  Neutron powder diffraction studies of the crystal and magnetic structures of
the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2) have been carried out as a
function of temperature, applied magnetic field, and pressure. The data reveal
that there is only one transition observed over the entire range of variables
explored, which is a combined magnetic and structural transformation between
the paramagnetic to ferromagnetic phases (Tc~255 K for this composition). The
structural part of the transition is associated with an expansion of the
hexagonal unit cell in the direction of the a- and b-axes and a contraction of
the c-axis as the FM phase is formed, which originates from an increase in the
intra-layer metal-metal bond distance. The application of pressure is found to
have an adverse effect on the formation of the FM phase since pressure opposes
the expansion of the lattice and hence decreases Tc. The application of a
magnetic field, on the other hand, has the expected effect of enhancing the FM
phase and increasing Tc. We find that the substantial range of
temperature/field/pressure coexistence of the PM and FM phases observed is due
to compositional variations in the sample. In-situ high temperature diffraction
measurements were carried out to explore this issue, and reveal a coexisting
liquid phase at high temperatures that is the origin of this variation. We show
that this range of coexisting phases can be substantially reduced by
appropriate heat treatment to improve the sample homogeneity.
