高压多晶X 射线衍射方法的发展及其在地球深部研究中的应用

Research

Deep Matter &Energy—Review

Development of High-Pressure Multigrain X-Ray Diffraction for Exploring the Earth’s

Interior

Li Zhang a ,?,Hongsheng Yuan a ,Yue Meng b ,Ho-Kwang Mao a ,c

a

Center for High Pressure Science and Technology Advanced Research (HPSTAR),Shanghai 201203,China b

HPCAT,X-Ray Science Division,Argonne National Laboratory,Argonne,IL 60439,USA c

Geophysical Laboratory,Carnegie Institution of Washington,Washington,DC 20015,USA

a r t i c l e i n f o Article history:

Received 3September 2018Revised 16January 2019Accepted 1February 2019Available online 6April 2019Keywords:High pressure

Synchrotron X-ray Multigrain

Diamond anvil cell Minerals Petrology

Earth’s interior

a b s t r a c t

The lower mantle makes up more than a half of our planet’s volume.Mineralogical and petrological experiments on realistic bulk compositions under high pressure–temperature (P –T )conditions are essen-tial for understanding deep mantle processes.Such high P –T experiments are commonly conducted in a laser-heated diamond anvil cell,producing a multiphase assemblage consisting of 100nm to submicron crystallite grains.The structures of these lower mantle phases often cannot be preserved upon pressure quenching;thus,in situ characterization is needed.The X-ray diffraction (XRD)pattern of such a multi-phase assemblage usually displays a mixture of diffraction spots and rings as a result of the coarse grain size relative to the small X-ray beam size (3–5l m)available at the synchrotron facilities.Severe peak overlapping from multiple phases renders the powder XRD method inadequate for indexing new phases and minor phases.Consequently,structure determination of new phases in a high P –T multiphase assemblage has been extremely dif?cult using conventional XRD techniques.Our recent development of multigrain XRD in high-pressure research has enabled the indexation of hundreds of individual crystallite grains simultaneously through the determination of crystallographic orientations for these individual grains.Once indexation is achieved,each grain can be treated as a single crystal.The combined crystallographic information from individual grains can be used to determine the crystal structures of new phases and minor phases simultaneously in a multiphase system.With this new development,we have opened up a new area of crystallography under the high P –T conditions of the deep lower mantle.This paper explains key challenges in studying multiphase systems and demonstrates the unique capabilities of high-pressure multigrain XRD through successful examples of its applications.

ó2019THE AUTHORS.Published by Elsevier LTD on behalf of Chinese Academy of Engineering and Higher Education Press Limited Company.This is an open access article under the CC BY-NC-ND license

(https://www.360docs.net/doc/f518283883.html,/licenses/by-nc-nd/4.0/).

1.Introduction

Various high pressure–temperature (P –T )experimental tech-niques have been developed in order to study mantle minerals existing in the Earth’s interior.Under increasing P –T conditions that correspond to those of the deep Earth,the crystal structures of minerals that are stable under low P –T conditions are replaced by denser structures.The pyrolitic composition of the lower man-tle has been tested using high P –T experiments [1,2].At least ?ve oxide components are required to describe the mantle system.Even in a single bulk composition (e.g.,mid-ocean ridge basalt

(MORB)or pyrolite),phase changes are associated with element partitioning within the assemblage under high P –T conditions.Interactions of subducting slabs with the mantle system further complicate their phase relations [3,4].For example,some amounts of water may be stored in nominally anhydrous lower mantle phases,but the presence of water components in subducting slabs may also stabilize hydrous phases under P –T conditions.Petrolo-gical experiments have been widely conducted in a multi-anvil apparatus up to the P –T range of the top of the lower mantle (<30GPa)[5],and the P –T range can be extended to the middle of the lower mantle ($50GPa)when sintered diamond anvils are used [2].

The ambition to explore the whole interior of the Earth has pro-moted extensive use of the laser-heated diamond anvil cell (DAC).Diamond is the hardest known material and is transparent to

?Corresponding author.

E-mail address:zhangli@https://www.360docs.net/doc/f518283883.html, (L.Zhang).

X-rays.Typically,a sample assembly less than50l m in diameter and about10l m in thickness is compressed between the tips of two diamond anvils and laser-heated from both sides of the sample[6].Megabar pressure was?rst achieved40years ago, allowing the simulation of deep-Earth P–T conditions[7].Through development of the DAC and laser-heating techniques,experiments have been conducted to study the structure of iron(Fe)under the inner-core P–T conditions of up to377GPa and5700K[8].These experiments have mostly been limited to a single mineral phase or a simple system(e.g.,MgO–SiO2).However,interpretations of key enigmatic features in the deep lower mantle—namely,the sharp upper boundary of the D00layer[9],the seismic anisotropy [10],and the large low-shear-velocity provinces in the bottom third of the lower mantle[11]—require information from high P–T petrological experiments on a realistic lower mantle multicomponent system,which produces an assemblage consisting of multiple phases under high P–T conditions in a laser-heated DAC.

In a multiphase assemblage,most diffraction peaks overlap in a powder X-ray diffraction(XRD)pattern;even without overlap, powder XRD alone is inadequate for studying new structures. Single-crystal XRD is regarded as the ideal method for structural analysis,and has been widely developed for high-pressure research in a DAC[12–14].However,single crystals of around 5m m in size,as required for conventional single-crystal XRD experi-ments,are unachievable in a realistic mantle assemblage under the corresponding high P–T conditions.Instead,a polycrystalline multiphase sample is always obtained in phase equilibrium experi-ments corresponding to the deep mantle.In this paper,we review the applications of the multigrain XRD technique for studying multiphase systems under high P–T conditions,including in situ characterization of new phases in a multiphase assemblage,crystal structure determination of selected individual grains in a polycrys-talline sample,and two-dimensional(2D)mapping through the precise determination of unit-cell parameters for individual phases at steps of a few microns.

2.Challenges in experimental petrology under high P–T conditions

Phase identi?cation and structural analysis in a multiphase assemblage have long been recognized as a challenge in high-pressure research.Frost and Fei[15]pointed out dif?culties in identifying the hydrous phases D,F and G,of dense hydrous mag-nesium silicates from powder XRD patterns of recovered samples when the synthesis pressure was as low as22GPa.However,if such a multiphase assemblage contains individual crystals with grain sizes greater than5l m and the structure can be preserved under ambient conditions,these grains can be picked out for single-crystal studies in the recovered sample.

Higher pressure is generated in an apparatus at the expense of sample volume.The sample volume decreases from a few milli-meters in dimension in a multi-anvil apparatus to tens of micrometers at above60GPa in a DAC.Furthermore,most structures obtained at ultrahigh pressures,such as MgSiO3or(Mg,Fe)SiO3post-perovskite (pPv),cannot be preserved under ambient conditions[16–18]. Meanwhile,the grain size in a polycrystalline sample generally decreases with increasing pressure.For example,the grain size of a hydrous phase in a mantle assemblage could be as small as about 100nm under the high P–T conditions of the deep lower mantle [4].The grain size of the hydrous d-phase decreases from about 2l m at68GPa to about200nm at128GPa after heating treat-ment at a similar temperature[4],indicating that pressure has a dominant effect on grain size.Other factors may also in?uence grain size;for example,lower temperature tends to decrease grain size[19].The recovery of tiny crystals(<1l m)for single-crystal diffraction thus becomes practically infeasible.In conclusion, in situ characterization is needed in order to study phase changes and structural details in a multiphase system under the P–T condi-tions of the deep lower mantle.

High P–T conditions produce a wealth of new structures.In a high P–T experiment,a few new peaks often appear in a powder XRD pattern;however,a perfect match to a known structure is not easy to?nd.One type of new structure is due to atom rear-rangement into a denser structure,such as the B1–B2structural transition in NaCl.Another type involves new pressure-induced stoichiometry,such as NaCl3and Na3Cl[20].In situ structure deter-mination of a high-pressure phase can be very challenging when the chemical composition is not known.In a petrological experi-ment,phase transitions are often associated with compositional changes under high P–T conditions.For example,in a high P–T experiment on a natural MORB composition,a four-phase assemblage of the MgSiO3-rich pPv phase,a-PbO2-type SiO2, Ca-perovskite,and CaFe2O4-type Al-phase was identi?ed by a com-bination of in situ powder XRD and ex situ transmission electron microscopy(TEM)under the P–T conditions of the deep lower mantle,in which the pPv phase has a high Na2O content and a high ratio of Fe3+/total Fe[21].Solving the complex structures of the pPv phase under realistic deep lower mantle conditions is beyond the capabilities of existing powder XRD methods.Furthermore,a minor phase will be overshadowed by the diffraction peaks of coexisting phases in a powder XRD pattern of a multiphase assem-blage,making phase identi?cation of a minor phase extremely dif-?cult using the powder XRD technique.

In summary,conventional single-crystal or powder diffraction techniques are inadequate for studying multiphase systems under high P–T conditions.New methods are required in order to explore phase changes and their detailed structures occurring in the deep lower mantle.

3.In situ multigrain synchrotron XRD

Synchrotron radiation X-rays have been regarded as an ideal probe for in situ studies of materials under high P–T conditions in a laser-heated DAC[22–24].Heat treatment often promotes crystal growth,and spotty powder XRD patterns of mantle phases are obtained under high P–T conditions.Fig.1shows a representative spotty powder XRD pattern of(Mg,Fe)SiO3pPv[25].Such a spotty pattern is a combined result of the small X-ray beam size ($6?8l m2full width at half maximum)and crystal growth upon heating at about2400K.The recent development of high-pressure beamlines has enabled reduction of the X-ray beam to the current 2–3l m[22],which can help to reduce the level of overlapping spots in powder XRD patterns.

The recent achievement of multigrain XRD in high-pressure research has made it possible to index hundreds of individual crys-tallite grains simultaneously through the determination of crystal-lographic orientations for these individual grains[25,26].Once indexation is achieved,a multiphase assemblage can then be trea-ted as an aggregate of thousands of individual single crystals.An accurate and ef?cient indexing algorithm is required to process such datasets.We have found that GrainSpotter[27],which is part of the FABLE package[28],is an ef?cient algorithm that can be readily applied to a rotation-scan dataset for indexing grains in an assemblage contained in a DAC[25,29,30].In some cases,experi-enced crystallographers can index a few grains manually in a high-pressure rotation dataset and solve their crystal structures[6,31]. However,only by applying the multigrain XRD method to search and index tens to hundreds of grains for each individual phase simultaneously,it is possible to systematically study crystal struc-tures of each individual phase including a minor phase.

442L.Zhang et al./Engineering5(2019)441–447

Fig.2shows the setup for high-pressure multigrain XRD data collection.Similar to the ‘‘rotation method”used in conventional single-crystal crystallography,diffraction patterns in step-scans are collected on a rotation stage up to the maximum angular access allowed by the DAC design.We have achieved a good balance between an angular access of 50°–60°for X-rays and the mainte-nance of stable high pressures up to the core-mantle boundary (CMB)at 136GPa.We used angular steps as small as 0.2°to improve the signal-to-noise ratio in XRD patterns.We used an exposure duration ranging from 1to 10s per frame in our experi-ments,and tested the exposure before each collection until the strongest diffraction spots were near intensity saturation.Software packages,including CrysAlis [32],XDS [33],and GSE_ADA/RSV [34],were used to calculate the intensities of the re?ections in a high-pressure dataset.The structure solution and re?nement steps were performed by conventional single-crystal programs,such as,the SHELX package [35].

The synthesis of high-quality individual crystallites within the assemblage is the prerequisite for applying the high-pressure multigrain XRD technique.It has been found that a quasi-hydrostatic sample environment and stable heating signi?cantly promote crystal growth under high P –T conditions in a laser-heated DAC [29].In mantle petrological experiments,phase chem-istry is frozen after temperature quenching;most structures are temperature-quenchable but often not pressure-quenchable,so a multigrain sample is often indexed in situ at high pressure and after temperature quenching.

Spotty diffraction patterns enable multigrain indexation,but such spottiness can result in unreliable intensity measurements in an integrated powder XRD pattern,leading to unreliable estima-tion of phase proportions in an assemblage.Spottiness implies crystal growth in the assemblage,in contrast to the smooth rings that are displayed in an XRD pattern of a ?ne-grained sample.In a sample synthesis,the high-temperature conditions required for crystal growth should facilitate thermodynamic equilibrium.On the other hand,the kinetics in phase transitions in a coarse-grained starting sample can be much slower than that occurring in a ?ne-grained sample under similar P –T conditions,because the higher surface area in a ?ne-grained sample can facilitate the nucleation for a phase transformation.Spotty XRD patterns are commonly observed in high P –T experiments conducted in a laser-heated DAC,and more attention should be paid to the kine-tics issues in phase transitions occurring in coarse-grained samples.4.Applications of multigrain XRD in high-pressure experiments Multigrain XRD enables the phase identi?cation and structural analysis of tiny individual grains in a multiphase assemblage con-tained in a DAC.Here,we summarize the applications of in situ multigrain synchrotron XRD for the phase identi?cation of unknown phases and minor phases,the determination of high-pressure crystal structures,and the re?nement of unit-cell para-meters by merging re?ections from multiple grains for each individual phase.

4.1.Phase identi?cation in a multiphase assemblage

The unique capability of multigrain XRD has been demonstrated for the phase identi?cation of an unknown phase in a mixture.When Fe-bearing bridgmanite (Bdg)decomposes to a nearly Fe-free MgSiO 3Bdg and a Fe-rich phase with a hexagonal structure (H-phase),only three peaks above 2.4?were speci?c to the H-phase,with many peaks of the Bdg phase overlapping with other peaks of the H-phase [29].By searching for the orientation matri-ces of individual grains,154grains of the H-phase were indexed from one rotation XRD dataset.A grain is indexed when tens to hundreds of re?ections are shown to satisfy the stringent geomet-rical angles determined by its particular orientation matrix.Each re?ection has three variables (x ,g ,and 2h )to constrain the orien-tation matrix,where x ,g ,and h represent the rotation,azimuth,and Bragg angles (Fig.2),respectively.Furthermore,154individual crystallites,each with its particular orientation matrix,redundantly con?rmed the same lattice.

Powder XRD patterns only contain d -spacing information,and successful indexing of several obvious peaks for a new phase can sometimes lead to an incorrect unit cell.For example,the recently discovered hexagonal hydrous phase (HH-phase)in (Fe,Al)OOH was examined using the multigrain XRD,and its lattice parameters of a =10.5803(6)?and c =2.5897(3)?were obtained at 110GPa [36].However,if the weak peaks of (211)and (311)in the powder XRD pattern were not selected for indexing,a small orthorhombic lattice was obtained instead,with the lattice parameters

of

Fig. 1.A representative spotty powder XRD pattern of (Mg,Fe)SiO 3pPv.The diffraction pattern was collected at a ?xed x angle at 120GPa after temperature quenching from 2400K,showing the spottiness of the XRD pattern.Ne:neon.Reproduced from Ref.[25]with permission of PNAS,ó

2013.

Fig.2.Experimental setup for high-pressure multigrain XRD data collection in a DAC.The sample is aligned to the rotation center,and a set of 2D XRD patterns are collected at small incremental steps,e.g.,0.2°,to the angular access of a DAC,typically 50°–60°,where x ,g ,and h represent the rotation,azimuth,and Bragg angles of each re?ection,respectively.

L.Zhang et al./Engineering 5(2019)441–447443

a =4.03(2)?,

b =3.699(5)?,and

c =2.78(1)?.However,this orthorhombic lattice di

d not pass th

e three-dimensional (3D)multigrain XRD test because most o

f the re?ections predicted from a speci?c orientation matrix were not found in the measured rota-tion dataset.It was only when the hexagonal lattice was input for searchin

g that 104re?ections were observed at the predicted angles for one of the crystallites and 27individual crystallites were found,eac

h with its particular orientation matrix,con?rming the correct lattice of the HH-phase [36].

The unique power of multigrain XRD has been further demon-strated by its capability to identify a minor phase in an assemblage.Most peaks of a minor phase are overshadowed by the diffraction of major phases in a powder XRD pattern.In a previous experi-ment,seifertite (SiO 2)was found to exist as a minor phase in a lower mantle assemblage;only peak (110)was visible without overlapping with the peaks from the major phase pPv [30],as shown in Fig.3(a).Still,over 100grains of seifertite were con-?rmed by applying multigrain XRD,although most grains showed

weak diffraction due to their tiny grain sizes.The proportion of a minor phase to a major phase can be roughly estimated by the relative intensities in the integrated powder diffraction peaks (Fig.3(a)).In a multigrain XRD dataset,sometimes only a small number of grains can be indexed for a minor phase.Nevertheless,a tiny grain from a minor phase can still be identi?ed and treated as a single crystal,which is otherwise impossible using conventional XRD techniques (Fig.3(b)).Thus,the development of high-pressure multigrain XRD enables these previously impossible tasks and drives forward laboratory exploration of the Earth’s entire interior.4.2.Crystal structure determination

The (Mg,Fe)SiO 3-pPv phase has been a focus of study since its discovery in 2004as a dominant mineral in the bottom of the lower mantle [16–18].Compositional variations in pPv can induce changes in its density and structure;thus,even subtle structural changes have signi?cant implications for interpretations of the seismic and dynamic features observed near the CMB.For example,the pPv phase synthesized in a MORB composition was enriched in Na 2O and had a high ferric content [21].On the other hand,the pPv phase may have become very Fe-enriched at the CMB in contact with the liquid iron core [18].Chemical composition in individual phases can be measured by ex situ TEM on a recovered sample;however,determination of structure changes induced by chemical impurities in the pPv phase is beyond the existing capabilities of powder XRD and theoretical methods.In addition,the pPv struc-ture cannot be preserved after recovery under ambient conditions.Thus,in situ crystal structure determination is required.A high-quality multigrain sample of (Mg,Fe)SiO 3-pPv was synthesized in a quasi-hydrostatic neon (Ne)environment (Fig.1),and in situ crystal structure determination was performed on an individual grain of pPv selected by the multigrain method [25].The structure of this Fe-depleted pPv shows a nearly identical structure to that of the MgSiO 3predicted by theory [16].To the best of our knowledge,this is the ?rst single-crystal structure determination of the pPv phase;it demonstrates the feasibility of in situ crystal structure determination of submicron crystallites above megabar pressures.This technique can then be applied to investigate the structure changes induced by compositional variations in high-pressure minerals in the future.

A DAC has a limited angular access;in a multigrain sample,several grains with different orientations can be scaled and merged to increase the coverage of a structure for the structure determination [30].Clearly,re?ection conditions combined from multiple grains can provide a more de?nite determination of the space group of a high-pressure phase.The intensities of the re?ections from each selected grain were calculated using the XDS software,and com-patibility between the data sets was checked by merging them using the XSCALE software in the XDS package [33].For example,a total of 613re?ections were combined from the merged datasets of six grains to provide a redundancy of 6.6and data completeness of 92%in the d -spacing range down to 0.72?,which allowed high-quality structure re?nement for seifertite at 129GPa [30].Note that CrysAlis and other softwares may also be used for integration and scaling purpose.In situ crystal structure determination enabled by high-pressure multigrain synchrotron XRD opens up a new area of megabar crystallography by providing crystallographic solutions that are otherwise extremely dif?cult to achieve using conventional XRD techniques [25,30].

4.3.Determination of unit-cell parameters in a multiphase assemblage The determination of unit-cell parameters for individual phases in an assemblage is a common task in mineralogical and petrolog-ical studies.To ensure a high-quality sample and reliable

pressure

Fig.3.XRD data of a phase assemblage including a dominant pPv phase and a minor phase of seifertite (SiO 2).(a)An integrated powder XRD pattern showing the weak proportion of SiO 2at 129GPa and after temperature quenching;(b)the multigrain XRD pattern including two Friedel pairs of {221}from one selected grain of seifertite.S:silica.Reproduced from Ref.[30]with permission of GeoScienceWorld,ó2016.

444L.Zhang et al./Engineering 5(2019)441–447

determination,a pressure medium and/or thermal insulator,a pressure calibrator,and sometimes a laser absorber are loaded together with the sample into a DAC chamber of only tens of micrometers in diameter.During high P–T treatment or after tem-perature quenching,most peaks from these materials overlap in a powder XRD pattern.Only a few peaks can be selected for the unit-cell determination of the sample or the pressure scale,which leads to large uncertainties.Fortunately,multigrain XRD makes it possi-ble to constrain the unit-cell parameters from many re?ections belonging to an individual grain.The3D orientation and geometri-cal relationships in the multigrain dataset allow the separation of re?ections with close https://www.360docs.net/doc/f518283883.html,ing seifertite as an example, the uncertainties in the unit-cell parameters were gradually reduced by increasing the number of re?ections merged from mul-tiple grains in the re?nement[30].For this reason,uncertainties of the unit-cell parameters from the multigrain dataset are mainly de?ned by the resolution of the monochromator within0.1%.

Conversely,precise unit-cell parameters can be used to evaluate subtle compositional effects on the mineral phases.Iron enters the crystal structures of all dominant lower mantle phases—silicate Bdg,pPv,and ferropericlase[37,38].Multigrain XRD was applied to calculate the unit-cell parameters of the coexisting pPv and H-phase in(Mg,Fe)SiO3at119GPa[39].Because the uncertainties of the unit-cell parameters were reduced by increasing the number of re?ections in the re?nement(>150re?ections),the re?ned unit-cell parameters were able to resolve small differences of the Fe content across the center area(20–30l m)of a laser-heated area, while the sample pressure remained unchanged in the Ne quasi-hydrostatic environment.The unit-cell volumes of the pPv phase decreased by only0.16%over a10l m distance from the heating center to a colder region,while the unit-cell volume of the H-phase decreased by0.54%over the same distance,indicating that both phases were more Fe-enriched in the slightly hotter center [39].In contrast to the Soret effect that drives heavy elements,such as Fe,from hot to cold regions,we conclude that the observed Fe distribution was caused by a temperature effect on the element partitioning.Thus,high-pressure multigrain XRD can be applied to calculate the unit-cell parameters of each individual phase precisely that allows2D mapping of element partitioning or phase distributions.

https://www.360docs.net/doc/f518283883.html,bination of in situ XRD and ex situ TEM techniques:An example

In high P–T petrological experiments,ex situ chemical analysis on recovered samples is required to connect compositional changes associated with structural transformations determined by in situ XRD.The focus ion beam(FIB)has been widely used to cut and lift out a thin cross-section in a laser-heated sample in a DAC[40].The thin section can then be analyzed in a TEM.Ex situ TEM analysis combining electron diffraction and energy dispersive spectrometer(EDS)chemical analysis can be used to verify the phases under ambient conditions.The consistency between in situ multigrain XRD and ex situ TEM analysis should be carefully checked.To ensure consistency,we always prepared a TEM speci-men from the heating center where in situ XRD data were mea-sured under high P–T.Meanwhile,2D XRD scans were performed to record the phase distribution under high P–T as well as after recovery to ambient conditions.Note that most high-pressure structures are unquenchable to ambient conditions,and the recov-ered sample can sometimes be contaminated in the sample prepa-ration for a TEM thin section.

Here,we provide an example to demonstrate the identi?cation of hydrous phases in a multiphase assemblage through the combi-nation of in situ synchrotron XRD and ex situ TEM.The identi?ca-tion of hydrous phases in realistic lower mantle compositions is essential in order to understand the deep-water storage.We used a hydrous gel as our starting material with a composition of 30mol%Al2O3–10mol%Fe2O3–60mol%MgSiO3containing about 7wt%H2O.As shown in Fig.4,the XRD pattern contains a domi-nant pPv phase and a hydrous d-phase,with only one weak peak indicating the presence of the cubic pyrite-type phase(py-phase).After multigrain indexing,we unambiguously con?rmed the py-phase by sorting out42grains with13–18re?ections con-sistent with the Pa-3space group.Fig.4(b)shows a composition-mapping image of a thin cross-section from the recovered sample prepared by FIB,showing several grains belonging to a very Fe-rich phase.EDS analysis veri?ed only the elements Fe and oxygen(O)in the grains,while hydrogen(H)is undetectable if present.The grains range from50to200nm in size.From the2D element map-ping,the volume proportion of the py-phase is estimated to be less than3vol%in the assemblage.The chemical composition of these Fe-rich grains and their volume proportion in the assemblage are consistent with the in situ XRD observation of the py-phase under the high P–T conditions.

In this experiment,we used a higher Al2O3content than realis-tic compositions in order to enhance the diffraction peaks of the d-phase in the XRD patterns.In general,volatile-bearing phases exist as minor phases in a mantle assemblage.Regardless,this example demonstrated that high-pressure multigrain XRD is capable of identifying individual tiny grains of a minor hydrous phase with less than3vol%in an assemblage through a combination of in situ phase identi?cation and ex situ chemical analysis.

6.Conclusions

This paper provides an overview of the high-pressure multi-grain XRD technique,along with examples of its successful applications in studying multiphase systems under the P–T conditions of Earth’s deep lower mantle.The crystal size in a multiphase assemblage can be as small as about100nm when the sample is synthesized under megabar pressures,making it infeasible to recover individual grains;thus,in situ characteriza-tion is needed.Meanwhile,peak overlapping renders the powder XRD method inadequate for investigating unknown phases and minor phases.

In situ multigrain XRD relies on an intense synchrotron X-ray beam,DAC rotation,and a fast indexing algorithm to identify tens to hundreds of crystallites belonging to each individual phase in a multiphase assemblage;in this way,it enables robust phase identi?cation and structure determination in a multiphase assemblage.Examples were presented to illustrate the unique capabilities of the high-pressure multigrain XRD technique, including detecting the breakdown of iron-bearing Bdg into an iron-rich H-phase and Fe-free Bdg,identifying the HH-phase (Fe,Al)OOH,determining the crystal structures of(Mg,Fe)SiO3-pPv and seifertite(SiO2)under megabar pressures,measuring the unit-cell parameters of seifertite to a precision level of0.1%, and examining subtle variations of Fe content in the coexisting pPv and H-phase over a temperature gradient.In some cases, the in situ high-pressure multigrain XRD technique can be combined with ex situ TEM analysis in order to connect structural transformation with compositional changes,which was demon-strated by the discovery of the py-phase FeOOH coexisting with the pPv phase in a model composition of subducted slabs under high P–T conditions.Through these effective illustrations,we have explained key challenges in studying multiphase systems and showed the important role of high-pressure multigrain XRD in exploring the Earth’s deep interior.

L.Zhang et al./Engineering5(2019)441–447445

Acknowledgements

The authors thank the anonymous reviewers for their construc-tive comments.This work was supported by the National Natural Science Foundation of China (41574080and U1530402).Portions of this work were performed at High Pressure Collaborative Access Team (HPCAT)(Sector 16),Advanced Photon Source (APS),Argonne National Laboratory.HPCAT operations are supported by the Department of Energy (DOE)/National Nuclear Security Adminis-tration (NNSA)’s Of?ce of Experimental Sciences.The APS is a US DOE Of?ce of Science User Facility operated for the DOE Of?ce of Science by Argonne National Laboratory (DE-AC02-06CH11357).Portions of this work were performed at 15U1,Shanghai Synchrotron Radiation Facility (SSRF).Compliance with ethics guidelines

Li Zhang,Hongsheng Yuan,Yue Meng,and Ho-Kwang Mao declare that they have no con?ict of interest or ?nancial con?icts to disclose.

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Engineering 2 (2016) xxx–xxx

Research

Deep Matter & Energy—Review

高压多晶X 射线衍射方法的发展及其在地球深部研究中的应用

张莉 a, *, 苑洪胜 a , 孟悦 b , 毛河光 a, c

a

Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai 201203, China b

HPCAT, X-Ray Science Division, Argonne National Laboratory, Argonne, IL 60439, USA c

Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA

a r t i c l e i n f o

摘要

Article history:

Received 3 September 2018Revised 16 January 2019Accepted 1 February 2019Available online 6 April 2019

下地幔占地球体积一半以上。在高温高压下对下地幔真实组分所开展的矿物学和岩石学实验是了解深部地幔演化过程的必要途径。激光加热的金刚石对顶压砧（laser-heated diamond anvil cell, LH-DAC ）是开展这类高温高压实验最常用的工具，实验产物通常包括从上百纳米到数微米尺寸不一的大量晶粒所组成的多相集合体。这些下地幔相的晶体结构往往不能在卸压后保存下来，因此必须对它们进行原位表征。相对于同步辐射光源设备中可用的聚焦X 射线光斑尺寸（3~5 μm ），晶粒尺寸要小一个量级，所得到的X 射线衍射（X-ray diffraction, XRD ）图谱通常显示为衍射斑点和衍射环的混合。由于多相衍射峰重叠严重，粉晶X 射线衍射法无法对新相和弱相进行鉴定，因此采用传统XRD 技术测定高温高压下多相混合物非常困难。我们最近在高压研究中所发展的同步辐射多晶X 射线衍射法可以通过测定多相组合中成千上百个晶粒各自的晶面取向，使多相体系中单晶的指标化成为可能。一旦完成指标化，多晶混合物中的每一个晶粒都可以看作是单晶。因此，我们可以同时确定多相体系中新相和弱相的晶体结构。随着这一新方法的发展，我们开辟了深下地幔高温高压条件下的结晶学新领域。本文阐述了多相体系研究中的关键技术挑战，并通过高压多相X 射线衍射法的成功应用实例，论证了该方法的独特能力。

? 2019 THE AUTHORS. Published by Elsevier LTD on behalf of Chinese Academy of Engineering and Higher

Education Press Limited Company This is an open access article under the CC BY-NC-ND license

(https://www.360docs.net/doc/f518283883.html,/licenses/by-nc-nd/4.0/).

关键词高压

同步辐射X 射线多晶

金刚石对顶压砧矿物学岩石学地球内部

1.引言

多种高温高压实验技术被开发出来以用于研究地球内部的矿物性质。地球内部的温度、压力条件随深度增加而增加，矿物在较低温度、压力条件下稳定的晶体结构将会被更致密的结构所取代。通过高温高压实验对下地幔火成岩组分进行了验证[1,2]。描述下地幔体系至少需要5种氧化物组分。即使在单一的成分组成 [如大洋中脊玄武岩（mid-ocean ridge basalt , MORB ）或地幔岩]中，矿物相的变化及其组分也与高温高压条件下的元素分配

相关。而俯冲板块与地幔之间的相互作用则使其相关系进一步复杂化[3,4]。例如，一部分水可能储存在名义上无水下地幔相中，但是俯冲板块中的水还可能储存于在该温度、压力条件下稳定的含水相中。岩石学实验在大压机装置中已经得到广泛开展，其可达到的温度、压力范围与下地幔顶部（<30 GPa ）相当[5]，当使用烧结金刚石压砧取代碳化钨压砧时[2]，大压机装置所能获取的温度、压力范围则可扩展至下地幔中部（50 GPa ）。

探索整个地球内部的目标促使激光加热的金刚石对顶压砧（diamond anvil cell , DAC ）得到了广泛的应用。

* Corresponding author.

E-mail address: zhangli@https://www.360docs.net/doc/f518283883.html,

(L. Zhang).

Contents lists available at ScienceDirect

Engineering

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钻石是已知的最坚硬的物质，又是透明的，允许X射线通过。通常，将直径小于50 μm以及厚度约为10 μm的初始矿物样品放在两个金刚石压砧尖端的微小台面之间进行加压，激光可穿过金刚石压砧对样品进行双面加热[6]。在40年前首次报道了静高压实验达到了兆巴压力，这使得模拟地球超深部成为可能[7]。随着DAC和激光加热技术的发展，铁在内核条件下的结构得到了研究，在这一实验中，温度和压力分别达到了5700 K和377 GPa[8]。然而这些实验大多局限于单一的矿物相或简单的二元体系（如氧化镁-二氧化硅），想要解释深下地幔的复杂结构，如D′′层锋利的上边界[9]、地震各向异性[10]，以及占据下地幔三分之一的剪切波低速带[11]等主要特征，我们需要在真实的下地幔多组分体系中开展高温高压岩石学实验，实验产物通常为多相混合物。

在多相混合物中，大多数衍射峰在粉晶X射线衍射（X-ray diffraction, XRD）图谱中重叠。即使没有重叠，单靠粉晶X射线衍射也不足以研究新的结构。单晶XRD 被认为是较为理想的结构分析方法，在运用DAC的高压实验研究中得到了广泛的发展[12–14]。然而，传统的单晶XRD实验要求单晶尺寸在5 μm左右，这样的单晶无法在相应的高温高压实验条件下、在真实的地幔组分中合成。相反，在与深部地幔相对应的相平衡实验中，总是得到多晶多相样品。本文将回顾用于高温高压下多相体系中的多晶XRD技术，包括：多相混合物中新相的原位表征、多晶样品中选定的单个晶粒的晶体结构测定以及通过二维扫描精确确定晶胞的晶格常数。2.高温高压实验岩石学面临的挑战

多相混合物中相的识别和结构分析一直是高压研究中的一个难题。Frost和Fei[15]指出，当合成压力低至22 GPa时，从回收样品的粉晶XRD图谱中很难识别出致密含水镁硅酸盐相D、F和G的差别。但是，如果这种多相混合物含有颗粒尺寸大于5 μm的单晶晶粒，并且其结构在常温常压条件下可以保存下来，我们则可以在回收样品中挑选出这些颗粒进行单晶研究。

然而在更高的压力下，样品体积会显著减小。在DAC中，当压力大于60 GPa时，样品腔的尺寸减小到了几十微米。此外，大多数在超高压下获得的结构不能在常压下保存下来，比如MgSiO3和(Mg,Fe)SiO3的后钙钛矿结构（post-perovskite, pPV）[16–18]。同时，多晶样品中晶粒尺寸也通常随压力增加而减小。在深下地幔的高温高压条件下，地幔组分中含水相的晶粒尺寸仅为100 nm左右[4]。在相同温度下，含水相D在68 GPa时晶粒尺寸约为2 μｍ，而在128 GPa时则减小至200 nm左右[4]，这表明压力是晶粒尺寸变化的主要影响因素。其他因素也可能影响晶粒尺寸，例如，在较低的温度下生成的晶粒尺寸通常较小[19]。对微晶颗粒（<1 μm）进行回收并开展单晶X射线衍射，在实际操作中几乎是不可能的。综上所述，为了研究深下地幔温压条件下真实多相体系的相变和晶体结构，原位表征是必不可少的。

高温高压条件下大量新结构产生。在高温高压实验中，粉晶XRD图谱中经常会出现一些新的衍射峰，然而要找到能与之相匹配的结构并不容易。高温高压新结构有两种类型：一种是原子重新排列形成更致密的结构，如NaCl中的B1-B2结构转变；另一种则涉及压力导致的化学计量变化，如NaCl3和Na3Cl [20]。在不知道化学成分的情况下，高压相的原位结构测定非常具有挑战性。另外，在岩石学实验中，高温高压相变也伴随组分变化。例如，在一个使用自然条件下的MORB组分作为初始样品的高温高压实验中，在深下地幔温压条件下，该实验获得了一个由富镁pPv、PbO2型二氧化硅、钙钙钛矿和CaFe2O4型铝相组成的四相混合物。该四相混合物通过原位的粉晶XRD技术以及非原位的透射电镜（transmis-sion electron microscopy, TEM）被分辨出来，其中，后钙钛矿相在Na2O中富集，并且Fe3+在总体铁元素中的占比也较高[21]。现有的粉晶XRD技术不能在实际的深下地幔条件下求解pPv相的复杂结构，此外，在多相混合物的粉晶XRD图谱中，弱相衍射峰通常被强衍射峰或背底所掩盖，使得利用粉晶XRD技术对弱相进行识别极为困难。

总而言之，不论是单晶X射线衍射还是粉晶X射线衍射技术都不足以用于研究高温高压下的多相系统。研究深下地幔中发生的相变以及这些矿物相的具体结构需要发展新的方法。

3.原位多晶同步辐射X射线衍射

在激光加热DAC高温高压实验中，同步辐射X射线被认为是最理想的探测工具[22–24]。通常，高温处理会促进晶体生长，在高温高压实验中，我们获取斑点状的粉晶XRD图谱。图1所示为具有代表性的(Mg,Fe)SiO3 pPv的斑点状粉晶XRD图谱[25]。这种斑点图案是由相对小尺寸的X射线束（约6 μm×8 μm半高宽）照在经过

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2400 K左右高温加热而导致晶粒迅速生长的多晶样品所

导致的。近年来随着高压同步辐射线站的技术发展，X

射线束尺寸已经缩小至2~3 μm [22]，这将有助于降低衍

射斑点在XRD图谱中的重叠程度。

高压多晶XRD技术的发展使得同时确定几百个晶粒

各自的晶面取向成为可能[25,26]。一旦指标化完成，多

相混合物就可以看作是成千上万个单晶晶粒的聚合体。

处理这类数据集需要一种精确、高效的算法。我们发现

Fable程序中的Grain Spotter软件包[27,28]可直接应用于

通过旋转法所收集的DAC中多晶样品的高压X射线衍射

数据，同时分辨出成百上千个晶粒的晶面取向及其基本

晶体学信息分析[25,29,30]。在某些情况下，经验丰富

的晶体学家可以在高压旋转扫描数据集中手动确定几个晶粒，并解出其晶体结构[6,31]。然而，只有利用多晶XRD方法分析属于每个相的数十至数百个晶粒，才有可能系统地研究混合相中包括弱相在内的每个相的晶体结构。

图2为高压多晶XRD数据采集装置的示意图。与传统单晶晶体学中使用的“旋转法”类似，一组多晶衍射图谱是在DAC设计开口角度允许的范围内、样品围绕旋转中心的同时以一定角度为步长所收集的。综合考虑维持稳定的下地幔高压（高达136 GPa）并获得尽可能大的X射线入射角，我们选择了开口大小为50°~60°的DAC。为了提高XRD衍射图谱的信噪比，我们采用的最小角度步长仅为0.2°。在实验中，所使用的曝光时间为每帧1~10 s，并在每次收集之前对曝光时间进行测试，以使最强的衍射点强度接近饱和为标准。接下来，可以采用CrysAlis [32]、XDS [33]、GSE_ADA/RSV [34]等软件包来计算高压衍射数据的衍射强度，然后采用传统的单晶结构分析程序如SHELX [35]进行结构求解和精修。

合成高质量晶粒是应用高压多晶XRD技术的前提。研究发现，晶体在准静水压和稳定加热的高温高压条件下可以生长得更好[29]。在地幔岩石学实验中，高压相的结构通常能在温度淬火后保存下来，而不能在卸压之后保存下来，因此我们通常在加热之后、卸压之前采集多晶X射线衍射数据。

斑点状的衍射图谱使得多晶衍射分析方法成为可能，但这些斑点的存在使得粉晶XRD图谱中衍射峰的强度测量不可靠，从而导致对多相混合物中相比例的估计不准确。斑点状衍射图谱意味着样品中的某些晶粒生长很好，而与之相反，光滑的衍射圆环则意味着晶粒非常细小。在样品合成中，晶体生长所需的高温条件有利于热力学平衡。另一方面，从相变动力学的角度来考虑，在类似的温压条件下粗晶粒为起始样品的相变可能比细小晶粒样品慢得多，因为细晶粒样品中较大的表面积可以促进相变的成核。在激光加热DAC中开展高温高压实验时，我们经常可以观察到斑点状的XRD图谱，此外，高温高压相变中的动力学问题应得到更多的关注。4. 多晶XRD技术在高压实验中的应用

多晶XRD方法能够对DAC中多相样品的微小晶粒进行相鉴别和结构分析。本文总结了原位多晶XRD技术在以下三个方面的应用：未知相和弱相的相识别、高压

图1. 具有代表性的(Mg,Fe)SiO3 pPv的斑点状粉晶XRD图谱。该XRD图谱采集于120 GPa、2400 K淬火之后的样品中，旋转角ω固定（Reproduced from Ref. [25] with permission of PNAS, ? 2013）。图2. DAC中多晶XRD数据采集的实验装置示意图。样品放置在旋转

中心，以0.2°的步长收集一组2D XRD图谱，DAC开口角度一般为50°~60°，其中ω、η和θ分别代表旋转角、方位角和布拉格角。

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晶体结构的确定以及通过合并多个晶粒的衍射信息来确定其晶胞参数。

4.1. 多相混合物中的相识别

多晶XRD技术在从混合相中识别未知相上展示了其独特的优越性。当含铁布里奇曼石（bridgmanite, Bdg）分解为几乎不含铁的MgSiO3 Bdg和富铁的六方相（H相）时，Ｈ相中只有3个高于2.4 ?的峰是特定的，其他峰基本都与Bdg相衍射峰重叠[29]。在一套高压XRD旋转数据中，通过确定每个晶粒特定的晶面取向矩阵，找到了属于H相的154个晶粒。当晶粒的数十到数百个衍射斑点满足由其特定取向矩阵确定的严格几何角度时，晶粒的存在就可以被确定。每个衍射点由三个角度（ω, η, 2θ）来约束，其中，ω,η, θ分别代表旋转角、方位角以及布拉格角（图2）。此外，154个具有特定取向矩阵的晶粒重复地验证了同一晶格。

粉晶XRD图谱只包含晶面间距d的信息，有时仅对新相的几个主要衍射峰进行拟合会导出不正确的晶胞。以最近发现的(Fe,Al)OOH中六方含水相（hexagonal hy-drous phase, HH相）为例，采用多晶XRD对其进行分析，在110 GPa下得到了它的晶格常数：a = 10.5803(6) ?，c = 2.5897(3) ?[36]。然而，如果不选取粉晶XRD图谱中的弱峰(211)和(311)，则会导出到一个较小的正交晶胞，晶格参数为a = 4.03(2) ?，b = 3.699(5) ?，c = 2.78(1) ?。这个晶格不能通过三维（three-dimensional, 3D）多晶XRD检测，因为根据该晶胞及其特征取向矩阵预测得到的多数衍射斑点没有出现在旋转衍射XRD图谱中。当输入六方相晶格参数时，其中符合某一个晶粒取向矩阵的衍射点就有104个，并且同时找到了27个取向矩阵不同的晶粒，它们重复验证了HH-phase的六方相晶胞。

多晶XRD在一个多相混合物中确定一个弱相展示了其独特优越性。在粉晶XRD图谱中，主相的衍射峰通常会遮掩大多数弱相的衍射峰。之前的实验发现在下地幔混合物中，赛石英（seifertite）作为一个弱相存在，可见的衍射峰里只有（110）与主相pPv的衍射峰没有重叠[30]，如图3（a）所示。尽管如此，我们还是利用多晶XRD方法找到了100多个晶粒，但由于晶粒尺寸较小，多数晶粒的衍射信号较弱。另一方面，通过对比粉晶衍射谱中衍射峰的相对强度，可以粗略估计出弱相与主相的比例[图3（a）]。在多晶XRD数据里，有时只能确定弱相的少数几个晶粒。然而，属于弱相的微小颗粒仍然可以被识别并作为单晶处理，这在传统的XRD技术中是

不可能实现的[图3（b）]。高压多晶XRD的发展完成了以前不可能完成的任务，并且推动了探索地球深部的实验研究的发展。

4.2. 晶体结构测定

(Mg,Fe)SiO3-pPv相是下地幔底部的主导矿物，自2004年被发现以来一直是研究的热点[16–18]。pPv相的成分变化可引起其密度和结构的变化。因此，研究pPv相的化学组成及其结构变化对解释核幔边界（core-mantle boundary, CMB）附近观测到的地震和动力特征具有重要意义。例如，在MORB组分中合成的pPv相发现Na2O 富集并且铁含量也较高[21]。另一方面，pPv 相可能在

图3. 由主相pPv和弱相SiO2组成的混合相的XRD数据。（a）在129 GPa、温度淬火后采集的粉晶XRD图谱显示SiO2比例很低；（b）多晶XRD图谱，包括某个选定的赛石英晶粒中来自{2,2,1}晶面的两个弗里德对（Reproduced from Ref. [30] with permission of GeoScienceWorld, ? 2016）。

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CMB与液态铁核接触时变得富铁[18]。利用非原位透射电镜（ex situ TEM）可以在回收样品中测定各个相的化学组分。然而现有的粉晶XRD方法以及理论计算并不能检测由pPv中的杂质成分所引起的结构变化，并且pPv 结构也不能在常压下保存下来，因此需要进行原位晶体结构测定。之前的研究在氖（Ne）气包围样品的准静水压条件下合成了高质量的(Mg,Fe)SiO3-pPv多晶样品（图1），并用多晶分析方法对所选的pPv单晶进行了原位晶体结构的测定[25]，这种贫铁pPv的结构与理论预测的MgSiO3的结构几乎相同[16]。据我们所知，这是pPv 相单晶结构的首次实验测定，证明了在超高压下原位测定亚微米晶粒的晶体结构的可行性。该技术可用于研究高压矿物成分变化引起的结构变化。

DAC的开口角度有限，在多晶样品中，可以将多个取向不同的晶粒进行合并，以增加结构的完备性，有助于晶体结构的测定[30]。显然，将多个晶粒的反射条件结合在一起，可以更加明确高压相的空间群。每个选定晶粒的衍射强度由XDS软件计算，检查数据集之间的兼容性以及合并数据集则由XDS程序包中的XSCALE程序来完成[33]。例如，在这套赛石英在129 GPa的多晶数据中，来自6个晶粒的总共613个衍射点被组合在一起，使得这套数据的丰度达到6.6倍，完整度达到92%，获得d 值范围低至0.72 ?的高质量高压晶体结构数据，因此可以对赛石英在129 GPa时的结构进行高质量精修[30]。此外，CrysAlis和其他软件也可以用于这类数据的处理。高压多晶XRD原位晶体结构的测定开辟了兆巴压力晶体学的新领域，提供了传统XRD技术难以实现的晶体学解决方案[25,30]。

4.3. 多相混合物中晶胞参数的测定

在矿物学和岩石学研究中，确定多相混合物中各相的晶胞参数是一个共同的任务。为了获得高质量的样品以及可靠的压力测量，经常将传压介质和（或）绝热层、压标，有时还有吸收激光的物质，与样品一起装入直径只有几十微米的DAC样品腔中。在高温高压实验过程中或者温度淬火之后，这些物质的衍射峰通常在粉晶XRD 图谱上重叠，仅有少数几个衍射峰可以用于定压和测定样品的晶胞参数，因此存在很大的不确定性。幸运的是，应用多晶XRD技术，能通过属于单个晶粒的多个d 值导出样品的晶胞参数。多晶数据所具有的三维旋转和几何关系使来自相近d值的衍射峰得以分离。以赛石英为例，通过增加参与拟合的衍射峰数量，晶胞参数的不确定性能够大大降低[30]。因此，从多晶数据拟合得到的晶胞参数的不确定性主要取决于单色仪在0.1%以内的分辨率。

精确的晶胞参数可以用来评估矿物组分变化对矿物相的微妙影响，铁能进入包括硅酸盐Bdg、pPv和铁方镁石在内的所有下地幔主要相的晶体结构中[37,38]。多晶XRD方法曾用于计算在119 GPa下共存于(Mg,Fe)SiO3中的pPv和H相的晶胞参数[39]。晶胞参数的不确定性可以通过在精修过程中增加衍射峰的个数而减小。因为样品腔中传压介质Ne的存在使得样品腔中间部分的压力不变，因此精修过的晶格常数可以用来探究样品在激光加热区域铁组分随温度梯度的变化。在距离加热中心10 μｍ的地方，pPv的晶胞体积下降了0.16%，而H相的晶胞体积减少了0.54%，这表明两个相都在较热的区域更加富铁[39]。与驱动铁等重元素从热区向冷区转移的Soret效应相反，我们认为所观测到的铁分布是由温度影响元素分配的结果。因此，利用高压多晶XRD方法可以精确地计算出每个相的晶胞参数，从而根据晶胞参数对元素或相的分布进行二维扫描。

5. 原位XRD和非原位TEM技术的结合实例

在高温高压岩石学实验中，需要对回收的样品进行化学分析，并将原位XRD测定的样品结构变化与组分变化联系起来。利用聚焦离子束（focus ion beam, FIB）对DAC中高压下激光加热过后的样品回收后进行切割和打磨[40]，就可以获取用于透射电镜中分析的样品薄片。电子衍射和能谱仪（energy dispersive spectrometer, EDS）化学分析与非原位TEM分析方法相结合可以对常温常压下的相进行验证。实验中应仔细检查原位多晶XRD与非原位TEM分析结果是否具有一致性。为了保证其一致性，我们总是从加热中心选取样品用于制备TEM薄片。同时，可以采用XRD进行二维扫描来记录高温高压条件下以及恢复到常温常压后样品中的相分布。值得注意的是，大多数高压结构在常温常压条件下不能保存下来，此外在制备TEM薄片样品时，回收的样品有时还可能受到污染。

在此，我们提供了一个实例来演示通过原位XRD 和非原位TEM的结合对多相混合物中的含水相进行识别。为了了解地球深部储水，确定真实的下地幔成分中的含水相是必不可少的。我们使用了一种含水约7%（质量分数）的60% MgSiO3-30% Al2O3-10% Fe2O3（摩尔分

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数）的胶体作为起始材料。如图4所示，主要相为pPv 和含水δ相，只有一个微弱的衍射峰符合立方黄铁矿结构FeOOH相（py相，pyrite-type phase）。通过多晶XRD 分析，我们找到42颗晶粒，观测到有13~18个衍射点符合Pa-3空间群，因此确定了py相的存在。图4（b）为FIB制备的回收样品薄截面的成分分布图像，显示有几个属于富铁相的颗粒。EDS分析确认了颗粒中的铁和氧元素，而EDS分析检测不了氢元素是否存在。图中颗粒大小从50 nm到200 nm不等。从二维元素分布可以看出，py相的体积占总体积的比例小于3%。在高温高压条件下，这些富铁颗粒的化学成分及其在多晶混合物中的体积比与原位XRD观测到的py相一致。

在本实验中，我们使用了比真实地幔成分更高的Al2O3含量来增强XRD图谱中δ相的衍射峰强度。在地幔混合物中，含挥发分矿物相一般作为弱相存在。无论如何，这个例子表明，高压多晶XRD技术能够通过结合原位相XRD和非原位化学分析，识别出混合物中体积比小于3%的含水相的单个细小晶粒。

6. 结语

本文对高压多晶XRD技术进行了综述，并给出其在深下地幔温压条件下多相体系研究中的成功应用实例。当样品在兆巴压力下合成时，多相混合物中的晶体尺寸

图4. 多相混合物中包括主要相pPv、含水δ相和次要相py相（FeOOH）。TEM测试结果（a、b）显示少量富铁晶粒，这与原位XRD（c）显示的少量py相的存在相一致。

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小至100 nm，因此无法回收单个晶粒，需要对其进行原位表征。同时，衍射峰的重叠使得粉晶XRD方法不能很好地研究未知相和弱相，原位多晶XRD方法能依靠同步辐射X射线束、DAC旋转和快速算法来识别多相混合物中属于每个相的数十至数百个晶体。通过这种方法，可以在多相混合物中实现可靠的相识别和晶体结构测定。文中实例论证了多晶XRD技术的独特优越性：应用多晶XRD技术探测到含铁的布里奇曼石分解为富铁的H相以及不含铁的布里奇曼石，发现了HH相(Fe,Al)OOH，首次确定了赛石英（SiO2）和pPv (Mg,Fe)SiO3的晶体结构，在兆巴的压力下测量赛石英的晶胞参数并且精度达0.1%，还研究了和pPv共存的H相中铁含量随温度梯度的细微变化。在某些情况下，原位高压多晶XRD技术还可以与非原位TEM分析相结合，来探究结构转变和成分变化之间的关系，应用这一方法，在高温高压条件下的俯冲板模型组分中发现了py相FeOOH与pPv共存。通过这些有效的例证，我们阐明了研究多相体系的关键挑战，并展示了高压多相XRD方法在探索地球深部的重要作用。

致谢

作者感谢匿名评论者的建设性意见。本研究由国家自然科学基金（41574080和U1530402）资助。部分工作是在HPCAT（sector-16）、先进光子源（APS）、阿贡国家实验室进行的。HPCAT的运作得到了美国能源部（DOE）/国家核安全管理局（NNSA）实验科学办公室的支持。先进光子源是美国能源部科学用户办公室的设施，由美国阿贡国家实验室根据编号为DE-AC02-06CH11357的合同为美国能源部科学办公室运作。部分工作在上海同步辐射光源（SSRF）的15U1进行。Compliance with ethics guidelines

Li Zhang, Hongsheng Yuan, Yue Meng, and Ho-Kwang Mao declare that they have no conflict of interest or finan-cial con?icts to disclose. References

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