Eightfold superstructure in K2Gd2Sb2Se9 and K2La2Sb2S9 caused by three-dimensional ordering of the 5s(2) lone pair of Sb3+ ions

The new isostructural compounds, K2Gd2Sb2Se9 acid K2La2Sb2S9, were discover ed by the molten polychalcogenide salt method. They crystallize in the orth orhombic space group Pbam with a 11.4880(3)Angstrom b = 17.6612(Angstrom);, 4.2201(1) A, and Z = 2 for K2Gd2Sb2Se9 and a =11.2080(5) Angstrom, b = 16. 8781(8) Angstrom, c = 4.2419(2) Angstrom, and Z = 2 for K2La2Sb2S9. The com pounds have a three-dimensional [M(2)Sb(2)Q(9)](2-) framework (M= Gd, La; Q = Se, S) with K+-ion-filled channels running along the c axis. The coordin ation geometry around the rare earth atom is best described as a bicapped t rigonal prism. The Gd3+/La3+ centered trigonal prisms share triangular face s with neighboring prisms forming one-dimensional columns along the c axis, The columns are connected to each other to form sheets by sharing Se/S ato ms on the capping sites of the trigonal prisms. Sb3+ ions are stabilized in distorted octahedral sites, The SbQ(6)(Q = Se, S) octahedra share edges wi th neighboring octahedra making an infinite chain along the c axis and brid ging Gd/La layers together to make the whole framework three-dimensional. I n both compounds, Sb atoms appear to be positionally disordered over two cr ystallographically different sites with half occupancy. This disorder was r emoved upon elucidation of a 2ax2bx2e superstructure, which more accurately describes the positional ordering of Sb atoms in the structure, The supers tructure of K2Gd2Sb2Se9 was refined in the monoclinic space group C2/m with a 22.8783(4) Angstrom, b = 8.4052(2) Angstrom, 20,970(1);i, beta = 123,022 (1)degrees, and Z = 8, These compounds are semiconductors with band gap val ues of 1.33eV for K2Gd2Sb2Se9 and 2.20 eV for K2La2Sb2S9. Magnetic suscepti bility measurements indicate no apparent magnetic coupling between the Gd3 centers showing Curie-Weiss behavior with mu(eff),,= 7.96 B.M, K2Gd2Sb2Se9 melts congruently at 598 degrees C while K2La2Sb2S9 decomposes gradually a bove 400 degrees C, The Raman spectra show the diselenide stretching vibrat ion in K2Gd2Sb2Se9 at 266 cm(-1) and the disulfide stretching vibration in K2La2Sb2S9 at 473 cm(-1), (C) 1999 Academic Press.