The influence of transformation twins on the seismic-frequency elastic and anelastic properties of perovskite:: dynamical mechanical analysis of single crystal LaAlO3

The low-frequency mechanical properties of single crystal LaAlO3 have been investigated as a function of temperature, frequency and applied force using the technique of dynamical mechanical analysis (DMA) in three-point bend geometry. LaAlO3 undergoes a cubic to rhombohedral phase transition below 550degreesC. The mechanical response in the low-temperature rhombohedral phase is shown to be dominated by the viscous motion of transformation twin domain walls, resulting in a factor of 10 decrease in the storage modulus relative to the high-temperature cubic phase (super-elastic softening) and a significant increase in attenuation. Super-elastic softening is observed down to 200degreesC, below which the mobility of the domain walls decreases markedly, causing a rapid increase in storage modulus and a pronounced peak in attenuation (domain wall freezing). The frequency dependence of the storage modulus close to the freezing temperature is accurately described by a modified Burgers model with a Gaussian distribution of activation energies with mean value 84.1 (1) kJ/mol and S.D. 10.3(l) kJ/mol. This activation energy suggests that domain walls are pinned predominantly by oxygen vacancies. Detailed analysis of the dynamic force-deflection curves reveals three distinct regimes of mechanical response. In the elastic regime, the domain walls are pinned and unable to move. The elastic response is linear with a slope determined by the intrinsic stiffness of the lattice, the initial susceptibility of the pinning potential and the bending of twin walls between the pinning sites. In the super-elastic regime, the domain walls unpin and displace by an amount determined by the balance between the applied and restoring forces. The value of the apparent super-elastic modulus is shown to be independent of the spontaneous strain and hence independent of temperature. At high values of the applied force, adjacent domain walls come into contact with each other and prevent further super-elastic deformation (saturation). The strain in the saturation regime scales with the spontaneous strain and the resulting modulus is strongly temperature dependent. The possible effects of domain wall motion on the seismic properties of minerals are discussed. It is concluded that, if these results are directly transferred to mantle-forming (Mg, Fe)(Si, AI)O-3 perovskite, the strain amplitude of a typical seismic wave would be sufficient to cause super-elastic softening. However, pinning of domain walls by oxygen vacancies leads to very short relaxation times at mantle temperatures. If translated to (Mg, Fe)(Si, Al)O-3, these would be too short to amount to significant seismic attenuation. Increased pinning of ferroelastic domain walls by defects, impurities and grain boundaries in real mantle perovskite, or a significant positive activation volume for oxygen vacancy diffusion, would be sufficient to increase the relaxation time to values resulting in seismic wave attenuation, however. (C) 2002 Elsevier Science B.V. All rights reserved.