METASTABLE MAGNESIUM TITANATE PHASES SYNTHESIZED IN NANOMETRIC SYSTEMS

When heating mixtures of conventional magnesia and titania powders, ge ikielite (MgO.TiO2) appears first at about 600 degrees C; only above 1 200-1300 degrees C is either qandilite (2MgO.TiO2) or karrooite (MgO.2 TiO(2)) formed, depending on whether MgO or TiO2 is in excess. A diffe rent behaviour pattern is observed when starting from nanometre precur sors, consisting of hydroxides of magnesium and titanium, which lose w ater on heating to about 400 degrees C, yielding nanocrystalline oxide s. Coprecipitated nanometre precursors yield at 600 degrees C qandilit e, geikielite or karrooite, depending on the relative stoichiometries of magnesia and titania in the precursor powder. At higher temperature s, up to 1200 degrees C geikielite is the only titanate formed, and on ly above 1300 degrees C does qandilite or karrooite reappear, dependin g on the stoichiometry of the original mixture. Only karrooite is form ed when independently prepared nanometre precursors of magnesia and ti tania are mixed together and heated to temperatures as low as 400 degr ees C and up to 600 degrees C, the stoichiometric ratio of the mixture notwithstanding. Estimation of surface area changes taking place when nanometre particles coalesce with each other to form magnesium titana tes show that substantial reductions in surface energy take place, eve n if the products remain of nanometre size. Such reductions depend on the absolute and relative particle sizes of the reacting oxides and th e stoichiometry of the final product. The contribution of diffusion pr ocesses, surface energy and misfit volume to the formation of the vari ous magnesium titanates from the oxides is discussed. All three magnes ium titanates synthesized from nanometre precursors show a stable diel ectric constant epsilon over a wide range of frequencies, in contrast with an equivalent commerical powder used in the manufacture of microc ondensers. Furthermore, the quality factor Q of magnesium orthotitanat e (geikielite) synthesised from nanometre precursors is higher by abou t one order of magnitude than that of the commercial product.