Abstract
We report the unexpected appearance of a new zone center phonon branch in
PbTe on warming. In a solid the number of phonon branches is strictly determined
by the size and symmetry of the unit cell. The appearance of the new
mode unambiguously indicates a crossover to a lower symmetry structure with
increasing temperature. No structural transition is seen crystallographically
but the appearance of the new mode in inelastic neutron scattering coincides
with the appearance of local Pb off-centering dipoles in the local structure.
1
The observed behavior is intrinsic but cannot be explained within the framework
of conventional phase transition theories such as soft-mode theory. It is
an example of a new class of transition between phases, which we refer to as
emphanitic, where hidden, local broken symmetries emerge on warming.
