The relationship between vacancy ordering and magnetism in ${\mathrm{TlFe}}_{1.6}{\mathrm{Se}}_2$ has been investigated via single crystal neutron diffraction, nuclear forward scattering, and transmission electron microscopy. The examination of chemically and structurally homogeneous crystals allows the true ground state to be revealed, which is characterized by Fe moments lying in the ab plane below 100 K. This is in sharp contrast to crystals containing regions of order and disorder, where a competition between $c$ axis and ab plane orientations of the moments is observed. The properties of partially disordered ${\mathrm{TlFe}}_{1.6}{\mathrm{Se}}_2$ are, therefore, not associated with solely the ordered or disordered regions. This contrasts the viewpoint that phase separation results in independent physical properties in intercalated iron selenides, suggesting a coupling between ordered and disordered regions may play an important role in the superconducting analogues.

• Received 29 February 2012
• Publisher error corrected 4 September 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.077003