  The anharmonic vibrational dynamics in nearly stoichiometric La2CuO4+delta is
studied by means of anelastic and ^{139}La NQR relaxation. In the absorption
component of the elastic susceptibility as well as in the nuclear relaxation
rate a peak is detected as a function of temperature, and a relaxation time tau
= 1.7 10^{-12} exp [(2800 K)/T] s is derived. The relaxation processes are
attributed to tilt motion of the CuO6 octahedra in doublewell potentials, whose
cooperative character increases the effective energy barrier to the observed
value. The analysis of the relaxation mechanisms has been carried out by
reducing the dynamics of the interacting octahedra to a one-dimensional
equation of motion. The soliton-like solutions correspond to parallel walls
separating domains of different tilt patterns and give rise to pseudo-diffusive
modes which appear as a central component in the spectral density of the motion
of the octahedra. The tilt waves may be considered to correspond to the
dynamical lattice stripes observed in La-based and Bi-based high-Tc
superconductors.
