Direct ultrasonic evidence for heterophase oscillations near the 186 K transition in KMnF3

KMnF₃ is one of the first systems in which a dynamic ``central peak'' was observed by inelastic neutron scattering in the vicinity of a structural phase transition. The origin of this peak, however, whether intrinsic or extrinsic, was never fully resolved. In the present paper, we report combined dielectric and ultrasonic results on KMnF₃, pure and lightly doped with lithium, which provide direct evidence for the presence of intrinsic ``heterophase (cubic-tetragonal) oscillations'' near the transition. These oscillations are seen as a ``second sound'' wave, akin to the density wave observed in liquid helium near the superfluidity transition.