Effects of La³⁺ addition on the phase transition, microstructure, dielectric and piezoelectric properties of Ba_0.9Ca_0.1Ti_0.9Zr_0.1O₃ ceramics prepared by hydrothermal method

Lead-free Ba_0.9Ca_0.1Ti_0.9Zr_0.1O₃–xLa (x = 0.000, 0.005, 0.010, 0.015, 0.020, 0.025) ceramics were prepared by hydrothermal method and were assisted by fast microwave sintering. The effects of La³⁺ addition on the phase transition, microstructure, dielectric and piezoelectric properties were investigated. Single orthorhombic phase was observed in the composition of x ≤ 0.010 and the coexistence of orthorhombic and tetragonal phase were detected at x = 0.015 and 0.020, which is characterized by the broadening of (002)/(200) peaks at around 2θ ~ 45°. The ceramics show cubic phase when the x was increased to 0.025. The grain size was affected significantly by the La³⁺ content, reaching a maximum value of ~8.2 μm at x = 0.010. The dielectric constant decreases with the addition of small amount of La³⁺ and then increases sharply at x ≥ 0.015, exhibiting a PTC behavior at x = 0.020, while no T _C was observed at x = 0.025. The Curie temperature (T _C ) always decrease with the introduction of La³⁺ and the dielectric loss shows a minimum value at x = 0.015. The piezoelectric constant (d ₃₃ ) measured at 50 °C of the ceramics were increased after the partial substitution of La³⁺ because of the enhancement of coexistence of orthorhombic and tetragonal phases at this temperature. Ferroelectric and piezoelectric properties can not be detected when x was increased to 0.02 while they were observed again with further addition.