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

Abstract

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 μeff=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.