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Thermal Vibrations (thermal + vibration)
Selected AbstractsRefinement of crystal structural parameters and charge density using convergent-beam electron diffraction , the rhombohedral phase of LaCrO3ACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2002Kenji Tsuda Atom positions and anisotropic Debye,Waller factors of the rhombohedral phase of LaCrO3 have been refined simultaneously with the low-order structure factors, using a structure-analysis method of convergent-beam electron diffraction (CBED) proposed by Tsuda & Tanaka [Acta Cryst. (1999), A55, 939,954]. The method is based on the least-squares fitting between full dynamical calculations and energy-filtered intensities of two-dimensional higher-order Laue-zone (HOLZ) and zeroth-order Laue-zone (ZOLZ) CBED patterns. The positions of the oxygen atoms have been determined with a high precision. Clear anisotropy of the thermal vibrations of the oxygen atoms has been successfully determined by the CBED method for the first time. The charge transfer from the La and Cr atoms to the O atoms has been found from the deformation charge-density map. [source] Resonant X-ray diffraction: `forbidden' Bragg reflections induced by thermal vibrations and point defectsACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2000V. E. Dmitrienko In general, the local atomic environment becomes less symmetric owing to point defects and thermal vibrations of atoms in crystals. It is shown that, as a result of this phenomenon, an additional anisotropy of the resonant scattering factors can occur and the forbidden Bragg reflections can be excited near absorption edges. Examples of crystals are presented (Ge, K2CrO4, C-15 type) where such thermal-motion-induced (TMI) and point-defect-induced (PDI) reflections can be observed. The tensor structure factors of both types of reflection are computed. Owing to their resonant character, the PDI reflections allow both impurity atoms and host atoms of different types to be studied separately. The considered phenomena can provide a very sensitive method to study point defects because only the atoms distorted by defects produce contributions to the PDI reflections. [source] Apparent shortening of the Csp3,Csp3 bond analysed via a variable-temperature X-ray diffraction study in racemic 1,1,-binaphthalene-2,2,-diyl diethyl bis(carbonate)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009Susanta K. Nayak Crystal structure determination at room temperature [292,(2),K] of racemic 1,1,-binaphthalene-2,2,-diyl diethyl bis(carbonate), C26H22O6, showed that one of the terminal carbon,carbon bond lengths is very short [Csp3,Csp3 = 1.327,(6),Å]. The reason for such a short bond length has been analysed by collecting data sets on the same crystal at 393, 150 and 90,K. The values of the corrected bond lengths clearly suggest that the shortening is mainly due to positional disorder at two sites, with minor perturbations arising as a result of thermal vibrations. The positional disorder has been resolved in the analysis of the 90,K data following the changes in the unit-cell parameters for the data sets at 150 and 90,K, which appear to be an artifact of a near centre of symmetry relationship between the two independent molecules in the space group P at these temperatures. Indeed, the unit cell at low temperature (150 and 90,K) is a supercell of the room-temperature unit cell. [source] | ||||||||||