Polar and nonpolar atomic motions in the relaxor ferroelectric Pb1-3x/2LaxZr0.2Ti0.8O3 from dielectric, anelastic and NMR relaxation

Dielectric, anelastic, and 139La NMR relaxation measurements have been made on the relaxor ferroelectric Pb1-3x?2LaxZr0.2Ti0.8O3 (PLZT) with x=0.22. The dielectric susceptibility exhibits the frequency dispersive maximum due to the freezing of the polar degrees of freedom around T?250 K. The anelastic and especially NMR relaxation, besides this maximum, indicate an intense and broad component at lower temperatures, attributed to rotational modes of the O octahedra, weakly coupled to the polar modes. It is discussed why such short range rotational instabilities, known to occur in the Zr-rich rhombohedral region of the PLZT phase diagram, might appear also in the Ti-rich region.