Non-Stoichiometry in Bismuth Perovskite Solid Solutions Public Deposited

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  • The role of A-site non-stoichiometry was investigated in lead-free piezoelectric ceramics based on compositions in the 1-x(Bi₀.₅Na₀.₅TiO₃)-xBaTiO₃ system near the morphotropic phase boundary where x = 0.055, 0.06 and 0.07. The samples were prepared by a conventional solid state mixed oxide route with the A- site stoichiometry modified to incorporate donor-doping (through Bi-excess) and acceptor-doping (through Na-excess). While no change in the crystal structure was observed via donor-doping, acceptor-doping was found to promote rhombohedral distortions. A significant improvement in dielectric properties was observed in donor-doped compositions and, in contrast, a degradation in properties was observed in acceptor-doped compositions. Compared to the stoichiometric composition, the acceptor-doped compositions displayed a significant increase in coercive field (E[subscript c]) which is an indication of domain wall pinning as found in hard piezoelectrics such as Pb(Zr[subscript x]Ti[subscript 1-x])O₃ (PZT). This result was further confirmed via polarization hysteresis studies including PUND tests and remanent P-E hysteresis analyses. Moreover, all A-site acceptor-doped compositions also exhibited an increase in mechanical quality factor (Q[subscript m]) as well as a decrease in piezoelectric coefficient (d₃₃), dielectric loss (tan δ), remanent polarization (P[subscript r]) and dielectric permittivity, which are all the typical characteristics of the effects of "hardening". The mechanism for the observed hardening in A-site acceptor doped BNT-based systems is linked to changes in the long-range domain structure and defect chemistry. Impedance spectroscopy was utilized to analyze the effects of A-site non- stoichiometry on the conduction mechanisms. An electrically heterogeneous microstructure was observed in both the stoichiometric and Na-excess compositions. In addition, the Na-excess compositions exhibited lower resistivities (ρ ~ 10³ Ω-cm) with characteristic peaks in the impedance data indicating ionic conductivity similar to recent observations of oxide ion conduction in (Bi₀.₅Na₀.₅)TiO₃. In contrast, Bi- excess compositions resulted in an electrically homogeneous microstructure with an increase in resistivity by ~3-4 orders of magnitude and an associated activation energy of 1.57 eV which was close to half of the optical band gap. Long-term annealing studies were conducted at 800°C to identify changes in crystal structure and electrical properties. The results of this study demonstrates that the dielectric and electrical properties of (1-x)BNT-xBT ceramics at the compositions near the MPB are very sensitive to Bi/Na stoichiometry.
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