R. Guinebretière

A high-resolution X-ray diffractometer for the study of imperfect materials

A high-resolution X-ray diffractometer devoted to the study of imperfect materials (mainly oxides and ceramics) is presented. It is based on a rotating anode generator, a four-bounce monochromator, a five-movement sample holder and a curved position-sensitive detector (PSD). This setup allows rapid acquisition of a reciprocal-space map (in less than 10 h) even for very poorly diffracting materials. The two-dimensional instrumental profile is calculated taking into account each optical element in the beam path. The one-dimensional instrumental profiles corresponding to widely used scans (ω scan, θ–2θ scan, rocking curve and powder scan) are also calculated. In the three former cases, the setup exhibits an excellent angular resolution (0.003°), whereas in the latter case the resolution is lowered by one order of magnitude at the benefit of a strong increase in the collected intensity. The possibilities of this diffractometer are illustrated with three examples: an epitaxic layer, a microstructured single crystal and a powder.

Tetragonal zirconia powders from the zirconium n-propoxide-acetylacetone-water-isopropanol system

Abstract As a part of a work concerning densification and toughening of silicate ceramic products, this paper describes the preliminary characterization of a zirconia precursor fabricated through a sol-gel route in the zirconium n-propoxide-acetylacetone-water-isopropanol system. When the molar ratio R = [acac]/[Zr] increases from R = 0 to R = 0.8, the precursor changes from a colloidal precipitate to a polymeric gel with an increasing gelation time. Drying and firing the precipitates leads to the monoclinic stable form of zirconia while the first crystalline phase obtained beyond 500°C from the gel is the metastable tetragonal one. Successive steps of the reactions are investigated by small angle X-ray scattering, differential thermal analysis, thermogravimetry and X-ray diffraction.

Reflection Asymmetric Powder Diffraction with Flat-Plate Sample using a Curved Position-Sensitive Detector (INEL CPS 120)

The reflection flat-sample asymmetric geometry is an attractive set-up for powder diffraction measurements, as compared to either conventional Debye–Scherrer or symmetric Bragg–Brentano geometries. Combined with grazing- or fixed-incidence-angle X-rays, it is suitable for surface-structure or thin-film texture analysis. Furthermore, it allows curved position-sensitive detectors to be used for in situ experiments. In this paper, the major systematic error sources in connection with this geometry are reviewed. It is confirmed that experimental linearization of the curved position-sensitive detector and sample positioning have to be very carefully performed. Simple and automatable procedures are proposed. Then, it is shown that peak-location accuracy leads to satisfactory cell-parameter determination as checked by the use of a silicon international standard reference sample and that structure refinement by the Rietveld method can be performed in very good conditions, as observed with an NaBi2Sb3O11 compound.

XRD and TEM study of heteroepitaxial growth of zirconia on magnesia single crystal

Abstract Zirconia thin films have been deposited on magnesia (001) single-crystal substrates using a sol–gel precursor route. Thermal treatment at 600°C induces the crystallization of polycrystalline thin films containing randomly oriented nanocrystals. Annealing at higher temperature gives rise to the appearance of a progressive heteroepitaxy and breakup of the film into islands. Polycrystalline tetragonal zirconia thin films, several tens nanometer thick, have been studied with an incident X-ray beam angle equal to several tenth degrees. The epitaxial growth has been characterized by XRD under controlled incidence angles and concurrently imaged by TEM on cross-sectional samples. The development of heteroepitaxied single-crystal islands results from abnormal growth of interfacial grains having a lower orientational free energy.

Shape, size and composition of mullite nanocrystals from a rapidly sintered kaolin

The characteristics of mullite crystallites from a kaolin raw material were determined at the early stage of crystallisation by X-ray diffractometry. The equipment was a Debye–Scherrer system equipped with a curved position sensitive detector. We measured the size and shape of mullite crystallites in relation to the temperature (1100–1150°C) and the temperature rate (3–20°C/min). In particular, a temperature rate influence was found on crystallites anisotropy. This result was satisfactorily correlated with TEM observations of sintered materials. The mullite cells constants were also measured, they vary significantly with sintering conditions. Cell constants were correlated with mullite stœchiometry using the Cameron relationship. Results indicated a narrow composition domain (62–65% Al2O3) for all thermal cycles. In each case, the orthorhombic phase was identified, in spite of the low temperature range studied and the relatively high alumina content.

Effect of tensile and compressive strains on the transport properties of SmNiO3 layers epitaxially grown on (001) SrTiO3 and LaAlO3 substrates

This paper deals with the role of epitaxial strain on the structure and electronic transport properties of metastable SmNiO3 layers grown by metal-organic chemical vapor deposition onto SrTiO3 and LaAlO3 substrates. The characterization of these layers is carried out by high resolution x-ray diffraction and four-probe resistivity measurements. It is found that the SmNiO3 phase is stabilized by in-plane compressive strain whereas in-plane tensile strain induces the creation of oxygen vacancies that induces an annihilation of the metal-insulator transition and a huge increase of the resistivity.

Phenomenological analysis of heterogeneous strain fields in epitaxial thin films using x-ray scattering

A model that allows the quantitative analysis of heterogeneous strain fields in epitaxial thin films using x-ray diffraction (XRD) is presented. Particular emphasis is laid on the modelling of the two-component XRD profiles (i.e. profiles made of the superposition of a narrow coherent Bragg peak and a broad diffuse scattering profile) encountered in the XRD investigation of epitaxial thin films containing localized strain fields. The spatial properties of the strain field are included in a correlation function based on phenomenological parameters such as the defect correlation length ξ and the level of disorder σ∞. No assumption regarding the nature of the defect is hence required. The statistical properties of the strain field are described by means of Levy-stable distributions which allow us to account for profile shapes ranging between the Gaussian and profiles exhibiting pronounced power law-type tails, as well as for asymmetrical profiles. The effects of finite size of the domains (crystallites) over which diffraction is coherent are rigorously taken into account by calculating the auto-correlation function of the crystallite shape including the size distribution effects. The effects of each parameter are presented and discussed in detail and the applicability of the model is illustrated with two examples.

SrBi2Nb2O9 Ferroelectric Powders and Thin Films Prepared by Sol-Gel

Pure SrBi2Nb2O9 powders and thin films were obtained using sol-gel synthesis from mixtures of niobium ethoxide, bismuth and strontium 2-ethylhexanoates. Powders crystallized for 2 hours at 700°C had grain sizes of about 100–150 nm. SrBi2Nb2O9 thin films were prepared by spin-coating a stable precursor solution onto Si/SiO2/TiO2/Pt substrates. Crystallization of pure SrBi2Nb2O9 phase occurred at about 500–550°C. Randomly oriented 0.3 μm-thick crack-free films were obtained after 10 successive depositions and heating at 700°C for 2 hours. P-E hysteresis loops have confirmed the ferroelectric behavior of the films: they show a remanent polarization of 2.5 μC/cm2 (5 V, 8 ms). No fatigue was observed up to 109 full switchings.

X-ray diffuse scattering from stacking faults in thick 3C-SiC single crystals

Stacking faults in thick (001)- and (111)-oriented 3C-SiC single crystals are studied by high resolution x-ray diffraction. The authors demonstrate that the analysis of the diffuse scattering intensity distribution can be used as a nondestructive means to accurately determine the densities of Shockley-type stacking faults. The diffuse scattering intensity is simulated with a scattering model based on a difference-equation description of faulting in fcc materials. It is shown that the (001) SiC crystals exhibit an anisotropic fault distribution, whereas the (111) SiC crystals exhibit an isotropic fault distribution, in excellent quantitative agreement with transmission electron microscopy observations.

Reciprocal-space mapping of epitaxic thin films with crystallite size and shape polydispersity

A development is presented that allows the simulation of reciprocal-space maps (RSMs) of epitaxic thin films exhibiting fluctuations in the size and shape of the crystalline domains over which diffraction is coherent (crystallites). Three different crystallite shapes are studied, namely parallelepipeds, trigonal prisms and hexagonal prisms. For each shape, two cases are considered. Firstly, the overall size is allowed to vary but with a fixed thickness/width ratio. Secondly, the thickness and width are allowed to vary independently. The calculations are performed assuming three different size probability density functions: the normal distribution, the lognormal distribution and a general histogram distribution. In all cases considered, the computation of the RSM only requires a two-dimensional Fourier integral and the integrand has a simple analytical expression, i.e. there is no significant increase in computing times by taking size and shape fluctuations into account. The approach presented is compatible with most lattice disorder models (dislocations, inclusions, mosaicity, ...) and allows a straightforward account of the instrumental resolution. The applicability of the model is illustrated with the case of an yttria-stabilized zirconia film grown on sapphire.