Rietveld Refinement Of Crystal Structure Research Articles

Nanostructured Niobium and Titanium Carbonitrides as Electrocatalyst Supports.

Nanostructured niobium-titanium carbonitrides, (Nb,Ti)C1-xNx, with the cubic-rock salt structure are prepared without the use of reactive gases via thermal treatment (700-1200 °C) under nitrogen of mixtures of guanidine carbonate and ammonium niobate (V) oxalate hydrate, with addition of ammonium titanyl oxalate monohydrate as a titanium source. The bulk structure and chemical composition of the materials are characterized using powder X-ray diffraction (XRD) and powder neutron diffraction, elemental homogeneity is studied using energy dispersive spectroscopy (EDS) mapping using transmission electron microscopy (TEM), and surface chemical analysis is examined using X-ray photoelectron spectroscopy (XPS). Nanoscale crystallites of between 10 and 50 nm are observed by TEM, where EDS reveals the homogeneity of metal distribution for the mixed-metal materials. Titanium carbonitrides are found to be air sensitive, reacting with air under ambient conditions, while titanium-niobium carbonitrides are found to degrade in aqueous sulfuric acid. The niobium carbonitrides, however, show some stability toward acidic solutions. Materials are produced with composition NbC1-xNx with x between 0.35 and 0.45, and more carbon-rich materials (x ≈ 0.35) are found as the synthesis temperature is increased, as proven by Rietveld refinement of crystal structure against powder neutron diffraction data. Despite phase purity seen by diffraction and negligible bulk carbon content, XPS shows a complex surface chemistry for the NbC1-xNx materials, with evidence for Nb2O5-like oxide species in a carbon-rich environment. The NbC1-xNx prepared at 900 °C has a surface area around 50 m2 g-1, making it suitable as a catalyst support. Loading with iridium provides a material active for the oxygen evolution reaction in 0.1 M sulfuric acid, with minimal leaching of either Nb or Ir after 1000 cycles.

Yb3+/Er3+ co-doped Gd2Te4O11 nanosheets with intrinsic polarity: One-step hydrothermal synthesis and upconverted optical temperature measuring ability

The digadolinium tellurite phosphors of Gd2Te4O11(GTO):Yb3+/Er3+ have been successfully synthesized as upconversion luminescence (UCL) materials via one-step hydrothermal method. The crystal structure, morphology, and upconversion luminescence property were systematically characterize by XRD, SEM, and spectroscopy techniques. The Rietveld refinements of crystal structure were carried out on the XRD patterns and the feature of crystal structure was analyzed. Under the 980 nm NIR excitation, these materials showed very bright upconverted emissions. The concentrations of Yb3+ and Er3+ were optimized and the strongest upconverted emissions were achieved in the GTO:15%Yb3+/1%Er3+. The possible energy transfer mechanism of UCL was proposed based upon the analysis of power-dependent UCL and fluorescence kinetics. Furthermore, the fluorescence intensity ratio (FIR) derive from the two thermally coupled energy levels (2H11/2 and 4S3/2) of Er3+ was employed as indicator for temperature measurement. The maximum absolute sensitivity can be achieved to be 7.34 × 10−3 K−1 at 501 K. This material exhibited good reliability and repeatability in optical temperature measurement, which could be a novel promising candidate for noncontact temperature sensors.

Synthesis and Characterization of Novel Nanoparticles of Lithium Aluminum Iodate LiAl(IO3)4, and DFT Calculations of the Crystal Structure and Physical Properties.

Here we report on the non-hydrothermal aqueous synthesis and characterization of nanocrystalline lithium aluminum iodate, LiAl(IO3)4. Morphological and compositional analyses were carried out by using scanning electron microscopy (SEM) and energy-dispersive X-ray measurements (EDX). The optical and vibrational properties of LiAl(IO3)4 have been studied by UV-Vis and IR spectroscopy. LiAl(IO3)4 is found to crystallize in the non-centrosymmetric, monoclinic P21 space group, contrary to what was reported previously. Theoretical simulations and Rietveld refinements of crystal structure support this finding, together with the relatively high Second Harmonic Generation (SGH) response that was observed. Electronic band structure calculations show that LiAl(IO3)4 crystal has an indirect band gap eV, in agreement with the experimental optical band gap eV. The complex relative permittivity and the refraction index of LiAl(IO3)4 have also been calculated as a function of energy, as well as its elastic constants and mechanical parameters. LiAl(IO3)4 is found to be a very compressible and ductile material. Our findings imply that LiAl(IO3)4 is a promising material for optoelectronic and non -linear optical applications.

Effect of laser irradiation on Ag3.54In10.62Sb49.56Te36.28

Ag3.54In10.62Sb49.56Te36.28 alloy has been prepared by melt-quench method. X-ray diffraction (XRD) and Raman spectroscopy have been used to investigate phase homogeneity, crystal structure and effect of laser irradiation. The muliphase nature of Ag3.54In10.62Sb49.56Te36.28 alloy and the presence of AgIn3Te5 and Sb8Te3 phases have been revealed by Rietveld refinement of crystal structure. Raman spectroscopy combined with optical microscopy has been used to understand the vibrational modes and the distribution of the phases identified on the surface of the samples. The results obtained from spectroscopic analysis have substantiated the results of Rietveld refinement. Interaction of 20 mW laser beam with samples has also been investigated using Raman measurements. The results revealed that the regions with large phase fraction of AgIn3Te5 become amorphous during laser-sample interaction, but the starting phase remains nearly the same after interaction. Regions with AgIn3Te5 and nearly equal or larger amount of Sb8Te3 have shown significant growth of α-Sb2O3 during and after laser-sample interaction. These observations have been explained based on the maximum temperature rise at different regions during laser-sample interaction.

Phase homogeneity and crystal structure of Sb88Te12 synthesized by melt-quench method

Phase homogeneity and crystal structure of Sb88Te12 alloy synthesized by melt-quench method have been analyzed using X-ray diffraction (XRD) and Raman spectroscopy. Rietveld refinement of crystal structure of Sb88Te12 has revealed the formation of Sb8Te3 and Sb phases respectively with 11R and A7 structures (space group is $$R\bar 3m$$ ). Phase fractions of the phases Sb8Te3 and Sb have been determined to be 44.44% and 55.56% respectively. Raman spectrum of Sb88Te12 has showed a highest intensity peak at 156 cm−1 and another peak at 151.1 cm−1, which correspond to A1g modes of Sb8Te3 and element Sb, respectively. Analysis of Raman spectrum has substantiated the results obtained from structure refinement regarding the presence of two different phases.

High pressure phase transitions in the rare earth metal erbium to 151 GPa

High pressure x-ray diffraction studies have been performed on the heavy rare earth metalerbium (Er) in a diamond anvil cell at room temperature to a pressure of 151 GPa and Erhas been compressed to 40% of its initial volume. The rare earth crystal structure sequence fcc (hcp: hexagonal close packed; fcc: face centered cubic; dhcp: double hcp) isobserved in Er below 58 GPa. We have carried out Rietveld refinement of crystalstructures in the pressure range between 58 GPa and 151 GPa. We have examinedvarious crystal structures that have been proposed for the distorted fcc (dfcc)phase and the post-dfcc phase in rare earth metals. We find that the hexagonalhR 24 structure is the best fit between 58 and 118 GPa. Above 118 GPa, a structural transformation fromhR 24 phase to a monoclinicC 2/m phase is observed witha volume change of − 1.9%. We have also established a clear trend for the pressure at which a post-dfcc phase isformed in rare earth metals and show that there is a monotonic increase in this pressurewith the filling of 4f shell.

High-pressure phase transitions in rare earth metal thulium to 195 GPa

We have performed image plate x-ray diffraction studies on a heavy rare earth metal,thulium (Tm), in a diamond anvil cell to a pressure of 195 GPa and volume compressionV/Vo = 0.38 at room temperature. The rare earth crystal structure sequence, distorted fcc, is observed in Tm below 70 GPa with the exception of a pure fcc phase. Thefocus of our study is on the ultrahigh-pressure phase transition and Rietveld refinement ofcrystal structures in the pressure range between 70 and 195 GPa. The hexagonalhR- 24 phase is seen to describe the distorted fcc phase between 70 and 124 GPa. Above124 ± 4 GPa, a structuraltransformation from hR 24 phase to a monoclinic C 2/m phase is observed with a volume change of − 1.5%. The equation of state data shows rapid stiffening above the phase transition at 124 GPaand is indicative of participation of f-electrons in bonding. We compare the behavior of Tmto other heavy rare-earths and heavy actinide metals under extreme conditions of pressure.

Synthesis and effect of Sr substitution on fluorescence of new Ba 2−xSr xZnS 3: Eu 2+ red phosphor: Considerable enhancement of emission intensity

A series of 0.5 mol% Eu 2+-activated Ba 2− x Sr x ZnS 3 phosphor materials were synthesized using precursors prepared by the polymerizable complex method and their fluorescent properties were studied for the first time. It was found that Sr substitution for Ba leads to the considerable improvement of internal quantum efficiency and emission intensity in these materials compared to Ba 2ZnS 3, while emission peak wavelength exhibits a blue shift from 680 to 660 nm. Rietveld refinement of crystal structure of sample with x=0.7 suggests that Sr ions preferentially occupy one of two Ba sites in this compound. Such a structural re-arrangement might be responsible for the observed quantum efficiency dependence on Sr concentration.

Piezoelectric properties of Fe 2O 3-doped (1 − x)BiScO 3– xPbTiO 3 ceramics

Effects of Fe substituents on bismuth scandium lead titanate ceramics (1 − x)BiScO 3– xPbTiO 3 (BSPT) at x = 0.64 were investigated in order to improve the piezoelectric properties. The density of Fe-doped BSPT ceramics tends to slightly decrease when Fe contents increase; except at 1.50 mol% Fe at which the highest density value about 7.43 g/cm 3 is achieved. The Rietveld refinements of crystal structure of all samples show that they are of pure tetragonal phase. The Curie temperature ( T c) increases with doping Fe content and the highest T c of 451 °C is reached for 2 mol% Fe-doped BSPT sample. All these ceramics yield higher Curie temperatures ( T c) than that of PbZr 0.52Ti 0.48O 3 (386 °C), the standard piezoceramics. Meanwhile, the dielectric constant and piezoelectric properties of the Fe-doped BSPT samples were significantly improved compared with those of the undoped sample. Electron spin resonance spectra clearly show that the iron is in the valence state 3+, which indicates that Fe 3+ ions replace either Ti 4+ or Sc 3+ at B-sites. This gives rise to oxygen vacancies to maintain the charge balance and results in three Fe 3+ ESR signals. The first and the third asymmetric signals at around 100 and 300 mT clearly arise from octahedral sites. The second ESR broad peak at around 190 mT implies that the neighboring O 2− vacancies are randomly distributed. It is evident that the 1 mol% Fe-doped sample yields the best over all piezoelectric properties. Furthermore, these new enhanced-piezoelectric materials can effectively reduce at least 36 mol% of the lead content in the ceramics employed in the high power applications so that they are more environmentally friendlier.

Crystal structure refinement at high pressures using angle-dispersive powder diffraction techniques

Abstract The development of area detectors such as the imaging plate (IP) allows the use of angle-dispersive techniques for powder diffraction studies at high pressure. Integration of the 2-d pattern greatly improves the powder averaging and signal-to-noise ratio, making it possible to carry out full Rietveld refinements of crystal structures in high-pressure phases. An IP system is being developed at the Synchrotron Radiation Source (SRS), Daresbury, and the current status of this system -hardware and software -is described. Recent results on La2CuO4 and lnSb demonstrate the effectiveness and advantages of the techniques.

Angle-dispersive powder diffraction at high pressure using an image-plate area detector

Abstract An imaging-plate system designed for the full Rietveld refinement of crystal structures at high pressure is described. Emphasis is given to techniques that have been developed to obtain data free from contaminating diffraction peaks and to a general method of processing the diffraction data. The advantages of using pressure cells that allow full diffraction patterns to be collected are also described. Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991

Angle-dispersive powder-diffraction techniques for crystal structure refinement at high pressure

An imaging-plate system designed for the full Rietveld refinement of crystal structures at high pressure is described. Emphasis is given to techniques that have been developed to obtain data free from contaminating diffraction peaks. Initial results from studies of III–V semiconductors and La2CuO4 are given.

Structural Refinements of ZrO<sub>2</sub> and HfO<sub>2</sub> Treated at 600°C⋅6GPa

The Rietveld refinements of crystal structure were undertaken on powder samples of ZrO2 and HfO2 quenched from 600°C⋅6GPa. The results on orthorhombic ZrO2 were consistent with those of a single crystal obtained under pressure. The compressibility of this phase was calculated as 4.65×10-3GPa-1. The results on HfO2 were also conformable to those on ZrO2.