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Abstract
The effect of simultaneous aliovalent cation substitution on a pyrochlore lattice in (Sm,Zr)2-x(Sc,Ce)xO7 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) was investigated by X-ray diffraction in ambient and high-temperature conditions, electron microscopy, Raman spectroscopy and impedance spectroscopy. The samples were prepared by a conventional high temperature ceramic route. Due to comparable sizes of the ions involved, both the substituent ions diffused randomly into both cation sites leading to an order-to-disorder transition. The lattice thermal expansion coefficient decreases continuously with increased aliovalent cations substitutions which is mainly attributed to the increased diatomic bond energy of the average cation-oxygen bond, unlike the usual trends observed in the systems having phase transition from ordered pyrochlore to disordered fluorite system. The trends in ionic conductivity suggest that it is more governed by the co-operative behaviour of the mobile charge carriers. in the higher substitutions leading to higher activation energies. Hence in the case of pyrochlore type composition, in addition to the lattice disordering, co-operative behaviour of the mobile charge carriers and ionization energy of the individual bond play an important part in determining the electrical behaviour of the system.
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