  Brillouin spectroscopic measurements have been carried out on relaxor
ferroelectric Pb(Zn1/3Nb2/3)0.85Ti0.15O3 (PZN-PT) single crystal over the
temperature range 300-585 K. The longitudinal acoustic phonon begins to soften
below 650 K, which is attributed to the Burns temperature (TB). On the other
hand, the line width of the longitudinal acoustic (LA) phonon mode exhibits a
sharp Landau-Khalatnikov-like maximum and an accompanying anomaly in the LA
mode frequency around 463 K, the tetragonal-cubic phase transition temperature
(Ttc). In addition, a broad central peak, found below the characteristic
intermediate temperature T* ~ 525 K, exhibits critical slowing down upon
approaching Ttc indicating an order-disorder nature of the phase transition.
The relaxation time of polar nano regions estimated from the broad central peak
is found to be same as that obtained for LA phonon mode suggesting an
electrostrictive coupling between strain and polarization fluctuations. The
activation energy for the PNRs relaxation-dynamics is found to be ~236 meV.
Polarized nature of the central peak suggests that the polar nano regions have
the tendency to form long-range polar ordering.
