Physical Properties Of (Mg, Fe) SiO₃ And Al₂O₃ At High Temperatures And High Pressures

The structural change in solid materials under high pressures would affect their elastic,electrical and optical properties.To promote our understanding for the natural law,it is of importance to study these high-pressure phase transitions and corresponding variations in physical properties.Two parts are included in this thesis.The first part is to measure Hügoniot sound velocities for the sample of (Mg_0.92,Fe_0.08)SiO₃ enstatite using shock compression technique,so as to explore the thermo-elasticity and the possible phase transitions in Pbnm-pervoskite occurred at the temperature-pressure conditions relevant to -1700-2300 km depths in the Earth's lower mantle.This could have profound implications for probing into the origin of low seismic velocity anomaly,observed seismically in the middle part of the Earth's lower mantle,and constraining the geophysical and geochemical models for the Earth's lower mantle.In the second part,first-principles calculations are used to study the effects of the high-pressure structural phase transitions in Al₂O₃ on its band gap (the width of the forbidden band) and optical absorption as well as the influences of the vacancy point-defects on its optical absorption under high pressures through artificially making oxygen & aluminum vacancies in perfect Al₂O₃,in order to explore the physical mechanisms responsible for the transparency loss and the abrupt increase in electrical conductivity,observed during shock compression.In addition,based on the structural analogue between Al₂O₃ CaIrO₃ and MgSiO₃ post-perovskite,the variation in band gap,induced by the perovskite to post-povskite transition in MgSiO₃,is also studied by the first-principles calculations for revealing the mechanisms of the observed change in the length of Earth's day on a decadal timescale.The main results are as follows: 1) By using two-stage light-gas gun as shock loading device and optical analyzer technique as diagnostic means,three supplementary shots on Hügoniot sound velocity measurement for the samples of (Mg_0.92, Fe_0.08SiO₃ enstatite are performed,ranging from -60 to -90 GPa.The newly published Hiigoniot equation of state (EOS) parameters of the same enstatite [Geophys.Res.Lett.,3(2004) L04616] within about 40-140 GPa are used in data processing. We also utilized these new Hugoniot parameters to retreat the previous five shots' data [Chin.Phys.Lett., 16(1999)695].We use above-mentioned eight data points to draw a Hugoniot sound velocity vs. shock pressure plot. Interestingly,this demonstrates that a -21% sudden increase in sound velocity occurs at -64 GPa and a -23% sudden decrease in sound velocity appears at -83 GPa.2) Further analyses show that the first sound discontinuity may be attributed to the phase transition from enstatite to Pbnm-perovskite (this result is consistent with that obtained from Hugoniot equation of state measurements by some others),while the second one is likely caused by a Pbnm-perovskite to tetragonal-perovskite transition, accompanied by material strength softening due to the melting of oxygen sublattices.This strength softening evidence is obtained first from our shock wave experiments.In addition,because the pressures of the softening region are roughly in according with those of the seismically observed low sound velocity anomaly,located in -1700-2300 km depths of the Earth's lower mantle,this strength softening phase transition might be a main origin that creates the low seismic velocity domain.3) Based on the plane-wave ultra-soft pseudopotential methods in the frame-work of the density function theory and the local density approximation,the electronic energy-band structures for three structural phases of perfect Al₂O₃ (corundum,Rh₂O₃(â…¡) and CaIrO₃) up to 220 GPa are calculated,and the pressure dependence of the band gap for three structural phases is obtained.The key results are:(i) the corundum-Rh₂O₃(â…¡) transition causes a 7-8% band-gap reduction,and the Rh₂O₃(â…¡)-CaIrO₃ transition yields an 18-20% band-gap reduction;(ii) the band gap decreases slightly with pressure in the CaIrO₃ phase region but increases quickly in corundum and Rh₂O₃(â…¡) phase regions.Further analyses indicate that the behavior of an increase in electrical conductivity due to the corundum-Rh₂O₃(â…¡) transition supports the conjecture proposed by Lin et al.[Nat.Mater.,3(2004)389],while the behavior of an increase in electrical conductivity due to the Rh₂O₃(â…¡)-CaIrO₃ transition could qualitatively explain the shock-induced decrease in resistivity observed by Weir et al.[J.Appl.Phys.,80(1996)1522].4) Using above-mentioned calculation schemes,the optical absorption in perfect Al₂O₃ are studied to 220 GPa.Results show that in this pressure range the optical absorption coefficients of Al₂O₃ are zero within the wavelength range of -250-1000 nm.The two high-pressure structural phase transitions in alumina (a property in atomic scale) might not be responsible for its optical transparency degradation observed by shock experiments in the above-mentioned pressure range.These results do not support Lin et al.'s and Oganov et al.'s conjectures [Nat.Mater., 3(2004)389;PNAS,102(2005)10828].On the other hand,these results give an indirect support for the adiabatic shear banding mechanism (a property in meso-scale) proposed by Hare et al.[Phys.Rev., B66(2002)014108].In addition,using the same method but adopting the generalized gradient approximation,the optical absorption of A1₂O₃ with the neutral or charged oxygen and aluminum vacancies at 131.2 GPa are investigated.Results indicate that the obvious heterogeneous optical absorption, induced by above various vacancies except -3 charged aluminum vacancy,appears within the visible-light region.However,in comparison with the measured optical absorption coefficient at -130 GPa and -633 nm [Zhang et al,J.Synthetic Crystal,36(2007)531],it is indicated that the calculated datum only for 2+ charge oxygen vacancy at this wavelength is similar to the measured value (the calculated data for other vacancies are far from the measured result),i.e.,the heterogeneous absorption in the visible-light region,induced by the shock-produced +2 charge oxygen vacancy,would be a possible origin of the optical transparency loss.This result supports partly Weir et al.'s conjecture [J. Appl.Phys.,80(1996)1522].5) Using the plane-wave ultra-soft pseudopotential method in the frame-work of the density function theory and the local density approximation,the electronic energy-band structures for two strucrural phases of perfect MgSiO₃ (perovskite and post-povskite) within 40-131.4 GPa are calculated,and the pressure dependences of the band gap for these structural phases are obtained.It is found that:(i) the band gaps of post-perovskite are -21-27% lower than those of perovskite at 83.7-131.4 GPa;(ii) the band gap decreases slightly with pressure in the post-perovskite phase region but evidently increases in the perovskite phase region.Experimental study indicated that a perovskite to post-povskite transition in MgSiO₃ occurs at pressure-temperature conditions of Earth's D" layer (-125 GPa and -2500 K) [Murakami et al, Science,304(2004)855].According to the solid theory,the conductivity increase (â–³lnσ),produced by the band-gap reduction (â–³E_g) due to the perovskite to post-perovskite transition in MgSiO₃ at pressure-temperature conditions of Earth's D" layer,may be estimated through a relationship:â–³lnσ=-â–³E_g/(2k_BT)-6.81 (k_B and T represent Boltzmann constant and temperature,respectively).If this datum is compared with the calculated result for sapphire,together with its measured data,we may conclude that the electrical conductivity of MgSiO₃ post-perovskite should be one order of magnitude higher than that of MgSiO₃ perovskite.According to the estimated data of the electrical conductivity of perovskite,we may judge that post-perovskite has high conductivity.This confirms Ono et al.'s conjecture and has important implications for exploring the physical mechanisms of the observed change in the length of Earth's day on a decadal timescale [Ono et al,Earth and Planet.Sci.Lett.,246(2006)326].