Influence of disorder-to-order transition on lattice thermal expansion and oxide ion conductivity in (CaxGd1−x)2(Zr1−xMx)2O7 pyrochlore solid solutions

Abstract

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.

Thermal annealing of natural, radiation-damaged pyrochlore

“…The local structure of synthetic pyrochlore-type materials have been extensively studied by Raman spectroscopy and the observed Raman signals could be assigned to certain phonon modes on the basis of model calculations [6,7,10,45,48,49,52,[58][59][60][61][62][63][64][65][66][67][68][69][70][71].…”

Electrical transport properties of manganese containing pyrochlore type semiconducting oxides using impedance analyses

“…The reason for this behavior is the incorporation of lower ionic radii Mn 2+ ion in place of larger Ce 3+ ion. The decrease of lattice parameter can also be attributed to progressive ordering of both anions and cations of the pyrochlore structure [25]. Oxygen x parameter also found to be decreased with progressive substitution of Mn and the values are within the range reported for ordered pyrochlore structure.…”

“…This is in agreement with the XRD results. This is a clear indication of the ordering in the lattice with Mn substitution [25] as the x parameter is more sensitive to even small amount of disorder in the lattice than the lattice parameter.…”

“…The broadening of this F 2g mode with increasing Mn content can be correlated with increasing oxygen vacancy concentration[30], since we have substituted Mn 2+ in place of Ce 3+ . The mode at $830 cm À1 may be due to local disorder, vacancies and defects present in the pyrochlore lattice[25].…”

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

“…5), respectively. The latter is indicative of the number of charge carriers available for conduction [26][27][28]. The calculated values of the activation energy and pre-exponential factor are presented in Table 1.…”