Crystallization of alkaline earth zirconates and niobates from compositionally flexible aqueous solution-gel syntheses

Barium and strontium zirconates and niobates were prepared by means of an economic and ecologic aqueous solution-gel route with high compositional flexibility. The synthesis of precursor solutions and gels, and thermal analysis of the gels are presented. The crystallization of the different oxides was studied in situ using high-temperature X-ray diffraction measurements (HT-XRD). Zirconate crystallization was observed between 600 °C and 700 °C. Small amounts of Ba(II)- and Sr(II)-carbonate crystallized before the oxide did, and decomposed between 700 °C and 800 °C. More sensitive Fourier transform infrared spectra, showed that very small traces of carbonate remain at 900 °C. The crystallization behavior of strontium niobates was strongly dependent on the Sr:Nb ratios, as observed in HT-XRD. Phase pure SrNb2O6 was obtained at low temperature (600 °C). Sr2Nb2O7 and Sr5Nb4O15 were formed at 900 °C from an intermediate phase, with a minor secondary phase. Nb2Sr4O9 crystallized at 600 °C, but transformed to a mixture of phases at high temperature (1000 °C). By off-line furnace annealing, phase pure strontium niobates of all four compositions were obtained. The presented work demonstrates the high compositional flexibility of the aqueous solution-gel route, allowing fast synthesis of different multimetal oxides with high purity.