New Uranium Chalcoantimonates, RbU₂SbS₈ and KU₂SbSe₈, with a Polar Noncentrosymmetric Structure

The new compounds, RbU2SbS8 and KU2SbSe8, were prepared as golden-black, blocklike crystals by the polychalcogenide molten flux method. RbU2SbS8 crystallizes in the monoclinic space group Cm with a = 7.9543(9) Å, b = 11.0987(13) Å, c = 7.2794(10) Å, β = 106.030(2)°, and Z = 2. The compound has a two-dimensional character with layers running perpendicular to the c-axis. The coordination geometry around the U4+ atoms is best described as a bicapped trigonal prism. The trigonal prisms share triangular faces with neighboring prisms, forming one-dimensional columns along the a-axis. The columns are then joined to construct sheets by sharing capping S atoms. Sb3+ ions are sitting at the center of a slightly distorted seesaw coordination environment (CN = 4). Rb+ ions are stabilized in 8-coordinate bicapped trigonal prismatic sites. KU2SbSe8 crystallizes in the monoclinic space group Cm with a = 11.5763(2) Å, b = 8.2033(1) Å, c = 15.2742(1) Å, β = 112.22(2)°, and Z = 4. KU2SbSe8 has essentially the same structure as RbU2SbS8. However, Sb3+ and K+ ions appear disordered in every other layer, resulting in a different unit cell. RbU2SbS8 is a semiconductor with a band gap of 1.38 eV. The band gap of KU2SbSe8 could not be determined precisely due to the presence of overlapping intense f−f transitions in the region (0.5−1.1 eV). The Raman spectra show the disulfide stretching vibration in RbU2SbS8 at 479 cm-1 and the diselenide stretching vibration in KU2SbSe8 at 252 cm-1. Magnetic susceptibility measurements indicate the presence of U4+ centers in the compounds. The compounds do not melt below 1000 °C under vacuum.