Influence of phase transition from order to disorder and Philip's ionicity on the thermal expansion coefficient of pyrochlore type compositions with a multivalent environment

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, RE₂Y_2/3Zr_2/3Nb_2/3O₇ (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(Y_1/3Zr_1/3Nb_1/3)₂O₇ at 1023 K. The minimum thermal expansion coefficient is obtained at the phase boundary for Gd₂Y_2/3Zr_2/3Nb_2/3O₇ 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.