E. Zolotoyabko

Determination of the degree of preferred orientation within the March–Dollase approach

An analytic expression has been derived connecting the degree of preferred orientation in a polycrystalline material to the March parameter. The latter defines the spread of angular distribution of crystallite inclinations in the March–Dollase approach [March (1932). Z. Kristallogr. 81, 285–297; Dollase (1986). J. Appl. Cryst. 19, 267–272]. In turn, the March parameter can be extracted from experimental data using either the Rietveld refinement of the entire diffraction pattern or the measurement of two diffraction intensities originating in the selected crystallographic planes. Working examples taken with two different types of samples are presented.

On the structure of aragonite

High-resolution synchrotron powder diffraction measurements were carried out at the 32-ID beamline of the Advanced Photon Source of Argonne National Laboratory in order to clarify the structure of geological aragonite, a widely abundant polymorph of CaCO(3). The investigated crystals were practically free of impurity atoms, as measured by wavelength-dispersive X-ray spectroscopy in scanning electron microscopy. A superior quality of diffraction data was achieved by using the 11-channel 111 Si multi-analyzer of the diffracted beam. Applying the Rietveld refinement procedure to the high-resolution diffraction spectra, we were able to extract the aragonite lattice parameters with an accuracy of about 20 p.p.m. The data obtained unambiguously confirm that pure aragonite crystals have orthorhombic symmetry.

The effect of AlN buffer layer on GaN grown on (111)-oriented Si substrates by MOCVD

GaN films were grown by metal organic chemical vapor deposition on (1 1 1)Si substrates, using AlN as a buffer layer. The influence of the AlN buffer layer growth temperature and growth duration on the morphology and preferred orientation of GaN films was studied. Drastic enhancement of epitaxial registration was observed with increasing buffer growth temperature. A sharp transition in the growth mode occurred at 760°C. For that temperature, an optimal buffer layer growth duration was found. The use of March parameter as a figure of merit in X-ray diffraction testing of textured GaN films is proposed.

Microstructure of natural plywood-like ceramics: a study by high-resolution electron microscopy and energy-variable X-ray diffraction

The three-dimensional microstructure of a Strombus decorus persicus seashell was studied by means of high-resolution electron microscopy and energy-variable X-ray diffraction on synchrotron beam lines. Energy variation in small steps allows the X-ray penetration depth to be changed precisely and, on this basis, for a non-destructive microstructural analysis with depth resolution to be developed. This technique enabled determination of depth-resolved microstructural parameters, such as the degree of the preferred orientation, the lamella size, and average microstrain fluctuations in both the prismatic and the crossed-lamellar layers of these seashells. The X-ray results were in good agreement with direct observations made by electron microscopy. A detailed study of the shell microstructure shed additional light on the relationship between the structural characteristics and superior mechanical properties of seashells.

Biomineralization of calcium carbonate: structural aspects

We analyze the observed structural distinctions between calcium carbonate crystals of biogenic and non-biogenic origin in terms of interaction between organic macromolecules and growing mineral. We conclude that rather strong forces are imposed by organic macromolecules on mineral crystallites, which cause lattice strains of about 0.2%. According to the developed model, these strains arise primarily throughout the amorphous-crystalline phase transformation at early stages of biomineralization.

HIGH-TEMPERATURE PHASE TRANSFORMATION IN TI-DIFFUSED WAVEGUIDE LAYERS OF LINBO3

Phase formation processes accompanying high-temperature Ti diffusion in thin waveguide layers of LiNbO3 were studied by grazing-incidence x-ray diffraction and complementary techniques. The development and decay of a rutilelike phase (Li0.25Nb0.75O2)1−x (TiO2)x was directly observed as a function of annealing time. A solid-state reaction is proposed, which clarifies the mechanism of phase transformation, relating it to cation replacements.

Protein-induced, previously unidentified twin form of calcite

Using single-crystal x-ray diffraction, we found a formerly unknown twin form in calcite crystals grown from solution to which a mollusc shell-derived 17-kDa protein, Caspartin, was added. This intracrystalline protein was extracted from the calcitic prisms of the Pinna nobilis shells. The observed twin form is characterized by the twinning plane of the (108)-type, which is in addition to the known four twin laws of calcite identified during 150 years of investigations. The established twin forms in calcite have twinning planes of the (001)-, (012)-, (104)-, and (018)-types. Our discovery provides additional evidence on the crucial role of biological macromolecules in biomineralization.

Non-destructive microstructural analysis with depth resolution: application to seashells

Energy-variable X-ray diffraction at a synchrotron beamline has been used to control the X-ray penetration depth and thus to study structural parameters in polycrystalline and textured materials with depth resolution. This approach was applied to the investigation of the depth evolution of microstructure in the nacre layer of bivalvia seashells. According to conventional X-ray diffraction and scanning electron microscopy, the nacre layer in the seashells of Acanthocardia tuberculata under investigation consists of large [001]-oriented lamellae packed nearly parallel to the inner shell surface. In this paper, attention is focused on the microstructural information that can be extracted from the shapes of diffraction profiles (line profile analysis) measured at X-ray energies that are varied by small steps. Depth dependences of the thickness of the lamellae and the average microstrain fluctuation are revealed.

STRUCTURAL MODIFICATIONS IN HE-IMPLANTED WAVEGUIDE LAYERS OF LINBO3

He-implanted and heat-treated optical waveguide layers in Y-cut LiNbO3 wafers were studied by means of high-resolution X-ray diffraction. The depth-resolved profiles of the lattice parameter were derived from the measured diffraction spectra as a function of annealing temperature. Using these data, the spatial distribution of the electron density related to the post-implantation lattice swelling was determined, which shed light on the waveguide formation in damaged layers. Heat treatments at temperatures T > 200°C resulted in the deterioration of waveguide properties due to annealing of the implantation-induced damage. Lattice recovery was found to be a thermally activated process with an activation energy of Ea=0.32 ± 0.03 eV. This is accompanied by defect clustering, giving rise to diffuse scattering components in the diffraction spectra.

Hydrogen content and density in nanocrystalline carbon films of a predominant diamond character

Nanocrystalline carbon films possessing a prevailing diamond or graphite character, depending on substrate temperature, can be deposited from a methane hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While at a temperature of ∼880 °C, following the formation of a thin precursor graphitic film, diamond nucleation occurs and a nanodiamond film grows, at higher and lower deposition temperatures the films maintain their graphitic character. In this study the hydrogen content, density and nanocrystalline phase composition of films deposited at various temperatures are investigated. We aim to elucidate the role of hydrogen in nanocrystalline films with a predominant diamond character. Secondary ion mass spectroscopy revealed a considerable increase of the hydrogen concentration in the films that accompanies the growth of nanodiamond. It correlates with near edge x-ray adsorption spectroscopy measurements, that showed an appearance of spectroscopic features associat...