Analysis And Simulation Calculation Study On Phase Transition And Hydrogen Defects In Ultrahigh-Pressure Minerals

UHP（Ultrahigh-Pressure,abbreviated as UHP）minerals are formed by ultrahigh-pressure metamorphism.They are generated from crust rocks going into to the deep interior of the earth during subduction,and metamorphosing under ultrahigh pressure,then exhumating onto the earth surface with UHP metamorphic rocks.The in situ analysis of the complex structure information of UHP minerals at the ultra-micro scale is of great significance for the exploration of the crystal structure,deformation mechanism,defect properties,and H activity of minerals.In this paper,the zircon,coesite andα-PbO₂ type TiO₂ in UHP metamorphic rocks from the Dabie orogenic belt were used as the main research objects.Using the combining the modern material testing techniques and first-principles simulation calculations,the phase transitions between these UHP mineral polymorphs and their hydrogen structural defects were investigated in detail.The paper mainly carried out the following research work:1.The zircon samples from Shuanghe and Bixiling,Dabie mountains were characterized by X-ray diffraction（XRD）and Fourier transform infrared（FTIR）analyses.The crystal structure data of zircon was obtained by XRD.The zircon crystals showed the tetragonal crystal structure.The crystallinity of mineral samples was good.The structural water content of zircon containing Hydrogen defects in Shuanghe and Bixiling was recorded using the FTIR technology.The structural water content of zircon in Shuanghe was in the region of 3-56 ppm and its average value was approximately 29ppm.The structural water content in Bixiling was 4-41 ppm,and the average level was about 15 ppm.The structural water content of zircon in different regions was significantly different.The structural water content of zircon in Shuanghe was about double that of Bixiling.The distribution of structural water content among different zircon grains in the same region wasn’t also uniform.It may be related to factors such as uneven distribution of hydrogen defects,initial H concentration,H binding ability and diffusion rate depending on temperature and pressure conditions.The typical infrared absorption peaks of zircons in the two regions were:（I）3385-3414 cm^–1,（II）3261-3281 cm^–1,（III）2918–2929 cm^–1,and there are still（IV）Infrared absorption peaksat 2850-2855 cm^–1 in Bixiling.The infrared absorption peaks at（I）,（II）and（III）are related to the stretching vibration of the substitution O-H of the Si vacancy.2.Transmission electron microscope（TEM）tests were conducted on the coesite samples in Shima,Dabie Mountains.The single crystal electron diffraction patterns were used to calculate and analyze the crystal structure data of coesite.The result indicated the coesite structure was ascribed to the monoclinic system.According to the analysis of micrograph of coesites,they were accompanied by degenerated quartz.Dislocations are evident in quartz,and there was no dislocation in coesite.The convex surfaces of coesite and degenerated quartz boundaries face coesite,It shows that quartz was grown by the consumption of coesite.The phase transition from coesite to degenerated quartz belongs to the reconstruction phase transition,which was on the basis of the analysis of the composite electron diffraction pattern at the boundary of the two phases.Through FTIR analysis,the structure water content of coesite grains in Shima area was 15-52 ppm,and its average value was³2 ppm.The main infrared absorption peaks of coesite were（I）3561-3580 cm^–1,（II）3433-3462 cm^–1,and（III）3412-3425 cm^–1.The infrared absorption peaks at（I）,（II）,and（III）corresponded to the stretching vibration of OH in hydrogen defects,and the infrared absorption peaks at（I）and（II）were derived from hydrogarnet substitutions(SiO₄^4-<=>（O4H4）^4-).3.The first-principles local density approximation（LDA）and generalized gradient approximation（GGA）methods based on density functional theory（DFT）were used to simulate the unit cells of zircon with geometric optimization.The theoretical values of parameters,volume,bulk modulus,and Si-O and Zr-O bond lengths of zircon decreased to some extent from the experimental values,but the theoretical calculation results of the GGA method were closer to the experimental ones.The high-pressure compression properties of zircon were calculated using the GGA method in the pressure range of0.1-20 GPa.The results showed that the cell axis length（a,b,c）and volume of zircon decreased gradually with increasing pressure,and the axial angleα,β,andγfluctuate up and down within a small range.When the pressure reached 20 GPa,the zircon cell volume was compressed from 257.80?³ to 250.48?³,and was reduced by about 4%.The Si-O,Zr-O（I）and Zr-O（II）bond lengths were compressed at 1.7%,0.8%,and 0.9%,respectively.The bulk modulus of zircon changes a lot and increases by approximately26.5%.Using an approximation in thermodynamics（temperature setting 0 K）and GGA,this work obtained the Gibbs free energy（ie,the enthalpy value）of zircon and reidite inthe pressure range of 0.1 to 20 GPa.The phase enthalpy differences determined that the zircon→reidite phase transformation pressure threshold to¹3.89 GPa.The defect formation energies of（4H）_Si,（AlH）_Si and（YH）_Zr supercell models in hydrogen doped zircon are 3.7 eV,11.54 eV and 16.12 eV,respectively.According to the principle of energy minimization,it was easiest to form the hydrogen defects in the（4H）_Si model.The 4（OH）-<=>SiO₄^4-binding mechanism may be one of the most preferred modes of hydrogen doping.In the（4H）_Si model,there were Raman scattering peaks at 3431,3538and 3571 cm^–1,where the scattering peak corresponded to the substitution mechanism of hydrogarnet.In the（AlH）_Si and（YH）_Zr models,3278 and 3220 cm^–1 Raman scattering peaks appeared.4.Using the first-principle GGA method,the length and volume of the coesite crystals were calculated to decrease with increasing pressure in 0-10 GPa,but the axial angleβand bulk modulus increased.At a pressure of 10 GPa,the volume decreases by approximately 7.8%,while theβand bulk modulus increased by approximately 3%and7.8%,respectively.The larger change in the value of the crystal axis angleβreflected the distortion of the coesite crystal lattice at high pressure.The coesite and quartz phase transitions only considered the pressure factor,and the temperature was fixed at 0 K.Using the method of enthalpy difference between the two phases,the coesite→degenerated quartz phase transition pressure threshold was⁴.1 GPa in 0-10 GPa.The defect formation energies of（4H）_Si and（AlH）_Si supercell models（2×1×1）in hydrogen doped coesite could be-4.92 eV and-3.10 eV,respectively.It was the lower energy of the（4H）_Si defect model,and the defect model was easier to form by hydrogen doping.The 4（OH）substituted Si bonding mechanism was the preferred mode in the coesite structure.The Raman scattering peaks of 3135,3385,3474 and 3509 cm^–1 existed in the（4H）_Si defect model.The（4H）_Si defect structure corresponds to the hydrogarnet substitution mechanism,which is Si⁴⁺+4O^2-<=>4OH^–.The Raman scattering peaks of 3198,3487,3526 and 3682 cm^–1 have appeared in the（AlH）_Si defect model.The Raman scattering peaks of the hydrogen bonding mechanism of the two（4H）_Si and（AlH）_Si crystals deviate from the experimental results of infrared spectroscopy.There may be the cations of other elements bound to hydrogen in coesite,causing absorption peaks.There are still more experiments and theoretical discussions to be conducted.5.The first-principles GGA method was used to calculate the energy band and electronic density of rutile,ultrahigh-pressure mineralα-PbO₂-type and baddeleyite-type TiO₂ as UHP minerals.The energy band of rutile TiO₂ is a direct bandgap with 1.84 eV,which is 1.19 eV smaller than that of the experiment.The energy bands ofα-PbO₂-type and baddeleyite-type TiO₂ are indirect band gaps,and their values are 2.47 eV and 2.04 eV,respectively.Their energy bands are smaller than that of the normal rutile.The crystalline activity of the UHP phase after rutile phase transition is lower.In the range of 0-8 GPa,the theoretical pressure threshold for rutile→α-PbO₂-type TiO₂ phase transition is 2.87 GPa.In the range of 0-14 GPa,the theoretically calculated pressure threshold for the UHP phase transition ofα-PbO₂-tye→baddeleyite-type TiO₂ is 10.08 GPa,which is inconsistent with the experimental values.This may be related to the introduction of thermodynamics and quantum mechanics approximation.The method is used in simulation calculations.