摘要
高压下固体物质的结构改变会对它的力学、电学以及光学等性质产生影响。研究这些高压相变及其物性变化特征，将促进人们对自然规律有更深入的认识。本学位论文内容主要包括两个部分。第一部分是以冲击压缩技术为研究手段，从测量(Mg_(0.92)，Fe_(0.08))SiO_3顽火辉石(下地幔中的一种主要候选组分)样品的Hügoniot声速入手，结合它的Hügoniot状态方程数据，探索了在下地幔大约1700-2300 km深度区的温压环境下(Mg_(0.92)，Fe_(0.08))SiO_3斜方晶系钙钛矿相的热弹性及可能存在的相变现象，这对揭示存在于下地幔中部的地震波低速带的起因及构建下地幔的地球物理和地球化学模型具有重要的意义。第二部分是基于高压下的第一性原理计算，研究了蓝宝石的结构相变对其能隙(禁带宽度)和光吸收性的影响，以及通过在蓝宝石理想晶体中人为预置氧或铝空位，探讨了空位点缺陷对其光吸收性的影响，这些研究的目的是探索在冲击实验中观测到的蓝宝石电导率突增以及光学透明性降低现象的物理机理。另外，根据Al_2O_3 CaIrO_3相与MgSiO_3后钙钛矿相的同构性，也研究了MgSiO_3从钙钛矿结构向后钙钛矿结构转变时对其能隙所产生的变化，该结果对揭示在十年时间尺度上地球物理观测到的地球日长度变化的物理机理具有重要意义。
本文的主要结果如下：
1)以二级轻气炮作为加载手段，用光分析技术在三个压力点(大约在60-90 GPa的压力范围内)上补充测量了(Mg_(0.92)，Fe_(0.08))SiO_3顽火辉石样品的Hügoniot纵波声速，实验数据处理中采用了该顽火辉石样品在～40-140GPa压力范围的新的Hügoniot状态方程参数[Geophys.Res．Lett.,3(2004)L04616]。同时，利用这个新的Hügoniot状态方程参数还重新计算了过去测量的五个压力点的Hügoniot声速数据[Chin.Phys. Lett.,16(1999)695]。基于以上总共八个压力点的Hügoniot纵波声速数据，构建了Hügoniot声速和冲击压力的关系。结果发现：在冲击压力约为64 GPa处，出现了一个幅度约为21％的纵波声速正跃变；在约83 GPa处，出现了一个幅度约为23％的纵波声速负跃变。
2)进一步的分析表明，第一个声速间断可归功于(Mg_(0.92)，Fe_(0.08))SiO_3从顽火辉石相(低压相)到斜方晶系钙钛矿相(高压相)的相变。此结果与先前多位作者对顽火辉石样品的Hügoniot线测量的分析结果相一致。第二个声速间断可能源于(Mg_(0.92)，Fe_(0.08))SiO_3钙钛矿相从斜方结构到正方结构相变时伴随的氧原子亚晶格熔化所导致的材料强度软化。此种材料强度软化现象是我们首次从冲击波实验中观测到的。另外，由于该软化区的压力大体上与地震学观测到在下地幔约1700 km到2300 km深度范围的地震波低速带位置所处的压力环境一致，所以这个强度软化相变可能是该地震波低速带形成的一个主要起因。
3)基于密度泛函理论框架下的第一性原理平面波超软赝势方法，结合局域密度近似(LDA)，计算了Al_2O_3理想晶体的三个结构相(corundum相、Rh_2O_3(Ⅱ)相及CaIrO_3相)在220 GPa压力范围内的电子能带结构,给出了三个结构相的能隙随压力变化关系。结果表明，(i)在从corundum相向Rh_2O_3(Ⅱ)相转变时其能隙减小约7-8％，在从Rh_2O_3(Ⅱ)相向CaIrO_3相转变时其能隙降低约18-20％；(ii)在CaIrO_3相区，能隙随压力减小很缓慢，但在corundum和Rh_2O_3(Ⅱ)相区的能隙随压力则呈快速增加。进一步分析表明，第一个结构相变伴随的电导率突增行为支持Lin等人的猜测[Nat．Mater.,3(2004)389]；第二个结构转变中伴随的电导率突增行为可以定性地解释Weir等人通过冲击实验观测到的蓝宝石电阻率突降现象[J．Appl．Phys．，80(1996)1522]。
4)采用上述计算方案，研究了在220 GPa压力范围内Al_2O_3理想晶体的光吸收性。结果表明，该压力区内以及在-250-1000 nm的光波段范围内，Al_2O_3的光吸收系数均为零，即在该压力区内用冲击压缩实验观测到的蓝宝石光学透明性降低现象与它的结构相变(一种原子尺度的性质)无关，这个结果不支持Lin等人[Nat．Mater．，3(2004)389]和Oganov等人[PNAS，102(2005)10828]提出的猜测。但从另一方面看，该结果间接支持Hare等人[Phys．Rev．，B66(2002)014108]提出的蓝宝石发光的绝热剪切带机制(一种介观尺度的性质)。另外，本文还用同样方法但在广义梯度近似(GGA)下，研究了131．2 GPa处在Al_2O_3理想晶体中含有电中性或带电的氧和铝空位点缺陷时的光吸收性。结果表明，除3-带电铝空位外，其它类型的空位点缺陷都在可见光范围内诱导了非均匀的光吸收，但与张岱宇等人在-130 GPa和-633 nm处测量得到的光吸收系数比较[人工晶体学报，36(2007)531]，仅有2+带电氧空位在该波长处的计算数据与测量值相近(其它空位的计算数据与测量结果相差很大)，即，蓝宝石在冲击压缩下所诱导的2+带电氧空位点缺陷在可见光范围内引起的非均匀光吸收可能是导致其光学透明性降低的一个原因。此结果部分地支持Weir等人的猜测[J．Appl．Phys．，80 (1996)1522]。
5)基于密度泛函理论框架下的第一性原理平面波超软赝势方法，结合局域密度近似(LDA)，本文还计算了MgSiO_3理想晶体的两个结构相(钙钛矿和后钙钛矿相)在40-131.4 GPa压力范围内的电子能带结构，并得到了该两个结构相的能隙随压力变化关系。结果发现：(i)在83．7-131．4 GPa的压力范围内，后钙钛矿相的能隙比钙钛矿相的能隙大约低21-27％；(ii)在后钙钛矿相区，能隙随压力增大而微弱地减小，但在钙钛矿相区的能隙随压力增大而明显地增加。实验观测到的MgSiO_3从钙钛矿到后钙钛矿的相变发生在下地幔D”层的温压条件下(-125 GPa和-2500 K)[Murakami et al．，Science，304(2004)855]。根据固体理论，可以估算出在下地幔D”层的温压条件下该相变诱导的能隙降低所引起的电导率突增值为：△lnσ=-△E_g/(2k_BT)-6.81(其中,σ、E_g、k_B和T分别表示电导率、能隙、玻尔兹曼常数和温度)。把这个数据与蓝宝石的计算数据比较，并结合蓝宝石的测量结果，得出MgSiO_3后钙钛矿相的电导率比其钙钛矿相的电导率高一个数量级的结论。根据钙钛矿相的电导率结果，可以推断后钙钛矿相具有高的电导率。这个结果证实了Ono等人的猜测，并对探索在十年时间尺度上地球物理观测到的地球日长度变化的物理机理具有重要意义[Ono etal.,Earth and Planet．Sci．Lett．，246(2006)326]。
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))SiO_3 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 Al_2O_3 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 Al_2O_3,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 Al_2O_3 CaIrO_3 and MgSiO_3 post-perovskite,the variation in band gap,induced by the perovskite to post-povskite transition in MgSiO_3,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_(0.08)SiO_3 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 Al_2O_3 (corundum,Rh_2O_3(Ⅱ) and CaIrO_3) 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-Rh_2O_3(Ⅱ) transition causes a 7-8% band-gap reduction,and the Rh_2O_3(Ⅱ)-CaIrO_3 transition yields an 18-20% band-gap reduction;(ii) the band gap decreases slightly with pressure in the CaIrO_3 phase region but increases quickly in corundum and Rh_2O_3(Ⅱ) phase regions.Further analyses indicate that the behavior of an increase in electrical conductivity due to the corundum-Rh_2O_3(Ⅱ) 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 Rh_2O_3(Ⅱ)-CaIrO_3 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 Al_2O_3 are studied to 220 GPa.Results show that in this pressure range the optical absorption coefficients of Al_2O_3 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 A1_2O_3 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 MgSiO_3 (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 MgSiO_3 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 (△lnσ),produced by the band-gap reduction (△E_g) due to the perovskite to post-perovskite transition in MgSiO_3 at pressure-temperature conditions of Earth's D" layer,may be estimated through a relationship:△lnσ=-△E_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 MgSiO_3 post-perovskite should be one order of magnitude higher than that of MgSiO_3 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].
引文
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