Neutron diffraction on functional materials under electric field or mech. load

In this contribution neutron diffraction studies on functional materials under special environmenal conditions will be presented. In particular, studies of ferroelectric ceramics under high electric fields as well as shape memory alloys under mechanical load will be highlighted. The investigations were carried out at the high-resolution neutron powder diffractometer SPODI (FRM II / Garching n. Munich) which offers special sample environmenal tools for electric fields, mechanical load etc. In-situ studies on ferroelectrics under the influence of high electric fields enable to establish correlations between the macroscopic poling behaviour and corresponding structural changes. The investigations were carried out on technologically applied lead zirconate titanate based samples and on a bismuth sodium titanate based system. A self-designed device allows the investigation of large bulk samples under different orientations of the electric field. This method allows to analyze the poling mechanisms in technical ferroelectrics, such as piezoelectric effect, domain reorientation and phase transformations. In the system Bi0.5Na0.5TiO3 - BaTiO3 - K0.5Na0.5NbO3 the large recoverable field-induced strain could be attributed to a reversible field-induced phase transition from an almost non-polar, pseudocubic tetragonal phase to a distorted, ferroelectric active phase [1]. Polycristalline monoclinic nickel-titanium shape memory alloys have been investigated under mechanical load to analyze their stress-strain behaviour and to derive the elastic constants. A novel tensile rig allows to orient the load axis in a Eulerian cradle like manner. The elastic constants tensor could be calculated based on a series of diffraction patterns under different sample orientations in the initial state and under 0.6 % strain. Furthermore the contributions of elastic deformation (lattice dilatation) and inelastic deformation (orentation of twins) to the total strain could be separated.

Superionic phase transitions in anti-fluorite structures

Fast ion conductors attract continuous and increasing interest in view of possible applications in battery technology. Early examples of superionic phase transitions in anti-fluorite type structures, where the small cations reside in a tetrahedral cage of large anions include Ag2Te [1]. At elevated temperatures anharmonic atom thermal displacements induce cation diffusion towards the large void formed by the central anion octahedron. Of the anti-fluorite structure type compounds Li2 X, where X=(O, S, Se, Te), the compounds Li2O and Li2S showed diffuse transitions to a superionic phase. Very recent advances in battery technology of these compounds [2] motivated us to investigate the end member Li2Te [3] by temperature dependent neutron powder diffraction. The quasi-harmonic temperature dependence of the Li thermal displacement factor shows a distinct steepening of slope around 400⁰C, indicating a phase transition to a superionic phase. Analysis of derived probability density functions and atom potentials again reveal a corresponding increase of anharmonic, anisotropic Li-ion motion towards the octahedral void. This indicates opening up of Li-ion diffusion pathways at the phase transition. The superionic phase transitions of the Li2X anti-fluorite type structures are steered by their cation-anion distance ratio, which in turn determines their respective transition temperatures. The superionic phase transitions mark the onset of cation sublattice melting, where these transition temperatures are proportional to the melting temperatures of the entire compounds.

Hierarchical structure of CaCO₃ biominerals - mesocrystals and functionalization

Skeletal parts and teeth of marine organisms, avian eggshells, trilobite and isopod eyes, and many more biomineralized tissues consist of bio-calcite or bio-aragonite crystals. We explore the nano- to micro-scale architectures of these materials by electron backscatter diffraction (EBSD) and complementary techniques. In contrast to their inorganic cousins the biogenic "crystals" are hybrid composites with small amounts of organic matrix controlling morphogenesis and critically improving mechanical performance or other functions. For the biominerals meso-crystal-like structures are ubiquitous, consisting of co-oriented nano-blocks with a mosaic-spread of a few degrees, depending on the organism and on the size of the mesocrystal entity[1, 2, 3]. The nano-mosaic can be attributed to growth by nano-particle accretion from an amorphous or gel-like precursor, where relics of organic matrix cause misorientations between the crystallized nano-blocks. Recently we were able to reproduce this feature in gel-grown calcite [Nindiyasari et al., Crystal Growth and Design, in press]. The mesocrystal-co-orientation spreads on to the micro- and even millimeter-scale, frequently with a fractal nature of co-oriented hierarchical units [Maier et al., Acta Biomaterialia, accepted for publication]. The hierarchically structured morphology of the composite crystal or polycrystal is always directed by organic matrix membranes. Sea urchin teeth show a multiplex composite crystal architecture, where different subunits of engineered shapes, Mg-contents, and small misalignments are essential prerequisites for self-sharpening [1]. The figure shows an EBSD map of dendritic interdigitating calcite crystals in an avian egg shell (color coding for crystal orientation) with an misorientation profile along the grey line.

Teaching crystallography and material science focusing on large scale facilities

Modern crystallography makes intense use of large scale facilities: neutron reactors, synchrotron sources, free electron lasers, where sources, optics and detectors allow for a wide range of possible experiments putting forwards the limits of the analysis of the structure and dynamics of matter and materials. Giving the large scale facilities a major role in the teaching of crystallography and material science, allowing for intense practice, requires to gather different skills which is more often done through summer school or intensive programs at PhD or junior scientist level. The Erasmus Mundus Master Course MaMaSELF (Master in Material Science Exploring Large Scale Facilities) is a unique European master program focused on the use of large scale facilities to investigate intimate nature of matter and materials where the five consortium higher education institutions (University of Rennes 1, France; Technische Universität München and Ludwig Maximilian University in München, Germany; University of Torino, Italy and University of Montpellier 2, France) have managed, together with Large Scale Facilities partners (ESRF, ILL, FRMII, DESY, LLB, SOLEIL, PSI) and third country partners spread out all over the world (Brazil, India, Japan, Russia, Switzerland , USA), to offer to the students a two years program at the master level including large amount of crystallography and spectroscopy teaching and an intensive summer-school totally dedicated to large scale facilities and including a large proportion of lessons and labs taught by experts, as well as many internship and master thesis opportunities at the large scale facilities.