Class Note for GEOS 306 at UA
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Date Created: 02/06/15
Synchrotron X ray Study of ron at High Pressure and Temperature S K Saxena L S Dubrovinsky P H39aggkvist Y Cerenius G Shen H K lVlao X ray synchrotron experiments with in situ laser heating of iron in a diamond anvil cell show that the high pressure a phase a hexagonal close packed hop structure trans forms to another phase possibly a polytype double layer hop at a pressure of about 38 gigapascals and at temperatures between 1200 and 1500 kelvin This information has implications for the phase relations of iron in Earth s core Recent studies on iron 1 3 have led to greater understanding of the behavior of iron under the extreme pressure temperature conditions of Earth s core For example the melting curve of iron is now known to a pressure of nearly 2 Mbar and it is recognized that iron may occur in additional poly morphs The structures of four phases of iron are well known or body centered cubic bcc y face centered cubic fcc 8 hcp and the high temperature form 5 bcc Boehler 1 and Saxena et al 2 presented experimental evidence for the occurrence of a new iron phase which was tentatively called 3 by Saxena et al Fig 1 This de termination was made by heating iron under pressure in a diamond anvil cell with a Nd yttrium aluminum garnet YAG laser and plotting the laser power against the corre sponding temperature measured by thermal radiation spectroscopy A change in slope on such a plot is considered to signify a phase transition or melting Although the laser power temperature technique may be used successfully in determining melting and phase transformations 1 2 4 structural information on the iron phase transforma tion is necessary to understand the state of iron in the core Here we report the result of an x ray study on phase transformation of 8 hcp iron into a new phase The result shows that one or more new iron phases may be present in Earth s core but to recognize them great care is needed in designing the experiments To obtain x ray data we used the syn chrotron X 17C beam line at the Brookhaven National Laboratory We load ed an iron foil 999 pure in a Mao Bell diamond anvil cell with predried periclase MgO as the pressure medium and the stan dard for pressure measurement and heated the iron sample with an 18 W laser in con tinuous wave TEMGOO mode As the purpose of the study was to establish the presence of S K Saxena L S Dubrovinsky P Haggkvlst Y Cere nius Institute of Earth Sciences Uppsala University 8752 36 Uppsala Sweden G Shen and H K Mao Geophysical Laboratory Carn egie Institution of Washington 5251 Broad Branch Road NW Washington DC 20015 1305 USA SCIENCE e the new iron phase at any possible temper ature we made no attempt to measure the temperature The pressure was generally be tween 35 and 40 GPa At such pressures iron should change to the y fcc phase if the temperature ever exceeded 1400 K Fig 1 As we did not observe any fcc phase either during heating or after the sample was quenched while maintaining pressure the temperature in our study must not have exceeded 1500 K Our other experiments have shown that the fcc phase can be quenched if the pressure is maintained 5 At pressures of 10 GPa Fig 1 we could recognize the fcc phase quite clearly both during in situ heating and in the quenched product at high pressure At higher pressures between 35 and 40 GPa Fig 1 upon heat ing we observed phase transformation of 8 hcp to a new phase Figs 2 and 3 al though this new phase was not noted on unheated spots or where the laser absorption was poor it was noted at all other spots both during in situ heating and after quenching at high pressure The simulation of the x ray diffraction for the new phase is on the basis of a superlattice of hcp We modeled the new phase as a four layer close packed hcp struc ture analogous to that of Ce and some other metals 7 The calculated diffraction pat terns Fig 2 are based on the assumptions 2000 5 1500 39y B 9 3 1200 D E g 800 E 2 400 8 or O l 0 10 20 30 40 50 60 Pressure GPa Fig 1 Part of the iron phase diagram showing the pressuretemperature fields of our study On laser heating iron at lower pressures was largely con verted to the fcc phase Temperatures shown are approximate At high pressures the iron shows a distinct phase transformation from hop to possibly a dhcp form of iron According to Saxena er al 2 4 3 iron would be stable at 1400 K between 85 and 40 GPa The 8 phase is not shown VOL 269 22 SEPTEMBER 1995 3 O O 1 Q 3 ii J 39 J may feel 3 1 Errc 3 L 01 O q x39Kjff d 9 a its rs we flu w 100 Intensity arbitrary units 01 x O 1 l I 155 155 175 185 195 025 30 35 EkeV Fig 2 Energy dispersive x ray data on iron A The top curve shows the x ray diffraction energy dispersive 20 21 pattern for the unheated iron which is all hcp at pressures between 85 and 40 GPa The middle curve shows similar data for iron after laser heating during which temperatures must have reached 1000 to 2000 K As no fcc peaks are visible we estimate that the average temperature was below 1500 K The middle curve displays a clear development of a new peak below 185 keV arrow The bottom curve shows a cal culated pattern for a mixture of MgO hcp iron and the new phase assumed to be the dhcp phase of iron B The data shown are similar to those in A The peaks are less distinctive than those in A but a comparison of the data for heated and unheated iron shows a clear develop ment of the two additional peaks arrows in this energy range which would belong to the dhcp phase that the phase mixture is ideal that is primary orientation of any one phase is absent ii the diffraction peaks are de scribed by pseudo Voigt functions and iii x ray intensity does not depend on energy range The background is taken from the experimental data The data in the low energy range are well simulated by this ap proach Fig 2A At the high energy range Fig 2B the patterns are not as distinct but when we compare the heated and un heated samples there remains little doubt that we are looking at the transformation of 8 hcp to a new double layer hcp dhcp phase We checked our sample for the presence Fig 3 Phases of iron A v foo B 8 hop and C the four layered phase dhcp of any other phase formation resulting from reaction between the iron and the pressure medium Although the sample contained only MgO and iron small traces of oxygen and water can never be totally ruled out We watched the sample on the monitor and observed no visible reaction during heating After the experiment the pressure was re leased and the iron completely reverted to the bcc phase None of the x ray peaks in Fig 2 correspond to any iron oxides mag netite hematite or wustite Although there are additional peaks that do not be long to 8 hcp iron the analysis is compli cated by the fact that the x ray pattern includes peaks from the untransformed hcp iron this is because the laser beam heats only part of the iron while the xrays pass through the entire thickness of the sample Therefore the dhcp peaks necessarily in volve the peaks from the th iron as well The calculated lattice parameters 38 GPa and 300 K are a 2396 A and c 3814 for the 8 hgp phase and a 2427 A and c 766 A for the dhcp phase Fig 3 The molar volumes for the 8 and dhcp phases are 571 and 589 cm3mol respectively with unknown errors these values are consistent with the thermody namic assessment of such data 4 if the new phase is considered as B iron Table 1 shows the calculated diffraction pattern of a sample heated for 35 min and after tem perature quench pressure maintained Many of these peaks correspond to the 8 hcp phase and not necessarily to the new phase The calculated molar volume of the heated sample is 6246 cm as compared with 592 cm3 for the quenched sample under a pressure of 35 to 40 GPa The x ray study confirms that iron oc curs as at least four different crystallograpl ic structures Because the 8 hcp iron was the only phase recognized as a suitable Table 1 Calculated xray diffraction pattern for dhcp a 2427 A c 7666 A Mo K radiation P 85 to 40 GPa T 800 K relative intensity O nk d A i 100 21010 0 101 20270 45 004 19105 33 102 10429 100 103 10233 12 104 14102 5 105 12307 9 110 12135 21 100 10910 10 021 10412 4 114 10253 22 202 10135 12 023 09719 3 107 09712 3 000 09502 3 025 00000 2 200 00110 5 1 704 SCIENCE 0 high pressure phase for Earth s core all geo physical models of the core have been based on properties of the 8 iron We must now consider that one or more additional iron phases in Earth s core are possible Note added in proof Recent x ray diffrac tion laser heating measurements also provide evidence for a new phase in a pres sure temperature range similar to that stud ied here REFERENCES AND NOTES 1 R Boehler Nature 363 584 1998 2 S K Saxena G Shen P Lazor Science 260 1812 1998 8 O L Anderson in HighPressure Science and Tech nology 7998 AlP Conference Proceedings Amer ican Institute of Physics 1998 vol 809 p 907 4 S K Saxena G Shen P Lazor Science 264 405 1994 5 J Zhang and S K Saxena conducted in situ heating 3 reakng n xeta tien ef Spree and bre MEquot UMquot eghm experiments with SAM 85 equipment from the Cen ter for HighPressure Research CHiPR State Uni versity of New York at Stony Brook at X 17B at the Brookhaven National Laboratory during 1998 and measured the pressure and temperature transfor mation of bcc to fcc 6 The sample area studied was about 85 um by 80 um in size The cold pressure variation across the sam ple chamber was 84 to 40 GPa The pressure mea surement was based on the 200 d spacing of llgO The x ray beam was 7 pm by 15 pm in size which was similar to the size of the laser beam diameter 15 um The pressure variation within the beam size area was less than 05 GPa however there was a significant Gaussian temperature distribution over the beam size area 7 W D Pearson The Crysta Chemistry and Physics of Metas and Aioys Pergamon New York 1972 C Y Yoo et ai Science in press We thank J Hu for assistance at the Brookhaven National Laboratory and colleagues at CHiPR for many useful discussions Supported by grants from the Swedish Natural Science Research Council NFR and Wallenberg s Foundation 000 8 May 1995 accepted 28 June 1995 50500 Conveeten Robert E Ecke Yuchou l lu Ronnie llainieri Guenter Ahlers Spiral defect populations in low Prandtl number Rayleigh B nard convection with slow rotation about a vertical axis were measured in carbon dioxide at high pressure The results indicate that spirals act like thermally excited defects and that the winding direction of a spiral is analogous to a magnetic spin Rotation about a vertical axis the spiral analog of the magnetic field breaks the zerorotation chiral symmetry between clockwise and counterclockwise spiral defects Many properties of spiral defect statistics are well described by an effective statistical mechanical model The discovery of spiral defect chaos SDC in Rayleigli Benard convection l was completely unexpected and challenged long standing theoretical ideas 2 about the possible states and dynamics of convec tion Since the initial experimental obser vations numerical simulations 3 and ex perimental work 4 5 have confirmed the robust nature of the spiral state Fig 1A One aspect of spiral defects that differenti ates them from defects in other systems 6 is that they are not constrained to be cre ated in pairs The spiral defect state displays individual spirals with clockwise or coun terclockwise winding Fig l B and C targets Fig 1D dipoles with the same or R E Ecke Materials Science and Technology Division and Center for Nonlinear Studies Los Alamos National Laboratory Los Alamos NM 87545 USA Y Hu Materials Science and Technology Division and Center for Nonlinear Studies Los Alamos National Lab oratory Los Alamos NM 87545 USA and Department of Physics and Center for Nonlinear Science University of California Santa Barbara CA 98106 USA R llainieri Theoretical Division Los Alamos National Laboratory Los Alamos NM 87545 USA C Ahlers Department of Physics and Center for Nonlin ear Science University of California Santa Barbara CA 98106 USA To whom correspondence should be addressed VOL 269 9 22 SEPTEMBER 1995 opposite windings Fig l E and F and multiple armed spirals Fig l G and H The mechanism for the creation of such a variety of forms is unknown but as dem onstrated by numerical simulations 3 a crucial element is the strength of the mean drift field which plays an important role in low Prandtl number convection In many areas of physics an external field is helpful in probing the state of a given system For example a magnetic field allows for a determination of magnetization and magnetic susceptibility which are im portant characteristics of systems with mag netic spins Similarly rotation about a ver tical axis is useful in probing the SDC state Analogies between phenomena in nonequi librium and thermodynamic systems for example between bifurcations and phase transitions and concepts from condensed matter physics such as orientational order have been helpful before in the analysis of nonlinear nonequilibrium systems 7 We have found that a thermal excitation de scription of spiral defect pOpulations works very well Rotation about a vertical axis breaks the chiral symmetry between clock wise and counterclockwise spirals in anal
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