Sol‐Gel Route to Ferroelectric Layer‐Structured Perovskite SrBi ₂ Ta ₂ O ₉ and SrBi ₂ Nb ₂ O ₉ Thin Films

Abstract

Precursors for layer‐structured perovskite thin films of SrBi 2 Ta 2 O 9 (SBT) and SrBi 2 Nb 2 O 9 (SBN) were prepared by the reactions of a strontium‐bismuth double methoxyethoxide and tantalum or niobium methoxyethoxide in methoxyethanol, followed by partial hydrolysis. Several spectroscopic techniques, such as 1 H‐, 13 C‐, and 93 Nb‐NMR (nuclear magnetic resonance), and Fourier‐transform infrared spectroscopy were used to analyze the arrangement of the metals and oxygen in the precursor molecules. The precursors contained Sr‐O‐M (where M is Ta or Nb) bonds (i.e., a strontium is connected to two MO 6 octahedra) and Sr‐O‐Bi bonds with a bismuth atom bonded to the oxygens of the MO 6 octahedron. The arrangement of metals and oxygens was considered to be similar to the layer‐structured perovskite crystal sublattice. As a result, the sol‐gel‐derived SBT thin films crystallized, by rapid thermal annealing in an oxygen atmosphere below 550°C, and they exhibited preferred (115) orientation. The crystallinity improved and the crystallite size increased with temperature up to 700°C. In the case of SBN thin films, a low heating rate (2°C/min) was necessary for the control of the crystallographic (115) orientation, whereas a rate of 200°C/s (rapid thermal annealing) produced films that exhibited c ‐axis orientation. The (115) SBT thin film, heated to 700°C, exhibited improved ferroelectric properties.