In situ Investigation of Structural Stability and Reversible Conformational Changes in Gd2(WO4)3 at High Temperatures

• In situ PXRD reveals anisotropic thermal expansion along the b and c axes • In situ Raman detects reversible local conformational changes near 523 K • Combined PXRD-Raman analysis links long-range stability to local vibrational dynamics In this work, Gd 2 (WO 4 ) 3 samples were synthesized by the hydrothermal method and subsequently calcined at 923 K, being characterized at room temperature by X-ray diffraction, UV-Vis spectrophotometry and Raman spectroscopy, in addition to in situ PXRD and Raman measurements under variable temperature conditions. Rietveld refinement confirmed the exclusive formation of the monoclinic phase (space group C2/c ). The in situ PXRD analyses, performed between 298 K and 773 K, revealed high long-range structural stability, evidenced by anisotropic thermal expansion that is more pronounced along the b (11.9(4) × 10 -6 K -1 ) and c (10.3(4) × 10 -6 K -1 ) axes than the a (2.2(6) × 10 -6 K -1 ) axis. The b/c ratio exhibited a non-linear trend: a continuous increase followed by an abrupt decrease around 523 K during both heating and cooling cycles, suggesting a subtle conformational modification within the crystal lattice. Consistently, in situ Raman spectroscopy indicated reversible local changes in the same temperature range (538 K), variation in intensity, as well as, shifts of vibrational modes associated with WO 4 2- tetrahedra. These effects were attributed to enhanced phonon-phonon anharmonicity and rearrangements of GdO 8 polyhedra. The results demonstrate that although the global structure of Gd 2 (WO 4 ) 3 remains monoclinic throughout the investigated temperature range, reversible local reorganizations occur, revealing an intrinsic thermal sensitivity of the lattice and supporting its potential use in thermally stable photonic and optoelectronic applications, such as Raman-based temperature sensors, high-temperature optical components, and rare-earth-doped laser host materials.