Ferroelectric ceramics are widely used as sensors and actuators for their electro-mechanical properties, and in electronic applications for their dielectric properties. Domain switching--the phenomenon wherein the ferroelectric material changes from one spontaneously polarized state to another under electrical or mechanical loads--is an important attribute of these materials. However, this is a complex collective process in commercially used polycrystalline ceramics that are agglomerations of a very large number of variously oriented grains. As the domains in one grain attempt to switch, they are constrained by the differently oriented neighbouring grains. Here we use a combined theoretical and experimental approach to establish a relation between crystallographic symmetry and the ability of a ferroelectric polycrystalline ceramic to switch. In particular, we show that equiaxed polycrystals of materials that are either tetragonal or rhombohedral cannot switch; yet polycrystals of materials where these two symmetries co-exist can in fact switch.

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SynopsisA new facility for microdiffraction strain measurements and microfluorescence mapping has been developed at the Advanced Light Source. Details of the mechanics and performance of the beamline and endstation will be given. AbstractA new facility for microdiffraction strain measurements and microfluorescence mapping has been built on beamline 12.3.2 at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory (LBNL).This beamline benefits from the hard x-radiation generated by a 6 Tesla superconducting bending magnet (superbend). This provides a hard x-ray spectrum from 5 keV to 22 keV and a flux within a 1 µm spot of ~ 5 · 10 9 photons per seconds (0.1% bandwidth at 8 keV). The radiation is relayed from the superbend source to a focus in the experimental hutch by a toroidal mirror. The focus spot is tailored by two pairs of adjustable slits, which serve as secondary source point. Inside the lead hutch, a pair of Kirkpatrick-Baez (KB) mirrors placed in a 2 vacuum tank re-focuses the secondary slit source onto the sample position. A new KB-bending mechanism with active temperature stabilization allows for more reproducible and stable mirror bending and thus mirrorfocusing. Focus spots around 1 µm are routinely achieved and allow a variety of experiments, which have in common the need of spatial resolution. The effective spatial resolution (~0.2 µm) is limited by a convolution of beam size, scan-stage resolution and stage stability. A 4-bounce monochromator consisting of 2 channel-cut Si(111) crystals placed between the secondary source and KB-mirrors allows for easy changes between whitebeam and monochromatic experiments while maintaining a fixed beam position. High resolution stage scans are performed while recording a fluorescence emission signal or an x-ray diffraction signal coming from either a monochromatic or a white focused beam. The former allows for elemental mapping, whereas the latter is used to produce 2-dimensional maps of crystal-phases,-orientation, -texture and -strain/stress. Typically achieved strain resolution is in the order of 5 · 10 -5 strain units. Accurate sample positioning in the x-ray focus spot is achieved with a commercial laser-triangulation unit. A Si-drift detector serves as a high-energy-resolution (~150 eV FWHM) fluorescence detector. Fluorescence scans can be collected in continuous scan mode with up to 300 pixels per second scan-speed. A CCD area detector is utilized as diffraction detector. Diffraction can be performed in reflecting or transmitting geometry. Diffraction data are processed using XMAS, an in-house written software package for Laue and monochromatic microdiffraction analysis.

Ferroelectric materials, such as BaTiO(3), have piezoelectric properties that make them attractive for microelectronic and sensing applications. It is well known that the application of mechanical stress or electric field can alter the domain structure in ferroelectrics. Indeed, the constitutive behaviour of a ferroelectric is largely governed by the formation, movement and interaction of its domains. Therefore, it is crucial that the micromechanics of domains and their effect on internal stresses in ferroelectrics be understood. Here we show that the emerging technique of scanning X-ray microdiffraction can be used to measure directly, for the first time, the local triaxial strain fields around 90 degrees domains in single-crystal BaTiO(3). Specifically, residual strain maps in a region surrounding an isolated, approximately 40 microm wide, 90 degrees domain were obtained with 3 microm resolution, revealing significant residual strains. This information is critical for accurate micromechanical modelling of domain behaviour in ferroelectrics.

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