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Tetragonal Distortion (tetragonal + distortion)

Distribution by Scientific Domains
Chemistry75%

Selected Abstracts

Vibrational spectroscopic and force field studies of copper(II) chloride and bromide compounds, and crystal structure of KCuBr3

JOURNAL OF RAMAN SPECTROSCOPY, Issue 1 2008
Liubov V. Stepakova
Abstract Vibrational spectroscopic and force field studies have been performed of 15 related copper(II) chloride and copper(II) bromide compounds, including hydrated salts crystallizing in ternary aqueous systems with alkali and ammonium halides. For halocuprates with distorted octahedral coordination characteristic stretching Raman wavenumbers, corresponding to symmetric stretching CuIIX modes in the equatorial plane, were found in the ranges 247,288 cm,1 for X = Cl, and 173,189 cm,1 for X = Br, while the low-wavenumber stretching modes for the weaker axial CuX interactions varied considerably. The tetrahedral coordination for Cs2CuCl4 and Cs2CuBr4 leads to somewhat lower CuX symmetric stretching wavenumbers, 295 and 173 cm,1, respectively. The assignments of the copper,ligand stretching vibrations were performed with the aid of normal coordinate calculations. Correlations between force constants, averaged CuX stretching wavenumbers and bond distances have been evaluated considering the following aspects: (1) Jahn,Teller tetragonal distortion (axial elongation) of the octahedral copper(II) coordination environment, (2) differences between terminal and bridging halide ligands (3) effects of coordinated water and the influence of outer-sphere cations. Force constant ratios for terminal and bridging metal,halide bonds reveal characteristic differences between planar and tetrahedrally coordinated M2X6 species. In the hydrated copper(II) halide complexes, the halide ligands are more strongly bound than coordinated water molecules. The crystal structure of KCuBr3 (K2Cu2Br6), which was determined to provide structural information for the force field analyses, contains stacks of planar dimeric [Cu2Br6]2, complexes held together by weak axial CuBr interactions. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Effect of lanthanum substitution on the Raman spectra of barium titanate thin films

JOURNAL OF RAMAN SPECTROSCOPY, Issue 2 2007
P. S. Dobal
Abstract Thin films of Ba1,xLaxTiO3 on platinum substrates were synthesized using the sol,gel method for x values of 0.0, 0.03, 0.05, and 0.10, and the effect of trivalent La3+ substitution on the structural and dielectric properties was studied. Using X-ray diffraction, structural analysis of these compositions revealed a slight increase in the tetragonal distortion of the unit cell with increase in La content. Accordingly, an increase in the tetragonal to cubic transition temperature TT/C was detected by temperature-dependent Raman spectroscopy in the range of 70,500 K. Unlike the results from Raman scattering for the La-doped BaTiO3 films, the dielectric measurements showed broad and diffused dielectric maxima, making the estimation of the transition temperature merely qualitative. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Exciton,phonon interaction and Raman spectra of [(CH3)2NH2]5Cd2CuCl11 crystals

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2004
V. Kapustianik
Abstract Temperature evolution of the exciton,phonon interaction (EPI) in ((CH3)2NH2)5Cd2CuCl11 solid solution was studied on the basis of absorption spectroscopy data. The obtained values of effective phonon energies were compared with the data of Raman spectroscopy. It is shown that the (T) and E, parameters of Urbach's rule show the continuous anomalous change characteristic of the second-order phase transition at T1 = 176 K. The anomalous behaviour of the EPI and other spectral parameters at T0 = 310,315 K was related to the complex co-operative effect involving weakening of the hydrogen bonds and variation of the Jahn,Teller distortion of metal,halogen polyhedra with temperature. This process takes place only within the copper,chlorine sublattice and due to this would be hardly related to the usual phase transition. At the same time, the considered temperature change of the tetragonal distortion of the metal,halogen octahedra is followed by nonfulfillment of Urbach's rule in the temperature range TT0. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Review of polyhedral distortions as a multi-scale minimization of the electric polarization and their correlations with physical properties

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2006
Abderrahim Benabbas
The Jahn,Teller (JT) and non-Jahn,Teller polyhedral distortions are reviewed within the same context, based on a multi-scale minimization of the electric polarization by handling formal ionic valences and valence electron density. This model is applied to tetragonal distortions of octahedra, particularly in K2NiF4 structures with different formula types, along with doping. The predictions are always in good agreement with the observed data. In particular, the ferrodistortive order of JT distortions is obtained from formal charge polarizations, while the antiferrodistortive one is adopted when only the valence electron density is involved. The correlations between physical properties and octahedral elongations through the crystal structures on one side and chemical compositions on the other side are discussed according to this model for high- Tc cuprate superconductors and CMR manganites. [source]