Samples with various compositions around BaNbSe 3 were prepared. X-ray analysis revealed that the compound is stable at the composition of BaNb 0.80 Se 3 , which formally requires Nb 5Ψ ions (d 0 ). The single-phase region extends to the composition BaNb 0.95 Se 3 . A semiconductor-to-metal transition was observed at & &140 K in BaNb 0.95 Se 3 . Seebeck measurements showed that the dominant carriers were electrons, which are considered to originate from Nb 4Ψ ions (d 1 ) coexisting with Nb 5Ψ ions. On the basis of detailed phase study, the transition was revealed to be intrinsic to BaNb 0.95 Se 3 with a very slight Ba deficiency. BaNb 0.95 Se 3 with a slight Ba excess shows a semiconductor-tosemiconductor transition at 200-300 K. Seebeck measurements showed that the dominant carriers are electrons above the transition and are holes below the transition. A new compound, BaNb 2 Se 5 with a BaTa 2 S 5 -type structure, was found to show superconductivity at 2.5 K. 1997 Academic Press
EXPERIMENTS
Samples with various compositions around BaNbSe were prepared as follows. Mixtures of the desired ratios of Ba metal, Nb powder, and Se powder sealed in evacuated silica tubes (8 mm inner diameter) were gradually heated up to 800Β°C at a rate of 100Β°C/h and were annealed at this temperature for 5 days. To prevent the reaction of Ba metal with the silica tubes, the insides of the tubes were coated 188