Effect of lanthanum and tungsten co-substitution on the structure and properties of new Aurivillius oxides Na_0.5La_0.5Bi ₂Nb_2-xW_xO₉

New Aurivillius ferroelectrics Na_0.5La_0.5Bi ₂Nb_2-xW_xO₉ (NLBN-W, x = 0, 0.01, 0.02, 0.03, and 0.05) by lanthanum (La³⁺) and tungsten (W ⁶⁺) co-substitution were prepared by using a solid-state reaction process. The crystal structure of Na_0.5La_0.5Bi ₂Nb_1.98W_0.02O₉ (NLBN-W-0.02) was refined by the Rietveld method with powder X-ray diffraction at room temperature and it was confirmed to be a two-layer Aurivillius oxide with an orthorhombic space group A2₁am (a = 5.48754(8) Å, b = 5.47967(9) Å and c = 24.8523(4) Å). The ferroelectric to paraelectric phase transition temperature (T_c) of NLBN-W-0.02 is around 501 °C, with prominent relaxation behavior. The NLBN-W-0.02 ceramics have a high piezoelectric coefficient (d₃₃ = 31 pC N^-1) and large remnant polarization (P_r = 15.1 μC cm^-2). The decrease in P_r for the NLBN-W-0.02 ceramics after 10⁷ switching cycles at 40 and 120 °C is only 2 and 9%, respectively.