X‐Ray Diffraction and Raman Spectroscopic Investigation on the Phase Relations in Yb₂O₃‐ and Tm₂O₃‐Substituted CeO₂

Abstract

Powder X‐ray diffraction (XRD) and Raman spectroscopic studies on Ce1−xYbxO2−x/2 and Ce1−xTmxO2−x/2 (0.0≤x≤1.0) are being reported in this manuscript. It has been observed that ceria when heavily doped with ytterbia and thulia get transformed from single‐phasic F type to single‐phasic C type through the formation of a biphasic mixture. The critical concentrations at which the phase transition takes place have also been ascertained from XRD measurements and verified using Raman spectroscopy. From XRD studies, it has been found that in the case of Ce1−xYbxO2−x/2, pure F‐type lattice continues until x=0.4 and then the biphasic region comprising F‐type and C‐type solid solutions starts, which is followed by single‐phasic pure C‐type region above x=0.9. For Ce1−xTmxO2−x/2, XRD studies reveal that the biphasic region comprising C‐type and F‐type lattice is formed from x=0.5 onward. However, it has been confirmed from Raman spectroscopic studies that the C‐type lattice is formed even in the sample at x=0.4. The discrepancy is believed to have originated because of the small domain size (smaller than the coherence length of X‐rays) of the C‐type lattice that are inhomogeneously distributed within the F‐type solid solution in the composition Ce1−xTmxO2−x/2 (x=0.4), which could not be directly detected from XRD measurements.

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“…Moreover, owing to large coherence length of X-rays, it is not often easy to detect parasitic micro domains of altogether different new phases amidst a bulk phase using XRD technique [11]. Raman spectroscopy becomes useful in such cases and promise better results because of its sensitivity to both oxygen displacement due to large polarizability [12] and intermediate range order without long-range periodicity [2,13]. In particular, the Raman spectrum of the perfect crystal lattice undergoes drastic changes in the presence of any disorder in the system and proves to be an excellent probe for local disorder.…”

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“…2(f) and (g). Laser Raman spectroscopy is a powerful tool to investigate micro-domains in ceramic materials [14]. Fig.…”