Tetragonal-to-orthorhombic structural phase transition at 90 K in the superconductor Fe(1.01)Se.

In this Letter we show that superconducting ${\mathrm{Fe}}_{1.01}\mathrm{Se}$ undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that nonsuperconducting ${\mathrm{Fe}}_{1.03}\mathrm{Se}$ does not. High resolution electron microscopy at low temperatures further reveals an unexpected additional modulation of the crystal structure of the superconducting phase that involves displacements of the Fe atoms, and that the nonsuperconducting composition shows a different, complex nanometer-scale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.