The effect of simultaneous substitutions of Ca at A site and Nb or Ta at B site in pyrochlore-type solid solutions: (Ca(x)Gd(1-x))(2)(Zr(1-x)M(x))(2)O(7) (x = 0.1, 0.2, 0.3, 0.4, 0.5 and M = Nb or Ta) were studied by powder X-ray diffraction (XRD), FT NIR Raman spectroscopic techniques and transmission electron microscopy. The solid solutions were prepared by the conventional high-temperature ceramic route. The XRD results and Rietveld analysis revealed that the defect fluorite structure of Gd(2)Zr(2)O(7) progressively changed to a more ordered pyrochlore phase by simultaneous substitutions at A and B sites. Raman spectroscopy reveals the progressive ordering in the anion sublattice with simultaneous doping. High-resolution images and selected-area electron diffraction patterns obtained from TEM confirms the XRD and Raman spectroscopic results. High-temperature XRD studies show that the lattice expansion coefficient in these pyrochlore oxides is of the order of 10(-6) K(-1). Lattice thermal expansion coefficient increases with increase of disorder in pyrochlore oxides, and hence the variation of thermal expansion coefficient with composition is also a good indicator of disordering in pyrochlore-type oxides. The ionic conducting properties of the samples were characterised by impedance spectroscopy, and it was found that Nb-doped compositions show a considerable change in conductivity near the phase boundary of disordered pyrochlore and defect fluorite phases.

No abstract

A series of quaternary powellites with varying cerium contents in a Ca–Ce–Nb–M–O (where M=Mo or W) system has been synthesized through the solid‐state route. Powder X‐ray diffraction analysis shows that these compounds crystallize in a tetragonal powellite structure with the I41/a space group. The structure was further confirmed by Raman spectroscopic analysis. The electrical conductivity studies reveal that they exhibit negative temperature coefficient (NTC) thermistor characteristics. The characteristic parameter of the thermistor (β) is found to be in the range of 5000–7000 K, and these values can be tuned with the cerium concentration. The electrical conductivity in these compounds is due to the presence of Ce3+, which remains in the reduced state without being oxidized to Ce4+ due to the structural stabilization. The X‐ray photoelectron spectroscopy analysis corroborates the presence of Ce in the 3+ state.

Complex ceramic oxides, CeTiMO6 (M=Nb or Ta) having aeschynite type mineral structure were prepared by the conventional ceramic route. Complex impedance analysis in the frequency range 10 Hz–1 MHz over a wide temperature range (30–600 °C) indicates the presence of grain boundary effect along with the bulk contribution and also confirms the presence of non-Debye type of multiple relaxations in the material. The frequency dependent conductivity plots exhibit double power law dependence suggesting three types of conduction mechanisms: low frequency (10 Hz–1 kHz) conductivity owing to long range translational motion of electrons (frequency independent), mid-frequency conductivity (1–10 kHz) due to short-range hopping, and high frequency (10 kHz–1 MHz) conduction due to localized orientation hopping mechanism. The hopping model can explain the nature of the conduction mechanism completely. The electrical conductivity measurements with temperature suggest the negative temperature coefficient of resistance behavior. The activation energy studies allow insight into the nature of the conduction mechanisms.

No abstract