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Stereochemical Activity (stereochemical + activity)
Selected AbstractsThallium Halides , New Aspects of the Stereochemical Activity of Electron Lone Pairs of Heavier Main-Group ElementsCHEMINFORM, Issue 16 2007Anja-Verena Mudring Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2010Ian Swainson Synchrotron powder diffraction data from methylammonium tin bromide, CH3NH3SnBr3, taken as a function of temperature, reveal the existence of a phase between 230 and 188,K crystallizing in Pmc21, a = 5.8941,(2), b = 8.3862,(2), c = 8.2406,(2),Å. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s2 lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost). [source] Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50,GPa and crystal structure of the high-pressure phaseACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010Alexandra Friedrich The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi2Ga4O9, was determined up to 30.5,(5),GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3,(2), 6.2,(3), 8.9,(1) and 14.9,(3),GPa for the low-pressure phase, and at 21.4,(5) and 30.5,(5),GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50,GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50,GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemcial activity of the lone electron pair of Bi is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure. [source] | ||||||||||