Local Static Disorder Research Articles

Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals.

We show that formamidinium-based crystals are distinct from methylammonium-based halide perovskite crystals because their inorganic sublattice exhibits intrinsic local static disorder that coexists with a well-defined average crystal structure. Our study combines terahertz-range Raman scattering with single-crystal X-ray diffraction and first-principles calculations to probe the evolution of inorganic sublattice dynamics with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal structural dynamics and phase transitions at higher temperatures.

Gem-Quality Tourmaline from LCT Pegmatite in Adamello Massif, Central Southern Alps, Italy: An Investigation of Its Mineralogy, Crystallography and 3D Inclusions

In the early 2000s, an exceptional discovery of gem-quality multi-coloured tourmalines, hosted in Litium-Cesium-Tantalum (LCT) pegmatites, was made in the Adamello Massif, Italy. Gem-quality tourmalines had never been found before in the Alps, and this new pegmatitic deposit is of particular interest and worthy of a detailed characterization. We studied a suite of faceted samples by classical gemmological methods, and fragments were studied with Synchrotron X-ray computed micro-tomography, which evidenced the occurrence of inclusions, cracks and voids. Electron Microprobe combined with Laser Ablation analyses were performed to determine major, minor and trace element contents. Selected samples were analysed by single crystal X-ray diffraction method. The specimens range in colour from colourless to yellow, pink, orange, light blue, green, amber, brownish-pink, purple and black. Chemically, the tourmalines range from fluor-elbaite to fluor-liddicoatite and rossmanite: these chemical changes occur in the same sample and affect the colour. Rare Earth Elements (REE) vary from 30 to 130 ppm with steep Light Rare Earth Elemts (LREE)-enriched patterns and a negative Eu-anomaly. Structural data confirmed the elbaitic composition and showed that high manganese content may induce the local static disorder at the O(1) anion site, coordinating the Y cation sites occupied, on average, by Li, Al and Mn2+ in equal proportions, confirming previous findings. In addition to the gemmological value, the crystal-chemical studies of tourmalines are unanimously considered to be a sensitive recorder of the geological processes leading to their formation, and therefore, this study may contribute to understanding the evolution of the pegmatites related to the intrusion of the Adamello pluton.

Does Z' equal 1 or 2? Enhanced powder NMR crystallography verification of a disordered room temperature crystal structure of a p38 inhibitor for chronic obstructive pulmonary disease.

The crystal structure of the Form A polymorph of N-cyclopropyl-3-fluoro-4-methyl-5-[3-[[1-[2-[2-(methylamino)ethoxy]phenyl]cyclopropyl]amino]-2-oxo-pyrazin-1-yl]benzamide (i.e., AZD7624), determined using single-crystal X-ray diffraction (scXRD) at 100 K, contains two molecules in the asymmetric unit (Z' = 2) and has regions of local static disorder. This substance has been in phase IIa drug development trials for the treatment of chronic obstructive pulmonary disease, a disease which affects over 300 million people and contributes to nearly 3 million deaths annually. While attempting to verify the crystal structure using nuclear magnetic resonance crystallography (NMRX), we measured 13C solid-state NMR (SSNMR) spectra at 295 K that appeared consistent with Z' = 1 rather than Z' = 2. To understand this surprising observation, we used multinuclear SSNMR (1H, 13C, 15N), gauge-including projector augmented-wave density functional theory (GIPAW DFT) calculations, crystal structure prediction (CSP), and powder XRD (pXRD) to determine the room temperature crystal structure. Due to the large size of AZD7624 (ca. 500 amu, 54 distinct 13C environments for Z' = 2), static disorder at 100 K, and (as we show) dynamic disorder at ambient temperatures, NMR spectral assignment was a challenge. We introduce a method to enhance confidence in NMR assignments by comparing experimental 13C isotropic chemical shifts against site-specific DFT-calculated shift distributions established using CSP-generated crystal structures. The assignment and room temperature NMRX structure determination process also included measurements of 13C shift tensors and the observation of residual dipolar coupling between 13C and 14N. CSP generated ca. 90 reasonable candidate structures (Z' = 1 and Z' = 2), which when coupled with GIPAW DFT results, room temperature pXRD, and the assigned SSNMR data, establish Z' = 2 at room temperature. We find that the polymorphic Form A of AZD7624 is maintained at room temperature, although dynamic disorder is present on the NMR timescale. Of the CSP-generated structures, 2 are found to be fully consistent with the SSNMR and pXRD data; within this pair, they are found to be structurally very similar (RMSD16 = 0.30 Å). We establish that the CSP structure in best agreement with the NMR data possesses the highest degree of structural similarity with the scXRD-determined structure (RMSD16 = 0.17 Å), and has the lowest DFT-calculated energy amongst all CSP-generated structures with Z' = 2.

Local Static Distortion of Rare Earth Ions in ROs4Sb12 (R = La, Ce, Pr, Nd, Sm) by Extended X-ray Absorption Fine Structure

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on ROs 4 Sb 12 (R = La, Ce, Pr, Nd, Sm) to study the motion of rare earth ions accommodated in the Sb icosahedron cage. The temperature dependences of the thermal (Debye–Waller) factors were analyzed from 15 to 300 K. The Einstein temperature obtained from EXAFS analysis of SmOs 4 Sb 12 is the lowest of the compounds, while there are no significant differences among the other compounds. The local static disorder obtained from the temperature dependence of EXAFS shows good correlation with the space inside the Sb icosahedron cage. Effective static distortion is expected to occur when the cage space is larger than about 0.5 A.

Structural and transport characteristics of the LAMOX family of fast oxide-ion conductors, based on lanthanum molybdenum oxide La2Mo2O9

A new family of fast O2− conductors, which exhibit anionic conductivity comparable to that of stabilised zirconia, is presented. The parent compound of this new family, hereafter called LAMOX, is lanthanum molybdate La2Mo2O9. Various substitutions have been attempted: on the lanthanum site (La2 − xAx)Mo2O9 with A = Sr, Ba, K, or Bi; on the molybdenum site La2(Mo2 − xBx)O9 with B = Re, S, W, Cr and V; and on the oxygen site with fluorine. Most of these substitutions suppress the phase transition which occurs in La2Mo2O9 around 580 °C from a low temperature α form to a high temperature (more conducting) β form, and stabilise the β form at room temperature. Several members of the LAMOX series are studied through X-ray and neutron diffraction, and conductivity measurements. Large O2− thermal factors and local static disorder agree well with the anionic nature of the conductivity. Partly vacant sites with short inter-site distances suggest a most probable conduction path with tridimensional character.

New Opportunities in the Soft X-Ray Absorption to Characterize the Adsorbate Bonding

One of the important applications of XAS in the soft X-ray regime is the ability to measure the EXAFS of light elements, like carbon, nitrogen, etc. up to sulfur and chlorine. These elements, adsorbed on metal surfaces, are the most important constituents for the investigation of gas-metal interactions. One decade ago, the surface EXAFS world was harmonic, the nearest neighbor distances were determined within the harmonic approximation, R did not change with temperature, and the coordination number N was determined from measurements at a single temperature. Recently, SEXAFS data were taken as a function of T and at various angles of X-ray incidence. This advance, in combination with multiple scattering analysis and full simulation computer codes, allows to routinely determine the pair distribution function up to the third moment. We relate in general terms the Einstein temperature, the local static disorder and thermal expansion to the ionic versus covalent bonding, shape of effective pair potentials and adsorbate induced surface reconstruction. Furthermore, these low Z systems are ideal candidates to detect unambiguously the photoelectron backscattering by interstitial charge densities (AXAFS).

Local structure and dynamics of disordered systems studied by EXAFS

Abstract EXAFS spectroscopy has been used to study and compare the local structure in systems of various kinds and degrees of disorder: c-Ge, a-Ge, AgI and AgI-doped superionic glasses. The non-standard cumulant analysis of EXAFS has allowed an accurate characterization of local disorder of both thermal and static origin. Non-Gaussian distributions of interatomic distances and the corresponding anharmonic pair potentials have been reconstructed in a model-independent way from experimental results. Harmonic and anharmonic contributions to thermal disorder have been singled out, leading to refined comparisons with harmonic vibrational models. A strong correlation of vibrational motion of silver and iodine nearest-neighbour ions has been found in both β-and α-AgI. In amorphous systems the cumulant method allows the accurate characterization of local static disorder; as for a-Ge, interatomic distance and coordination number are in very good agreement with neutron diffraction results.

Treatment of thermal vibrations and local static disorder within tensor LEED.

A general formalism is developed to include thermal vibrations and local static disorder into tensor-LEED calculations. The given expression for the averaged t matrix is based on a cumulant expansion for the probability distribution of atomic displacements. The method is a generalization of the concept of temperature-dependent scattering phase shifts and allows an efficient description of both anisotropy and anharmonicity of thermal motions, which are important in particular for adsorbate atoms and substrate atoms in the topmost layers.

Surface exafs study of the crystallization of a metallic glass

The crystallization of an amorphous solid on high-temperature annealing may occur progressively from the surface into the bulk or uniformly throughout the whole sample. We have studied crystallization by means of surface EXAFS in the metallic glass Fe 40Ni 32Cr 8P 12B 8. Samples were annealed at various temperatures and times ranging from 240°C for 10 min to 400°C for 42 h; X-ray diffraction showed the gradual formation of crystalline phases. EXAFS measurements were made from the surface to about 4000 Å deep, with a depth resolution of a few hundred Å. The EXAFS results were quite sensitive to the crystallization, an effect quantified by model fitting in which the Debye-Waller factor σ 2 was varied as a parameterization of the local static disorder. We find that for partly crystallized samples σ 2 increases at the surface compared to deeper inside the sample. Since σ 2 is a direct measure of local static disorder (thermal vibrations are not important here as all measurements were made at room temperature), in this sytem these results imply that crystallization begins at the surface before it occurs in the bulk.