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 +xNd2/3TiO3 (BNT) ceramics sintered in air below 1650 K have been carried out. X-ray powder diffraction studies indicate apparent phase singularity for compositions withx 0.08 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 +xNd2/3TiO3 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 eeff ≳ 104 with tanδ < 0.1 and nearly flat temperature capacitance characteristics. The grain-size dependence of eeff, variations of eeff 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 eeff and rise in tanδ, are explained on the basis of an intergranular (internal boundary layer) dielectric characteristic of these ceramics.