Phase relations and dielectric properties of BaTiO₃ ceramics heavily substituted with neodymium

Abstract

Investigations on the phase relations and dielectric properties of (1 -x)BaTiO₃ +xNd_2/3TiO₃ (BNT) ceramics sintered in air below 1650 K have been carried out. X-ray powder diffraction studies indicate apparent phase singularity for compositions withx < 0.3. Nd₂Ti₂O₇ is detected at higher neodymium concentrations. The unit cell parameter changes continuously with neodymium content, and BaTiO₃ is completely cubic at room temperature withx ≌ 0.0525, whereas electron diffraction studies indicate that the air-sintered BNT ceramics withx > 0.08 contain additional phases that are partly amorphous even to an electron beam. SEM observations reveal that BaTiO₃ grains are mostly covered by a molten intergranular phase, and show the presence of randomly distributed Nd₂Ti₂O₇ grains. Energy dispersive X-ray analysis shows the Ba-Nd-Ti ternary composition of the intergranular phase. Differential thermal analysis studies support the formation of a partial melt involving dissolution-precipitation of boundary layers of BaTiO₃ grains. These complex phase relations are accounted for in terms of the phase instability of BaTiO₃ with large cation-vacancy concentration as a result of heavy Nd³⁺ substitution. The absence of structural intergrowth in (1 -x)BaTiO₃ +xNd_2/3TiO₃ under oxidative conditions leads to a separation of phases wherein the new phases undergo melting and remain X-ray amorphous. BNT ceramics with 0.1 <x < 0.3 have ε_eff ≳ 10⁴ with tanδ < 0.1 and nearly flat temperature capacitance characteristics. The grain-size dependence of ε_eff, variations of ε_eff and tanδ with the measuring frequency, the non-ohmic resistivities, and the non-linear leakage currents at higher field-strengths which are accompanied by the decrease in ε_eff and rise in tanδ, are explained on the basis of an intergranular (internal boundary layer) dielectric characteristic of these ceramics.