Multidataset refinement resonant diffraction, and magnetic structures

Abstract

The scope of Rietveld and other powder diffraction refinements continues to expand, driven by improvements in instrumentation, methodology and software. This will be illustrated by examples from our research in recent years. Multidataset refinement is now commonplace; the datasets may be from different detectors, e.g., in a time-of-flight experiment, or from separate experiments, such as at several x-ray energies giving resonant information. The complementary use of x rays and neutrons is exemplified by a recent combined refinement of the monoclinic superstructure of magnetite, Fe 3 O 4 , below the 122 K Verwey transition, which reveals evidence for Fe 2+ /Fe 3+ charge ordering. Powder neutron diffraction data continue to be used for the solution and Rietveld refinement of magnetic structures. Time-of-flight instruments on cold neutron sources can produce data that have a high intensity and good resolution at high d -spacings. Such profiles have been used to study incommensurate magnetic structures such as FeAsO 4 and β–CrPO 4 . A multiphase, multidataset refinement of the phase-separated perovskite (Pr 0.35 Y 0.07 Th 0.04 Ca 0.04 Sr 0.5 )MnO 3 has been used to fit three components with different crystal and magnetic structures at low temperatures.

Charge and orbital order in frustrated Pb3Mn7O15

“…This un-derscores the importance of determining the relationship between electronic degrees of freedom and crystal structure in this emerging class of materials. Here a comprehensive re-investigation of the structure of Pb 3 Mn 7 O 15 by combined Rietveld refinement 15 against synchrotron X-ray and multiple wavelength neutron powder diffraction is reported. This complementary use of techniques is particularly suitable to charge and orbital ordering problems, where microtwinning, extinction, and multiple scattering, as well as subtle lattice distortions, hinder single crystal approaches 16,17 .…”