High temperature Aurivillius piezoelectrics: The effect of (Li, Ln) modification on the structure and properties of (Li, Ln)_0.06(Na, Bi)_0.44Bi₂Nb₂O₉ (Ln = Ce, Nd, La and Y)

High temperature Aurivillius piezoelectrics, (Li, Ln)_0.06(Na, Bi)_0.44Bi₂Nb₂O₉ (NBN-LiLn), with Li and Ln (Ce, Nd, La and Y) co-substitution at the A site, were prepared using a solid-state reaction process. Taking cation disordering between A site and the (Bi₂O₂)²⁺ layers into consideration, the crystal structure of (Li, Nd)_0.06(Na, Bi)_0.44Bi₂Nb₂O₉ (NBN-LiNd) was refined by using the Rietveld method with powder X-ray diffraction, which was confirmed to be a two-layer Aurivillius oxide with an orthorhombic space group, A2₁ am [a = 5.48666(9) Å, b = 5.46046(8) Å, c = 24.9122(4) Å and Z = 4], at room temperature. LiNd substitution induced a decrease in cation disorder and an increase in orthorhombic distortion. The ferroelectric to paraelectric phase transition temperature (T_c) of the NBN-LiLn (Ln = Ce, Nd, La and Y) ceramics ranged from 751 to 842 °C and the T_c increased as the radii of the Ln³⁺ (Ce³⁺, Nd³⁺, La³⁺ and Y³⁺) was decreased. The NBN-LiCe and NBN-LiNd had a d₃₃ of 31 and 29 pC N⁻¹, respectively, which is much higher than the reported d₃₃ values of other Aurivillius ceramics with a high T_c (T_c > 650 °C, d₃₃ < 20 pC N⁻¹). With increasing annealing temperature, a significant degradation in d₃₃ was observed for NBN-LiCe, while no drop in d₃₃ was observed up to 650 °C for NBN-LiNd. The inhomogeneous domain structures determined the dissimilar piezoelectric behaviors of NBN-LiCe and NBN-LiNd.