Elastic relaxations associated with the – transition in LaAlO3: III. Superattenuation of acoustic resonances

Abstract

Resonant ultrasound spectroscopy has been used to characterize elastic softening andanelastic dissipation processes associated with the transition in single crystal and ceramic samples ofLaAlO3. Softening of the cubic structure ahead of the transition point is not accompanied by anincrease in dissipation but follows different temperature dependences for the bulk modulus,, and the shear components, and C44, as if the tilting instability contains two slightly different criticaltemperatures. The transition itself is marked by the complete disappearanceof resonance peaks (superattenuation), which then reappear below∼700 K in spectra from single crystals. Comparisons with low frequency, high stress data fromthe literature indicate that the dissipation is not due to macroscopic displacement of needletwins. An alternative mechanism, local bowing of twin walls under low dynamic stress, ispostulated. Pinning of the walls with respect to this displacement process occurs below∼350 K. Anelasticity maps, analogous to plastic deformation mechanism maps, are proposed todisplay dispersion relations and temperature/frequency/stress fields for different twinwall related dissipation mechanisms. These allow comparisons to be made ofanelastic loss mechanisms under mechanical stress with elastic behaviour observed bymeans of Brillouin scattering at high frequencies which might also be related tomicrostructure.