Investigation of the phase formation and dielectric properties of Bi7Ta3O18

• Synthesis condition of Bi7TaO3O18 had been determined.
• Recombination of intermediate BiTaO4 and Bi3TaO7 phases are required for the Bi7TaO3O18 phase formation.
• Stable material as confirmed by thermal and structural analyses.
• Typical ferroelectric showing high dielectric constants and low losses.
• Resonance and thermal activated polarisation processes are responsible for the excellent dielectric characteristic.

Polycrystalline Bi7Ta3O18 was synthesised at the firing temperature of 950 °C over 18 h via conventional solid state method. It crystallised in a monoclinic system with space group C2/m, Z = 4 similar to that reported diffraction pattern in the Inorganic Crystal Structure Database (ICSD), 1-89-6647. The refined lattice parameters were a = 34.060 (3) Å, b = 7.618 (9) Å, c = 6.647 (6) Å with α = γ = 90° and β = 109.210 (7), respectively. The intermediate phase was predominantly in high-symmetry cubic structure below 800 °C and finally evolved into a low-symmetry monoclinic structured, Bi7Ta3O18 at 950 °C. The sample contained grains of various shapes with different orientations in the size ranging from 0.33–22.70 μm. The elemental analysis showed the sample had correct stoichiometry with negligible Bi2O3 loss. Bi7Ta3O18 was thermally stable and it exhibited a relatively high relative permittivity, 241 and low dielectric loss, 0.004 at room temperature, ∼30 °C and frequency of 1 MHz.