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The magnetic properties of the isomorphous compounds α-CrPO4 and α-CrAsO4 have been investigated by magnetic susceptibility and low-temperature powder neutron diffraction methods. These materials order antiferromagnetically below 9.1 ± 1.0 and 7.7 ± 0.8 K, respectively, and the magnetic structure of α-CrPO4 at 2 K reveals both ferromagnetic and antiferromagnetic Cr-O-Cr interactions, arising from different linkages of CrO6 octahedra. The magnetic structure of α-CrAsO4 at 5 K differs from that of α-CrPO4 in that the ferromagnetic interactions are frustrated. This is ascribed to the Cr-O-Cr interactions in α-CrAsO4 being weaker than those in α-CrPO4, relative to competing antiferromagnetic interactions through the XO4 group, as the relevant Cr-O bonds are longer in the former structure. © 1989 American Chemical Society.

The crystal structure of highly stoichiometric magnetite (Fe3O4) below the Verwey transition has been refined from high-resolution neutron and synchrotron x-ray powder-diffraction data. The refined model has a monoclinic P2/c symmetry cell with orthorhombic Pmca pseudosymmetry constraints on the atomic positions, and contains four independent octahedral B site iron atoms. Charge ordering is evidenced by the presence of expanded and contracted BO6 octahedra, and by the distribution of B-B distances resulting from unequal Coulombic repulsions between the different B site charges. The B-B distances are inconsistent with dimer formation. Competition between the B-O and B-B interactions results in polar displacements of two of the B site cations. The charge ordering has a predominant [001] density modulation, which relieves a nesting instability in the electronic density of states, but a second [00 1/2] phase modulation also occurs. The monoclinic distortion at the Verwey transition is consistent with a macroscopic rhombohedral magnetostriction, driven by the localization of orbitally degenerate Fe2+, coincident with the microscopic charge ordering distortions that have an orthorhombic lattice symmetry.

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, Fe3O4, 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 FeAsO4 and β-CrPO4. A multiphase, multidataset refinement of the phase-separated perovskite (Pr0.35Y0.07Th0.04Ca0.04Sr0.5)MnO3 has been used to fit three components with different crystal and magnetic structures at low temperatures.