Stephen P. Thompson

The structure and thermal expansion behaviour of ikaite, CaCO3.6H2O, from T = 114 to T = 293 K

Abstract The hydrous calcium carbonate mineral ikaite (CaCO3 ·6H2O) forms in nature at low temperature in carbonate- and Ca-rich waters. Ikaite crystallizes in the spacegroup C2/c, and consists of CaCO3 ·6H2O units with Ca ions coordinated by eight oxygens, six from H2O and two from the carbonate group. Hydrogen bonding links CaCO3 ·6H2O moieties to form the crystal structure. We have used synchrotron X-ray powder diffraction at T = 243 K to refine the monoclinic structure of ikaite, and have measured unit-cell parameters of ikaite between T = 114 K and T = 293 K. Anisotropic thermal expansion in ikaite is evident, with the smallest relative increase occurring along the b direction parallel to 2-fold axes. The contribution of hydrogen bonding to thermal expansion is assessed by comparison of our data with previously published data for deuterated ikaite, ice and gypsum. Ikaite exhibits a coefficient of volume expansion intermediate between that of ice (Ih) and of deuterated gypsum (CaSO4 ·2D2O) between T = 114 K and T = 293 K.

The role of residual stress in the fracture properties of a natural ceramic

The role of residual stress in enhancing the fracture properties of the shell Ensis siliqua (razor shell) is reported. Using energy-variable synchrotron X-ray diffraction it is shown that the calcium carbonate crystals are preferentially orientated as a function of depth. Using scanning electron microscopy the structure is found to be mainly divided into two regions, namely crossed lamellar and prismatic. The latter are shown to be at a higher residual compressive stress than the former which gives rise to delaminations of the structure upon mechanical deformation. The role of residual stress in the structure is also shown to be critical to the fracture properties of the material and this has implications for the fabrication of man-made laminate ceramics.

Software for automatic calibration of synchrotron powder diffractometers.

An automatic procedure to calibrate angular-dispersive monochromatic diffraction instruments has been developed at the Daresbury Synchrotron Radiation Source. The procedure uses a macro Language to control the powder diffraction instruments to locate Bragg reflections and perform peak-centre refinement from a standard reference material. The information obtained is used to refine the wavelength of the radiation used and the angular offset of the detector arm. The concept and implementation of the new software are described with applications to demonstrate its viability. The results of a reliability and accuracy study are also presented.

The characterisation of residual strain in Ensis siliqua shells

This study reports the variation of residual strains within the posterior ventral area of the Ensis siliqua mollusc shell, as determined using glancing incidence synchrotron X-ray diffraction. The outer layer of this structure exhibits a tensile strain, in contrast to a compressive strain observed within the inner layer. Fluctuations in unit cell parameters for the inner layer have been determined, showing that the microscopic prismatic layer of the structure exhibits a compressive strain orientated parallel to the surface of the shell. This is thought to enhance the crack deflection properties of this layer, and aid in resisting catastrophic failure. Further analysis of residual strains has been performed using the same method, throughout several stages of compressive testing of the anterior dorsal region of the shell. This identified no variation in residual strains at various levels of loading, and it is therefore proposed that load may be transferred via the organic matrix of mollusc shell structures. A Raman spectroscopic investigation, comparing whole and powdered shell with non-biogenic aragonite, has shown that residual strains are also present in this analagous material which is devoid of organic content. This indicates that the observed strain is not entirely due to the organic matrix.

A high-resolution synchrotron powder diffraction study of substituted gallium ferrites using flat-plate fixed angle of incidence geometry on beamline I11 at Diamond

Very high resolution powder diffraction structural studies of the potentially room temperature multiferroic Ga2−xFexO3 solid solution series (x = 0.7–1.3, 0.1 steps) have been undertaken using a fixed angle of incidence geometry. The applied absorption correction was seen to improve the goodness of fit (χ2) of the Rietveld refinements from an average of 1.41 to 1.06. The correction also resulted in an increased mean isotropic displacement parameter from 0.5 to 0.65. The mean difference in the fractional coordinates of the atoms between the refined models from the corrected and uncorrected data was 0.0007 A, compared with the mean fractional coordinate error of 0.0003 A. It is concluded that the final crystal structures refined from the corrected and uncorrected data are not significantly different. The number of reflections in each data set was over 2700, and the average peak half-width was 0.018° with the data binned in 4 millidegree steps. The data quality allowed bond length and angle determinations of sufficiently high accuracy to measure significant metal site distortions to an average precision of ±0.007 A. A lattice parameter nonlinearity was observed on either side of the x = 1 composition; this was attributed to local distortions, primarily of the Fe1 and Ga2 sites.