Tetragonal-cubic phase boundary in nanocrystalline ZrO2–Y2O3 solid solutions synthesized by gel-combustion

By means of synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy, we have detected, in a series of nanocrystalline and compositionally homogeneous ZrO2–Y2O3 solid solutions, the presence at room temperature of three different phases depending on Y2O3 content, namely two tetragonal forms and the cubic phase. The studied materials, with average crystallite sizes within the range 7–10 nm, were synthesized by a nitrate–citrate gel-combustion process. The crystal structure of these phases was also investigated by SXPD. The results presented here indicate that the studied nanocrystalline ZrO2–Y2O3 solid solutions exhibit the same phases reported in the literature for compositionally homogeneous materials containing larger (micro)crystals. The compositional boundaries between both tetragonal forms and between tetragonal and cubic phases were also determined.

Research highlights
► Gel-combustion synthesis yields compositionally homogeneous, single-phased ZrO2–Y2O3 nanopowders, that exhibit the presence at room temperature of three different phases depending on Y2O3 content, namely two tetragonal forms (t′ and t″) and the cubic phase.
► Phase identification can be achieved by synchrotron XPD (SXPD) and Raman spectroscopy since the tetragonal forms and the cubic phase can be distinguished by these techniques.
► The crystallographic features of ZrO2–Y2O3 nanopowders were determined by SXPD. They are similar to those reported by Yashima and coworkers for compositionally homogeneous materials containing larger (micro)crystals. However, the lattice parameters are slightly different and the axial ratios c/a of our t′ samples are smaller than those reported by these authors.
► Compositional t′/t″ and t″/cubic phase boundaries are located at (9 ± 1) and (10.5 ± 0.5) mol% Y2O3, respectively.
► For the whole series of nanocrystalline ZrO2–Y2O3 solid solutions studied in the present work, no evidences of the presence of a mixture of phases – as reported by Yashima and coworkers for microcrystalline solid solutions – were detected.