New layer-structured ferroelectric polycrystalline materials, Na_0.5Nd_xBi_4.5-xTi₄O₁₅: Crystal structures, electrical properties and conduction behaviors

New layer-structured ferroelectric polycrystalline materials, Na_0.5Nd_xBi_4.5-xTi₄O₁₅ (x = 0-2.5; NNBTO-x), were prepared, and the tailoring effects of neodymium (Nd) were investigated thoroughly. A relaxation in orthorhombic distortion was caused by the Nb substitution, and the compositions with high x value modification tended to crystallize in tetragonal space group. Rietveld structure refinements revealed that NNBTO-0.5 and NNBTO-1.5 crystallized in the orthorhombic space group A2₁am at room temperature, and Nd/Bi combinations occupied the cation sites of the (Bi₂O₂)²⁺ layers. The Nd substitution had strong impacts on the electrical properties of NNBTO-x. Ferroelectric to paraelectric phase transition temperature (T_c) was lowered from about 670 °C to below 200 °C. Ferroelectric (remnant) polarization (P_r), piezoelectric activity (d₃₃) and field-induced strain (S₃₃) first increased, and then decreased drastically owing to a significant degradation in spontaneous polarization (P_s). In addition, the high temperature resistances of the NNBTO-x specimens were discussed and their conduction mechanisms were explored. The NNBTO-0.5 with a T_c of 670 °C had a P_r of 10.8 μC cm^-1, a d₃₃ of 24.5 pC N^-1 and a S₃₃ of about 0.04%, due to the decrease in oxygen vacancy concentration and the local lattice distortion.