On the mechanism of the zircon-reidite pressure induced transformation

We report first principles results of a detailed investigation directed to elucidate mechanistic aspects of the zircon-reidite phase transition in ZrSiO₄. The calculated thermodynamic boundary is located around 5 GPa, and the corresponding thermal barrier, estimated from temperatures at which the transition is observed at zero and high pressure, is 133 kJ/mol. Under a martensitic perspective, we examine two different transition pathways at the thermodynamic transition pressure. First, the direct, displacive-like, tetragonal I4₁/a energetic profile is computed using the c/a ratio as the transformation parameter, and yields a very high activation barrier (236 kJ/mol). Second, a quasi-monoclinic unit cell allows us to characterize a transition path from zircon (β=90^°) to reidite (β=114.51^°) with an activation barrier of around 80 kJ/mol at β=104^°. This energy is somewhat lower than our previous estimation and supports the reconstructive nature of the transformation at the thermodynamic transition pressure.