Elastic relaxations associated with the Pm(3)over-barm-R(3)over-barc transition in LaAlO3: II. Mechanisms of static and dynamical softening

Brillouin spectra have been collected in situ at temperatures up to similar to 1000 K for different crystallographic directions from two single crystal plates of LaAlO3 perovskite. Elastic moduli derived from these, together with heat capacity, spontaneous strain and Raman data from the literature, have then been used to calibrate the coefficients in a classical Landau free energy expansion for the second order Pm (3) over barm <-> R (3) over barc phase transition at T-c = 817 K. The static strain/order parameter coupling model provides a quantitative description of elastic softening between room temperature and similar to 700 K, but from similar to 700 K up to Tc additional elastic softening correlates with the development of a central peak in the Brillouin spectra. The presence of quasi-elastic scattering, which reaches maximum intensity similar to 5-15 K below Tc, implies a strong dynamical component to the phase transition. Relaxation times estimated from the width of the central peak are of the order of similar to 10-100 ps and appear to be more or less constant between similar to 700 and 800 K, which is consistent with an intrinsic origin associated with phonon density fluctuations. Central peak width variations and an irregular pattern of acoustic velocity variations in a 20 K temperature interval below Tc are interpreted in terms of flipping of clusters of tilted octahedra between different < 111 >, < 011 > and < 001 > tilt axes. The additional softening beyond that expected from the classical strain/order parameter coupling model can be understood in terms of coupling of acoustic modes with the central peak mode(s).