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) Å, b=17.6612(1) Å, c=4.2201(1) Å, and Z=2 for K₂Gd₂Sb₂Se₉ and a=11.2080(5) Å, b=16.8781(8) Å, c=4.2419(2) Å, 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 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 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) Å, b=8.4062(2) Å, c=20.970(1) Å, β=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 μ_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.