Polymorphism of solids

Hirata, S. Fast electron-correlation methods for molecular crystals an application to the a, /h, and /j2 polymorphs of solid formic acid. J. Chem. Phys. 2008, 129, 204104. 

Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) are the techniques most widely used for the characterization of crystallinity and polymorphism of solid lipid particles. Although DSC is usually more sensitive in detecting crystalline material, XRD is much more reliable in determining the type of polymorph present in the dispersions because it provides structural data. In contrast, DSC can detect the type of polymorph only indirectly via the transition temperatures and enthalpies. Because these parameters may be different from those observed in the bulk material, particularly for small colloidal particles [1,62], assigmnent of polymorphic forms in DSC curves should be supported by x-ray data. 

In preformulation, IR may be applied to the study of polymorphism of solid crystals. Polymorphs pose different IR characteristics, and they may be used as a tool for fingerprint identification. In certain cases, the absorbance at a selected wavelength is proportional to the amount of specific polymorph present. This relationship may be used for quantitative determination in solid states for a given polymorph. IR has the ability to differentiate isomer groups such as cis-trans double bond compounds as well as position isomers in an aromatic ring. 

A complete set of thermodynamic data for the polymorphs of solid selenium, the liquid, and the gas molecules Se (g), = 1 to 8, was derived in the review by Mills [74MIL]. This work has been a valuable source of information, but several important later studies have appeared. A compilation of data for the equilibrium between liquid selenium and saturated selenium vapour was made by Rao [82RAO]. The work was mainly based on the contribution by Rau [74RAU] and the selection of data sources was limited. 

A New Conformational Polymorph of Solid Tetramesityldisilene Mes2Si=SiMes2, found by Raman, UV-Vis and Fluorescence Spectroscopy... 

A New Conformational Polymorph of Solid Tetramesityldisiiene Mes2Si=SiMes2,... 

H. H. (2012) Geometric hydrogen bond H/D isotope effects and isotopic polymorphism of solid... 

The complexity of the typical solid deformation response can be further compounded by the presence of one or more polymorphic phase transformations, and a host of other phenomena typical of solids. Table 1.1 lists a number of such phenomena. 

The ideas developed in this chapter are descriptive of shock waves in fluids. Solids have many significant features that distinguish them from liquids and gases, such as shear strength, polymorphic phase transformations, heterogeneous structure, anisotropy, and viscoplastic behavior. The influences of these special properties of solids on shock compression are the topics of several of the other chapters, and for the most part are ignored in this introduction to the basic principles of shock compression. 

The recent book by Young [91Y02], Phase Diagrams of the Elements, presents an authoritative and comprehensive account of the influence of pressure on polymorphic and solid-liquid transitions. Both theoretical and experimental work are succinctly summarized. 

Boggs, J.L. Prentice, K.J. Kraeutle J.E. Crump, The Role of the Scanning Electron Microscope in the Study of Solid Propellant Combustion , inavwepsceiiu ir h/zo yiyoy) do) u,u, Graber, F.C. Rauch A.J. Fanelli, Observation of Solid-Solid Polymorphic Transformation in 2,4,6-Trinitro Toluene , JPhChem 73, (10), 3514—15 (1969) 39) J.E. Crump, J.L. Prentice K.J. Kraeutle Role of Scanning Electron Microscopy in the Study of Solid Propellant Combustion. Part 11—Behavior of Metal Additives , NavWepsCentr TP-5142-PT-2 (1969) 40) J.A. Markham A.R. Cox, Applications... 

The following section deals with the crystallization and interconversion of polymorphic forms of polymers, presenting some thermodynamic and kinetic considerations together with a description of some experimental conditions for the occurrence of solid-solid phase transitions. 

In the penultimate section, it is shown, with some examples, that other characterization technique (in particular FTIR and solid-state NMR), besides diffraction techniques, can be useful in the study of the polymorphism of polymers at the molecular level. 

In the following, some examples of applications of Fourier transform infrared (FTIR) Spectroscopy and of solid-state nuclear magnetic resonance (NMR) to the study of polymorphism in polymers are described. 

Table 1. 13-C chemical shifts (relative to TMS) from CP/MAS NMR spectra of solid cellulose polymorphs Ref.15>...

In 1933, Bernal and Crowfoot [1] reported on the solid state polymorphism of p-azoxyanisole. They found two crystalline modifications of this compound, a stable yellow form and an unstable white polymorph. Krigbaum et al. [31 reexamined the crystal structure of the stable yellow form. The compound shows an imbricated structure which is the basic packing required for nematic behaviour according to Gray [132]. 

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

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