Quaternary Lead-Niobium-Tungsten Oxides Based on the Tetragonal Tungsten Bronze Structure

The first Na-containing quaternary rare-earth thioantimonate, Na9Gd5Sb8S26, was synthesized from the reaction of Gd metal, Sb, and S in Na2S flux at 750°C. Black plate-shaped crystals were obtained and their crystal structure was determined by the single-crystal X-ray diffraction method. This compound crystallizes in the orthorhombic space group Pnma (No. 62) with a unit cell of a=11.406(3) ?, b=23.765(2) ?, c=16.923(5) ?, and Z=4. Na9Gd5Sb8S26 has a complex three-dimensional structure that consists of SbS3, SbS4, GdS6, and GdS7 units. In the structure, three crystallographically different Gd atoms form a staircase-shaped building block, where two Gd atoms share disulfide ions, S2-2. The formula of the compound can be described as (Na+)9(Gd3+)5(Sb3+)8(S2-)22(S2-2)2. Measurement of the optically diffuse reflectance indicates that Na9Gd5Sb8S26 is a semiconductor with a band gap of 1.63 eV.