Abstract:
The present study involves a detailed investigation on interrelationships among Philip’s ionicity, thermal
expansion coefficient and ionic conductivity in a new series of pyrochlore type compositions,
RE2Y2/3Zr2/3Nb2/3O7 (RE = La, Nd, Sm, Gd, Dy, Y) using high temperature X-ray diffraction studies, a
transmission electron microscope, a Raman spectroscopic technique, electrical conductivity measurements
and crystal chemistry principles. The system undergoes structural transition from an ordered pyrochlore
structure to a disordered defect fluorite structure in the series with a phase boundary upon Gd substitution.
The ionicity difference of M–O bonds follows the thermal expansion trend in the fluorite phase but
opposes it in the pyrochlore phase. The ionicity of the A–O bond contributes more to the thermal
expansion coefficient in the pyrochlore system unlike in perovskites where it is due to the cumulative
effect of the A–O and B–O bonds. On the contrary, the ionic conductivity increases with the decrease in
the ionicity difference associated with the charge carrier concentrations. The best conductivity obtained in
the series is 7.9356 10 6 S cm 1 for Y2(Y1/3Zr1/3Nb1/3)2O7 at 1023 K. The minimum thermal expansion
coefficient is obtained at the phase boundary for Gd2Y2/3Zr2/3Nb2/3O7 and the value is 5.2 10 6 K 1.
Furthermore, the anomalous behaviour of Sm and Nd compounds is explained based on the ionicity of
M–O bonds. These results demonstrate that the ionicity difference plays a great role in determining the
thermal expansion and ionic conductivity of pyrochlore based materials.
