Crystal chemistry, structure and magnetic properties of the Cu(Mo_xW_1−x)O₄ solid solution series

Abstract

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of a crystal as a homogeneous solid formed by a repeating, three-dimensional pattern of a unit cell is in principle no longer applicable, taking the statistical chemical distribution of the substituting ions into account.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 W_1−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 W_1−x )O₄ powder samples are characterized with high-resolution synchrotron diffraction, magnetization measurements and neutron diffraction. Cu(Mo _x W_1−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.

Keywords:

• diffraction
• magnetic oxides
• magnetic structure
• transmission electron microscopy

Acknowledgements

We would like to thank Dr. C. Meingast and Dr. W. Knafo from the Forschungszentrum Karlsruhe for the heat capacity measurements. Financial support by the Deutsche Forschungsgemeinschaft (WE 1542/5-3) is gratefully acknowledged.

Notes

Notes

1. Beside the investigations on crystal growth 9,10, optical, electrical and electrochemical characteristics 11–13, hydrogen reduction 14, photoelectrochemical investigations 15,16, etc. also the magnetic properties of CuWO₄ were investigated extensively 17–24.

2. Only the most important parameters are mentioned here.

3. Overlap by the 1 0 0- or 0 0 1-reflections can be avoided by judicious choice of the rotation angle Φ of the crystal so that their exciting wavelengths lie outside the incident wavelength range.

4. The formation of superstructures accompanied with changes in physical properties as a consequence of ‘chemical stress’ induced by an elemental substitution is also well known for manganates A_1−x B _x MnO₃ (A: rare earth or alkaline earth metal ion; B: transition metalion) 53.