High-Temperature Isostructural Phase Transition in Ce ₂ (MoO ₄ ) ₃ : A Rare Phenomenon Investigated through X-ray Diffraction and Raman Scattering

The rare-earth compound cerium-(III) molybdate (Ce₂(MoO₄)₃) has gained attention due to its diverse industrial applications, such as photocatalysis, corrosion inhibition, self-repair of protective layers, as well as antiviral and antibacterial properties. However, its response to extreme temperature conditions remains insufficiently explored. This study employs ambient powder X-ray diffraction (PXRD), UV-vis diffuse reflectance spectroscopy, scanning electron microscopy (SEM), and Raman spectroscopy techniques to confirm the successful hydrothermal synthesis of a crystalline sample of the Ce₂(MoO₄)₃ compound. Subsequently, in situ temperature-dependent (13-973 K) XRD and Raman scattering (293-998 K) studies were conducted. Rietveld analysis of diffraction patterns reveals a stable low-temperature phase (13-303 K) and anomalies in the high-temperature evolution (<i>T</i> > 583 K) of lattice parameters and the sample's microstrain and crystallite size. In the high-temperature (<i>T</i> > 848 K) Raman spectra of Ce₂(MoO₄)₃, an additional band emerges at 452 cm^-1; the observed anomalies are attributed to an isostructural phase transition (IPT). This type of phase transition is among the rarest phenomena reported in the literature. Our findings enhance the understanding of the physical properties of Scheelite-type compounds and emphasize the importance of investigating these uncommon phenomena. This knowledge can potentially drive the development of novel materials and expand their applications in materials science.