  We report on the synthesis of superconducting single crystals of FeSe, and
their characterization by X-ray diffraction, magnetization and resistivity. We
have performed ac susceptibility measurements under high pressure in a
hydrostatic liquid argon medium up to 14 GPa and we find that TC increases up
to 33-36 K in all samples, but with slightly different pressure dependences on
different samples. Above 12 GPa no traces of superconductivity are found in any
sample. We have also performed a room temperature high pressure X-ray
diffraction study up to 12 GPa on a powder sample, and we find that between 8.5
GPa and 12 GPa, the tetragonal PbO structure undergoes a structural transition
to a hexagonal structure. This transition results in a volume decrease of about
16%, and is accompanied by the appearance of an intermediate, probably
orthorhombic phase.
