Study of optical phonon lifetimes near the diffuse phase transition in relaxor ferroelectric thin films

We have studied the lattice dynamical properties of oriented Pb(Sc0.5Nb0.25Ta0.25)O-3 (PSNT) thin films by micro-Raman spectroscopy in the temperature range of 80-450 K. An anomalous temperature dependence of the integrated intensity and Raman linewidths was found near the diffuse phase transition. The phonon lifetime analysis indicates strong damping of A(1g) mode phonons near the transition. This analysis falls into two regimes: below the transition an increase in the lifetime with very small activation energy (E-a similar to 0.001 eV) is found, and above the transition an increase in the lifetime with higher activation energy (E-a similar to 0.018 eV) is fitted. Although the activation energy calculated from the dielectric loss spectra matched within a factor of 3 with the phonon lifetime energy calculations at all temperatures, we interpret these data as non-equilibrium phenomena due to a relaxation bottleneck in a glassy system (Littlewood and Rammal 1988 Phys. Rev. B 38 2675). Whenever activation energy E << kT is fitted, one should suspect such processes, and they are known in polyvinylidene fluoride (Zhang 1998 Science 280 2101; Scott 2006 J. Phys.: Condens. Matter 18 7123). The new data provide a new way of characterizing non-equilibrium processes in relaxors.