Formation of complex alkoxides to control layer structure in Sr-Bi-M-O (M: Ta or Nb) perovskite thin films

Precursors for layer-structured perovskite thin films of Sr₂Bi₂Ta₃O_12.5 or SBT223, and Sr₂Bi₂Nb₃O_12.5 or SBN223 were prepared by the reactions of Sr-Bi (2 : 2) double methoxyethoxide and Ta or Nb methoxyethoxide, followed by partial hydrolysis. By several analytical techniques, such as ¹H-, ¹³C- and ⁹³Nb-NMR (nuclear magnetic resonance) and FT-IR (Fourier transform infrared) the molecular structure of the precursors was found to be similar to the layer-structured perovskite crystal sub-lattice. As a result, the onset of crystallization in the sol-gel derived SBT223 thin films was at a low temperature of 550°C. By rapid thermal annealing in an oxygen atmosphere, the single-phase perovskite films exhibited preferred (1 1 7) orientation. In contrast to the SBT system, the observed number of the pseudo-perovskite layers of TaO₆ octahedral units between Bi₂O₂²⁺ layers in the SBT223 was three. Despite the elongation of the unit cell along the c-axis, 700°C-heated SBT223 thin film did not exhibit a saturated hysteresis loop due to the small crystallite size.