High-pressure elasticity of stishovite and the P4(2)/mnm reversible arrow Pnnm phase transition

A Landau free energy expansion has been developed to describe the elastic c onstant variations of stishovite (SiO2) associated with the P4(2)/mnm (ruti le-type) reversible arrow Pnnm(CaCl2-type) phase transition as a function o f pressure. The transition appears to display classical second-order charac ter, with an equilibrium transition pressure which is renormalized by coupl ing of the soft optic mode with spontaneous strain. Lattice parameter data from the literature show that the symmetry breaking strain is large, while the nonsymmetry breaking strains are small. These have been used to constra in the numerical values of the strain/order parameter coupling coefficients , which in turn have been used to constrain values for the Landau coefficie nts. When substituted into expressions for all the individual elastic const ants, taking values for experimental and calculated values of the bare elas tic constants from the literature, a clear view of the pattern of elastic c onstant variations is produced. Variations of P and S wave velocities deriv ed from the calculated elastic constants for a second-order transition show a dip as the transition point is approached from high pressure and from lo w pressure but do not show a discontinuity. The velocity anomaly provides a signature for the presence of free silica in the lower mantle.