Effect of Bond Covalency on the Lattice Stability and Fatigue Behavior of Ferroelectric Bismuth Transition-Metal Oxides

Abstract

The effects of bond covalency on the chemical bonding nature and lattice stability of ferroelectric bismuthbased 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, Bi 3 TaTiO 9 shows a weaker strength of (Ti IV -O) bonds than Bi 4 Ti 3 O 12 , 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 3 TaTiO 9 and Bi 4 Ti 3 O 12 . In contrast, there is only a slight spectral change in the Ta L III -edge XANES spectra of Bi 3 TaTiO 9 and SrBi 2 Ta 2 O 9 upon the chemical reduction, underscoring the higher stability of more covalent (Ta V -O) bonds than (Ti IV -O) bonds. Alternatively, an energy difference between the peaks corresponding to Ta 2p 3/2 f Ta 5d t 2g and Ta 2p 3/2 f Ta 5d e g transitions appears to be greater for Bi 3 TaTiO 9 than for SrBi 2 Ta 2 O 9 , indicating that the bond competition between collinear (Ta-O) and (Ti-O) bonds has a greater influence on the crystal field of TaO 6 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 SrBi 2 Ta 2 O 9 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.

Electric Fatigue of Lead‐Free Piezoelectric Materials

“…Utilizing X‐ray absorption spectroscopy to study the nature of the chemical bonds in Bi 3 TaTiO 9 (BTT), Bi 4 Ti 3 O 12 (BIT) and SBT, it was found that upon chemical reduction the Ti–O environment is changed much more than the Ta–O environment . This renders the Ti–O containing materials weaker compared to unreduced materials and possibly more prone to fatigue degradation due to the easy generation of electronic charges . This finding is in accordance with a study of the fatigue behavior of Sr 0.3 Bi 3.7 Ti 2.7 Ta 0.3 O 12 (SBTT), BIT, and SBT bulk ceramics .…”

“…Nb‐doped thin films not only show higher polarization values compared to SBT films but also fatigue resistance up to 10 9 switching cycles . Utilizing X‐ray absorption spectroscopy to study the nature of the chemical bonds in Bi 3 TaTiO 9 (BTT), Bi 4 Ti 3 O 12 (BIT) and SBT, it was found that upon chemical reduction the Ti–O environment is changed much more than the Ta–O environment . This renders the Ti–O containing materials weaker compared to unreduced materials and possibly more prone to fatigue degradation due to the easy generation of electronic charges .…”

Atomic Layer Deposition of Lithium Tantalate Solid-State Electrolytes

“…The peak at the L 3 edge arises from Ta 2p 3/2 to unoccupied Ta 5d states via dipole transition, and its intensity is expected to be higher in LiTaO 3 and exhibits a blue shift since Ta is in a higher oxidation state (d charge depletion). In Figure , it can be seen that all the lithium tantalate thin films have very similar Ta L 3 -edge spectrum to reference LiTaO 3 , except that the lithium tantalate thin films exhibit one broad peak at ∼9886.5 eV, instead of two well-resolved peaks at 9884.5 and 9886.8 eV for reference crystalline LiTaO 3 . This slight difference is attributable to the amorphous state of the lithium tantalate thin films. , In Figure , it is also obvious that the Ta L 3 -edge XANES of the lithium tantalate thin films are remarkably different from that of pure Ta metal as expected.…”

“…In Figure 4, it can be seen that all the lithium tantalate thin films have very similar Ta L 3 -edge spectrum to reference LiTaO 3 , except that the lithium tantalate thin films exhibit one broad peak at ∼9886.5 eV, instead of two wellresolved peaks at 9884.5 and 9886.8 eV for reference crystalline LiTaO 3 . 50 This slight difference is attributable to the amorphous state of the lithium tantalate thin films. 51,52 In Figure 4, it is also obvious that the Ta L 3 -edge XANES of the lithium tantalate thin films are remarkably different from that of Compositions of the lithium tantalate thin films were analyzed by XPS, and the results are displayed in Figure 5.…”

A Crucial Role of Bond Covalency Competition in Determining the Bandgap and Photocatalytic Performance of Silver Oxosalts

“…For example, the Ag−O bond covalency is a main factor determining the overall band structure of silver oxosalts, Ag x (XO y ) z , in which O atom in the XO y group makes bonds with both X and Ag to form Ag−O−X linkages (see Figure S1 of Supporting Information). It is noted that the Ag−O bond of each Ag−O−X linkage is strongly affected by the neighboring X−O bond through the covalency competition effect; 21 an increase (a decrease) in the X−O bond covalency decreases (increases) the covalent character of the neighboring Ag−O bond, hence influencing the band structure of the silver oxosalts. Since the X−O bond covalency is sensitive to the charge-to-size (Z/r) ratio of the central X atom, 22 it would be informative to correlate this Z/r ratio of the central atom with the bandgap and the photocatalytic activity of the silver oxosalts.…”