Pressure-induced structural symmetrization in nanocrystalline (Sm,Nd)3(Nb,Ta)O7 weberite

Multicomponent (Sm,Nd) 3 (Nb,Ta)O 7 was synthesized by high-energy mechanical milling and investigated from mechanochemical formation to compression up to 50 GPa. After 22 h of milling, X-ray diffraction reveals a highly nanocrystalline orthorhombic weberite described by the C 222 1 space group, with a mean crystallite size of approximately 20 nm. Annealing at 800 °C for 3 h increases the mean crystallite size to approximately 26 nm, enhances structural coherence, and sharpens the Raman response while preserving the average weberite structure. In situ synchrotron X-ray diffraction shows anisotropic compression of the C 222 1 phase, with the b axis more compressible than a and c , and a second-order Birch–Murnaghan bulk modulus B 0 = 145(3) GPa up to 19.5 GPa. Above ∼21 GPa, weakening and broadening of characteristic weberite reflections, together with increasing peak overlap and diffraction-pattern simplification, mark a progressive structural reorganization extending to ∼31 GPa. The high-pressure state remains crystalline to 50 GPa and can be described by a Cmcm -compatible higher-average-symmetry orthorhombic metric, although the broadened diffraction patterns do not permit an unequivocal space-group assignment or a reliable high-pressure equation of state. Raman spectroscopy independently reveals progressive broadening and coalescence of framework-related bands, whereas high-frequency (Nb,Ta)–O stretching features remain detectable to 50 GPa. The combined results indicate pressure-induced structural symmetrization in which long-range weberite distortions are suppressed while local structural heterogeneity persists.