The pressure-induced phase transition(s) of ZrSiO 4 : revised: Experimental proof for the existence of a new high-pressure polymorph of zircon

The existence of a new high-pressure low-symmetry (HPLS) ZrSiO 4 phase (space group I4 ¯ 2 d), which has been predicted by density-functional-theory (DFT) calculations (Stangarone et al. in Am Mineral, 2019b), is experimentally confirmed by in situ high-pressure Raman spectroscopic analysis up to 25.3 GPa. The new ZrSiO 4 polymorph is developed from zircon via a soft-mode-driven displacive phase transition. The Cochran-law-type pressure dependency of the soft-mode wavenumber reveals a zircon-to-HPLS critical pressure pc = 20.98 ± 0.02 GPa. The increase in the phonon compressibilities of the zircon hard mode near 202cm-1 at p> pr= 10.0 GPa as well as of the reidite hard mode near 349cm-1 at p< pr marks the pressure above which zircon becomes thermodynamically metastable with respect to reidite; the experimentally determined value of pr is in good accordance with the equilibrium zircon–reidite transition pressure derived from DFT simulations. However, at room temperature, there is not enough driving force to rebuild the atomic linkages and the reconstructive transition to reidite happens ∼ 1.4 GPa above pc, indicating that at room temperature, the HPLS phase is a structural bridge between zircon and reidite. The pressure dependencies of the phonon modes in the range 350--460cm-1 reveal that the reconstructive phase transition in the ZrSiO 4 system is triggered by energy resonance and admixture of hard modes from the parent and resultant phase.