Interplay of Spins, Orbitals, Charges, and Lattice Instabilities in Complex Oxides

Multinary oxides constitute a remarkably versatile and prolific class of materials. They have continued to play a major role in the fields of high temperature superconductivity (HTSC) and colossal magneto resistivity (CMR), or, more recently, of multiferroics and spintronics. Although a lot of effort has gone into unraveling the phenomena of HTSC in cuprates and of CMR in manganates, no fully consistent and conclusive microscopic explanation has become available yet. In the first place, this unpleasant state is due to the high complexity of the problems resulting among others from strong electron correlation, and coupled charge, spin and orbital ordering in collective systems. Moreover, virtually all oxide materials showing HTSC or CMR include severe structural disorder, even decay into multiphase systems (phase separation, stripe formation), a fact that has impaired theoretical analyses commonly relying upon translational invariance, and has blurred experimental observations.
Against this background, it would be highly desirable to employ fully periodic and chemically well-defined materials as model systems for studying charge, spin and orbital ordering, either coupled or independent. We have tried to tackle this issue by providing new families of structurally and compositionally well-defined oxide based functional materials displaying

• Frustrated magnetic exchange interactions and orbital ordering on trigonal lattices in silver nickelates.
• Wigner crystallization vs. charge density waves in quasi one dimensional, mixed valent cuprates and manganates.
• Lattice instabilities and competing spin structures in 3d-5d double perovskites.

Besides full characterizations with respect to structures, physical properties and electronic structure analyses, particular emphasis has been given to aspects of solid state synthesis. In order to achieve well defined valence states and structural perfection, special, in part newly developed, synthesis protocols, e.g. the "azide/nitrate route", needed to be employed.