SYNTHESIS, CRYSTAL-STRUCTURE AND OPTICAL ABSORPTIONS OF THE 2-DIMENSIONAL SULFIDE NACU2NBS4

The new quaternary compound NaCu2NbS4 was obtained as yellow platelets single crystals (0.5x0.14x0.04 mm) by reacting a mixture of Na2S3,Cu, Nb and S in a 4:1:1:8 ratio at 300 degrees C. The crystal structure i s characterized by two-dimensional [Cu2NbS4](-) layers parallel to (01 0) which are isolated by Na cations. The Na+ is coordinated by nine S atoms within a distorted tricapped trigonal prism. The Na-S distances are extremely long compared to those in other sodium and sulfur contai ning compounds. As a consequence large anisotropic displacement compon ents for the Na cation are observed indicative for a statically disord er. Low temperature X-ray investigations give no hints for an ordering of Na+. From optical investigations a band gap E-g of 2.46 eV is esti mated.