Structural feature controlling superconductivity in compressed BaFe ₂As₂

Superconductivity can be induced with the application of pressure but it disappears eventually upon heavy compression in the iron-based parent compound BaFe₂As₂. Structural evolution with pressure is used to understand this behavior. By performing synchrotron X-ray powder diffraction measurements with diamond anvil cells up to 26.1 GPa, we find an anomalous behavior of the lattice parameter with a S shape along the a axis but a monotonic decrease in the c-axis lattice parameter with increasing pressure. The close relationship between the axial ratio c/a and the superconducting transition temperature T_c is established for this parent compound. The c/a ratio is suggested to be a measure of the spin fluctuation strength. The reduction of T_c with the further increase of pressure is a result of the pressure-driven weakness of the spin-fluctuation strength in this material.