Anomalous compressibility effects and superconductivity of EuFe ₂As₂ under high pressures

The crystal structure and electrical resistance of structurally layered EuFe₂As₂ have been studied up to 70 GPa and down to a temperature of 10 K, using a synchrotron x-ray source and designer diamond anvils. The room temperature compression of the tetragonal phase of EuFe ₂As₂ (I4/mmm) results in an increase in the a-axis length and a rapid decrease in the c-axis length with increasing pressure. This anomalous compression reaches a maximum at 8 GPa and the tetragonal lattice behaves normally above 10 GPa, with a nearly constant c/a axial ratio. The rapid rise in the superconducting transition temperature (T_c) to 41 K with increasing pressure is correlated with this anomalous compression, and a decrease in T_c is observed above 10 GPa. We present P-V data or the equation of state for EuFe₂As₂ both in the ambient tetragonal phase and in the high pressure collapsed tetragonal phase up to 70 GPa.