Dielectric and Impedance Spectroscopy of Barium Orthoniobate Ceramic

Barium orthoniobate (Ba3Nb2O8), a derivative of the perovskite family, was prepared using a high-temperature solid-state reaction technique (calcination temperature = 1425A degrees C and sintering temperature = 1450A degrees C for 4 h). Preliminary x-ray structural analysis with room-temperature x-ray diffraction data confirmed the formation of a single-phase compound with hexagonal crystal structure. Study of the microstructure of a gold-coated pellet by scanning electron microscopy (SEM) showed that the sample has well-defined grains that are distributed uniformly throughout the surface of the sample. Detailed studies showed that the dielectric parameters (epsilon (r) and tan delta) of the compound at three different frequencies (10 kHz, 100 kHz, and 1000 kHz) are almost constant in the low-temperature region (from room temperature to about 200A degrees C). An anomaly in the relative permittivity (epsilon (r)) (similar to 357A degrees C) suggests the possible existence of a ferroelectric-paraelectric phase transition of diffuse type in the material. Detailed studies of impedance and related parameters show that the electrical properties of the material are strongly dependent on temperature, showing good correlation with its microstructure. The bulk resistance (evaluated from impedance studies) is found to decrease with increasing temperature. This shows that the material has negative temperature coefficient of resistance (NTCR), similar to that of semiconductors. Studies of electric modulus indicate the presence of a hopping conduction mechanism in the system with nonexponential-type conductivity relaxation. The nature of the variation of the direct-current (dc) conductivity with temperature confirms the Arrhenius and NTCR behavior in the material. The alternating-current (ac) conductivity spectra show a typical signature of an ionic conducting system and are found to obey Jonscher's universal power law.