Temperature X-ray Diffraction Research Articles

Unveiling the key factor for the phase reconstruction and exsolved metallic particle distribution in perovskites

To significantly increase the amount of exsolved particles, the complete phase reconstruction from simple perovskite to Ruddlesden-Popper (R-P) perovskite is greatly desirable. However, a comprehensive understanding of key parameters affecting the phase reconstruction to R-P perovskite is still unexplored. Herein, we propose the Gibbs free energy for oxygen vacancy formation in Pr0.5(Ba/Sr)0.5TO3-δ (T = Mn, Fe, Co, and Ni) as the important factor in determining the type of phase reconstruction. Furthermore, using in-situ temperature & environment-controlled X-ray diffraction measurements, we report the phase diagram and optimum ‘x’ range required for the complete phase reconstruction to R-P perovskite in Pr0.5Ba0.5-xSrxFeO3-δ system. Among the Pr0.5Ba0.5-xSrxFeO3-δ, (Pr0.5Ba0.2Sr0.3)2FeO4+δ – Fe metal demonstrates the smallest size of exsolved Fe metal particles when the phase reconstruction occurs under reducing condition. The exsolved nano-Fe metal particles exhibit high particle density and are well-distributed on the perovskite surface, showing great catalytic activity in fuel cell and syngas production.

Experimental Charge Density Analysis and Electrostatic Properties of Crystalline 1,3-Bis(Dimethylamino)Squaraine and Its Dihydrate from Low Temperature (T = 18 and 20 K) XRD Data

Multipolar refinements of structural models fitting extensive sets of X-ray diffraction (XRD) data from single crystals of 1,3-bis(dimethylamino)squaraine [SQ, C8H12N2O2] and its dihydrate [SQDH, C8H12N2O2·2H2O], collected at very low T (18 ± 1 K for SQ, 20 ± 1 K for SQDH), led to an accurate description of their crystal electron density distributions. Atomic volumes and charges have been estimated from the experimental charge densities using the Quantum Theory of Atoms in Molecules (QTAIM) formalism. Our analysis confirms the common representation (in the literature and textbooks) of the squaraine central, four-membered squarylium ring as carrying two positive charges, a representation that has been recently questioned by some theoretical calculations: the integrated total charge on the C4 fragment is estimated as ca. +2.4e in SQ and +2.2e in SQDH. The topology of the experimental electron density for the SQ squaraine molecule is modified in the dihydrated crystal by interactions between the methyl groups and the H2O molecules in the crystal. Maps of the molecular electrostatic potential in the main molecular planes in both crystals clearly reveal the quadrupolar charge distribution of the squaraine molecules. Molecular quadrupole tensors, as calculated with the PAMoC package using both Stewart and QTAIM distributed multipole analysis (DMA), are the same within experimental error.

Extended theoretical analysis of crystallisation kinetics being studied by in situ XRD

ABSTRACTTheoretical simulations were used to study the consequences of simplifying the replacement of the step-wise in situ X-ray diffraction (XRD) temperature programme by simple linear heating (at corresponding effective heating rate) during the kinetic calculations based on the multivariate kinetic analysis. The simulations were performed for a large variety of step-wise non-isothermal in situ XRD temperature programmes, covering most practically used combinations of the temperature step magnitude ΔT, rate of heating, and duration of the isothermal hold Δt. To achieve the universal interpretation of the obtained results, the behaviour of the majority of crystallisation processes with commonly encountered kinetic profiles was explored: simulations were performed for single-process transformations with highly negative, symmetric and highly positive asymmetries; complex multi-process reactions with different degrees of sub-process overlaps and variable activation energy were analysed. It was found that the asymmetry and shape of the crystallisation peaks do not significantly influence the level of distortion of kinetic parameters. The main factors that increase the errors of in situ XRD kinetic evaluations are high Δt, high ΔT and high activation energy (with the latter two being most important). Findings were discussed for the accuracy of the corresponding kinetic predictions. Generalisation of the present conclusions towards their universal utilisation for optimisation of in situ XRD experiments was suggested.

Morphology, Opto-Electronic and Dielectric Properties of Vacuum Sublimed CuPc Based Thin Films

AC transport properties and dielectric response of sandwich geometry (Ag/CuPc/Ag) of CuPc(CuPc) thin films deposited using thermal evaporation technique have been studied within frequency range 1 Hz–10 KHz and in temperature range 303–383 K. Scanning electron microscope (SEM) investigations of these films reveal fiber like morphology. Crystalline natures of CuPc films have been characterized using X-ray diffraction for different temperatures. The molecular orientations in films for different substrate temperatures have been confirmed by Raman spectroscopy. The optical band gaps calculated from the UV–Visible absorption spectra is found to lie in the range 3.01–3.15 eV. Electrical conductivity of CuPc films increases with increase of temperature. The hole mobility values of CuPc films at different temperatures have been calculated using negative differential susceptance (–ΔB) technique. Both capacitance and dielectric constant have been found to decrease with the increase of frequency and temperature.

The Role of Nickel in the Formation of Phases of High Chromium Alloys of Cementite Composition upon Mechanical Synthesis and Subsequent Annealing

After medium and high temperature annealing, X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements are used to study phase transformations and patterns of the redistribution of Ni and Cr in high-chromium alloys of cementite composition produced via mechanical synthesis.

Characterization by Photoluminescence and Raman Spectroscopy of the Oxide Scales Grown on the PM2000 at High Temperatures

The microstructural change of a thermal oxidation on a PM2000 superalloy was investigated after a 48 h isothermal heat-treatment. The oxides (transitions aluminas, [Formula: see text]-alumina and mixed oxides) were characterized using the Raman spectroscopy, the Photoluminescence (PL) spectroscopy, MEB observations and the X-ray diffraction for temperatures between 600°C and 1200°C. The X-ray diffraction results under conventional incidence conditions make it possible to estimate the crystallites size and the specific surfaces of the [Formula: see text]-alumina. The PL analyses at various excitation wavelengths (200[Formula: see text]nm, 234[Formula: see text]nm, 326[Formula: see text]nm and 532[Formula: see text]nm) provide important information on the existing defects (intrinsic and extrinsic) according to the oxidation temperature. The Raman spectroscopy ([Formula: see text].18[Formula: see text]nm) gives the various signatures of the various alumina generated ([Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]-Al2O3).

Effect of La3+-donor substitution on structural, micro structural, dielectric and ferroelectric characteristics of BLNT-BZT solid solutions

ABSTRACTLanthanum (La)-modified 0.93(Bi0.5−xLaxNa0.5TiO3)–0.07(BaTi0.96Zr0.04O3) (Abbreviated as BLNT-BZT) ceramics were synthesized (close to MPB) by using conventional solid state route. The structural phase purity of ceramics was confirmed by using room temperature (RT) X-ray diffraction. The structural phases coexistence was analyzed with different duel phase model and bring out the amount of phase contribution quantitatively with the help of Rietveld refinement technique. The X-rays diffraction studies revealed that coexisting of minor tetragonal (P4mm) phase evolution along with major monoclinic structural (Cc) phase. Moreover, the above affirmation was clarified from Raman studies, observed that TiO6 reflection at 500–600 cm−1 splitting occurs resides the presence of MPB of ceramics. With the substitution of La in BNT–BZT induces the gradual change in the average grain size and non-uniform distribution was observed from the surface morphology. The dielectric anomalies of these BLNT-BZT ceramics show broad dielectric anomaly near the maximum dielectric constant, which reflects the diffuse phase transition behavior. The RT P-E loops were observed in slim and slanted shape, replicates the enhanced properties due to the phase additive behavior near the MPB.

Research on dielectric characterization of laterite ores under microwave radiation

Dielectric property of laterite ores plays a key role in the determination of its treatment effect by microwave. However, till date, to our knowledge, there is no article reported on the dielectric property of laterite ores up to the nickel–iron alloy extraction temperature of around 1200 °C, which impedes the efficient application of microwave power in the nickel–iron alloy extraction. This paper, in view of this, aims to provide the dielectric property variation of laterite ores treated by microwave radiation from room temperature up to 1250 °C. A custom-designed ridge waveguide-based system is applied to perform the measurement. To verify the reliability of the measured results and explain the experimental phenomenon, X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis are employed to conduct a thorough understanding on the variation of dielectric properties of laterite ores under different temperatures. The measured data are qualitatively verified by its temperature change tendency, SEM and XRD analysis. This work may be helpful to avoid thermal runaway, design more efficient microwave devices and optimize the existing production line of microwave treating laterite ores.

In situ temperature X-ray diffraction and mechanical study of the binary yellow 18-carat gold alloy AuCu

<i>In situ</i> temperature X-ray diffraction and mechanical study of the binary yellow 18-carat gold alloy AuCu

Synthesis, phase formation, and thermal expansion of sulfate phosphates with the NaZr2(PO4)3 structure

The NaFeZr(PO4)2SO4 and Pb2/3FeZr(PO4)7/3(SO4)2/3 sulfate phosphates with the NaZr2(PO4)3 (NZP) structure were synthesized and studied using X-ray diffraction, electron microprobe analysis, IR spectroscopy, and simultaneous differential thermal and thermogravimetric analysis. The phase formation and thermal stability of the compounds were studied by powder X-ray diffraction and DTA–TG. The Pb2/3FeZr(PO4)7/3(SO4)2/3 structure was refined by full-profile analysis. The structure framework is composed of randomly occupied (Fe,Zr)O6 octahedra and (P,S)O4 tetrahedra; the Pb2+ ions occupy extra-framework sites. The thermal expansion of Pb2/3FeZr(PO4)7/3(SO4)2/3 in the temperature range from–120 to 200°C was studied by temperature X-ray diffraction. In terms of the average linear coefficient of thermal expansion (αav = 1.7 × 10–6°C–1), this compound can be classified as having low expansion. The combination of different tetrahedral anions (a phosphorus and a smaller sulfur one) in the NZP resulted in a decrease in the framework size and cavities and enabled the preparation of low-expansion sulfate phosphate with a smaller extra-framework cation (cheap Pb) instead of larger cations (Cs, Ba, Sr) used most often in the monoanionic phosphates.

Structural aspects of the antiferroelectric-paraelectric phase transition in double perovskite Pb2MgWO6 at high pressures and temperatures

The crystal structure of antiferroelectric Pb2MgWO6 has been studied using neutron diffraction at high pressures to 5.4 GPa at room temperature and energy-dispersive X-ray diffraction at high pressures to 4 GPa in the temperature range 300–400 K. At normal conditions, in Pb2MgWO6, there is an antiferroelectric phase with the crystal structure described by the orthorhombic symmetry with space group Pnma. At temperature T = 313 K and normal pressure or at room temperature and pressure P ∼ 0.9 GPa, the crystal under-goes a structural phase transition to the cubic phase with space group $$Fm\bar 3m$$ (paraelectric phase). The temperature and pressure dependences of the lattice parameters, unit cell volume, and interatomic bond lengths have been obtained, and the thermal expansion coefficients and the bulk moduli have been calculated for the antiferroelectric and paraelectric phases of Pb2MgWO6.

New insights into the transitional behaviour of methyl-6-O-(n-dodecanoyl)-α-D-glucopyranoside using variable temperature FTIR spectroscopy and X-ray diffraction

The liquid crystalline behaviour of methyl-6-O-(n-dodecanoyl)-α-D-glucopyranoside, 1, has been characterised using X-ray diffraction and variable temperature Fourier transform infrared (FTIR spectroscopy). 1 exhibits a monotropic interdigitated smectic A phase consisting of bilayers in which the alkyl chains are overlapped. The crystal–isotropic transition is accompanied by a pronounced decrease in the strength of the hydrogen bonding network involving the sugar groups resulting in a marked change in the environment of the alkyl chains. The isotropic phase consists of disordered smectic-like domains stabilised via hydrogen bonding between the sugar groups. At the transition to the smectic A phase, a subtle change in hydrogen bonding is observed which is manifested by a change in the temperature dependence of the OH stretching peak position in the FTIR spectrum. On crystallisation, the strong hydrogen bonding network is re-established accompanied by a change in the conformational distribution of the alkyl chains. A model is proposed in which a combination of hydrogen bonding (enthalpic effects) and conformational arrangements (entropic effects) promotes initially the formation of smectic-like domains in the isotropic phase and subsequently stabilises the smectic A phase by inhibiting the microphase separation leading to the crystal phase.

Effect of Alkali Precipitateing Agent on the Activity and Stability of Copper-Manganese Mixed-Oxides for the Water Gas Shift Reaction

This paper reports the effect of alkali precipitating agents, i.e. KOH and NaOH, on activity and stability of Cu-Mn based catalysts for the water gas shift (WGS) reaction. Structural and texture characteristics of the catalysts were characterized by low temperature N2 adsorption, X-Ray Diffraction (XRD) and TPR techniques. The XRD and low temperature N2 adsorption results showed the two types of precipitators produced different Cu-Mn phases during precipitation and drying. The KOH precipitator resulted in partial precipitation of copper ion and formation of hydrate basic copper sulfate, Cu4SO4(OH)6•H2O after drying; while the NaOH precipitator led to a completed precipitation of Cu ion and formation of relatively stable Cu2+1O and Mn3O4crystals after drying. These phases formed after precipitation by the KOH and NaOH, however, experienced the same phase change during calcination and activation, namely, they first changed to Cu1.5Mn1.5O4in calcination followed by the formation of microcrystalline Cu and MnO phases during activation. The Cu-Mn WGS catalysts prepared by the NaOH was found to have larger surface area and more content of Cu and MnO phases after activation, which might be the reasons of higher activity and better stability of the Cu-Mn catalyst.

Emission and X ray diffraction in AlGaAs/ InGaAs Quantum wells with embedded InAs Quantum dots

ABSTRACTThe photoluminescence (PL), its temperature dependence and X-ray diffraction (XRD) have been studied in MBE grown GaAs/AlGaAs/InGaAs/AlGaAs /GaAs quantum wells (QWs) with InAs quantum dots embedded in the center of InGaAs layer in the freshly prepared states and after the thermal treatments during 2 hours at 640 or 710 °C. The structures contained two buffer (Al0.3Ga0.7As/In0.15Ga0.85As) and two capping (In0.15Ga0.85As / Al0.3Ga0.7As) layers. The temperature dependences of PL peak positions have been analyzed in the temperature range 10-500K with the aim to investigate the QD composition and its variation at thermal annealing. The experimental parameters of the temperature variation of PL peak position in the InAs QDs have been compared with the known one for the bulk InAs crystals and the QD composition variation due to Ga/Al/In inter diffusion at thermal treatments has been detected. XRD have been studied with the aim to estimate the capping/buffer layer compositions in the different QW layers in freshly prepared state and after the thermal annealing. The obtained emission and XRD data and their dependences on the thermal treatment have been analyzed and discussed.

Thermal degradation of microporous Sm2O3–MgO nanocomposites at isothermal conditions and surface chemical properties

The evolution of microstructural, morphological and surface chemical properties of Sm2O3–MgO nanocomposites are determined during and after isothermal heat treatments in the range of 500–1000 °C. The samples are investigated using (high temperature) X-ray diffraction, energy-filtered transmission electron microscopy including electron energy loss spectroscopy, nitrogen adsorption, and temperature programmed desorption of CO2. With small amounts of MgO the initial micropore fractions are low and mainly coarsening of Sm2O3 is observed. Large amounts of MgO result in high initial micropore fractions giving rise to enhanced densification. The different thermal degradation behaviors are explained by means of the respective characteristic diffusion distances which are determined by the volume phase fractions. The surface chemical properties of Sm2O3 and MgO remain qualitatively unchanged, however, the specific CO2 chemisorption capacities are increased through the nanocomposite formation.

A High-Resolution Diffraction and Spectroscopic Study of the Low-Temperature Phase Transformation of Hexagonal to Tetragonal GeO2 with and without Alkali Hydroxide Promotion

Pure and RbOH-impregnated hexagonal GeO2 were studied by high-resolution and -temperature X-ray diffraction, thermal gravimetric analysis coupled with mass spectrometry, and high-temperature Raman spectroscopy in the temperature range 313–1300 K for a direct comparison of the phase transformation from hexagonal to tetragonal GeO2. On both samples, the phase transformation was observed to follow through distinct reaction steps. At low temperatures, a partial transformation to tetragonal GeO2 occurred on both materials likely induced by the presence of strongly bound water. The removal of the latter is accompanied by substantial anisotropic lattice distortions and yields at maximum ∼12% tetragonal GeO2. This partial phase transformation appears to be almost independent of the alkali hydroxide addition. In contrast, the final high-temperature phase transformation around 1100 K results in full conversion to tetragonal GeO2 on the RbOH-impregnated GeO2 material, whereas, on pure hexagonal GeO2, only partial transformation to the tetragonal phase was obtained.

Emission related to exciton‐polariton coupling in porous SiC

AbstractThe paper presents the results of SiC nanocrystal (NC) characterization using the photoluminescence (PL), its temperature dependence and X‐ray diffraction (XRD) techniques. It is revealed that original n‐type 6H‐SiC wafers and porous 6H‐SiC layers consisted inclusions of 2H‐SiC, 4H‐SiC, and 8H‐SiC polytypes. The photoluminescence study has shown that in porous SiC layers the emission intensity of high energy PL bands enlarges. Temperature dependences of high energy PL bands testify that these PL bands related to excitons bounded at nitrogen donors in different SiC polytypes. The PL intensity enhancement of donor‐bound exciton PL bands in the big size (50‐250nm) SiC nanocrystals has been attributed to the realization of exciton weak confinement and exciton‐polariton coupling in SiC nanocrystals. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

ChemInform Abstract: Impedance Measurements of Amorphous AgIx×(Ag3PO4)1-x Over a Wide Composition Range.

Using a rapid quenching technique amorphous samples of AgIx(Ag3PO4)1−x have been obtained over a wide composition range up to x = 0.85. Above this composition the cooling rate of > 106 K s−1 was not sufficient to prevent partial crystallinity. All samples have been characterized both by X-ray diffraction and impedance spectroscopy for temperatures near room temperature. From impedance measurements the dc-conductivity σ has been determined which for amorphous materials varies from 7.6 · 10−5 ω−1 cm−1 (x = 0.5) to 4.4 · 10−2 ω−1 cm−1 (x = 0.85) at 298 K. A plot of log σ versus x yields a straight line which at x = 1 hits the value of the pure fast ionic conductor α-AgI extrapolated to 298 K.

SiC Nanowires Grown on 4H-SiC Substrates by Chemical Vapor Deposition

Growth of SiC nanowires on commercial 4H-SiC substrates by chemical vapor deposition is reported. The main objective was to explore a possibility of reproducing the substrate polytype in order to obtain SiC NWs specifically composed of the hexagonal 4H-SiC polytype. The growth experiments were conducted in a hot-wall CVD reactor with H2 as the carrier gas, SiCl4 as the silicon source, and CH3Cl as the carbon source. Vapor-liquid-solid (VLS) growth mode was enabled by using metal nano-particle on the surface of the 4H-SiC substrates. Formation of nanowires or bigger nano-cones was achieved depending on the temperature and the metal catalyst used. Only SiC phase with no presence of Si was confirmed by X-ray diffraction for the growth temperatures down to 1050°C. The low temperature photoluminescence spectra measured on as-grown NWs showed clear 4H-SiC nitrogen bound excitons in some of the samples, particularly when in-situ N2 doping was used. The density of stacking faults detected by TEM strongly depended on the growth conditions.

Thermal, mechanical and phase stability of LaCoO 3 in reducing and oxidizing environments

Thermal, mechanical, and phase stability of LaCoO 3 perovskite in air and 4% H 2/96% Ar reducing atmosphere have been studied by thermal mechanical analysis (TMA), high temperature microhardness, and high temperature/room temperature X-ray diffraction. The thermal behavior of LaCoO 3 in air exhibits a non-linear expansion in the 100–400 °C temperature range. A significant increase of coefficient of thermal expansion (CTE) measured in air both during heating and cooling experiments occurs in the 200–250 °C temperature range, corresponding to a known spin state transition. LaCoO 3 is found to be highly unstable in a reducing atmosphere. In case where LaCoO 3 was present as a powder, where surface reduction mechanism would prevail, the reduction starts as earlier as 375 °C with a formation of the metallic Co and La 2O 3 at 600 °C. In the bulk form, LaCoO 3 undergoes a series of expansion and contractions due to phase transformations beginning around 500 °C with very intensive chemical/phase changes at 800 °C and above. These expansions and contractions are directly related to the formation of La 3Co 3O 8, La 2CoO 4, La 4Co 3O 10, La 2O 3, CoO, and other Co compounds in the reducing atmosphere. Although LaCoO 3 is a good ionic and electronic conductor and catalyst, its high thermal expansion as well as structural, mechanical, and phase instability in reducing environments present a serious restriction for its application in solid oxide fuel cells, sensors or gas separation membranes.