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Physical Properties of （Mg, Fe） SiO3 and Al2O3 at High Temperatures and High Pressures

Author: HeLin
Tutor: JingFuQian
School: Southwest Jiaotong University
Course: Proceedings of the
Keywords: (Mg0.92, Fe0.08) SiO3 perovskite phase High-temperature high-pressure phase transition Hügoniot speed of sound experiment Lower mantle First-principles calculations Sapphire Energy gap Conductivity Reduced optical transparency The light absorption coefficient Oxygen and aluminum vacancies point defects MgSiO3 perovskite phase D layer of the lower mantle
CLC: O521
Type: PhD thesis
Year: 2008
Downloads: 250
Quote: 0
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Abstract


The structure of the solid material under high pressure to change will have an impact on the mechanical, electrical, and optical properties. To study the variation of these high-pressure phase transition and its physical properties, will promote a better understanding of the laws of nature. The content of this thesis consists of two main parts. The first part is based on shock compression techniques to study means of measurement (Mg 0.92 Fe 0.08 ) SiO 3 enstatite pyroxene (lower mantle in a major candidate component) the sample of Hügoniot the speed of sound start the to combine it Hügoniot equation of state data, explore the temperature and pressure environment of the lower mantle is about 1700-2300 km depth area (Mg 0.92 Fe 0.08 ) the SiO 3 thermoelastic orthorhombic perovskite phase and the possible existence of the phase transition, which under the mantle of central seismic waves reveal present in the low-velocity zone causes and build geophysical and geochemical models of the lower mantle has important significance. The second part is based on first-principles calculation under high pressure to study the structure of the sapphire phase change its energy gap (forbidden band width) and light absorption, as well as by human preset oxygen or aluminum vacancies in the crystal of sapphire ideal explore the vacancy point defects in its light absorbent, the purpose of these studies is to explore the sapphire impact experiments observed the conductivity sudden increase in optical transparency to reduce the phenomenon of the physical mechanism. In addition, according to the Al 2 O 3 CaIrO 3 phase MgSiO 3 post-perovskite phase with structural study MgSiO 3 back from the perovskite structure of perovskite structure transition energy gap changes, the results reveal geophysical observations on decadal time scales Earth Day length change of physical mechanism is of great significance. The main results are as follows: 1) stage light gas gun as a means of loading optical analysis techniques in three pressure points (about 60-90 GPa pressure range) supplementary measurement (Mg 0.92 < / sub> and Fe 0.08 ) SiO 3 enstatite pyroxene samples Hügoniot longitudinal wave velocity, the experimental data processing using the enstatite samples ~ 40-140GPa the pressure range Hügoniot equation of state parameter [Geophys.Res. Lett., 3 (2004) L04616]. At the same time, the use of of this new Hügoniot equation of state parameter also re-calculate the speed of sound data measured in the past five pressure points Hügoniot [Chin.Phys Lett, 16 (1999) 695]. Based on more than a total of eight the pressure points Hügoniot longitudinal wave velocity data, build a relationship the Hügoniot sound velocity and impact pressure. Found that: shock pressure of about 64 GPa at an amplitude of about 21% of the longitudinal wave velocity positive transition;, an amplitude of about 23% of the longitudinal wave velocity negative transitions at about 83 GPa. 2) Further analysis showed that the first sound velocity discontinuity can be attributed to (Mg 0.92 the and Fe 0.08 ) SiO 3 from enstatite pyroxene phase (low-pressure phase) to orthorhombic perovskite phase (high pressure phase) of the phase transition. This results with the previous number of enstatite samples Hügoniot line analysis of the measurement results are consistent. Second sound velocity discontinuity may be derived from (Mg 0.92 Fe 0.08 ) SiO 3 perovskite phase from the orthorhombic structure to the tetragonal structure phase accompanied in varying the strength of the materials caused by oxygen atom sublattice melting softening. The intensity of such material softening is the first time we observed from the shock wave experiments. In addition, due to the pressure of the softening zone is generally in line with the observation of seismology to the lower mantle depth range of about 1700 km to 2300 km of seismic waves at low speed with the position in which the pressure environment, so the strength softening phase transition may be the seismic low-velocity zone one of the main causes. 3) based on first-principles plane wave pseudopotential method under the framework of the density functional theory, combined with the local density approximation (LDA), calculated the Al 2 O 3 three ideal crystal structure (corundum phase, the Rh 2 O 3 (II) phase and CaIrO 3 phase) at 220 GPa pressure range within the electronic band structure of three structural phase energy gap varies with the pressure relationship. The results show that, (i) from the corundum opposing Rh 2 O 3 (II) the phase transition when its energy gap is reduced by approximately 7-8%, from Rh 2 O 3 (Ⅱ) opposite CaIrO 3 phase transition when the energy gap of about 18-20% lower; (ii) in CaIrO 3 phase region, the energy gap decreases with pressure very slow, but in the corundum and Rh 2 O 3 (II) phase region of the bandgap with pressure was increased rapidly. Further analysis indicated that the first structural phase transition behavior of the conductivity associated with the sudden increase in support speculation Lin et al [Nat. Mater, 3 (2004) 389]; the second structural changes accompanying the electrical conductivity of the sudden increase in behavior can be explained qualitatively sapphire resistivity Weir and others by the impact of the experimental observations to dump phenomenon [J. Appl. Phys. , 80 (1996) 1522]. 4) The above calculation program, studied Al 2 O 3 ideal crystal light absorbing in the 220 GPa pressure range. The results showed that the pressure region, and in -250-1000 nm light wavelength range, Al 2 O 3 of the light absorption coefficient is zero, i.e. in the pressure phase transition region with the shock compression experiments observed the sapphire optical transparency decreases phenomenon with its structure independent of (an atomic scale nature), this result does not support Lin et al. [Nat. Mater. , 3 (2004) 389] and Oganov and other people [PNAS, 102 (2005) 10828] raised speculation. On the other hand, the results indirectly support Hare et al [Phys. Rev. , B66 (2002) 014108] adiabatic shear band mechanism sapphire light (a mesoscopic scale nature). In addition, we also used the same method to study under the generalized gradient approximation (GGA) 131.2 GPa in Al 2 O 3 ideal crystal containing electrically neutral oxygen and aluminum vacancy point defects or charged when the light absorbability. The results show that, except 3 - charged aluminum vacancy, other types of vacancy point defects are induced in the visible range of the non-uniform light absorption, but by Zhang Dai Yu et al. -130 GPa and at -633 nm measured light absorption 2 charged oxygen vacancy point defects induced under compression caused by a non-uniform light in the visible range, absorption may be one of the reasons leading to the decrease of its optical transparency. This results partly support the speculation of Weir et al [J. Appl. Phys. , 80 (1996) 1522]. 5) Based on first-principles under the framework of the density functional theory (DFT) plane wave pseudopotential method, combined with the local density approximation (LDA), calculated MgSiO 3 ideal crystal structure of two-phase (perovskite and post-perovskite phase) in 40-131.4 GPa pressure range of the electronic band structure of the two structures bandgap phase with the pressure changes in the relationship. The results showed: (i) within a pressure in the range of 83.7-131.4 GPa, after the perovskite phase in the energy gap than the perovskite phase gap approximately Low 21-27%; (ii) in the post-perovskite ore phase region, the energy gap increases with pressure weak decreases with increasing pressure, but the energy gap in the perovskite phase zone and significantly increases. Observed experimentally MgSiO 3 perovskite to post-perovskite phase transition occurs in the lower mantle D \, Science, 304 (2004) 855]. can be estimated based on solid theoretical conductivity of the phase change induced by the temperature and pressure conditions of the lower mantle D \the g / (2k B T) -6.81 (where σ the E g, the k B and T are conductivity, the energy gap, the Boltzmann's constant and temperature). Data comparison of this data with the sapphire, and combined with a measurement result of sapphire, MgSiO 3 after the perovskite phase conductivity than the perovskite phase, high conductivity of one order of magnitude conclusions . According to the results of the conductivity of the perovskite phase may be inferred, the perovskite phase has a high conductivity. This result confirms the speculation of Ono et al, and explore the physical mechanism of geophysical observations to Earth Day length change on decadal time scales has important significance [Ono et al., Earth and Planet. Sci. Lett. , 246 (2006) 326].

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