The influence of excess K2O on the electrical properties of (K,Na)1/2Bi1/2TiO3 ceramics

The solid solution (K x Na 0.50-x )Bi 0.50 TiO 3 (KNBT) between Na 1/2 Bi 1/2 TiO 3 and K 1/2 Bi 1/2 TiO 3 (KBT) has been extensively researched as a candidate lead-free piezoelectric material because of its relatively high Curie temperature and good piezoelectric properties, especially near the morphotropic phase boundary (MPB) at x ∼ 0.10 (20 mol. % KBT). Here, we show that low levels of excess K 2 O in the starting compositions, i.e., (K y+0.03 Na 0.50-y )Bi 0.50 TiO 3.015 (y-series), can significantly change the conduction mechanism and electrical properties compared to a nominally stoichiometric KNBT series (K x Na 0.50-x )Bi 0.50 TiO 3 (x-series). Impedance spectroscopy measurements reveal significantly higher bulk conductivity (σ b ) values for y ≥ 0.10 samples [activation energy (E a ) ≤ 0.95 eV] compared to the corresponding x-series samples which possess bandgap type electronic conduction (E a ∼ 1.26-1.85 eV). The largest difference in electrical properties occurs close to the MPB composition (20 mol. % KBT) where y = 0.10 ceramics possess σ b (at 300 °C) that is 4 orders of magnitude higher than that of x = 0.10 and the oxide-ion transport number in the former is ∼0.70-0.75 compared to < 0.05 in the latter (between 600 and 800 °C). The effect of excess K 2 O can be rationalised on the basis of the (K + Na):Bi ratio in the starting composition prior to ceramic processing. This demonstrates the electrical properties of KNBT to be sensitive to low levels of A-site nonstoichiometry and indicates that excess K 2 O in KNBT starting compositions to compensate for volatilisation can lead to undesirable high dielectric loss and leakage currents at elevated temperatures.