Effects of neodymium substitution on the structural and the electrical properties of Bi₄Ti₃O₁₂ ceramics

Bismuth-layer-structured ferroelectric/piezoelectric Bi_4-xNd_xTi₃O₁₂ (BNdT) ceramics in which the Nd substitution was controlled from x = 0.25 to 1.00 were prepared by using the conventional solid-state-reaction method. Structural analysis by using X-ray diffraction (XRD) showed that all the BNdT ceramics had a polycrystalline structure without second phases, which was consistent with the data for Bi₄Ti₃O₁₂ (JCPDS No. 35-0795). We found that the BNdT ceramics had grains with elongated shapes. The change in the orientation ratio of a - b plane in the grains with increasing Nd substitution was found to be similar to the changes in their electrical properties. In particular, a notable improvement in the electrical properties was observed in the x = 0.75 - 0.90 composition ratio of the BNdT ceramics. For the Bi_3.1Nd_0.9Ti₃O₁₂ (x = 0.90) ceramics, the values of the dielectric constant and the piezoelectric coefficients exhibited maximum values of about 209 and 15 pC/N, respectively. On the other hand, with increasing Nd concentration, the phase transition temperature (T_c) continuously decreased from 613 °C (Bi_3.75Nd_0.25Ti₃O₁₂, x = 0.25) to 218 °C (Bi_3.00Nd_1.00Ti₃O₁₂, x = 1.00). These results suggest that controlling the Nd substitution in BNdT ceramics is essential for changing the dielectric constant and piezoelectric coefficients of bismuth-layer-structured piezoelectric materials.