Antiferromagnetism, structural properties, and electronic transport of BaCo0.9Ni0.1S1.8

The layered transition-metal compound BaCo0.9Ni0.1S1.8 is an antiferromagnetic insulator, TN = 280 K, which undergoes a unique insulator to metal transition at Ts = 200 K. By x-ray and neutron powder diffraction we explore the difference in crystal structure above and below Ts. The magnetic structure consists of Co moments of 1.8 ±0.5 μB with antiferromagnetic nearest-neighbor interactions. Below Ts long-range order disappears. Resistivity and thermoelectric power measurements indicate that above Ts this system is a narrow-band material with dominating nearest-neighbor hopping. Magnetoresistance to 12 T shows insensitivity of Ts to magnetic fields. The length changes at Ts as measured by dilatometry are much larger than for ordinary first-order phase transitions.