  Structural phase transitions drive several unconventional phenomena including
some illustrious ferroic attributes which are relevant for technological
advancements. With this note, we have investigated the structural transition of
perovskite-type trigonal Ba2ZnTeO6, across Tc ~ 150 K, which is also
accompanied by a para- to ferroelastic transition. With the help of Raman
spectroscopy and density-functional theory (DFT)-based calculations, here we
report new intriguing observations associated with the phase transition in
Ba2ZnTeO6 elucidating the lattice dynamics across the Tc. We have observed the
presence of a central peak (quasi-elastic Rayleigh profile), huge softening in
the soft mode, hysteretic phonon behavior, and signatures of coexistent phases.
The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in
the intensity of the Rayleigh profile in concomitant with the damping of the
soft mode near Tc, shedding light on the lattice dynamics during the phase
transition. While most of the phonon bands split below Tc confirming the phase
transition, we have observed thermal hysteretic behavior of phonon modes that
signifies the first-order nature of the transition and presence of coexisting
phases, which are corroborated by our temperature-dependent x-ray diffraction
and specific heat measurements. Further, an evidence of the concomitant
structural transition appears in the form of huge softening in the thermal
response of the soft phonon mode at ~ 31 cm-1 which is remarkable compared to
the hitherto known behavior of soft modes in well-known ferroelectrics. This is
further corroborated by our phonon calculations that show an unstable Eg-mode
in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and
Zn translation) which later condenses into the C2/m low-symmetry phase.
