Influence of Ce substitution on the order-to-disorder structural transition, thermal expansion and electrical properties in Sm2Zr2_xCexO7 system

Abstract

Cerium substituted rare earth zirconates of the form Sm2Zr2_xCexO7 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized via solid state reaction route. Structural, morphological and electrical characterizations were conducted using high temperature X-ray diffraction, FT Raman spectroscopy, thermo-mechanical analysis, scanning electron microscopy and impedance spectroscopy techniques. Addition of Ce ions into the crystal induced structural disorder to both cationic and anionic sublattices, gradually transforming the unit cell from an ordered pyrochlore to a defect fluorite structure. Substitution of bigger Ce ions to the Zr sites led to an increase in the lattice parameter and a decrease in thermal expansion coefficient of the material. The ionic disorder led to a decreased energy barrier for the thermally activated conduction process thereby increasing the overall conductivity of the materials. The maximum value of total conductivity observed in this study is 3.93 x 10-3 S cm-1 at 1023 K, which is higher than the recently reported values in a similar Sm2Zr2O7 system. Beyond x = 0.4, the ion–ion interaction in the disordered lattice began to dominate, leading to an increased activation energy and decreased total conductivity. The results demonstrate that the thermal expansion of these oxides is predominantly influenced by the B–O bond energy rather than the Madelung binding energy.