  Structural and physical properties determined by measurements on large single
crystals of the anisotropic ferromagnet MnBi are reported. The findings support
the importance of magneto-elastic effects in this material. X-ray diffraction
reveals a structural phase transition at the spin reorientation temperature
$T_{SR}$ = 90 K. The distortion is driven by magneto-elastic coupling, and upon
cooling transforms the structure from hexagonal to orthorhombic. Heat capacity
measurements show a thermal anomaly at the crystallographic transition, which
is suppressed rapidly by applied magnetic fields. Effects on the transport and
anisotropic magnetic properties of the single crystals are also presented.
Increasing anisotropy of the atomic displacement parameters for Bi with
increasing temperature above $T_{SR}$ is revealed by neutron diffraction
measurements. It is likely that this is directly related to the anisotropic
thermal expansion in MnBi, which plays a key role in the spin reorientation and
magnetocrystalline anisotropy. The identification of the true ground state
crystal structure reported here may be important for future experimental and
theoretical studies of this permanent magnet material, which have to date been
performed and interpreted using only the high temperature structure.
