  In this letter we show that superconducting Fe1.01Se undergoes a structural
transition at 90 K from a tetragonal to an orthorhombic phase but that
non-superconducting Fe1.03Se does not. Further, high resolution electron
microscopy study at low temperatures reveals an unexpected additional
modulation of the crystal structure of the superconducting phase involving
displacements of the Fe atoms, and that the non-superconducting material shows
a distinct, complex nanometer-scale structural modulation. Finally, we show
that magnetism is not the driving force for the phase transition in the
superconducting phase.
