High-Pressure Phase Transition Influenced By Natural Irradiation Induced Damage In Zircon

Zircon as a silicate mineral rich in U and Th,and has stable physicochemical property.During meteorite impact process,it will be transformed into reidite,the high-pressure phase of zircon,which is a very rare silicate mineral.It can be used to confirm the existence and indicate the pressure range of the meteorite impact.Temperature and pressure decide the formation of reidite.However,the microstructure of this material is affected byα-decay process in natural zircon,which influence the high-pressure phase transition.Though there are no systematic studies about the effects of nature radiation damage on the high-pressure phase transition of materials.Therefore,the effect of natural radiation damages influenced on high-pressure phase transition behavior of five nature zircon samples were studied systematically in this work.Combine with diamond anvil cell,the influence of radiation damage effect on the phase transition of zircon were independently analyzed by in-situ high pressure Raman and in-situ synchrotron XRD experiments.The result shows that the dilute irradiation dose promoted the high pressure phase transformation of zircon,and as radiation dose further increased,the reidite formation will be hampered.High resolution transmission electron microscope shows,zircon has completely transformed into the reidite,the intact lattice can be found in both the unirradiated and low dose samples after quenched from a similar pressure⁴1 GPa to 1 bar.High irradiation dose samples loss the intact of zircon lattice,and a mixture of zircon and reidite nanocrystals can be found in an amorphous matrix after pressurized to 42 GPa,suggesting that the formation of reidite were hampered upon the irradiations in high alpha doses.From the Rietveld refinement of X-ray diffraction data,the fraction of reidite content f_r can be obtained,which can be fitted as a function of pressure.During this process,the dependence of reidite formation onα-recoil damage can be quantified.The equation with two physical parameters,P₀ and P_1/2,P₀ is the pressure that reidite first appears and P_1/2/2 means the pressure at which half of zircon transforms into reidite.The trend of the two parameters is consistent with the qualitative results of XRD,Raman and HRTEM.There are two prerequisites that are critical to the transformation from zircon to reidite:1.the integrity of the zircon lattice to maintain the shear strain,which required to the martensitic transformation;2.minor atomic displacement are required for the readjustment into perfect scheelite structure.At low irradiation doses,theα-decay process produces a cascade of atomic displacements or movements,combine with the rotation of the[SiO₄]tetrahedron,which provide the minor atomic displacement.While the integrity of zircon lattice has not been destroyed.Thus,promote the transformation from zircon structure to a low-symmetry and high-density scheelite structure,under high pressures.Although as radiation dose increased,the intact of zircon lattice is destroyed,making it impossible to provide the shear strain that required for the martensitic transformation from zircon to reidite.In addition,the formation of nanocrystalline phases may also inhibit the occurrence of high pressure phase transition.