Structure-Spin-Transport Anomaly in Quasi-One-Dimensional Ba_9Fe_3Te_15 under High Pressure

Recently, a series of novel compounds Ba_3MX_5 (M =  Fe, Ti, V; X = Se, Te) with hexagonal crystal structures composed of quasi-1-dimensional (1D) magnetic chains has been synthesized by our research team using high-pressure and high-temperature methods. The initial hexagonal phases persist to the maximum achievable pressure, while spin configurations and magnetic interactions may change dramatically as a result of considerable reductions in interchain separations upon pressurization. These compounds therefore offer unique possibilities for studying the evolution of intrinsic electronic structures in quasi-1D magnetic systems. Here we present a systematic investigation of Ba_9Fe_3Te_15, in which the interchain separations between trimerized 1D chains (\sim 10.2 Å) can be effectively modulated by external high pressure. The crystal structure especially along the 1D chains exhibits an abnormal expansion at \sim 5 GPa, which accompanies trimerization entangled anomalous mixed-high-low spin transition. An insulator-metal transition has been observed under high pressure as a result of charge-transfer gap closing. Pressure-induced superconductivity emerges at 26 GPa, where the charge-transfer gap fully closes, 3D electronic configuration forms and local spin fully collapses.