SYNTHESIS AND CHARACTERIZATION OF NEW SOLIDS CONTAINING PERIODIC ARRAYS OF TRANSITION METAL OXIDE NANOSTRUCTURES

 

Wendy L. Queen and Shiou-Jyh Hwu

 

 

Over the last two decades, much research has been focused in the area of molecular-based magnetic solids, such as single-molecule magnets (SMMs) and single-chain magnets (SCMs).  These materials have organic based nonmagnetic ligands allowing structural confinement of the Transition-metal oxide (TMO) clusters and chains.  Due to the truly low-dimensional nature of these materials and large magnetic anisotropy, they often exhibit slow relaxation of magnetization resulting in hysteresis, a phenomenon that until the early 90’s was associated only with bulk magnetic materials.  

 

It is thought that magnetic properties, comparable to those of molecular magnets, can also be achieved in extended solids containing low-dimensional magnetic nanostructures. As a result we have focused our research efforts in the exploratory synthesis of new solids containing first-row TMOs.  In order to create structural and electronic confinement of the TMO units, rigid nonmagnetic inorganic ligands, XOn, (where X can be a fully oxidized early transition metal, such as V5+, or a neighboring main group element such as P, Si, or As) are utilized. Due to the high melting points and low solubility of these covalent oxides, crystal growth in this system sometimes suffers because of slow diffusion across the crystalline interface. In order to alleviate this problem alkali and alkaline-earth halides are employed as a high-temperature flux, allowing the growth of large single crystals.

 

Recent results have shown that the incorporation of molten salt into the products is sometimes inevitable and can lead to the formation of special frameworks. Although no reaction mechanism is known, one can imagine that once in the molten state,  the metal oxides are first “dissolved” in the corrosive molten salt and then, upon cooling, the covalent lattice aggregates around or within the inherent structure of the molten ionic salt.  The structural versatility within this system has led to a large number of interesting framework formations including those with high nuclearity TMO clusters and channelled structures.