Effect of bond covalency on the lattice stability and fatigue behavior of ferroelectric bismuth transition-metal oxides

The effects of bond covalency on the chemical bonding nature and lattice stability of ferroelectric bismuth-based transition-metal oxides have been investigated systematically through comparative X-ray absorption spectroscopic studies for several Aurivillius-structured materials and their chemically reduced derivatives. According to Ti K-edge X-ray absorption near-edge structure (XANES) analysis, Bi₃TaTiO₉ shows a weaker strength of (Ti^IV-O) bonds than Bi₄Ti₃O₁₂, which could be interpreted as a result of the bond competition with highly covalent (Ta^v-O) bonds. Also, it was found that a lithiation process gives rise to a remarkable variation of Ti local structure commonly for Bi ₃TaTiO₉ and Bi₄Ti₃O₁₂. In contrast, there is only a slight spectral change in the Ta L_III-edge XANES spectra of Bi₃TaTiO₉ and SrBi₂Ta ₂O₉ upon the chemical reduction, underscoring the higher stability of more covalent (Ta^v-0) bonds than (Ti^IV-0) bonds. Alternatively, an energy difference between the peaks corresponding to Ta 2p_3/2 → Ta 5d_t2g and Ta 2p_3/2 → Ta 5d_eg transitions appears to be greater for Bi₃TaTiO ₉ than for SrBi₂Ta₂O₉, indicating that the bond competition between collinear (Ta - O) and (Ti - O) bonds has a greater influence on the crystal field of TaOê octahedra than an interaction between perpendicularly aligned (Ta - O) and (Bi/Sr - O) bonds. On the basis of the present experimental findings, we are able to conclude that the highly resistive nature of the (Ta^v-O) bonds against the increase of electronic charge is responsible for the excellent cycle characteristics of the SrBi₂Ta₂O₉ phase. In this regard, the incorporation of highly covalent metal ions into the octahedral sites of the Aurivillius-structured lattice is supposed to be effective in improving the lattice stability and memory performance of this ferroelectric metal oxide.