Crystal chemistry, structure and magnetic properties of the Cu(Mo_xW_1−x)O₄solid solution series

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of a crystal as a homogeneous solid formed by a repeating, three-dimensional pattern of a unit cell is in principle no longer applicable, taking the statistical chemical distribution of the substituting ions into account.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 x W1−x )O4 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 CuWO4, but antiferromagnetically in the isostructural compound CuMoO4-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 the 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 x W1−x )O4 powder samples are characterized with high-resolution synchrotron diffraction, magnetization measurements and neutron diffraction. Cu(Mo x W1−x )O4 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.