Crystal structure of the R3Si1.25Se7 (R = Pr, Nd and Sm) compounds

The crystal structure of new ternary R₃Si_1.25Se₇ (R = Pr, Nd and Sm) compounds (Dy₃Ge_1.25S₇ structure type, Pearson symbol hP22.5, space group P6₃, a = 1.05268 (3) nm, c = 0.60396 (3) nm, R_I = 0.0897 for Pr₃Si_1.25Se₇; a = 1.04760 (3) nm, c = 0.60268 (3) nm, R_I = 0.0891 for Nd₃Si_1.25Se₇; a = 1.04166 (6) nm, c = 0.59828 (6) nm for Sm₃Si_1.25Se₇) was determined using X-ray powder diffraction. The nearest neighbours of the R and Si atoms are exclusively Se atoms. The latter form distorted trigonal prisms around the R atoms, octahedra around the Si1 atoms and tetrahedra around the Si2 atoms. Tetrahedral surrounding exists for Se1 and Se3 atoms. Six neighbours surround every Se2 atom.