TOWARD RATIONAL SYNTHESIS OF TRANSITION-METAL OXIDES

Transition metal oxides display structural diversity crystallizing into a variety of framework, layer and tunnel structures. The electronic and structural properties of these can be fine-tuned due to their ability to reversibly accept metal cations and pro tons into their internal void spaces and electron-charge-transfer equivalents into their conduction bands. It is essential to prepare them in suitable form (powders, thin film or single crystal) with well-defined stoichiometry and morphological characteristics to be employed as 'intelligent materials'. Traditional solid state reactions however lead to thermodynamically stable products with more compact structures (sometimes multiphasic) which are not suitable for different applications. The trend in modern solid state synthesis is therefore directed towards developing low-temperature reactions using appropriate molecular precursors that can react either as a molten metal or salt (the Aux method) or in a molecular solvent (the hydrothermal method). Mild hydrothermal reactions in the temperature range from 100-300 degrees C at low pH are found to be ideal conditions to crystallize novel vanadium, molybdenum and tungsten oxides and hydrates with layer or 3-D framework structures. The paper will focus mainly on the synthesis and structural properties of some selected materials. Effect of pH, time, temperature and nature of reactants on the structure of the products will be discussed. The approach used here is an attempt at synthesizing rationally designed transition metal oxides.