Real time observation of the hydrothermal crystallization of barium titanate using in situ neutron powder diffraction.

The hydrothermal crystallization of barium titanate, BaTiO3, has been studied in situ by time-resolved powder neutron diffraction methods using the recently developed Oxford/ISIS hydrothermal cell. This technique has allowed the formation of the ferroelectric ceramic to be followed in a noninvasive manner in real time and under genuine reaction conditions. In a first set of experiments, Ba(OD)2-8D2O was reacted with two different titanium sources, either crystalline TiO2 (anatase) or amorphous TiO2-H2O in D2O, at 100-140 degrees C and the reaction studied using the POLARIS time-of-flight neutron powder diffractometer, at the ISIS Facility. In a second series of experiments, the reaction between barium chloride and crystalline TiO2 (anatase) in NaOD/D2O was studied at temperatures between 100 and 200 degrees C and at different deuterioxide concentrations using the constant-wavelength D20 neutron powder diffractometer at the Institut Laue Langevin. Quantitative growth and decay curves were determined from analysis of the integrated intensities of Bragg reflections of starting materials and product phases. In both sets of experiments the rapid dissolution of the barium source was observed, followed by dissolution of the titanium source before the onset of crystallization of barium titanate. Using a nucleation-growth model we are able to simulate the growth curve of barium titanate at three temperatures. Our results indicate the predominance of a homogeneous dissolution-precipitation mechanism for the hydrothermal formation of barium titanate, rather than other possible mechanisms that have been discussed in the literature. Analysis of the line widths of the Bragg reflections in the neutron diffraction data indicates that the particle size of the BaTiO3 product phase prepared from the amorphous TiO2-H2O is smaller than that prepared from crystalline TiO2 (anatase).     

The Mechanism of Nonenzymatic Template Copying with Imidazole‐Activated Nucleotides

The emergence of the replication of RNA oligonucleotides was a critical step in the origin of life. An important model for the study of nonenzymatic template copying, which would be a key part of any such pathway, involves the reaction of ribonucleoside‐5′‐phosphorimidazolides with an RNA primer/template complex. The mechanism by which the primer becomes extended by one nucleotide was assumed to be a classical in‐line nucleophilic‐substitution reaction in which the 3′‐hydroxyl of the primer attacks the phosphate of the incoming activated monomer with displacement of the imidazole leaving group. Surprisingly, this simple model has turned out to be incorrect, and the dominant pathway has now been shown to involve the reaction of two activated nucleotides with each other to form a 5′–5′‐imidazolium bridged dinucleotide intermediate. Here we review the discovery of this unexpected intermediate, and the chemical, kinetic, and structural evidence for its role in template copying chemistry.     

A Note on Fracture Models Incorporating Surface Elasticity

Several recent papers that describe an approach to developing a theory of brittle fracture incorporating crack-surface elasticity have asserted that such models predict bounded stresses and strains at crack-tips in two dimensions. It is proved in this short note that the assertion is incorrect. While incorporating surface elasticity into fracture models in the setting of the linearized theory of elasticity does remove the square-root stress/strain singularity at crack-tips seen in classical theories that do not take account of crack-surface interfacial effects, it does not predict bounded stresses and strains but rather diminishes the singularity to being logarithmic.     

Generation of hollow crystalline tungsten oxide fibres

Hollow fibres of crystalline W₁₈O₄₉, arranged in a pine-tree-like structure, were generated by heating WS₂ powder in oxygen. SEM, TEM, X-ray and electron diffraction studies show that the individual fibres are composed of single-crystal tungsten oxide whiskers. One-dimensional growth, involving heat dissipation along the fibre axis, is proposed.     

Dynamics on the microsecond timescale in microporous aluminophosphate AlPO-14 as evidenced by 27Al MQMAS and STMAS NMR spectroscopy

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.     

Chemical activation of MgH2; a new route to superior hydrogen storage materials

We report the discovery of a new, chemical route for 'activating' the hydrogen store MgH2, that results in highly effective hydrogen uptake/release characteristics, comparable to those obtained from mechanically-milled material.     

Radical-carbanion cyclo-coupling in armed aromatics: overriding steric hindrance to ring closure

Omega-(2-Halophenyl)alkyl-2-oxazolines were prepared and reacted via base promoted intramolecular coupling of radical with carbanionic centres to yield 1-phenyl-1-oxazolino-indan and -tetralin derivatives containing quaternary C-atoms.     

Two chain gallium fluorodiphosphates: synthesis,structure solution,and their transient presence during the hydrothermal crystallisation of a microporous gallium fluorophosphate

Two novel gallium fluorodiphosphates have been isolated and their structures solved ab initio from powder X-ray diffraction data; the materials readily interconvert under hydrothermal conditions, and are metastable with respect to an open-framework zeolitic gallium fluorophosphate, during the synthesis of which they are present as transient intermediates.     

Novel nanotubes and encapsulated nanowires

2 mixtures. Prolonged electron irradiation of these nanowires leads to axial growth and to dynamic transformations. These observations suggest ways in which materials may be modified by microencapsulation and irradiation. Received: 31 July 1997/Accepted: 6 October 1997     

The microwave spectra of symmetric top polyacetylenes: 1,3,5-Heptatriyne,CH3(CC)3H and 1-cyano-2,4-pentadiyne,CH3(CC)2CN

The rotational spectra of 1,3,5-heptatriyne, CH₃(CC)₃H and 1-cyano-2,4-pentadiyne, CH₃(CC)₂CN have been studied in detail between 26.5 and 40.0 GHz. The molecules have long linear chains of heavy atoms and show characteristic C_3v symmetric top spectra consisting of groups of R-branch lines at regular intervals separated by approximately 2B₀. Six isotopic modifications of CH₃(CC)₂CN have been detected in natural abundance allowing r_s substitution structural data to be derived for this species. Long linear polyacetylenic chains are quite flexible and this dynamic property manifests itself in the appearance of extended sequences of complex vibrational satellites associated with the bending of the chain. The vibrational ground state spectra as well as several low frequency vibrational satellites have been analyzed yielding various vibration-rotation parameters. For CH₃(CC)₃H B₀ = 778.2445 ± 0.0007 and for CH₃(CC)₂CN B₀ = 778.040 ± 0.001 MHz.