Pressure response and phase stability of fergusonite type R NbO4 ( R = Sm, Ce)

The high-pressure structural behavior of fergusonite-type rare-earth orthoniobates RNbO4 (R = Sm, Ce) has been investigated using synchrotron powder x-ray diffraction up to ∼20 GPa and density-functional theory calculations. Ambient-pressure diffraction data confirm that both compounds crystallize in the monoclinic fergusonite structure (space group I2/a). Upon compression, no pressure-induced structural phase transition is observed in either compound up to the highest investigated pressure, demonstrating the remarkable stability of the fergusonite framework. Analysis of the pressure evolution of the lattice parameters reveals anisotropic compression, with the b-axis exhibiting the largest compressibility in both materials. The pressure–volume data were fitted using a third-order Birch–Murnaghan equation of state, yielding bulk moduli of 141(3) GPa for SmNbO4 and 136(4) GPa for CeNbO4. First-principles calculations reproduce the anisotropic compressional behavior and confirm the stability of the fergusonite phase in SmNbO4 up to 25 GPa. For CeNbO4, the optimized zero-temperature structure is found to be a pseudo-tetragonal scheelite phase that evolves into a fergusonite structure under compression. The results demonstrate that the compressibility of rare-earth orthoniobates is governed primarily by the rare-earth–oxygen polyhedra and provide insight into the structural stability and equation of state of fergusonite-type niobates under high-pressure.