We present a detailed study of the structural and magnetic phase transitions of the Heusler compound Co 2 NbSn. This material undergoes a structural transition at T S ϭ235 K from the cubic Heusler Fm3m hightemperature phase into an orthorhombic low-temperature lattice of Pmma symmetry. Further, the system exhibits a magnetic transition at T C ϭ116 K from para-to ferromagnetism. While on a macro-and mesoscopic scale Co 2 NbSn appears to be fully Fm3m ordered, from microscopic studies we find that on an atomic scale site disorder is present. We discuss the implications of these findings for the anomalous macroscopic physical properties and domain formation in the ferromagnetic state in Co 2 NbSn.

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