Local Atomic Environment (local + atomic_environment)

Distribution by Scientific Domains

Selected Abstracts

X-ray birefringence and dichroism obtained from magnetic materials

S. W. Lovesey
In the past decade, synchrotron radiation has triggered a surge in studies of the polarization dependence of X-ray beams passing through non-isotropic materials. A vast range of experimental results concerning polarization-dependent absorption (dichroism) and dispersion (birefringence, for example) are available from materials which are either magnetic or exhibit preferred directions due to the local atomic environment. This article aims to bring together the diversity of modern experiments in this field with established methods of optical calculus, in a way that highlights the simplicity of the underlying physics. A useful framework is formed when observable quantities, in the X-ray case, are related to atomic variables of the sample material. Atomic descriptions of absorption spectra with various levels of complexity are considered, and some well documented sum-rules are encountered. The framework is the most general allowed within the electric dipole approximation. By way of illustration, dichroic X-ray absorption by two materials with highly anisotropic properties and magnetic ions with different valence shells are considered; namely, a 3d -transition ion in ferrous niobate, and a lanthanide ion in dysprosium borocarbide. Both materials display interesting magnetic properties that are challenging to interpret at an atomic level of detail, and it is shown how absorption experiments can contribute to resolving some issues. [source]

Resonant X-ray diffraction: `forbidden' Bragg reflections induced by thermal vibrations and point defects

V. 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]

Resonant X-ray scattering in the presence of several anisotropic factors

E. N. Ovchinnikova
The general form of the X-ray susceptibility tensor near absorption edges is found when several anisotropic factors, such as the anisotropy of local atomic environment, magnetic ordering and orbital ordering, simultaneously exist in a crystal. Different phenomenological approaches are used to obtain the explicit form of the susceptibility tensor and to find the contributions from each anisotropic factor separately as well as `combined' terms owing to their simultaneous existence. The results of the theoretical treatment are applied to the resonant diffraction by La0.5Sr1.5MnO4 below the Néel temperature, where charge and orbital ordering coexist with anisotropy of local atomic environment and magnetic ordering. [source]

The vanadium Magnéli phases VnO2n -1

U. Schwingenschlögl
Abstract To compare the metal-insulator transitions (MITs) of VO2 and V2O3 we analyze the relations between the structural and electronic properties of the vanadium Magnéli phases. These materials set up the homologous series VnO2n -1 (3 , n , 9) and have crystal structures comprising typical dioxide-like and sesquioxide-like regions. As the MITs of the vanadium Magnéli phases are accompanied by structural transformations, we are able to discuss the effects of characteristic changes in the local atomic environments. The systematic investigation of the transport properties is based on a new and unifying description of the crystal structures of the Magnéli phases including VO2 and V2O3. Our results lead to a comprehensive understanding of the MITs in the Magnéli class and shed new light on the role of particular electronic states for the MIT of V2O3. [source]