  We report on a strain-induced martensitic transformation, accompanied by a
suppression of magnetic order in epitaxial films of chemically disordered FeRh.
X-ray diffraction, transmission electron microscopy and electronic structure
calculations reveal that the lowering of symmetry (from cubic to tetragonal)
imposed by the epitaxial relation leads to a further, unexpected,
tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type
lattice instability. The collapse of magnetic order is a direct consequence of
this structural change, which upsets the subtle balance between ferromagnetic
nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated
antiferromagnetic coupling among localized Fe moments at larger distances.
