Crystal Structural, Magnetic, and Transport Properties of Layered Cobalt Oxyfluorides, Sr₂CoO_3+<i>x</i>F_1–<i>x</i> (0 ≤ <i>x</i> ≤ 0.15)

The crystal structure of the layered cobalt oxyfluoride Sr₂CoO₃F synthesized under high-pressure and high-temperature conditions has been determined from neutron powder diffraction and synchrotron powder diffraction data collected at temperatures ranging from 320 to 3 K. This material adopts the tetragonal space group <i>I</i>4/<i>mmm</i> over the measured temperature range and the crystal structure is analogous to <i>n</i> = 1 Ruddlesden–Popper type layered perovskite. In contrast to related oxyhalide compounds, the present material exhibits the unique coordination environment around the Co metal center: coexistence of square pyramidal coordination around Co and anion disorder between O and F at the apical sites. Magnetic susceptibility and electrical resistivity measurements reveal that Sr₂CoO₃F is an antiferromagnetic insulator with the Néel temperature <i>T</i>_N = 323(2) K. The magnetic structure that has been determined by neutron diffraction adopts a G-type antiferromagnetic order with the propagation vector <i>k</i> = (1/2 1/2 0) with an ordered cobalt moment μ = 3.18(5) μ_B at 3 K, consistent with the high spin electron configuration for the Co³⁺ ions. The antiferromagnetic and electrically insulating states remain robust even against 15%-O substation for F at the apical sites. However, applying pressure exhibits the onset of the metallic state, probably coming from change in the electronic state of square-pyramidal coordinated cobalt