Anelasticity and transient creep in NaMgF3 perovskite at high pressure

Mechanical oscillation experiments of polycrystalline NaMgF3 perovskite were carried out at high pressure and temperatures across the orthorhombic to cubic phase boundary. Stress and strain of the sample were measured during the oscillations using X-ray diffraction. The accompanying attenuation and softening of the Young’s modulus peak near the phase boundary. The applied uniaxial stress induces flips the sample between the twin states of the perovskite. For grains oriented parallel to the unique stress axis, the twin state oscillates between (2 0 0) and (0 2 0) orientation, while the grains oriented parallel to radial stress axis align with the (1 1 2) twin state. The induced plastic strain in the polycrystal due to the ferroelastic effect is about 25% of the spontaneous strain. This implies, for the Earth’s lower mantle, that viscous flow of 1% strain can be accommodated by ferroelastic processes.