Comments on A new theory for X-ray diffraction

In an article entitled A new theory for X-ray diffraction [Fewster (2014). Acta Cryst. A70, 257–282], hereafter referred to as NTXRD, it is claimed that when X-rays are scattered from a small crystallite, whatever its size and shape, the diffraction pattern will contain enhanced scattering at angles of exactly 2θB, whatever the orientation of the crystal. It is claimed that in this way scattering from a powder, with randomly oriented crystals, gives rise to Bragg scattering even if the Bragg condition is never satisfied by an individual crystallite. The claims of the theory put forward in NTXRD are examined and they are found to be in error. Whilst for a certain restricted set of shapes of crystals it is possible to obtain some diffraction close to (but not exactly at) the Bragg angle as the crystallite is oriented away from the Bragg condition, this is generally not the case. Furthermore, contrary to the claims made within NTXRD, the recognition of the origin of the type of effects described is not new, and has been known since the earliest days of X-ray diffraction.

Morphology and molecular orientation of thin-film bis(triisopropylsilylethynyl) pentacene

As a modification to the insoluble and herringbone-structured pentacene, bis(triisopropylsilylethynyl) (TIPS) pentacene has two bulky side groups, leading to good solubility in common organic solvents and regular π–π stacking arrangements in the crystalline state. Solution processing of TIPS–pentacene thin films was investigated as a function of various process parameters in this work. Electron diffraction results suggested that TIPS–pentacene molecules tended to align with the acene unit edge on to the substrate, touching down with their bulky side groups. In a TIPS–pentacene polycrystalline film, the long axis of individual crystallite is [2 1 0], while the shorter axis is [1 ¯20]. High-resolution electron microscopy was used to study the local crystal structure and characteristic defects of TIPS–pentacene thin films. Due to the nonaromatic side groups, TIPS–pentacene was found to be significantly more sensitive to the electron beam (critical dose Jc= 0.05 C/cm2at 300 kV) than pentacene itself (Jc= 0.2 C/cm2at 100 kV).

The Stiffest Wire

We consider a polycrystal constituted from orthorhombic single crystals for which one particular principal axis of the crystallites is always oriented parallel to a particular direction; in the plane perpendicular to this direction the crystallites are randomly oriented. Bounds are found for the Young’s modulus in the axial direction. The lower bound on the Young’s modulus, which is realizable, is found to be that of the individual crystallite in the aligned direction. The upper bound determined is necessarily realizable when the single crystal elastic constants satisfy a certain condition. When this condition is not satisfied a bound is found; whether or not this bound is realizable must be examined using the specific elastic constants of the crystal being considered. For all physical examples considered the upper bound was indeed found to be realizable. Thus, generally speaking, a wire constituted as above, with the stiffest direction of the individual crystallites being along the wire, will have a higher Young’s modulus than the maximum modulus of the individual crystallites of which it is composed.

Analysis of Residual Stress States in Coarse Grained and Single Crystal Nickel-Base Superalloys

The single crystal measuring and evaluation technique allows the analysis of stress states in large grained and single crystalline materials. The technique is based on the analysis of the orientation of crystallites under study. Using a fouf-circle-diffractometer, the knowledge of the orientation matrix allows the calculation of the diffractometer setting for every given reflection (hkl). After experimental determination of the precise diffraction angles 20 for at least six reflections of an individual crystallite, its strain tensor components can be obtained by applying least-squares methods. The stress tensor referred to the crystal axes system is calculated using the single crystal elastic constants.The practical use of this measuring and evaluation technique is demonstrated on coarse grained and single crystal nickel-base superalloys and monocrystalline materials.