Molten hydroxides are exceptional solvents for the synthesis of metal oxides. The group has worked for a number of years in the preparation of new materials by dissolution and subsequent precipitation from molten alkali metal hydroxides. As an alternative to the conventional ceramic procedures, which require thermal treatments at high temperatures, the use of molten hydroxides provide a very basic environment at moderate temperatures and offers the possibility of stabilizing phases of metastable character and/or containing transition metals in an unusual or elevated oxidation state (Cu³⁺, Mn⁴⁺, Fe⁴⁺, etc.).

Synthesis of Complex Iron Oxides

Barium iron oxides are a widely studied class of solid-state materials; mainly because they have shown promise as oxidizing agents, memory devices and battery cathodes. The varied structural chemistry of the Ba-Fe-O system adds to their interest. We have developed a molten hydroxide synthetic route that yields large single crystals of barium iron oxides such as BaKFeO₃, Ba₂FeO₄, and Ba₃FeO₅. We are in the process of fully characterizing these compounds using single crystal x-ray diffraction, magnetic susceptibility measurements and Mössbauer spectroscopy.

Synthesis of Ni³⁺ Perovskite-Like Compounds

Recently, the report of metal-to-insulator transitions in RNiO₃ systems triggered our interest in the stabilization of Ni³⁺ in perovskite-like structures from molten hydroxides. These systems had traditionally been prepared under high oxygen pressure or high hydrostatic pressures, necessary to stabilize Ni in its trivalent state. Our work with molten hydroxides demonstrated that it is possible to isolate LaNiO₃ as a pure perovskite phase, rhombohedrally distorted. EDS, TG analysis and Rietveld refinements of XRD diffraction data show that La is partially replaced by Na/K, still increasing the oxidation state of Ni beyond 3+ (hole-doping). Several Ruddelsden-Popper materials containing mixed Ni²⁺-Ni³⁺ have been stabilized, such as (La,Nd,Na)₂NiO₄. The use of LiOH fluxes allowed us to isolate new phases of composition (La,Nd)₂Ni_0.5Li_0.5O₄, with layered K₂NiF₄ structure, containing Ni³⁺. In these last materials, each NiO₆ octahedron is surrounded by six LiO₆ octahedra, providing an optimal chemical environment for the successful stabilization of trivalent Ni.

• People

Jamie Delattre - Iron oxides
Dr. Marisol Martín-González - Ruthenates

• Collaborators

The molten hydroxide group focuses on the synthesis and characterization of new materials, and often collaborations are necessary for specialized characterization techniques.  We have worked with Prof. Gary J. Long (University of Missouri - Rolla) and Prof. Fernand Grandjean (University of Liege, Belgium) on the Mössbauer characterization of complex iron oxides.  Dr. Theo Siegrist (Bell Labs) and Dr. Victor G. Young, Jr. (Univ of Minnesota) have been valuable resources for assistance with difficult crystal structures.  And currently, Prof. Bill McCallum (Iowa State and Ames National Lab) is assisting us with high temperature magnetic characterization.