Structural and electrical properties of nonstoichiometric semiconducting pyrochlores in Ca–Ce–Ti–Nb–O system

A series of quaternary nonstoichiometric cubic pyrochlores with varying cerium content in Ca–Ce–Ti–Nb–O system has been synthesized through solid state route. The powder X-ray diffraction and Rietveld analysis indicate that smaller Ti cation occupies the A site by displacing the A and O′ ions causing displacive disorder in the cubic pyrochlore structure. The electrical conductivity measurements reveal that they exhibit semiconductor behavior in a broad temperature range (30–600 °C) and the conductivity increases with increase in Ce content. The X-ray photoelectron spectroscopy (XPS) analysis corroborates the presence of Ce in 3+ state. The impedance analysis reveals two types of conduction processes from the frequency dependent conductivity plots, i.e. low frequency conduction due to short range hopping and high frequency conduction due to the localized reorientation hopping mechanism. The relaxational properties indicate that hopping frequency is dependent on cerium concentration and is a measure of degree of disorder in the system.