Seeded crystal growth of the acentric organic nonlinear optical material methyl-p-hydroxybenzoate from the vapor phase

Using in situ differential interference contrast microscopy (DICM), growth morphology, structure, and step velocities of the vicinal hillocks on {110} and {111̅} faces of MHB crystal seeds growing from the vapor phase have been investigated over a supersaturation (σ) range of (0.2 < σ < 0.6). Under these conditions of supersaturation, a dislocation induced growth mechanism was identified. Ex situ atomic force microscopy (AFM) shows that some dislocation induced hillocks exhibit hollow cores. The general observations of the {110} and {111̅} surfaces reveal that these faces follow a classical mode of layer growth, continuous generation of new layers by dislocation outcrops, which subsequently bunch and spread to cover the entire facets. A tangential step velocity of the slow and fast sides of {110} and {111̅} growth hillocks show a linear dependence with supersaturation in the region of (0.2 < σ < 0.4). Analysis of this dependence leads to the respective growth parameters for the identified growth mechanism: the activation energies for the slow and fast step motion of a growth hillock (EaS and EaF) and the corresponding kinetic coefficients (βaS and βaF), for both faces. The growth from physical vapor transport (PVT) shows that for the title material, as with a number of other polar materials, solvent poisoning is not the cause of the highly differential growth rates and is an intrinsic feature of the crystal. The results suggest that in terms of the production of large single crystals of high perfection by PVT, the supersaturation range for dislocation growth should be between 0.2 and 0.4. These findings provide a foundation for the rational design of large MHB crystals that may find applications utilizing their high optoelectronic potential

Two new structures of 5-nitrouracil

The structure of monoclinic anhydrous 5-nitrouracil, C4H3N304, and of the solvate 5-nitrouracil dimethyl sulfoxide, C4H3N304.C2H6OS, are presented and compared with the previously known structures of the orthorhombic anhydrous form and the monohydrate

Cephalexin: a channel hydrate

The antibiotic cephalexin [systematic name: d-7-(2-amino-2-phenyl­acet­amido)-3-methyl-8-oxo-5-thia-1-aza­bi­cyclo­[4.2.0]oct-2-ene-2-carboxyl­ic acid] forms a range of isomorphic solvates, with the maximum hydration state of two water mol­ecules formed only at high relative humidities. The water content of the structure reported here (C16H17N3O4S·1.9H2O) falls just short of this configuration, having three independent cephalexin mol­ecules, one of which is disordered, and 5.72 observed water mol­ecules in the asymmetric unit. The facile nature of the cephalexin solvation/desolvation process is found to be facilitated by a complex channel structure, which allows free movement of solvent in the crystallographic a and b directions

Two new paracetamol / dioxane solvates - a system exhibiting a reversible solid state phase transformation

This work reports on the crystal structures of two dioxane solvates of paracetamol that are true polymorphs. The high temperature phase is an orthorhombic form, space group Pbca, Z = 8, a = 12.6078(3) î.., b = 12.1129(2) î.., c = 13.4138(3) î.., V = 2048.52(7) î..3, (at 295 K) and the low temperature form is monoclinic, space group P21/c, Z = 4, a = 12.325(6) î.., b = 11.965(4) î.., c = 13.384(6) î.., = 92.01°, V = 1972.6(14)î..3 (at 123 K). The structures of these polymorphs are described as is the interrelationship between the two structures. In addition to the structural interrelationship, it is shown that the two forms undergo a reversible phase transformation. Desolvation of either form generates the stable monoclinic phase of paracetamol

The selection and performance of diamond radiators used in coherent bremsstrahlung experiments

The bremsstrahlung emitted as a result of scattering electrons in thin diamond crystals provides a useful source of high energy photons for use in photonuclear experiments, since the coherent bremsstrahlung produced is linearly polarized. Techniques for selecting the most favorable diamonds have been investigated. These are optical polaroid analysis, X-ray topography and rocking curve measurements. The diamonds are characterized with a view to determining their performance as radiators, and bremsstrahlung spectra from a diamond radiator used at the Mainz MAMI-B facility are presented. The changes caused by high energy electrons to the crystal properties of the diamond and to the resulting coherent bremsstrahlung spectra are discussed