Abstract
The structure and chemical order of a Heusler alloy of non-stoichiometric composition
Ni-Mn-Ga were studied using constant-wavelength (1.538 Å) neutron diffraction at 363K and the
diffraction pattern was refined using the FullProf software. At this temperature the structure is
austenite (cubic) with Fm-3m space group and lattice constant of a = 5.83913(4) [Å]. The chemical
order is of critical importance in these alloys, as Mn becomes antiferromagnetic when the atoms are
closer than the radius of the 3d shell. In the studied alloy the refinement of the site occupancy
showed that the 4b (Ga site) contained as much as 22% Mn; that significantly alters the distances
between the Mn atoms in the crystal and, as a result, also the exchange energy between some of the Mn atoms. Based on the refinement, the composition was determined to be Ni1.91Mn1.29Ga0.8
