High-pressure and high-temperature experiments with Bi 2 O 3 using a 6-8 type multianvil device led to the formation of a metastable polymorph ͑HP-Bi 2 O 3 ͒ with noncentrosymmetric trigonal symmetry. This phase relaxes during the course of several months at ambient temperature or more rapidly via annealing, to a second intermediate modification ͑R-Bi 2 O 3 ͒. Upon further annealing finally the transformation back to the known ambient phase ͑␣-Bi 2 O 3 ͒ takes place. Both crystal structures were solved from high-resolution x-ray and neutron powder-diffraction data. The orientation and stereochemical activity of the Bi 3+ lone pairs ͑or inert pairs͒ is discussed in terms of crystal-chemical considerations and density-functional theory calculations. Whenever suitable, results were verified by experimental determination of the respective properties. The results of the theoretical analyses show that within the structure type of HP-Bi 2 O 3 , bismuth oxide exhibits a pronounced polarization and can be considered as ferroelectric.

The stable ambient‐temperature phase of tin(II) fluoride, α‐SnF2, was studied by 19F and 119Sn magic‐angle spinning (MAS) NMR spectroscopy. A large chemical shift anisotropy (CSA) is observed in the spectra of both nuclides, which is indicative of the presence of stereochemically active lone electron pair on tin. Symmetric satellite lines with low intensity have been detected in the 119Sn MAS spectra acquired under 19F decoupling, and are assigned to indirect J‐couplings between the magnetically active tin isotopes. The detection of J‐couplings is evidence for the existence of genuine covalent bonds in α‐SnF2, a fact which was deduced before from X‐ray crystallography, which revealed the presence of Sn–F distances smaller than the sum of the respective ionic radii.

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Page s198 s198 preferred orientation of the ferroelectric domains accompanied by a macroscopic polarization. In PZT highest piezoelectric response is found at the morphotropic phase boundary (MPB), separating a rhombohedral structure, in which the polarization is along the [111] c direction, from a tetragonal structure, in which the polarization lies along the [001] c direction [1]. The subscript "c" denotes the pseudocubic perovskite unit cell. A similar MPB with a rhombohedral and a tetragonal phase can be found in BNT doped with barium titanate (BT) [2]. In situ high resolution X-ray diffraction experiments with a specially developed sample environment for applied electric fields [3] were conducted at the MS beamline at the SLS [4]. The experiments were focused on texture analysis and revealed the microstructural reactions during poling and fatiguing of BNT and PZT based materials. The results indicate that poling and fatiguing of morphotropic ferroelectrics results in a combination of reversible phase transitions and complex texture formations. Reconstructed and inverse pole figures contribute additional informations to the interpretation of the complex microstructural processes during the application of an electric field. Texture analysis was performed using the program MAUD [5], which is especially designed for texture and structure analysis.[1] B. Jaffe, W. R. Cook, and H. Jaffe, Piezoelectric Ceramics, (Academic Press, London, 1971 . Using a large volume multi anvil type device we were able to identify two new metastable (quenched to ambient conditions) polymorphs of Bi 2 O 3 by means of X-ray and neutron powder diffraction. Structural analysis shows that in one of these modifications Bi 2 O 3 crystallizes in an entirely new structure type within the non centrosymetric space group P 3 1 c. The structure determining lone pairs for Bi (III) compounds are less pronounced on the 2b site and exhibit a highly polar orientation. These structural properties are giving raise to the assumption that the high pressure phase of Bi 2 O 3 is a potential ferroelectric. Upon annealing it relaxes to a second new modification at about 106 °C, before at about 310 °C the transformation towards the stable ambient α-modification is completed. This "relaxed" phase can be considered as an intermediate polymorph similar to the ambient. In contrast to the former it exhibits a two dimensional channel network with the lone electron pairs pointing into these channels. A weak ionic conduction was determined for this modification.[ Sillenites, Bi 12 MO 20 (M = Si, Ge, Ti), crystallize in the cubic non-centrosymmetric space group I 23. They are piezoelectric and optically active, they exhibit electro-optic effects and high photoconductivity, leading to applications using the photorefractive effect [1]. It is assumed that these properties are correlated with the stereochemical activity of the 6s 2 lone electron pair of Bi 3+. The lone electron pairs are hosted within large cavities, which are built up from four distorted BiO 5 pyramid...

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