Synchrotron X-ray powder diffraction and extended X-ray absorption fine structure spectroscopy studies on nanocrystalline ZrO₂–CaO solid solutions

Abstract

The crystal structure and the local atomic order of a series of nanocrystalline ZrO2–CaO solid solutions with varying CaO content were studied by synchrotron radiation X‐ray powder diffraction and extended X‐ray absorption fine structure (EXAFS) spectroscopy. These samples were synthesized by a pH‐controlled nitrate–glycine gel‐combustion process. For CaO contents up to 8 mol%, the t′ form of the tetragonal phase (c/a > 1) was identified, whereas for 10 and 12 mol% CaO, the t′′ form (c/a = 1; oxygen anions displaced from their ideal positions in the cubic phase) was detected. Finally, the cubic phase was observed for solid solutions with CaO content of 14 mol% CaO or higher. The t′/t′′ and t′′/cubic compositional boundaries were determined to be at 9 (1) and 13 (1) mol% CaO, respectively. The EXAFS study demonstrated that this transition is related to a tetragonal‐to‐cubic symmetry change of the first oxygen coordination shell around the Zr atoms.

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

“…6 displays this ratio, which exhibits a monotonous decrease with composition and became zero between 10 and 11 mol% Y 2 O 3 . This result indicates that the t /c transition in nanocrystalline ZrO 2 -Y 2 O 3 solid solutions occurs at (10.5 ± 0.5) mol% Y 2 O 3, in good agreement with the value independently determined by the above SXPD analysis following the intensity of the 1 1 2 Bragg peak and with the previous results reported by Yashima [22], a clearly linear behavior is observed over the whole composition range, thus confirming the compositionally homogeneous nature of the solid solutions prepared by the gel-combustion procedure [10][11][12][13][14][23][24][25][26][27][28][29]. The compositional homogeneity of materials synthesized by gel-combustion routes can be explained considering that the system remains homogeneous during the whole process and the final combustion step of the precursor gel proceeds very rapidly, thus preserving the compositional homogeneity in the as-reacted material.…”

“…The thermal parameters corresponding to Zr 4+ and Y 3+ atoms were assumed to be equal. 1 1 1) ratio by the following equation [12,13],…”

“…They also established the presence of a mixture of t and t for a solid solution with composition of ZrO 2 -14 mol% YO 1 In the last few years, we have investigated the presence of metastable tetragonal forms in compositionally homogeneous ZrO 2 -based solid solutions composed of small nanocrystals, instead of microcrystalline materials as studied by Yashima and coworkers. The studied nanocrystalline solid solutions were synthesized by gel-combustion routes [10][11][12][13][14]. Interestingly, we have demonstrated that similar tetragonal forms can be retained in nanocrystalline ZrO 2 -Y 2 O 3 [10], ZrO 2 -CeO 2 [11,12], ZrO 2 -CaO [13] and ZrO 2 -Sc 2 O 3 [14] systems, but the lattice parameters, axial ratio and oxygen displacement may change with the crystallite size and/or synthesis route.…”

“…For compositionally homogeneous ZrO 2 -based materials, the existence of three tetragonal forms, known as t, t and t , all belonging to the P4 2 /nmc space group have been reported [4][5][6][7][8][9][10][11][12][13]. The t-form is the stable one, limited to the solubility limit predicted by the equilibrium phase diagram.…”

Local atomic structure in tetragonal pure ZrO2nanopowders

“…Data reduction was performed using the WinXas code (Ressler, 1998) following the procedure described elsewhere (Fá bregas et al, 2006(Fá bregas et al, , 2008. Both pre-and post-edge backgrounds were subtracted from the raw data.…”