Vibrational pseudo-diffusive motion of the oxygen octahedra in La2CuO4 and Sr doped La2CuO4 from anelastic and 139La NQR relaxation

The vibrational dynamics in nearly stoichiometric La2CuO1+? has been studied by means of anelastic relaxation and by 139La NQR relaxation. Evidence of dynamical tilting of the octahedra in a double well local potential is obtained with both the experimental techniques. Anelastic relaxation shows a peak at T ~ 150 K for a measuring frequency in the kHz region, while the peak in the NQR relaxation at the quadrupole frequency VQ ~ 19 MHz occurs at T ~ 330 K. The data are analyzed in terms of a model of pseudo-diffusive cooperative motion. The measured effective barrier is 2800 K, higher than the one of the focal potential because of the cooperative character of the motion. Upon Sr doping the 139La relaxation is still dominated by the octahedra tilting waves for T > 100 K, while al low temperatures the magnetic relaxation mechanism related to Cu2+ spin fluctuation dominates. It is argued how the freezing of the tilts (occurring below 100 K in the undoped La2CuO4) might correspond to the formation of static stripes.