Structures, phase transformations, and dielectric properties of BiTaO₄ ceramics

Low (α)- and high-temperature (β) forms of BiTaO₄ have attracted much attention due to their dielectric and photocatalytic properties. In the present work, a third form, the so-called HP-BiTaO₄, was synthesized at high temperature and pressure. The phase evolution, phase transformations, and dielectric properties of α- and β-BiTaO₄ and HP-BiTaO₄ ceramics are studied in detail. β-BiTaO₄ ceramics densified at 1300 °C with the microwave permittivity ϵ_r ≈ 53, the microwave quality factor Q_f ≈ 12070 GHz, and the temperature coefficient of resonant frequency τ_f ≈ -200 ppm/°C. HP-BiTaO₄ ceramics were synthesized at 5 GPa and 1300 °C followed by annealing at 600 °C. In contrast with the α phase, HP-BiTaO₄ exhibited ϵ_r ≈ 195 at 1 kHz to 10 MHz, accompanied by a low dielectric loss of ∼0.004. The relation between structure and dielectric properties is discussed in the context of Shannon's additive rule and bond theory.