The crystal structure and phase transitions of the magnetic shape memory compound Ni 2 MnGa

High resolution neutron powder diffraction and single crystal measurements on the ferromagnetic shape memory compound Ni 2 MnGa have been carried out. They enabled the sequence of transformations which take place when the unstressed, stoichiometric compound is cooled from 400 to 20 K to be established. For the first time the crystallographic structure of each of the phases which occur has been determined. At 400 K the compound has the cubic L 2 1 structure, and orders ferromagnetically at T C ≈ 365 K. On cooling below ∼ 260 K a super-structure, characterized by tripling of the repeat in one of the ⟨110⟩ cubic directions, forms. This phase, known as the pre-martensitic phase, persists down to the structural phase transition at T M ≈ 200 K and can be described by an orthorhombic unit cell with lattice parameters a ortho = 1/√2 a cubic , b ortho = 3/√2 a cubic , c ortho = a cubic and space group Pnnm . Below T M the compound has a related orthorhombic super-cell with b ortho ≈ 7/√2 a cubic , which can be described within the same space group. The new modulation appears abruptly at T M and remains stable down to at least 20 K.