The structure of Ni
_{2.17}
Mn
_{0.83}
Ga has been investigated as a function of temperature using neutron powder diffraction. Above T = 310 K the compound has the cubic L2
_{1}
structure, with lattice parameter a = 5.79 Å, and with the excess nickel atoms occupying the vacant manganese sites. Magnetisation and resistivity measurements indicate a Curie temperature of T = 340 K with a further transition at T = 301 K associated with a structural phase transition. The martensitic phase was found to have a tetragonal structure with cell parameters a = b = 3.87 Å and c = 6.47 Å and space group I4/mmm. Although there is negligible change in atomic volume the lattice parameters a and b change by ∼ 5.5% and the c-axis by almost 12%. The large change in cell parameters and the relatively high transition temperature make the compound a promising candidate for applications.
