  Quest for new states of matter near an ordered phase is a promising route for
making modern physics forward. By probing thermal properties of a ferroelectric
(FE) crystal Ba1-xSrxAl2O4, we have clarified that low-energy excitation of
acoustic phonons is remarkably enhanced with critical behavior at the border of
the FE phase. The phonon spectrum is significantly damped toward the FE phase
boundary and transforms into glasslike phonon excitation which is reminiscent
of a boson peak. This system thus links long-standing issues of amorphous
solids and structural instability in crystals to pave the way to controlling
lattice fluctuation as a new tuning parameter.
