XDX – an initial solution to crystallization
Janet M. Newman, Edward H. Snell, Joe R. Luft, F. von Delft, Jochen Müller-Dieckmann, K. Taylor, D. Ratcliffe, Vincent J. Fazio · Acta Crystallographica Section A Foundations of Crystallography · 2011
Magnetic crystallography -description of crystal magnetic structures with the help of crystallographic concepts -is gradually acquiring the features of a full-fledged science.Physical models of an ordered magnetic state in crystals have been formulated and tested.The necessary experimental equipment based primarily on the diffraction of slow neutrons has been constructed and successfully operate.The main stages of determining the magnetic structure including the determination of the propagation vector of the structure, values and orientations of atomic magnetic moments have become clear.Finally, the algorithms for precise analysis of magnetic experimental diffraction data have been designed and implemented.Quantum models of magnetism are based on the concept of exchange interaction, the main of which for the consideration of the magnetic structure of crystals being the Heisenberg-Dirac-Van-Vleck model of interaction of localized magnetic moments.Magnetic structural experiments are mainly carried out on steady-state neutron sources using cold neutron beams.Accessible ranges of temperature and external pressure are very broad -a diamond anvil cell makes it possible to perform experiments at a pressure as high as 30 GPa and a temperature as low as 0.1 K.The TOF technique at high-flux pulsed neutron sources gives rise to some new opportunities.In particular, at TOF neutron diffractometers it is possible to register diffraction patterns with a relatively high resolution over a very wide range of d-spacings, which allows simultaneous refinement of both atomic and magnetic structures.The role of other nuclear techniques remains to be secondary, though μSR and Mössbauer spectroscopy could be very useful in complicated cases, especially if the local features of the magnetic structure are important.Of the two possible ways of crystallographic description of magnetic structures -"Symmetrical" based on Shubnikov or color groups and "Representative" based on irreducible representations of Fedorov groups -the center of gravity is gradually shifting towards the latter.This is manifested in the introduction to the daily practice of several software packages that allow very clear selection of magnetic structures, which are correct from the symmetrical point of view, and analysis of their consistency with the experimental data.Moreover, the approach based on irreducible representations secures a uniform analysis of the magnetic structure regardless of whether it is commensurate or incommensurate.Discussion of this particular aspect is the main content of the report.A few examples of neutron diffraction data analysis selected from the practice of recent years will be discussed in detail to illustrate the relationship of crystallographic and physical aspects in the formation of specific magnetic structures (commensurate and incommensurate).