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The Earth's lower mantle is considered to be composed of (Mg,Fe)SiO₃-perovskite, which is likely to have an orthorhombic structure in most of the lower mantle. Therefore, the rheological behaviour of orthorhombic perovskites should have an important influence on dynamic processes in the lower mantle. To investigate rheological behaviour of orthorhombic perovskites, an experimental study has been carried out on single crystals of YAlO₃ perovskite, which assumes the orthorhombic structure up to it's melting temperature (T_m = 1870°C). Compression tests were performed on single crystals along the <101> and <100> directions at temperatures of T =1490-1610°C (0.82-0.88T_m) and P_total = 0.1 MPa in a controlled atmosphere of fO₂ =10^-17 - 10^-4 MPa. Applied stresses were 70-300 MPa yielding strain rates of 10^-7 s^-1 to 10^-4 s^-1.

Our experimental results revealed a power-law creep behaviour for both <101> and <100> orientations with a stress exponent of n = 2.6-3.2. Creep activation energies, Q, varied from 380 kJ/mol (for the <101>- samples deformed at fO₂ <10^-8 MPa) to 719-859 kJ/mol (for the <100> and <101>-samples deformed at fO₂ >10^-8 MPa). A significant dependence of creep rate on oxygen fugacity was observed for the <101>-samples at fO₂ <10^-8 MPa, while creep rates of the other samples were insensitive to the change of oxygen fugacity. We have also observed a strong plastic anisotropy at fO₂ <10^-8 MPa. <100>-samples showed a significantly higher creep resistance than that of <101>-samples. Creep strengths of the <101>- and <100>-samples were comparable at fO₂ > 10^-7 MPa. TEM observations on <101>-samples showed significant dislocation interactions (b₁+b₂=b₃, b₁=[], b₂=[] and b₃=[] ). Stacking faults parallel to () and dislocation loops lying in (013) with b=[] were observed.

The obtained mechanical data and dislocation structures suggest a significant difference in creep mechanisms operating in orthorhombic perovskite from those in cubic perovskite, implying that crystal structure plays an important role in determining the rheological behaviour of perovskite in the lower mantle. Further transmission electron microscope investigations on the dislocation structures of deformed samples are being undertaken to provide interpretations of the observed rheological behaviour of orthorhombic perovskite.