The influence of transformation twins on seismic-frequency anelasticity in perovskite: an investigation using stroboscopic XRD-DMA
Abstract
Recent experimental and theoretical studies suggest that the lower mantle is significantly anelastic. The origin of this anelasticity remains uncertain, however. One possible cause is the viscous motion of twin domain walls. Here we use a combination of dynamical mechanical analysis (DMA) and stroboscopic X-ray diffraction (XRD) to study the effect of domain walls on seismic-frequency anelasticity in perovskite at high temperature. We apply the technique to single crystal LaAlO_3, a close structural analogue of the MgSiO_3 perovskite phase believed to make up more than 70% of the Earth's lower mantle. We demonstrate that superelastic behaviour associated with the motion of domain walls dominates the mechanical response over a temperature range spanning several hundred degrees. This is accompanied by a factor-of-ten decrease in the effective Young's modulus below the cubic to rhombohedral phase transition and a rapid increase in attenuation. Freezing of domain walls at lower temperatures leads to mechanical stiffening and a broad peak in attenuation (tand >0.9 at 200^oC and 1 Hz). The observations indicate that domain walls are strongly pinned by oxygen vacancies. The combination of DMA and XRD permits the response of domain walls to a dynamic force to be observed in-situ for the very first time. The XRD-DMA machine consists of a standard Perkin-Elmer DMA-7e in three-point bend geometry, combined with a conventional X-ray source and position sensitive detector. The X-ray source is movable, allowing X-rays to be focused on the underside of the sample at a wide range of incident angles. Measuring diffracted intensity as a function of incident angle results in a conventional rocking curve, permitting the twin microstructure to be quantified. During a DMA measurement, X-rays are binned into four separate spectra, which are synchronised to four different stages of the dynamic force cycle. In this way, changes in microstructure which occur directly in response to the dynamic force can be resolved on a timescale of 0.1-50 Hz. The first results of this technique applied to LaAlO_3 are presented. Clear differences in the rocking curves corresponding to the maximum and minimum dynamic force can be resolved in the temperature range where superelastic softening is observed. The changes correspond to a shift in the position of the diffraction peaks. This shift is thought to be caused by the compatibility strain which develops as the domain walls are forced to rotate away from their preferred orientations.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....5616H