Influence of phase transition from order to disorder and Philip's ionicity on the thermal expansion coefficient of pyrochlore type compositions with a multivalent environmentElectronic supplementary information (ESI) available. See DOI: 10.1039/c6nj02482c

• Main Authors: Renju, Ushakumari A, Prabhakar Rao, Padala, Vaisakhan Thampi, Divakaran S
• Format: Journal Article
• Language: English
• Published: 19.12.2016
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/c6nj02482c

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, RE 2 Y 2/3 Zr 2/3 Nb 2/3 O 7 (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 Y 2 (Y 1/3 Zr 1/3 Nb 1/3 ) 2 O 7 at 1023 K. The minimum thermal expansion coefficient is obtained at the phase boundary for Gd 2 Y 2/3 Zr 2/3 Nb 2/3 O 7 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. The pyrochlore to fluorite transition in the RE 2 Y 2/3 Zr 2/3 Nb 2/3 O 7 system allows us to understand the interrelationships among ionicity, thermal expansion and ionic conductivity.