Comment on "Structural transition and superconductivity in hydrothermally synthesized FeX (X = S, Se)" by U. Pachmayr, N. Fehn and D. Johrendt, Chem. Commun. , 2016, 52 , 194.

The occurrence of triclinic structural modification of β-FeSe was reported in the literature; in particular this polymorph was claimed to be observed at low temperature in samples prepared by hydrothermal synthesis. The establishment of triclinic symmetry was argued based on peculiar features characterizing some selected X-ray powder diffraction lines. Using high-resolution synchrotron X-ray powder diffraction, the same features were observed for an aged β-FeSe sample prepared by a solid state synthesis technique. Moreover, by refining the anisotropic microstrain parameters it was demonstrated that the diffraction pattern can be fairly fitted by applying the well-recognized orthorhombic structural model adopted for the low temperature phase of β-FeSe. This result indicates that the occurrence of the triclinic polymorph for β-FeSe can be ruled out.