efficient ultraviolet luminescence and a new application as scintillator.     

High pressure Raman spectroscopic study of spinel MgCr2O4

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr₂O₄ spinel up to pressures of 76.4 GPa. Results indicate that MgCr₂O₄ spinel undergoes a phase transformation to the CaFe₂O₄ (or CaTi₂O₄) structure at 14.2 GPa, and this transition is complete at 30.1 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr₂O₄ at 5.7-18.8 GPa, predicted by the theoretical simulation. This high pressure MgCr₂O₄ polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.     

Structural characterization of dense reduced BaTiO3 and Ba0.95La0.05TiO3 nanoceramics showing colossal dielectric values

BaTiO_3−x and Ba_0.95La_0.05TiO_3−x nanoceramics showing colossal permittivity values have been characterized. While starting powders are of cubic symmetry, X-ray and Neutron Diffraction techniques and Raman Spectroscopy measurements show that the one-step processed ceramics obtained by Spark Plasma Sintering (SPS) contain cubic and tetragonal phases. Rather large oxygen deficiency determined in such ceramics by Electron Micro Probe analysis and Electron Energy Loss Spectroscopy analyzes is explained by the presence of Ti³⁺, as evidenced by X-ray Photoelectron Spectroscopy measurements. Transmission Electron Microscopy and High Resolution Transmission Electron Microscopy show that these ceramics contain 50-300 nm grains, which have single-domains, while grain boundaries are of nanometer scale. Colossal permittivity values measured in our dense nanoceramics are explained by a charge hopping mechanism and an interfacial polarization of a large number of polarons generated after sample reduction in SPS apparatus.     

Structural stability of anhydrous proton conducting SrZr0.9Er0.1O3−δ perovskite ceramic vs. protonation/deprotonation cycling: Neutron diffraction and Raman studies

Long-term chemical and structural stability of an ion conducting ceramic is one of the main criteria for its selection as an electrolytic membrane in energy plant devices. Consequently, medium density SrZr_0.9Er_0.1O_3-δ (SZE) anhydrous proton conducting ceramic – a potential electrolyte of SOFC/PCFC, was analysed by neutron diffraction between room temperature and 900 °C. After the first heating/cooling cycle, the ceramic pieces were exposed to water vapour pressure in an autoclave (500 °C, 40 bar, 7 days) in order to incorporate protonic species; the protonated compound was then again analysed by neutron diffraction. This procedure was repeated two times. At each step, the sample was also controlled by TGA and Raman spectroscopy. These studies allow the first comprehensive comparison of structural and chemical stability during the protonation/deprotonation cycling. The results reveal good structural stability, although an irreversible small contraction of the unit-cell volume and local structure modifications near Zr/ErO₅[] octahedra are detected after the first protonation. After the second protonation easy ceramic crumbling under a stress is observed because of the presence of secondary phases (SrCO₃, Sr(OH)₂) well detected by Raman scattering and TGA. The role of crystallographic purity, substituting element and residual porosity in the proton conducting perovskite electrolyte stability is discussed.     

Characterization of mechanochemically synthesized imidazolates of Ag, Zn, Cd, and Hg: Solid state reactivity of nd cations

Silver, zinc, cadmium, and mercury imidazolates have been synthesized mechanochemically by milling imidazole and the metal oxides in an agate mortar. The reaction products were characterized by FTIR and XRPD techniques. The results obtained for the mechanochemical imidazolates have been compared with those obtained by precipitation reported in the literature. The mechanochemical Ag imidazolate has the same orthorhombic crystal structure as the precipitated one. The mechanochemical Zn imidazolate has a tetragonal structure with similar crystal parameters to those of Zn(Imz)₂H₂O, but no water molecules are present in the structure. This new anhydrous form is a polymorph of the one obtained by precipitation. The mechanochemical Cd imidazolate has a monoclinic structure which is the polymorph of the precipited orthorombic form. The mechanochemical Hg imidazolate presents a hexagonal structure which is a polymorph of the orthorombic structure obtained by precipitation. The influence of the nd¹⁰ electronic configuration of the cations on the mechanochemical reaction is discussed.     

Structural evolution of low-pressure polymerised C60 with polymerisation conditions

We show in this paper that characteristic features in the Raman spectra, especially the frequency of the pentagonal pinch mode, can give information about the polymeric structure of pressure polymerised C₆₀. High-pressure treatment at 1 GPa below 510 K for 3 h results in the formation of a low fraction of dimers only, while treatment at the same pressure and time above 540 K affords a fully polymerised material. In the latter case, different relative fractions of dimers and polymer chains are obtained depending on whether the final reaction conditions were reached by isobaric or isothermal path. We suggest that this difference results from different reaction dynamics in the two cases. The polymerisation rate depends on T and p and on the rotational and orientational states of the molecules. At 1 GPa no polymerisation is observed in sc C₆₀, while in “hexagon” oriented sc C₆₀ at 1.7 GPa dimers are already formed 175 K below the fcc–sc transition and a fully polymerised material is obtained just below the transition to the fcc phase.     

Densification mechanism of polyacrylonitrile-based carbon fiber during heat treatment

Polyacrylonitrile (PAN)-based carbon fibers were heat treated at various temperatures for varying durations to simulate the graphitization process in the manufacture of C/C composites. Densification of the resulting fibers was confirmed by density measurement. The composition and structure of the fibers were investigated by means of elemental analysis, X-ray diffraction and Raman spectroscopy. For specified isothermal heat treatment time, the structural parameters depended strongly on heat treatment temperature. The nitrogen content decreased with increased heat treatment temperature and extended time at constant temperature. Nitrogen loss was complete at temperatures above 1900 °C. The graphite crystallite size increased rapidly with increasing heat treatment temperature, and slowly with extended isothermal heat treatment time. At 2100 °C a more ordered graphitic structure appeared. Denitrogenation induced “puffing”, which made the fibers expand. Decrease in density in the heat treatment temperature range 1500-1900 °C originated from the abrupt evolution of nitrogen, and above 1900 °C the graphitization transition induced steadily increasing density. Densification of the carbon fibers was determined both by the rate of denitrogenation and the rearrangement of carbon atoms.