Elastic relaxations associated with the Pm3m-R3c transition in LaAlO(3): II. Mechanisms of static and dynamical softening.

Brillouin spectra have been collected in situ at temperatures up to ∼1000 K for different crystallographic directions from two single crystal plates of LaAlO(3) 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 [Formula: see text] 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 ∼700 K, but from ∼700 K up to T(c) 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 ∼5-15 K below T(c), implies a strong dynamical component to the phase transition. Relaxation times estimated from the width of the central peak are of the order of ∼10-100 ps and appear to be more or less constant between ∼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 T(c) 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).