Synthesis and crystal structure of the R3Fe0.1Ga1.6Se7 selenides (R – La, Ce, Pr, Nd)

Samples of the stoichiometric composition R3Fe0.1Ga1.6Se7 (R – La, Ce, Pr, Nd) were synthesized from high purity elements in evacuated quartz containers at a maximum synthesis temperature of 1100°C. The crystal structure of the selenides that was investigated by X-ray powder method found that they belong to the structural type La3CuSiS7 (S.G. P63). The lattice parameters are: a = 10.5358(8) Å, c = 6.3796(6) Å, V = 613.2(2) Å3, RI = 0.0846, Rp = 0.1575 (for La3Fe0.1Ga1.6Se7); a = 10.4379(5) Å, c = 6.3550(5) Å, V = 599.6(1) Å3, RI = 0.0878, Rp = 0.2137 (for Ce3Fe0.1Ga1.6Se7); a = 10.3431(4) Å, c = 6.3735(8) Å, V = 590.49(8) Å3, RI = 0.0846, Rp = 0.1575 (for Pr3Fe0.1Ga1.6Se7); a = 10.2817(4) Å, c = 6.3819(4) Å, V = 548.26(8) Å3, RI = 0.0939, Rp = 0.2409 (for Nd3Fe0.1Ga1.6Se7). Obtained chalcogenides are derived from the R3Ga1.67Se7 selenides by partial substitution of Ga atoms in the 2a site with divalent Fe atoms. Rare-earth atoms in the studied structures occupy the 6c site and, together with Se atoms, form trigonal prisms with one additional atom [R 7Se]. Trigonal prisms share edges to form blocks. Fe and Ga atoms are statistically distributed in the 2a site and have an octahedral environment. Octahedra [M 6Se] share faces to form spatial columns [M(Fe + Ga)Se6]n along the c axis, which are connected to the trigonal prisms by common edges. Part of Ga atoms is located in the center of the [Ga 4Se] tetrahedra in the 2b site.