  We report the synthesis of a new quasi one-dimensional (1D) iron selenide.
Ba9Fe3Se15 was synthesized at high temperature and high pressure of 5.5 GPa and
systematically studied via structural, magnetic and transport measurements at
ambient and at high-pressures. Ba9Fe3Se15 crystallizes in a monoclinic
structure and consists of face-sharing FeSe6 octahedral chains along the c
axis. At ambient pressure it exhibits an insulating behavior with a band gap
~460 meV and undergoes a ferrimagnet-like phase transition at 14 K. Under high
pressure, a complete metallization occurs at ~29 GPa, which is accompanied by a
spin state crossover from high spin (HS) state to low spin (LS) state. The LS
appears for pressures P >36 GPa.
