Crystal chemistry, structure and magnetic properties of the Cu(Mo,W)O4 solid solution series.
Résumé
For a solid solution series the common description of a crystal as a homogeneous solid formed by a repeating, three-dimensional pattern of a unit cell is in principle no longer applicable, taking the statistical chemical distribution of the substituting ions into account. The Cu(Mo_xW_{1-x})O_4 solid solution series represents an appropriate system to investigate how this chemical distribution affects the details of the crystal structure: Due to different coordination preferences of the isovalent diamagnetic ions W and Mo, a decisive magnetic exchange path couples ferromagnetically in CuWO_4, but antiferromagnetically in the isostructural compound CuMoO_4-III. From the investigations of the magnetic properties of the solid solution series it can be inferred for a certain range of stoichiometries that the Mo/W cation disorder of the solid solution series does not result in a corresponding disordered distribution of magnetic coupling constants but in a formation of a super structure of them. The magnetic superexchange here acts as a very sensitive probe for local bonding geometries. Consequently, in the solid solution cooperative structural processes dominate over individual coordination preferences. In the present work Cu(Mo_xW_{1-x})O_4 powder samples are characterized with high resolution synchrotron diffraction, magnetization measurements and neutron diffraction. Cu(Mo_xW_{1-x})O_4 single crystals are characterized by electron probe micro analysis, transmission electron microscopy, x-ray structure refinement and profile analyses, magnetization measurements and diffraction with 'white' and monochromatic neutrons.
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