Synthesis, Crystals Structures and Properties of Oxides with Unusual Structural Features

New synthetic solid state strategies for very air-sensitive, alkaline-rich transition metallates have enabled us to access unprecedented low coordination numbers (e.g. C.N. 2 and 3) as well as low oxidation states of the transition metals.

• Na₅[MO₂][CO₃] with M = Co^I, Ni^I, Cu^I in a two-fold coordination and non-coordinating complex anions (CO₃^2-), are examples of single square-lattices of magnetic ions. Our recent activities have been on spectroscopic characterization which leads to new insights into the lattice dynamics and electronic structures of the complex transition metals in low oxidation states in “unsaturated” low coordination;
• Na₁₀[Mn₃O₆][MnO₃]: Experimental realization of the (frustrated) star-lattice in magnetic oxides. Unprecedented low C.N.(Mn-O) of 3 for manganese(II) in oxides;
• Na₂ReO₃: First structural confirmation of a Re-Re triple bond in oxides with an exceptionally low C.N.(Re-O) 3;
• Na₂₆Mn₃₉O₅₅ The first T5 super-tetrahedron in oxide chemistry;

Reactivity of Solids

Ongoing research includes investigations related to the reactivity of solids. Of particular interest to us are classical solid state reactions (diffusion mechanisms) at lower temperatures, and redox reactions of in-situ activated oxides, e.g. ternary or polynary alkaline metal-rich oxides. A rather rare example for a syn-proportionation sublimation (chemical vapor transport mechanism and thermodynamic data) of In₂O₃ activated by manganese has been studied in detail. Currently, we are expanding our efforts of crystal growth into ionic flux methods, hydrothermal techniques or enhancements via slight vapor pressure of less ionic fluxes.

Structure-Property Relationships in Strongly Correlated Oxides

The focus of our research includes the synthesis and structural characterization of new compounds and addresses the inherent physical properties to achieve a more fundamental understanding of the mechanisms at work. Therefore, specifically selected compounds are investigated. In recent years, we have studied various phenomena related to dominantly low-dimensional magnetic interactions by several experimental techniques and combined our results in strong collaboration with experimental and theoretical physicists. Accomplishments are related to the interdisciplinary fields of synthesis, structural aspects, thermodynamic and transport properties, magnetism, spectroscopy, and diffraction techniques:

• AgCuVO₄: quasi one-dimensional S = ½ Heisenberg system, impact of frustrated inter-chain coupling on the ordering temperature;
• In₂VO₅: S = ½ zigzag chain with intra-chain frustration;
• TiOCl: Spin-Peierls material, magnetic properties, spectroscopy and pressure dependence;
• Magnetic nanoscale domains on the honeycomb lattice and finite size effects in InCu_2/3V_1/3O₃;
• (Sr, Ba)Ag₂M[VO₄] with M^II = Co, Ni, Cu: a new class of ferromagnetic insulators;
  • Tuning superexchange couplings via non-magnetic complex ions on a triangular lattice of magnetic ions: dependence on structural distortions and the spin system.