The temperature dependence of positron lifetimes in solid pivalic acid

Crystal polymorphism is encountered in all areas of research involving solid substances. Its occurrence introduces complications during manufacturing processes and adds another dimension to the complexity of designing materials with specific properties. Research on polymorphism is fraught with unique difficulties due to the subtlety of polymorphic transformations and the inadvertent formation of pseudopolymorphs. In this report, a summary of thermodynamic, kinetic and structural considerations of polymorphism is presented. A wide variety of techniques appropriate to the study of organic crystalline polymorphism and pseu-dopolymorphism is then surveyed, ranging from simple crystal density measurement to observation of polymorphic transformations using variable-temperature synchrotron X-ray diffraction methods. Application of newer methodology described in this report is yielding fresh insights into the nature of the crystallization process, holding promise for a deeper understanding of the phenomenon of polymorphism and its practical control.

Crystalline Polymorphism of Organic Compounds

Program system for analysing positron lifetime spectra and angular correlation curves

Controlling anisotropy is a key concept in the generation of complex functionality in advanced materials. For this concept, oriented attachment of nanocrystal building blocks, a self-assembly of particles into larger single-crystalline objects, is one of the most promising approaches in nanotechnology. We report here the two-dimensional oriented attachment of lead sulfide (PbS) nanocrystals into ultrathin single-crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by cosolvents, which alter nucleation and growth rates during the primary nanocrystal formation, and is finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photodetector device without further treatment.

https://science.sciencemag.org/content/sci/329/5991/550.full.pdf

Ultrathin PbS sheets by two-dimensional oriented attachment.

The past decade has seen the development of microporous materials (i.e., materials containing pores of dimensions <2 nm) derived wholly from organic components. Here we review this nascent area with a particular emphasis on amorphous polymers that possess intrinsic microporosity (IM), which is defined as microporosity that arises directly from the shape and rigidity of component macromolecules. Although IM can be readily identified within soluble non-network polymers and oligomers, for network polymers it is harder to differentiate IM from template effects that are responsible for the microporosity within hyper-cross-linked networks. The numerous examples of microporous polymers assembled from rigid monomers by the formation of rigid linking groups are surveyed and their IM assessed. The potential applications of these materials are highlighted.

Exploitation of Intrinsic Microporosity in Polymer-Based Materials

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

Two crystal structures of 2-4-6 trinitrotoluene (TNT) are given, the monoclinic form (a0 ) 1.49113 (1) nm, b0 ) 0.60340 (1) nm, c0 ) 2.08815(3) nm, â ) 110.365 (1)°, V ) 1.76137 (4) nm3, space group ) P21/a, T ) 100 K) and the orthorhombic form (a0 ) 1.4910 (2) nm, b0 ) 0.6031 (2) nm, c0 ) 1.9680 (4) nm, V ) 1.7706 (7) nm3, space group ) Pca21, T ) 123 K). Of these two forms, the most stable is the monoclinic and the less stable is the orthorhombic form. These two polymorphs are shown to be orientational, rather than configurational in character. Due to their restricted molecular motifs, no strong hydrogen bonding exists and the crystalline form is dominated by van der Waals type forces. The two structures are shown to be closely related and an analysis of the two structures shows that they are effectively large scale polytypes. Calorimetric studies show that the two polymorphs are monotropic and that the enthalpy of transformation is very low, concurring with the similarity shown by the diffraction data and calculated lattice energies. The thermal expansion coefficients are defined, and it is shown that both polymorphs have similar thermal expansions.

Polymorphism in 2-4-6 trinitrotoluene

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

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.

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

John N. Sherwood

Papers

The temperature dependence of positron lifetimes in solid pivalic acid

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

Polymorphism in 2-4-6 trinitrotoluene

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

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