Search results for " Methods"
showing 10 items of 4102 documents
Electronic properties and high-pressure behavior of wolframite-type CoWO4
2021
In this work we characterize wolframite-type CoWO4 under ambient conditions and under compression up to 10 GPa, with emphasis on its electronic structure. X-Ray diffraction and vibrational experiments, supported by ab initio calculations, show that CoWO4 is stable under high-pressure conditions, as no structural changes are detected in the studied pressure range. Interesting findings come from optical absorption spectroscopy. On the one hand, CoWO4 is confirmed to have one of the lowest band gaps among similar wolframites, around 2.25 eV. This makes CoWO4 suitable for use in applications such as the photocatalysis of organic pollutants and water splitting. Additionally, a monotonic decrease…
Effect of High Pressure on the Crystal Structure and Vibrational Properties of Olivine-Type LiNiPO4
2018
In this work, we present an experimental and theoretical study of the effects of high pressure and high temperature on the structural properties of olivine-type LiNiPO4. This compound is part of an interesting class of materials primarily studied for their potential use as electrodes in lithium-ion batteries. We found that the original olivine structure (α-phase) is stable up to ∼40 GPa. Above this pressure, the onset of a new phase is observed, as put in evidence by the X-ray diffraction (XRD) experiments. The structural refinement shows that the new phase (known as β-phase) belongs to space group Cmcm. At room temperature, the two phases coexist at least up to 50 GPa. A complete conversio…
High-pressure structural, elastic, and thermodynamic properties of zircon-type HoPO4 and TmPO4
2017
[EN] Zircon-type holmium phosphate (HoPO4) and thulium phosphate (TmPO4) have been studied by single-crystal x-ray diffraction and ab initio calculations. We report on the influence of pressure on the crystal structure, and on the elastic and thermodynamic properties. The equation of state for both compounds is accurately determined. We have also obtained information on the polyhedral compressibility which is used to explain the anisotropic axial compressibility and the bulk compressibility. Both compounds are ductile and more resistive to volume compression than to shear deformation at all pressures. Furthermore, the elastic anisotropy is enhanced upon compression. Finally, the calculation…
Polymorphism in Strontium Tungstate SrWO 4 under Quasi-Hydrostatic Compression
2016
The structural and vibrational properties of SrWO4 have been studied experimentally up to 27 and 46 GPa, respectively, by angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy measurements as well as using ab initio calculations. The existence of four polymorphs upon quasi-hydrostatic compression is reported. The three phase transitions were found at 11.5, 19.0, and 39.5 GPa. The ambient-pressure SrWO4 tetragonal scheelite-type structure (S.G. I41/a) undergoes a transition to a monoclinic fergusonite-type structure (S.G. I2/a) at 11.5 GPa with a 1.5% volume decrease. Subsequently, at 19.0 GPa, another structural transformation takes place. Our calculations indicate two possi…
Application of delta recycling to electron automated diffraction tomography data from inorganic crystalline nanovolumes
2013
δ Recycling is a simple procedure for directly extracting phase information from Patterson-type functions [Rius (2012). Acta Cryst. A68, 399-400]. This new phasing method has a clear theoretical basis and was developed with ideal single-crystal X-ray diffraction data. On the other hand, introduction of the automated diffraction tomography (ADT) technique has represented a significant advance in electron diffraction data collection [Kolb et al. (2007). Ultramicroscopy, 107, 507-513]. When combined with precession electron diffraction, it delivers quasi-kinematical intensity data even for complex inorganic compounds, so that single-crystal diffraction data of nanometric volumes are now availa…
Theoretical and structural studies of lithium cyclic amide conformations. Monomers and aggregates
2000
High level ab initio calculations on the conformations of unsolvated and solvated lithium piperidide, 1, and lithium morpholide, 2, were carried out. It was found that both monomers exhibit a global minimum for a chair structure with a planar nitrogen, and 2 shows an additional stable pseudo boat conformation. Dimers and amine–lithium amide mixed aggregates were also calculated including discrete solvation; the role of aggregation is clearly shown both by the changes in geometries and in the stabilization energies. Semiempirical calculations carried out on a recently synthesized tetrameric mixed aggregate give a geometry very similar to the structure determined by X-ray diffraction. The pre…
Using FOCUS to solve zeolite structures from three-dimensional electron diffraction data
2013
The programFOCUS[Grosse-Kunstleve, McCusker & Baerlocher (1997).J. Appl. Cryst.30, 985–995] was originally developed to solve zeolite structures from X-ray powder diffraction data. It uses zeolite-specific chemical information (three-dimensional 4-connected framework structure with known bond distances and angles) to supplement the diffraction data. In this way, it is possible to compensate, at least in part, for the ambiguity of the reflection intensities resulting from reflection overlap, and the program has proven to be quite successful. Recently, advances in electron microscopy have led to the development of automated diffraction tomography (ADT) and rotation electron diffraction (R…
Orthotropic plate dynamics by a novel meshfree method
2003
Publisher Summary This chapter deals with a novel meshfree method for the dynamic analysis of orthotropic plates under the Kirchhoff small deflection theory. The approach starts from a modified function whose stationarity conditions lead to the meshfree plate dynamic model through a discretization process—based on the use of orthotropic plate static fundamental solutions. The resolving system obtained is characterized by—frequency independent stiffness and mass matrices, which preserve the symmetry and definiteness properties of the continuum. Moreover, these operators are computed by boundary integrals of regular kernels. The method allows the application of standard numerical routines ava…
Instantaneous hourly and daily radiation on tilted surfaces
1991
A simple model to evaluate the irradiance incident on tilted surfaces of various orientations is proposed in this paper. It is based on the knowledge of the monthly daily average values of global and diffuse radiation on horizontal surfaces and their distributions over a period of a day. These distributions are approximated by a modified Gaussian expression with two parameters, which take into account the morning-to-afternoon asymmetries. Integrating the irradiance over a period of an hour or a day, values of monthly hourly average and daily on variously oriented surfaces have been calculated for the locations of Barcelona, Genoa, and Montreal. The calculated values are generally in good ag…
Magic numbers, excitation levels, and other properties of small neutral math clusters (N < 50)
2006
The ground-state energies and the radial and pair distribution functions of neutral math clusters are systematically calculated by the diffusion Monte Carlo method in steps of one math atom from 3 to 50 atoms. In addition the chemical potential and the low-lying excitation levels of each cluster are determined with high precision. These calculations reveal that the “magic numbers” observed in experimental math cluster size distributions, measured for free jet gas expansions by nondestructive matter-wave diffraction, are not caused by enhanced stabilities. Instead they are explained in terms of an enhanced growth due to sharp peaks in the equilibrium concentrations in the early part of the e…