Eightfold Superstructure in K2Gd2Sb2Se9 and K2La2Sb2S9 Caused by Three-Dimensional Ordering of the 5s2 Lone Pair of Sb3+ Ions

The new isostructural compounds, K2Gd2Sb2Se9 and K2La2Sb2S9, were discovered by the molten polychalcogenide salt method. They crystallize in the orthorhombic space group Pbam with a=11.4880(3) ?, b=17.6612(1) ?, c=4.2201(1) ?, and Z=2 for K2Gd2Sb2Se9 and a=11.2080(5) ?, b=16.8781(8) ?, c=4.2419(2) ?, and Z=2 for K2La2Sb2S9. The compounds have a three-dimensional [M2Sb2Q9]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 SbQ6(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 threedimensional. 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 2a×2b×2c 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) ?, b=8.4062(2) ?, c=20.970(1) ?, beta=123.022(1)°, and Z=8. These compounds are semiconductors with band gap values of 1.33 eV 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°C while K2La2Sb2S9 decomposes gradually above 400°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.