Elastic relaxations associated with the Pm(3)over-barm-R(3)over-barc transition in LaAlO3: IV. An incipient instability below room temperature

Resonant ultrasound spectroscopy has been used to characterize elastic softening and acoustic dissipation behaviour in single crystal and ceramic samples of LaAlO3 between 10 and 300 K. For the twinned R (3) over barc single crystals, average values of the cubic elastic moduli 1/2(C-11 - C-12) and C-44 were followed while the ceramic sample provided data for the bulk and shear moduli. A Debye-like dissipation peak occurs in the vicinity of 250 K, from which an activation energy of 43 +/- 6 kJ mol(-1) has been obtained. The mechanism for this is not known, but it is associated with C44 and therefore could be related in some way to the cubic <-> rhombohedral transition at similar to 817 K. Slight softening in the temperature interval similar to 220 -> 70 K of resonance peaks determined by shear elastic moduli hints at an incipient E-g ferroelastic instability in LaAlO3. The softening interval ends with a further dissipation peak at similar to 60 K, the origin of which is discussed in terms of freezing of atomic motions of La and/or Al away from their high symmetry positions in the R (3) over barc structure. LaAlO3 thus shows evidence of incipient structural instability at low temperatures which is potentially analogous with the phenomenologically rich behaviour of SrTiO3.