Structural and spectroscopic insights into the cation mediated phase transitions in samarium zirconates Sm2Zr2–O7–2 (x = 0 to 1)

Lanthanide zirconates have attracted recent attention as energy materials due to their desirable physical-chemical properties and excellent chemical stability. In this work, a series of Sm 2 Zr 2– x O 7–2 x ( x = 0–1, step = 0.125) compounds have been synthesized using a solution route and sintered at 1400 °C for 24 h. High-resolution synchrotron X-ray diffraction (SXRD) results indicate that Sm 2 Zr 2– x O 7–2 x samples transform from highly ordered pyrochlore ( x = 0), to disordered pyrochlores ( x = 0.125, 0.25, 0.375, 0.5, 0.625) and defect fluorites ( x = 0.75, 0.875, 1). The SXRD patterns show that pyrochlore superlattice peaks are present in samples ( x = 0–0.625) with SmO 1.5 50–59.3 mol.%, but are absent in samples ( x = 0.75–1) with SmO 1.5 61.5–66.7 mol.%, indicating a loss of long-range pyrochlore order. The short-range structural study by Raman spectroscopy reveals that with gradually increasing Sm mol.%, the structure transforms from ordered pyrochlore ( x = 0) to disordered pyrochlore ( x = 0.125–0.625) containing both pyrochlore and fluorite ( x = 0.5 and 0.625), then to defect fluorite ( x = 0.75–1). While SEM analysis shows the high compositional homogeneity, TEM-SAED results concur the SXRD data. In addition, diffuse reflectance spectroscopy reveals the absorption features reflecting the presence of Sm(III) ion. Overall, this work highlights the structural flexibility of lanthanide zirconates in terms of compositional variations and has important implications for energy applications such as solid-oxide fuel cells and nuclear waste management including waste forms for minor actinides.