Eightfold superstructure in K2Gd2Sb2Se9 and K2La2Sb2S9 caused by three-dimensional ordering of the 5s2 lone pair of Sb3+ ions

The new isostructural compounds, K2Gd2Sb2Se9 acid K2La2Sb2S9, were discovered by the molten polychalcogenide salt method. They crystallize in the orthorhombic 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 compounds 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 coordination geometry around the rare earth atom is best described as a bicapped trigonal prism. The Gd3+/La3+ centered trigonal prisms share triangular faces 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 atoms 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 with neighboring octahedra making an infinite chain along the c axis and bridging Gd/La layers together to make the whole framework three-dimensional. In both compounds, Sb atoms appear to be positionally disordered over two crystallographically different sites with half occupancy. This disorder was removed upon elucidation of a 2ax2bx2e superstructure, which more accurately describes the positional ordering of Sb atoms in the structure, The superstructure 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 values of 1.33eV for K2Gd2Sb2Se9 and 2.20 eV for K2La2Sb2S9. Magnetic susceptibility 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 above 400 degrees C, The Raman spectra show the diselenide stretching vibration in K2Gd2Sb2Se9 at 266 cm(-1) and the disulfide stretching vibration in K2La2Sb2S9 at 473 cm(-1), (C) 1999 Academic Press.