Origin of the crossover between a freezing and a structural transition at low concentration in the relaxor ferroelectric K 1-x Li x TaO 3
The origin of the relaxor behavior in K 1-x Li x TaO 3 (KLT) and other disordered perovskites is now recognized to be due to the reorientation of the polar nanodomains formed by the correlated dipoles of off-center ions. The collective dynamics of these systems evolve through several temperature stages. On decreasing temperature below the so-called Burns temperature T B , individual dipoles become correlated within nanosized regions. On further cooling, the slow dynamics of these polar regions allows local lattice distortions to take place and the formation of polar nanodomains at T* < T B . At still lower temperature, some relaxors undergo a phase transition while others do not. In KLT, there is a critical Li concentration x c = 0.022 above which the system undergoes a structural transition at T c , and below which it freezes in a dipole glass state at T f . To better understand the nature of this critical concentration, the changes that occur upon crossing it and the nature of the dipole glass state, the collective dynamics of KLT have been studied by dielectric spectroscopy and neutron diffraction for two Li concentrations (x = 0.026 and 0.018), close to but straddling the critical concentration x c . Two very different transitional behaviors are observed. Just below this critical concentration, KLT displays critical slowing down and the onset of freezing as seen in hydrogen-bonded molecular ferroelectrics, while just above this concentration, KLT undergoes a first-order structural transition.