STRUCTURE AND PHYSICAL-PROPERTIES OF THE BARIUM NIOBIUM SULFIDES BANBS3 AND BANB0.8S3-DELTA

BaNb0.8S3-delta and BaNbS3 were prepared by heating desired amounts of BaCo3 and Nb2O5 under CS2/N-2 flow. We found that nominal composition of BaNb0.8S has sulfur defect, and therefore should be written as BaNb0.8S3-delta XANES experiments of Nb K-edges and XPS experiments proved that the oxidation state of Nb is the same in BaNb0.8S3-delta and BaNbS3. To understand their structures, the electronic and physical properties of the BaMX(3) (M = V, Nb, X = S; M = Ta, X = S, Se) phases are compared. In spite of their structural similarity, they exhibit rather different physical properties. It has been known that BaVS3 is a metal and undergoes a metal to semiconductor transition at 130 K, and this phase shows the ferro antiferromagnetic transition at a lower temperature. However, BaNbS3 and BaTaS3 are diamagnetic semiconductors at room temperature. The major factor stabilizing the semiconducting state for BaTaX(3) and BaNbS3 may be a large spin-orbit coupling rather than a structural distortion. (C) 1995 Academic Press, Inc.