The rare earth metal polyselenides Gd8Se15, Tb8Se15-x, Dy8Se15-x, Ho8Se15-x, Er8Se15-x, and Y8Se15-x - Increasing disorder in defective planar selenium layers

摘要

Single crystals of the rare earth metal polyselenides Gd(8)Sel(15), Tb8Se15-x, Dy8Se15-x, Ho8Se15-x, Er8Se15-x, and Y8Se15-x, (0 < x <= 0.3) have been prepared by chemical transport reactions (1120 K -> 970 K, 14 days, I-2 as carrier) starting from pre-annealed powders of nominal compositions between LnSe(2) and LnSel.q. The isostructural title compounds adopt a 3 x 4 X 2 superstructure of the ZrSSi type and can be described in space group Amm2 with lattice parameters of a = 12.161(1) angstrom, b = 16.212(2) angstrom and c = 16.631(2) angstrom (Gd8Se15), a = 12.094(2) angstrom, b = 16.123(2) angstrom and c = 16.550(2) angstrom (Tb(8)Sel(15-x)), a = 12.036(2) angstrom, b = 16.060(2) angstrom and c = 16.475(2) angstrom (DY8Se15-x), a = 11.993(2) angstrom, b = 15.999(2) angstrom and c = 16.471(2) angstrom (Ho8Se15-x), a = 11.908(2) angstrom, b = 15.921(2) angstrom and c = 16.428(2) angstrom (Er8Se15-x), and a = 12.045(2) angstrom, b = 16.072(3) angstrom and c = 16.626(3) angstrom (Y8Se15-x), respectively. The structure consists of puckered [LnSe] double slabs and planar Se layers alternating along [001]. The planar Se layers contain a disordered arrangement of Se-2(2-) dimers, Se2- and vacancies. All compounds are semiconducting and contain trivalent rare earth metals (Ln(3+)).