J. N. Sherwood

The temperature dependence of positron lifetimes in solid pivalic acid

Abstract The temperature dependence of positron lifetimes in both the brittle and plastic phases of trimethylacetic (pivalic) acid 1 has been examined using the positron lifetime technique. In the plastic phase two long-lived components attributable to ortho-positronium (ortho-Ps) decay are observed. The longer of these (≈2.8 ns) increases with temperature and is believed to be characteristic of ortho-Ps trapped at defects, probably mono- and di-vacancies. A shorter lifetime component (≈1 ns) which shows little temperature dependence is also present in the brittle phase. Additionally in the brittle phase and close to the transition region a longer lifetime is detected. This may also be associated with trapping of ortho-Ps in thermally created defects, probably vacancies. In the plastic phase the intensity of a short-lived component associated with para-Ps is confirmed to be approximately one third of the total intensity of the two long-lived components associated with ortho-Ps, but in the brittle phase it higher than one third. On using data for several plastic crystals a relationship is established between the lifetime of ortho-Ps trapped in a vacancy and the vacancy volume.

Crystal growth and physical characterization of the semiorganic bis(thiourea) cadmium chloride

Single crystals of bis(thiourea) cadmium chloride (BTCC), a novel semiorganic nonlinear optical material, were grown from aqueous solutions. The grown crystals were characterized by recording the powder X-ray diffraction pattern and by identifying the diffracting planes. BTCC has a transmission that extends from 285 to 1900 nm. Differential thermal analysis carried out on BTCC indicates that the material does not sublime before it melts at 215°C. The grown crystals were estimated to have a moderate defect density. The mechanical hardness of BTCC was evaluated in the (0 1 0) plane.

Crystal growth and defects characterization of zinc tris (thiourea) sulfate: a novel metalorganic nonlinear optical crystal

Abstract Single crystals of the metalorganic nonlinear optical material zinc tris (thiourea) sulfate (ZTS) were grown from aqueous solution. The morphology of the crystals was indexed. The grown crystals were characterized by recording the powder X-ray diffraction pattern and by identifying the diffracting planes. Spectrophotometric studies on ZTS reveal that it has good transparency for the Nd: YAG laser fundamental wavelength. Differential thermal analysis of ZTS indicates that the material does not sublime before melting but decomposes immediately after melting. The defect content of the crystals was estimated using etching and X-ray topography. The mechanical hardness anisotropy was evaluated in the (100) plane, which indicates the presence of soft directions.

Assessment of the strain in small sodium chlorate crystals and its relation to growth rate dispersion

Abstract The growth rates of small (5–150 μm) crystals of sodium chlorate grown in solution from secondary nuclei have been measured by optical microscopy. Significant growth rate dispersion((0–7.81)×10-8 m s-1) was observed at constant supersaturation (0.35%). This dispersion was independent of crystal size and solution agitation. Zero growth rates were confined, however, to the smaller sized particles. Crystals removed from solution following growth were examined by Laue diffraction techniques using synchrotron X-radiation. The Laue patterns of separate crystals showed different degrees of radial asterism indicative of the presence of different degrees of lattice strain (mosaic spread) in the crystals. Analysis of these patterns confirmed that the growth rate of any crystal depended on the degree of lattice strain, the most strained crystals growing at the lowest rates. This correlation indicates that the strain concentrated in or at the surface of very small crystals during fracture or refacetting may well play a dominant role in growth rate dispersion.

The growth of single crystals of some organic compounds by the Czochralski technique and the assessment of their perfection

Abstract The application of the Czochralski technique to the growth of crystals of some organic materials is described. It is found that the low melting points, thermal conductivities and melt-surface tensions of these compounds are not a barrier to the use of this method of crystal growth. Crystals of dimensions up to 10 cm 3 can be pulled in periods of 1–2 days. The analysis of the perfection of the products by X-ray topography is described and indicates that the crystals contain substantial volumes (cm 3 ) which are essentially dislocation-free. This represents a considerable improvement on the crystals produced by the more commonly used Bridgman technique and compares favourably with the perfection achievable by growth from solution in much longer periods.

The growth and perfection of phenanthrene single crystals: 1. Purification and single crystal growth

A study has been made of the purification of phenanthrene, and the growth of large crystals from the melt by the Bridgman technique and their perfection. Of eight major impurities in the starting material four have segregation coefficients, k < 0.6 and segregate efficiently during zone refining. The remaining four which were identified as anthracene (k = 1.6), flourene (k = 0.95), dibenzothiophene (k = 0.8) and dihydrophenanthrene (k = 0.8) are inefficiently removed. When these are removed chemically, subsequent zone refining yields material in the centre part of the refined ingot of total impurity content ⪡ 1 ppm. When this material is used for crystal growth by the Bridgman technique, crystals of excellent quality can be produced. The influence of impurities on the growth process is outlined.

Crystal growth and characterization of semiorganic nonlinear optical material: sodium p -nitrophenolate dihydrate

Sodiump-nitrophenolate dihydrate (NPNa) is a new semiorganic nonlinear optical crystal. It has a deff of about 1.2 times that of potassium titanyl phosphate. Large single crystals of NPNa were grown from water and methanol solutions by isothermal solvent evaporation. The morphology of the grown crystals was indexed. The defect content of the methanol grown crystals was evaluated by chemical etching and synchrotron topography. The section topography carried out on a large uncut crystal revealed a considerable reduction in the defect density away from the seed. The improved optical transmission of NPNa crystals was achieved by purifying the starting materials. The mechanical hardness of the NPNa crystals in the 010 plane was evaluated by Vickers and Knoop indentations.

Investigation into the structures of some normal alkanes within the homologous series C13H28 to C60H122 using high-resolution synchrotron X-ray powder diffraction

High-resolution X-ray powder diffraction using synchrotron radiation has been used to determine the unit-cell parameters of some of the n-alkanes within the range C13H28–C60H122. These have, in general, been found to conform to the structural patterns predicted by Nyburg and Potworowski (Acta Crystallogr., Sect. B, 1973, 29, 347). Polymorphic behaviour has been observed for the n= even alkanes associated with increasing symmetry from a triclinic structure (space group P1), to a monoclinic structure (space group P21/a) and to an orthorhombic structure (mostly space group Pca21) as the chain length increases. The n-alkanes C38H78,C40H82 and C44H90 were found to crystallize in the polytypic orthorhombic polymorph (space group Pbca) proposed by Boistelle et al.(Acta Crystallogr., Sect. B, 1976, 32, 1240). The n= odd alkanes all conform to an orthorhombic structure (space group Pbcm). A comparative examination of the data, as a function of chain length, suggests the existence of a degree of structural disorder along the crystallographic c-axis for the longer homologues, which implies that incipient chain folding takes place.

Dislocation Slip Systems in Pentaerythritol Tetranitrate (PETN) and Cyclotrimethylene Trinitramine (RDX) [and Discussion]

An exam ination has been made of the orientation dependence of microhardness determined using a Knoop indenter on the principal habit faces of RDX ({210}) and PETN ({110}). The variations found (RDX , 32-44 kg mm-2; PETN, 13-24 kg mm-2) were consistent with the crystallographic symmetry of the solids and confirm previous estimates obtained using a Vickers indenter. On the basis that the principal deformation mechanism is dislocation slip and that the variation in hardness reflects the orientation of the dominant slip systems to the surface and hence to the faces of the indenter, a theoretical assessment was made using the models of F. W. Daniels and C. G. Dunn and C. A. Brookes, J. B. O’Neill and B.A.W. Redfern of the expected variation in hardness due to this cause. Satisfactory agreement with experiment was obtained using slip systems of the types (010) [001] and {021} [100] for RDX and (110) [111]-type for PETN . This is the first time that this type of analysis has been attempted for such highly anisotropic systems. These assignments were confirmed by etching and by X-ray topographic analysis of the distribution of dislocation alignments around the hardness impressions. The difference in mechanical behaviour of these solids and its possible role in sensitivity to detonation is discussed on the basis of the relative ease of migration of these dislocation systems.

Terahertz pulse generation in an organic crystal by optical rectification and resonant excitation of molecular charge transfer

Organic molecular crystals that are extremely efficient at terahertz-pulse generation are in- vestigated. Terahertz pulses produced by optical rectification at 800 nm in (−)2-(α-methylbenzyl-amino)-5-nitropyridine have an order of magnitude higher power than those generated in the commonly used inorganic crystal ZnTe. The organic molecular crystals were also found to generate terahertz pulses when excited on resonance at 400 nm. This may pave the way for studying ultrafast charge-transport dynamics in three dimensions.