Internal Barrier Layer Capacitance Effects in Neodymium Copper Tantalate Ceramics

A new perovskite material $Nd_{2//3}CuTa_4O_{12}$ was applied as a naturally formed internal barrier layer capacitor. The powder prepared by solid state synthesis and ball milling was pressed into pellets and sintered at 1180-1220°C. Dielectric properties of ceramic samples were characterized by impedance spectroscopic studies carried out in the temperature range from - 55 to 700C at frequencies 10 Hz ÷ 2 MHz. Two types of the dielectric response were revealed - a high frequency response attributed to grains which occurred at low temperatures, and a low frequency one related to grain boundaries which dominated at higher temperatures. Resistances and capacitances of grains were found to be two orders lower than those of grain boundaries. Two plateaus were observed in the dielectric permittivity versus frequency plots - a low frequency step corresponding to a high value of $10^4$ and a high frequency step at a level of 40. Scanning electron microscopy observations and energy dispersive spectroscopy analysis confirmed that $Nd_{2//3}CuTa_4O_{12}$ ceramics were composed of semiconducting grains and insulating oxygen-enriched grain boundaries. The formation of such an electrically heterogeneous system during the one step fabrication process in air leads to spontaneous internal barrier layer capacitance effects responsible for a high and relatively stable dielectric permittivity of the developed material.