Growth of Bi2SrTa2O9 Single Crystals Using the Self-Flux Method

The optimum heat-treatment process to grow single crystals of Bi2SrTa2O9 (BiSTa), which have been attracting much attention as a material for capacitors of ferroelectric nonvolatile memories, was determined using a Bi2O3-excess self-flux method. The starting mixture (Bi2O3:SrCO3:Ta2O5=79:10.5:10.5) was inserted into a double crucible made of platinum and alumina. Single crystals were grown at 1345°C and 1400°C in a vertical furnace. With a short hold time, the main product was α-Bi2O3 and with an excessively long hold time, SrTa4O11 and Sr2Ta2O7 were also synthesized. For the purpose of successfully obtaining BiSTa crystals in a single-phase state, complete vaporization of the excess Bi2O3 flux in the double crucible was essential under an appropriate heat-treatment condition, such as a hold time of 2h at 1400°C and a cooling rate of 2°C/h to 1150°C. The obtained crystals were transparent and rectangular thin plate, with the long side of 300μm-1mm. The compositional ratio of the crystal was 1.91±0.05:1.27±0.05:2.00 for Bi:Sr:Ta, suggesting that not only is the crystal Bi poorer but Sr richer, compared to the stoichiometric ratio.