High-pressure synthesis, structure and physical properties of two quasi-one-dimensional compounds Ba9Nb2.54Te15 and Ba9Ta1.89Te15

Two quasi-one-dimensional compounds Ba9Nb2.54Te15 and Ba9Ta1.89Te15 were synthesized under high pressure and high temperature conditions and systematically characterized by structural, transport and magnetic measurements. Both the two compounds crystallize into a hexagonal structure with the space group P-6c2 (No. 188). The structure consists of trimeric face-sharing octahedral Nb/TaTe6 chains separated by a large distance (>10 angstrom), thus presenting a strong one-dimensional crystal structure. The transport properties suggest that both the two compounds are semiconductors with a band gap similar to 0.15 eV for Ba9Nb2.54Te15 and similar to 0.22 eV for Ba9Ta1.89Te15. Magnetic properties characterization indicates that Ba9Nb2.54Te15 displays no long-range-order above 2 K, but with an effective moment mu(eff) similar to 1.8 mu B/f.u., while Ba9Ta1.89Te15 exhibits a diamagnetic behavior. Our results demonstrate that, in the sequence of Ba9V3Te15, Ba9Nb2.54Te15 and Ba9Ta1.89Te15, the occupation in the transition metal sites decreases, the semiconducting band gap increases, and the magnetism changes from ferromagnetism to diamagnetism.