Rietveld Refinement in the Characterization of Crystalline Materials (original) (raw)
Related papers
Elucidation of the pressure induced amorphization of tetrahydrofuran clathrate hydrate
The Journal of Chemical Physics, 2019
The type II clathrate hydrate (CH) THF⋅17 H 2 O (THF = tetrahydrofuran) is known to amorphize on pressurization to ∼1.3 GPa in the temperature range 77-140 K. This seems to be related to the pressure induced amorphization (PIA) of hexagonal ice to high density amorphous (HDA) ice. Here, we probe the PIA of THF-d 8 ⋅ 17 D 2 O (TDF-CD) at 130 K by in situ thermal conductivity and neutron diffraction experiments. Both methods reveal amorphization of TDF-CD between 1.1 and 1.2 GPa and densification of the amorphous state on subsequent heating from 130 to 170 K. The densification is similar to the transition of HDA to very-high-density-amorphous ice. The first diffraction peak (FDP) of the neutron structure factor function, S(Q), of amorphous TDF-CD at 130 K appeared split. This feature is considered a general phenomenon of the crystalline to amorphous transition of CHs and reflects different length scales for D-D and DO correlations in the water network and the cavity structure around the guest. The maximum corresponding to waterwater correlations relates to the position of the FDP of HDA ice at ∼1 GPa. Upon annealing, the different length scales for waterwater and water-guest correlations equalize and the FDP in the S(Q) of the annealed amorph represents a single peak. The similarity of local water structures in amorphous CHs and amorphous ices at in situ conditions is confirmed from molecular dynamics simulations. In addition, these simulations show that THF guest molecules are immobilized and retain long-range correlations as in the crystal.
Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization
Materials, 2016
The binary intermetallic clathrates K 8-x Si 46 (x = 0.4; 1.2), Rb 6.2 Si 46 , Rb 11.5 Si 136 and Cs 7.8 Si 136 were prepared from M 4 Si 4 (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb 11.5 Si 136 was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si 20 dodecahedra, was found in all compounds. SPS preparation of Na 24 Si 136 with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K 7.6 Si 46 and K 6.8 Si 46 in the temperature range 4-700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques.
Salts and Polymorph Screens for Bedaquiline
AAPS PharmSciTech
Bedaquiline is used to treat multi-resistant tuberculosis in adults. The fumarate salt is commercially available and used in the product Sirturo. To provide open access to bedaquiline molecule once the patent on the chemical substance expires, new salts were screened. This work offers additional information on the bedaquiline system, as new salts may present better pharmacokinetic properties. The current studies focus on the attempted isolation of the acetate, benzoate, benzenesulfonate, hydrobromide, succinate, hydrochloride, tartrate, lactate, maleate, malate, and mesylate salts of bedaquiline. Potential salts were screened using a unique combination of conventional screening, and small-scale experiments supplemented by crystallographic analysis and infrared microspectroscopy. Salts were prepared on a larger scale by dissolving 1:1 ratios of the individual salt formers and bedaquiline base (30 mg, 0.055 mmol) in different solvents and allowing the solutions to evaporate or crystal...
Nanomaterials, 2021
The large amounts of natural gas in a dense solid phase stored in the confined environment of porous materials have become a new, potential method for storing and transporting natural gas. However, there is no experimental evidence to accurately determine the phase state of water during nanoscale gas hydrate dissociation. The results on the dissociation behavior of methane hydrates confined in a nanosilica gel and the contained water phase state during hydrate dissociation at temperatures below the ice point and under atmospheric pressure are presented. Fourier transform infrared spectroscopy (FTIR) and powder X-ray diffraction (PXRD) were used to trace the dissociation of confined methane hydrate synthesized from pore water confined inside the nanosilica gel. The characterization of the confined methane hydrate was also analyzed by PXRD. It was found that the confined methane hydrates dissociated into ultra viscous low-density liquid water (LDL) and methane gas. The results showed ...
Pharmaceutics, 2022
There is an urgent need for new drugs to overcome the challenge of the ever-growing drug resistance towards tuberculosis. A new, highly efficient anti-tuberculosis drug, Perchlozone (thioureidoiminomethylpyridinium perchlorate, Pz), is only available in an oral dosage form, though injectable forms and inhalation solutions could be better alternatives, offering higher bioavailability. To produce such forms, nano- and micro-particles of APIs would need to be prepared as dispersions with carriers. We use this case study to illustrate the principles of selecting solvents and excipients when preparing such formulations. We justify the choice of water–THF (19.1 wt % THF) as solvent and mannitol as carrier to prepare formulations of Pz—a poorly soluble compound—that are suitable for injection or inhalation. The formulations could be prepared by conventional freeze-drying in vials, making the proposed method suitable for industrial scaling. A similar strategy for selecting the organic solve...
Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems
Proceedings of the National Academy of Sciences, 2012
There is interest in the role of ammonia on Saturn’s moons Titan and Enceladus as the presence of water, methane, and ammonia under temperature and pressure conditions of the surface and interior make these moons rich environments for the study of phases formed by these materials. Ammonia is known to form solid hemi-, mono-, and dihydrate crystal phases under conditions consistent with the surface of Titan and Enceladus, but has also been assigned a role as water-ice antifreeze and methane hydrate inhibitor which is thought to contribute to the outgassing of methane clathrate hydrates into these moons’ atmospheres. Here we show, through direct synthesis from solution and vapor deposition experiments under conditions consistent with extraterrestrial planetary atmospheres, that ammonia forms clathrate hydrates and participates synergistically in clathrate hydrate formation in the presence of methane gas at low temperatures. The binary structure II tetrahydrofuran + ammonia, structure ...
Journal of the …, 2007
A hydrogen-encapsulated inorganic clathrate, which is stable at ambient temperature and pressure, has been prepared in high yield. Na5.5(H2)2.15Si46 is a sodium-deficient, hydrogen-encapsulated, type I silicon clathrate. It was prepared by the reaction between NaSi and NH4Br under dynamic vacuum at 300°C. The Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. The type I clathrate structure has two types of cages where the guest species, in this case Na and H2, can reside: a large cage composed of 24 Si, in which the guest resides in the 6d crystallographic position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position. Solid-state 23 Na, 1 H, and 29 Si MAS NMR confirmed the presence of both sodium and hydrogen in the clathrate cages. 23
Nanomechanical Characterization of Enzyme Induced Carbonate Precipitates
Crystals
The mechanical properties of calcium carbonate minerals formed by enzyme-induced carbonate precipitation (EICP) were studied using nanoindentation. Two types of precipitates were considered: (i) a “baseline” precipitate, synthesized via urea hydrolysis in an aqueous solution of urease enzyme, urea, and calcium chloride; and (ii) a “modified” precipitate, synthesized from a similar solution, but with the inclusion of nonfat dry milk. While both precipitates predominantly comprised calcite, X-ray diffraction and Raman spectroscopy indicated broader peaks in the modified precipitate, implying differences in the crystal structure of the two precipitates. Both precipitates were polycrystalline and had a higher average indentation hardness (H) and a lower indentation modulus (M) compared with the values for single calcite crystals reported in the literature. The ductility of the precipitates was quantified by the ratio M/H. The modified precipitate had a higher average M/H, implying great...
Continuous Manufacture and Scale-Up of Theophylline-Nicotinamide Cocrystals
Pharmaceutics
The aim of the study was the manufacturing and scale-up of theophylline-nicotinamide (THL-NIC) pharmaceutical cocrystals processed by hot-melt extrusion (HME). The barrel temperature profile, feed rate and screw speed were found to be the critical processing parameters with a residence time of approximately 47 s for the scaled-up batches. Physicochemical characterization using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X-ray diffraction of bulk and extruded materials revealed the formation of high purity cocrystals (98.6%). The quality of THL-NIC remained unchanged under accelerated stability conditions.
The chemistry of low dosage clathrate hydrate inhibitors
Chemical Society Reviews, 2013
This review aims to introduce the chemistry of low dosage inhibitors of clathrate hydrate formation within the context of their role in the oil and gas industry. The review covers both kinetic hydrate inhibitors and anti-agglomerants from the point of view of structure-function relationships, focussing on recent refinements in mechanistic understanding and chemical design, and the consequently evolving and increasingly fine-tuned properties of these fascinating compounds. Clathrate hydrates Clathrate hydrates are crystalline, non-stoichiometric host-guest compounds comprising a hydrogen bonded water framework, into which small molecular guest species such as methane are included within cavities formed by the water cages. Because there are no strong directional interactions between guest and host the guests are free to vibrate and rotate but possess limited translational motion. 1 Typically common clathrate hydrates comprise 85 mol% water and 15 mol% guest(s) when all of the cavities are occupied. 2 These materials form when the components are subjected to ambient temperatures (generally less than 300 K) and moderate pressures (>0.6 MPa); conditions frequently found in oil and gas pipelines. 3 65 chemistry of low dosage clathrate hydrate inhibitors, their mechanism of action and the constraints placed upon performance by regulatory and environmental factors. Clathrate hydrate structures 70 Three common clathrate hydrate structures exist: structure I (cubic); structure II (cubic) and structure H (hexagonal); with the structure adopted being dependent predominantly upon the size of the guest(s). 12 Typically each cavity will accommodate one guest molecule, although when exposed to higher pressures (500 bar) 75 multiple occupancy can be reached. 13 A fourth general structure type has been reported, known as structure T (trigonal), which forms in the presence of dimethyl ether. In addition, an unusual complex structure is known comprising alternating stacks of structure H and structure II. 14 However, only structure I and 80 structure II will be discussed in detail herein, as these are the two most frequently observed within pipelines and hence are of importance in the context of inhibition chemistries. The cavities present within clathrate hydrates are supported by the repulsion 2 | Journal Name, [year], [vol], 00-00 This journal is © The Royal Society of Chemistry [year] forces imparted on the water molecules by the guest species. 15 Rodger has argued that such repulsion forces prevent the collapse of the expanded cavities (relative to ice, Ih), and that the attractive dispersion forces are unimportant in this context. 15