  The relationship between vacancy ordering and magnetism in TlFe1.6Se2 has
been investigated via single crystal neutron diffraction, nuclear forward
scattering, and transmission electron microscopy. The examination of chemically
and structurally homogenous crystals allows the true ground state to be
revealed, which is characterized by Fe moments lying in the ab-plane below
100K. 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 TlFe1.6Se2 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.
