Fig. 1 Relationship between d33 and Curie temperature Tc of piezoceramics with different crystal structures(Inset showing schematic diagrams of perovskite, tungsten bronze, bismuth layer and perovskite layer structures)

Fig. 2 Crystal structure of Sr2Nb2O7 viewed along a (a) and along c (b)Atoms are labelled in accordance with the nomenclature of Ref [23]

Fig. 3 (a) Sketch of the largest atomic displacements associated with the strongest instability mode obtained for the Cmcm phase of La2Ti2O7; (b) Sketch of a typical anti-ferrodistortive mode occurring in an ideal perovskite structure of BaTiO3. The arrows on the side represent the electric dipoles associated to the displacement of oxygens in different y-planes[34]

Fig. 4 Calculated valence-electron charge density for the monoclinic P21 La2Ti2O7. Contour lines differ by 0.05 e Å-3 [35] (1 Å=0.1 nm)

Fig. 5 SEM images of PLS high temperature piezoceramics fabricated with different techniques(a) Sr2Nb2O7 by solid state reaction method[42]; (b) Sr2Nb2O7 by spark plasma sintering[15]; (c) 0.3wt%ZnO-Sr2Nb2O7 by solid state reaction method[41]; (d) Ce2Ti2O7 at 4 GPa, 1100℃[26]

Fig. 6 Effect of thermal depoling on piezoelectric properties of Sr2-xBaxNb2O7 piezoceramics[45]

Fig. 7 Temperature dependence of d33 values of (1-x)Sr2Nb2O7-x(Na0.5Bi0.5)TiO3 piezoceramics after thermal annealing[51]