Pressure-induced Phase Transition In Hafnon And Its Dependence On Ion Irradiation

Hafnon has a same structure as zircon with the space group of I4₁/amd.Due to similarity,Hf is the most common compositional substitution for Zr.As the high-pressure phase of zircon,reidite,which is formed by meteorite impacts,is of great importance in high-pressure geology.Exploring the phase transition behavior of hafnon under high pressure is important for further understanding the phase transition mechanism of zircon-type ABO₄ minerals due to the similarity with zircon.In order to exclude the possible influence of impurities,the phase transition of synthetic zircon and hafnon was investigated in combination with ion irradiation.The in-situ high pressure synchrotron X-ray diffraction（XRD）experiments were used for the first time to identify the phase transition of hafnon from zircon structure to scheelite structure at 26.5 GPa.The Birch-Murnaghan equation of state yielded the zero-pressure bulk modulus of B₀=218（18）GPa and the cell volume of V₀=64.6（1）?³/f.u.Combined with theoretical calculations,the cell parameters of scheelite-type HfSiO₄ at ambient condition are:a=4.71?,c=10.38?,and space group I4₁/a.The pressure of phase transition of hafnon was verified again by the in-situ high pressure Raman experiment.A high-pressure low symmetry（HPLS）phase,similar to that of zircon-reidite phase transition process,was found at²6 GPa.Theoretical calculations indicated that the critical pressure of phase transition at 12.9 GPa for hafnon is significantly different from the experiment value,likely due to the kinetic hindrance.At 23 GPa,the zircon structure is dynamically unstable and shows a B_1u mode that changes to an A₁ mode in the HPLS phase with increasing pressure.The three structures indicate that the zircon structure shifts O^2-along the characteristic vector of the B_1uu mode,resulting in the HPLS phase structure.This result further suggests that the HPLS phase is a transition one to bridge the phase transformation from the structure of zircon to scheelite.The ion irradiation induced damage in mineral was further analyzed by in-situ high pressure Raman spectroscopy.A certain dose of ion irradiation can promote high pressure phase transformation.The deviation of the E_g symmetry of zircon from the original linear trend is not correlated with the presence or absence of defects within the lattice,but the magnitude of the deviation decreases with increasing dose.The effect of ion irradiation on the phase transition behavior of hafnon is consistent with that of zircon.Thus,ion irradiation can control the high-pressure phase transition in zircon-type minerals.