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Crystal Structure and Enhanced Piezoelectric Properties of Scandium and Bismuth-Excess Modified Aurivillius Oxides, Na0.5–xBi2.5+xNb2–xScxO9



Aurivillius piezoelectrics, Bi–Sc modified Na0.5–xBi2.5+xNb2–xScxO9 (x = 0.05 and 0.1) and Na-deficient and Bi-enriched Na0.5–xBi2.5+xNb2O9 (x = 0.05, 0.1, and 0.2), were prepared by using a solid-state reaction process. The Rietveld refinement verified that Sc3+ and excess Bi3+ were incorporated into the layered perovskite structure and that they occupied the B and A sites, respectively. The Na0.45Bi2.55Nb1.95Sc0.05O9 [0.05(BiSc)-modified] crystallized in the orthorhombic space group A21am [a = 5.49853(9) Å, b = 5.45976(9) Å, c = 24.98351(29) Å, and V = 750.021(20) Å3; Z = 4] at room temperature. The A-site Bi3+ donor substitution induced no obvious increase in the Curie point (Tc), but a further Sc3+ modification at the B site gave rise to a significant increase in Tc. The movements of non-180° domains contributed to the piezoelectric activities of the Bi-excess polycrystalline materials, whereas the excellent piezoelectric properties of the Bi–Sc-modified polycrystalline materials mostly originated from intrinsic lattice contributions, which are due to their single-180°-domain-wall structures. The Na0.45Bi2.55Nb1.95Sc0.05O9 [0.05(BiSc)-modified] polycrystalline materials have a high Tc of 811 °C and a high piezoelectric activity (d33) of 28 pC/N; interestingly, its d33 was very stable against temperature.