Crystal structure and enhanced piezoelectric properties of scandium and bismuth-excess modified aurivillius oxides, Na_0.5-xBi _2.5+xNb_2-xSc_xO₉

Aurivillius piezoelectrics, Bi-Sc modified Na_0.5-xBi _2.5+xNb_2-xSc_xO₉ (x = 0.05 and 0.1) and Na-deficient and Bi-enriched Na_0.5-xBi_2.5+xNb ₂O₉ (x = 0.05, 0.1, and 0.2), were prepared by using a solid-state reaction process. The Rietveld refinement verified that Sc ³⁺ and excess Bi³⁺ were incorporated into the layered perovskite structure and that they occupied the B and A sites, respectively. The Na_0.45Bi_2.55Nb_1.95Sc_0.05O ₉ [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) ų; Z = 4] at room temperature. The A-site Bi³⁺ donor substitution induced no obvious increase in the Curie point (T_c), but a further Sc³⁺ modification at the B site gave rise to a significant increase in T_c. 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 Na_0.45Bi _2.55Nb_1.95Sc_0.05O₉ [0.05(BiSc)-modified] polycrystalline materials have a high T_c of 811°C and a high piezoelectric activity (d₃₃) of 28 pC/N; interestingly, its d₃₃ was very stable against temperature. A Rietveld refinement ascertained that Sc³⁺ and excess Bi³⁺ were incorporated into the layered perovskite structure, and Na _0.45Bi_2.55Nb_1.95Sc_0.05O₉ crystallized in the orthorhombic space group, A2₁am. The high-temperature stability of the piezoelectric activity of the modified Na _0.45Bi_2.55Nb_1.95Sc_0.05O₉ results from its single-180°-domain-wall structure.