Mechanical properties and domain wall mobility of LaGaO 3 perovskite over a first order phase transition Claire Jakeways, Richard Harrison and Simon Redfern University of Cambridge

Aims What processes control seismic attenuation in the lower mantle? –Effect of anelasticity –What process causes this response? –How can this be measured? Lower mantle: perovskite, (Mg,Fe)(Si,Al)O 3, with a possible transition to a post-perovskite phase. Are the dominating processes driven by grain boundary sliding or transformation twinning? –comparison of polycrystalline and single crystals

Insights into first order transition effects in perovskite from La-Nd gallates La x Nd (1-x) GaO 3 perovskite LaGaO 3 and La 0.88 Nd 0.12 GaO 3 Czochralksi grown single crystals Highly twinned {110} pc and {112} pc Polycrystalline samples Synthesised from oxide starting materials Average porosity 10% Advantages? orthorhombic phase with twin domains GdFeO 3 structural group Disadvantages? opposite symmetry behaviour on increasing pressure

First-Order Phase Transition Pbnm R c Structures: L.Vasylechko et al. 2000, J.Marti et al respectively.

Dynamic Mechanical Analyser (DMA) 8.51mm 2. 86mm {110} p surface of LaGaO 3

Motion of the phase interface over a phase transition La 0.88 N 0.12 GaO 3 110mN F s 1Hz whilst heating over transition. Speed X3. LaGaO 3 110mN F s 2Hz whilst heating over transition. Speed X3.

Double peak in attenuation over the transition P1 and P2, Wang et al Comparison of single and polycrystalline samples Higher attenuation in the rhombohedral phase and single crystals Tan  ~ 0 in orthorhombic phase, and no twin motion is evident.

Characterisation and description of a first order phase transition Zhang et al. 1994, 1995a,b 0<n, l<1 Expected behaviour: Increase in internal friction peak height at transition with increasing ramp rate, decreasing frequency and force. Similar behaviour for polycrystalline and single crystals.

Frequency of Applied Force : Magnitude of Applied force: Heating rate:

Characterisation and description of a first order phase transition LaGaO 3 ln Single crystal / / Polycrystalline sample / /-0.093

Characterization of interface motion Effect of composition? Effect of frequency and force? –Is velocity a constant or exponential function? Initial linearity for La 0.88 Nd 0.12 GaO 3 irrespective of F, f.

Summary of first order phase transition observations Both single crystal and polycrystalline samples show behaviour consistent with previous work on first-order phase transitions. The motion of an interface between two phases is greatly influenced by the twin microstructure. The velocity of the interface between the two phases can be considered to be a function of temperature, applied force and frequency.

Are transformation microstructures important when considering seismic attenuation? Twin domain movement is not active in the orthorhombic phase until very close to the phase transition. When rhombohedral, the twins are then free to move. i.e. the attenuation in the rhombohedral phase is much higher than in the orthorhombic phase where the crystal response is elastic. The observed attenuation is much greater in single crystal samples than polycrystalline, implying that transformation twins play a large role in the measured anelasticity. The interface movement is associated with a large and sharp peak in attenuation observed at the first-order phase transition: we might expect similar effects at the first-order perovskite to post-perovskite phase transition at the base of Earth’s lower mantle.

Acknowledgements EPSRC PhD funding British Crystallographic Association (BCA), Institute of Minerals, Mining and Materials(IoM3) and The British Mineralogical Society for providing travel funds to this conference. Stuart Hayward (DSC) Savytskii et al. Lviv Polytechnic, Ukraine.