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January 2005 issue
Cover illustration: An experimentally derived plot of the Fermi hole mobility function (FHMF) for urea. The FHMF can be interpreted as a potential energy surface for electron transfer. This plot was obtained from a Hartree-Fock wavefunction constrained to fit measured X-ray diffraction structure factors, using the Tonto program. See Jayatilaka & Grimwood [Acta Cryst. (2004), A60, 111-119].
phase transitions
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The uses of Landau-type theories to describe structural, microstructural and mesostructural features of materials undergoing phase transitions are reviewed.
Molecular dynamics (MD) has evolved into a reliable technique for simulating incommensurate modulated phases. The application of a compensating external pressure tensor to organic materials allows a better fit to the experimental measurements. An overview of the MD method and its applications is presented here.
Typical experimental observations of structural phase transitions from and to the quasicrystalline state as well as their microscopic interpretation are reviewed.
Recent experiments on the magnetic properties of A–Mg–R (A = Zn and Mg; R = rare-earth elements) icosahedral quasicrystals are reviewed. Emphasis is placed on the origin of their spin-glass-like behavior; discussion is made based on existing data sets ranging from bulk magnetization to neutron diffraction.
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An overview is presented of inorganic compounds with low-dimensional electronic structures, and of X-ray diffraction as a tool to study the properties of charge-density waves (CDWs).
This review describes how the unique orientational properties of the C60 fullerene molecule influence the structure and phase transitions of the C60 monomer and polymer.
Oxidation/reduction reactions in three-dimensional or layered manganites and cuprates are accompanied by the formation of domain structures. The microstructure of several representatives was studied by means of electron diffraction and high-resolution electron microscopy.
The application to perovskite crystallography of group-theoretical methods, as implemented in the computer program ISOTROPY, is reviewed. The results have been used, in combination with high-resolution X-ray or neutron powder diffraction in fine temperature steps, to study structural phase transitions in perovskites.
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Recent X-ray diffraction studies of liquid crystals are reviewed. The topics are twist grain-boundary phases, antiferroelectric phases studied using resonant diffraction and smectic phases within gel structures.
obituaries
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research papers
The article presents a comprehensive theoretical description of the anomalous scattering method along with simulated numerical examples. The method is based on physical properties of parametric X-radiation.
An approximate analytical expression for the three-beam diffraction profile in a crystal plate subjected to elastic strain is derived.
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A direct method for transforming angle-dependent diffraction data into depth-dependent data by the use of regularization techniques is presented, including method validation and application to experimental data.
A drastic improvement of the original charge-flipping algorithm is presented. The phase space is explored more efficiently by a special treatment of weak reflections in the iteration process. Paradoxically, it is better not to use observed moduli of weak reflections: in the search for a solution, they create only unwanted constraints.
international union of crystallography
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