Abstract
A phase transition was observed in the tetragonal FeSe superconductor under high pressure at around 2.55 GPa using X-ray scattering and Raman spectrum. From X-ray powder diffraction (XRD) experiments, the slope of the unit cell volume versus pressure curve change slightly at around 2.55-4 GPa, and the Bragg peaks of FeSe tetragonal phase disappear when the pressure reaches 11.4 GPa. We also found the superconducting transition temperature T c increases and reaches a maximum value of ∼37 K at ∼3-4 GPa. The compressibility value (dV/dP) shows an abrupt change from 2.39(Å 3 /GPa) to 0.94(Å 3 /GPa) from pressure values of 0-2.55 GPa to 2.55-11.4 GPa. A similar phase transition was also observed in the Raman spectra under pressure. © 2009 Elsevier B.V. All rights reserved.