Microstructural features and domain formation in (Ba,Sr)2TiSi2O8 fresnoites

The formation and co-existence of crystallographically modulated and non-modulated regions in (Ba,Sr)2TiSi2O8 fresnoites is reviewed, particularly the dependence on local composition. It is shown that perturbations of the average fresnoite structure, determined from appreciable single crystals, are in some cases better described as nanometric domain intergrowths where departures from ideal stoichiometry are characteristics of incommensuration, while modulation is absent from volumes that are less perturbed chemically. Evidence for this differentiation is obtained from selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images. The domains are readily distinguished by their unique contrast in bright field electron micrographs. Fourier reconstructions of HRTEM images collected from areas with darker contrast show that modulation can change within relatively small volumes. Nearby areas with lighter contrast were found by SAED to be free of structural disorder and incommensurate reflections.

Fresnoite layers contain SiO4 and TiO5 polyhedra that undergo run rotation and tilting to create 3D, 4D and 5D structural modulations that can be directly observed by electron diffraction and high resolution imaging. Figure optionsDownload as PowerPoint slideHighlights
► (Ba,Sr)2TiSi2O8 fresnoites contain commensurate and incommensurate domains.
► Departures in stoichiometry promote the formation of nanostructured domains.
► Domains formation may provide a means for reducing strain in single crystals.