POST-ARAGONITE PHASE TRANSITIONS IN STRONTIANITE AND CERUSSITE—A HIGH-PRESSURE RAMAN SPECTROSCOPIC STUDY

PII: S0022-3697(96)00201-6

Pergamon @

J. Phvs, Chem Solids Vol 58. No. 6. m. 977–987. 1997 0“ 1997 ~k~vier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/97 $17.00 + O.(XI

POST-ARAGONITE PHASE TRANSITIONS IN STRONTIANITE AND CERUSSITE—A HIGH-PRESSURE RAMAN SPECTROSCOPIC STUDY CHUNG-CHERNG LIN* and LIN-GUN LIU Instituteof EarthSciences,AcademiaSinicaTaipei,Taiwan115,R.O.C. (Received 17 September 1996;accepted2 October 1996)

Abstract—By usingRaman spectroscopy,phase behaviorsof strontium and lead carbonates under quasihydrostaticconditionshave been studiedin a diamond-anvilcellup to 420kbar at room temperature. The aragonite-typeSrC03 (strontianite)and PbC03 (cerussite)were found to transform to their high-pressure polymorphsat w 350and w 170kbar, respectively.Mode softeningwas observedfor some of the Raman bands of both SrC03 and PbC03. For certain modes of both SrC03 and PbC03, furthermore, frequency shifts with the compressionand decompressionprocessesform a hysteresisloop. The causes of the mode softeningand hysteresisloophavebeensuggestedto correlatewiththe spatialhindranceon the movementof the C03 groups.Combinedresultsof presentand previousworksuggestthat all observedMC031++MC03 11transitions(M = Sr,Pb, and Ba)at roomtemperaturemayoperatewiththe samemechanism,and that the phase boundariesof these transitions possessa negativeClapeyronslope. Byextrapolation, a hydrostatic pressuregreater than -1000 kbar maybe requiredfor CaC03 to form the samepost-aragonitemodification at room temperature. ~ 1997ElsevierScienceLtd Keywords: A. inorganiccompounds,C. highpressure, C. Raman spectroscopy,D. phase transitions.

1. INTRODUCTION

The study of polymorphism of the alkaline-earth carbonates at high pressures and temperatures has been a subjectof interest recently.Exceptfor calciteH and III, it has been found that the aragonite structure is the common form of divalent metal carbonates at high pressures[1].This also parallels the general rule that for stoichiometriccompounds that are different in only one cation, the structure of the larger-cation compoundsis often a model structure for the smallercation compounds at high pressures[2].On the basis of this criterion, it would suggestthat the aragonitetype carbonates may possessthe samepost-aragonite structure at high pressures. This argument has been confirmedby a recent study of the phase behavior of strontianite (SrC03),cerussite(PbC03), and witherite (BaC03) at high pressuresand temperatures [3]. Unlike the Ca and Mg carbonates, whichare likely to be the most abundant carbonate minerals in the Earth, the phase behaior of SrC03 and PbC03 has attracted much less attention in the past. Similar to BaC03, strontianite also transforms to a disorderedcalcite form at high temperatures [4]. The aragonite ++disordered-calcite transition boundary of SrC03 has been followedup to 40kbar by Rapoport *Authorto whomcorrespondenceshouldbe addressed. 977

and Pistorius [5].The disordered-calcite-typePbC03 has not yet been found, though it has been suggested by Chang [6]that PbC03 may adopt such a form at ca. 800”Cand 10kbars. In previous studies, we have found that witherite transformsto another orthorhombicform (BaC03 II, the post-aragonite phase) either at ca. 1000°Cand above40kbar (perhapsevenat a lowerpressure)or at room temperature and above 80kbar [3, 7]. The quenched phase of BaC03 H is kinetically unstable at ambient conditions. Both SrC03 H and PbC03 II obtained from ca. 1000DCand high pressurespossess similar behavior to BaC03 II. This would suggest that both SrC03 H and PbC03 II may also be obtained at room temperature as is BaC03 II. In this work, therefore, we further study the high-pressurephase behaviorsof strontianite and cerussite at room temperature under quasi-hydrostatic conditions by means of Raman spectroscopy.Combining previous and present results, we also estimate the transition pressure for the post-aragonite transition in CaC03 at room temperature. 2. EXPERIMENTAL

The samplesused in the present work are natural strontianite from Harem, Westphalia, Germany and cerussite from Dundas, Tasmania, Australia. Both

978

C.-C.

LINandL.-G.LIU

samples are colorless and transparent. The compositions of these minerals has been describedin our previous work [3]. Due to the absence of any X-ray diffraction line corresponding to other phases, both carbonates are considered to be singlephase though they are solid solutions and may best be describedas (Sr~*~Ca~,J~)COJ and (pbo.dhACOJ. In addition to the natural strontianite, synthetic SrC03 powders were also used to check the effectof impurity on the phase transformation. The synthetic SrC03 powders were prepared by pouring a solution of SrC12o6H20 into a saturated solution of Na2C03 at 8O–1OO”C. The precipitates were then washed by deionizedwater and ethyl alcohol in turn, and finally dried at 110”Cfor 4h. An X-raydiffractionstudyindicated that the syntheticpowdersare pure strontianite. For quasi-hydrostatic experiments, both samples (either crystals of 20~40~m in size or synthetic SrC03 powders) and ruby powders (< 5pm in size) wereplaced insidethe hole (~150pm in diameter and 50–80pm in depth) in a hardenedstainlesssteelgasket in a standard diamond-anvilcell. The anvil culets are about 600pm in diameter. The pressure-transmitting medium is water, whichcrystallizesin icesVI and VII at pressures greater than 9.3kbar at room temperature. The wholeassemblywasthen sealedbycompressing the two diamond anvils.Pressuresweremeasured using the ruby fluorescence technique. In order to reduce the errors that may be caused by the pressure gradient acrossthe sample,both Raman spectraof the samples and the ruby fluorescent were measured at the same spot for all measurements. The Raman spectra were obtained using a microoptical spectrometer system (Renishaw) at room temperature. The 514.5nm line of an Ar+ laser was used for excitation in a backscattering (180°) geometry. The Raman frequenciesin the high-pressure experimentswererecordedin the range 100-1200en-1, but thiswasextendedto 1600cm-’for characterization of the starting samples at ambient conditions. The resolution is +1 cm-’. Raman spectra were recorded with a 25X UT Leitz microscopeobjectiveand three accumulations at 300s integration time with 160 (SrC03) and 120(PbCOJ mW power on the sample. The focused laser spot on the sample inside the diamond-anvil cell was estimated to be 2 ~4~m in diameter. The position of the Raman line in a spectrum was estimated by assuminga Lorentzian profile. The quality of Raman spectra obtained during compression is in general better than that obtained during decompression.This was attributed mainly to the incomplete relief of stress after decreasing the pressure and/or the formation of smaller crystals in the original crystal after a phase change at higher pressures.In all cases,however,the Raman data of the

lattice modes are alwaysmore scattered than those of the internal modes,and thus largerexperimentalerror occurred in the former.

3. RESULTSANDDISCUSSION

3.1. SrC03 3.1.1. Ambient Raman spectrum of srrontianite (SrC03Z). The ambient Raman spectrum of the strontianitecrystal in the range 100–1600cn-’ shows 10bands: 148(s), 183(s),248(m),263(w,sh), 701(m), 711 (W, sh), 107’3(VS), 1079(W, sh), 1446(W), 1546(W) cm-’ (wherew =weak, m=mediate, s= strong, VS= very strong in intensity, and sh = shoulder). For the syntheticSrC03 powders,an additional weak band at 216cm-’ was observed.The frequenciesobservedin the synthetic SrC03 are slightlylower than those of the natural crystal. This differencemay be attributed to the presence of a large amount of Ca2+ in the natural crystal. The Raman bands observed in the present samples are in good agreement with those reported in the literature [8, 9]. The crystal structure of strontianite is identical with that of aragonite. Thus the vibrational modes of strontianite are assigned by following those for aragonite based on the space group Prima (i.e. Pmcn (62)) [10, 11] as shown in Table 1. A shoulder at 1079cm-’ was not observed in the synthetic SrC03. The strontianite crystal contains some 14mole% of Ca. An X-ray diffractionstudy of these crystals did not reveal any discernible line correspondingto calcite or aragonite. Extrapolating the phase boundary reported by Chang [12]to room temperature suggeststhat the volubilityof CaC03 in SrC03mayexceed15mole%.Thereforeboth shoulders at 711and 1079cm–l may be the linesof strontianite, though one cannot totally exclude the presence of aragonite in the present sample.Among these modes, bands at 701, 711, 1072, 1079, 1446,and 1546cm-’ are the internal modes, and the others in the lowfrequency region are the lattice modes. The three shouldersat 263,711,and 1079cm-’ werenot observed throughoutthe wholecompressionand decompression processes, but reappeared after totally releasing the pressure. 3.1.2. Pressure dependence of Raman spectra of strontianite (SrC031). The Raman bands of SrC031 as a function of pressure are displayed in Fig. 1. Selected Raman spectra at various pressures and room temperature are shown in Fig. 2. Note that the three shoulders (263, 711, and 1079cm-’) did not appear in Fig. 1. In comparison with the ambient spectrum, only a few Raman bands of SrC03 were observedat highpressures.The absenceof someof the

Phase transitions in strontianite

979

Table 1,Assignmentsof Raman bands of the aragonite-typecarbonates at ambientconditions Mode$

Assignment

Bu AM

M trans (y) M trans (y) M, C03 trans (.x) M, C03 trans (2) M, C03 trans (x) C03 trans (y) COj rot fj) C03 trans (z) C03 rot (x)

B3g

O–C–Oin-planebending

BIg Alg B}g Alg B2g

Alg B2g

A]g

O–C–O in-planebending

B2g

O–C–Oin-planebending

B]g Alg Blg AIg Alg

O–C–O in-planebending O–C–O out-of-planebending O–C–O out-of-planebending C–O stretching C–O stretching

A]g

C–O stretching

Aragonite 112 142 152 161 206 214 248 272 284 701 705 717 721 853 907 1085 1462

Strontianite$

148[148] 183[181] [216] 248[246] 263[260]

Witherite~

Cerussitel

136 155 (161) 180 225

(62) (122) 131 150 176 (199) 219 245

701[699] 711[712]

691

1073[1072] 1079

1059

1446[1445]

1420

(643) 673? 682? (688)? 696? 839 (912)? 1054 1063 1372 1418 1474

C–Ostretching 1574 1546[1541] 1506 tData of aragoniteand witheriteare from Frech et al. [11]and Linand Liu [3],respectively.Cerussiteisassumedto havean aragonite lattice. $Basedon the space group Prima. If Pmcnis used, B,g,B2g,and B3gin this table shouldbe changedto F&,%8,andB]g, respectively.The coordinate system (yzx) used in this table corresponds to the (xyz) used in Pmcn; trans = translation, rot = rotation $Data in square brackets are the Raman bands of the syntheticSrC03 powders. ~Data in parentheseswereextrapolated from the P-v regressionsat zero pressure. B2g

?Theassignedmodesaretentative. Raman bands may be related to the orientation of crystal and/or weak intensity. The quality of the spectrumfor the syntheticsampleunder highpressure is much poorer than that for the natural crystals. Thus, most of the data obtained at high pressures were based on the study of the natural sample (i.e. those shownin Figs l(a), l(c) and 2). Exceptline3in Fig. l(c), all Raman bands of SrC03 I showed nonlinear dependenceon pressure. In fact, lines 1, 2, and 7 may also be well fitted by a linear regressionbelow 160kbar. Line6 shouldbest be fitted by a third-order polynomial. Above 160kbar, linear regressionsmay be fitted for both lines 2 and 7 up to 350kbar, whilst a quadratic regressionis needed for line 1.However,without a significantlossin reliability (i.e. R in Table 2), lines 1 and 7 may also be fitted by polynomials of degree of 3 and 2, respectively.The constants and coefficientsof the regressions are all listed in Table 2. Lines 1, 2, and 6 in Fig. 1 all show some mode softening at pressures greater than about 300kbar. Mode softening indicates that SrC03 I is unstable above 300kbar at room temperature. It may involve a phase changeto be discussedin the next section.The valuesof the ambientpressuredependenciesof Raman frequency,(8~i/OP)~ (or a’sin Table2),showthat the changes in frequencyfor the lattice modes are larger than those for the internal modes. This indicatesthat

the compressionof SrC03 I is mainlyattributed to the compression of the Sr2+ cation polyhedra. Similar results were observed in other carbonates [7, 13, 14] and silicates[15, 16]. As mentioned before, the Raman frequencies for the various modes of the natural strontianite are slightly greater than those of the synthetic SrC03 due to the substitution of Caz+ in the former. HOWever,the pressuredependenciesfor line 1are nearlythe same for both natural and syntheticsamples(Table 2 and Fig. 1). 3.1.3. Phase transformation in strontianite. As shown in Fig. 1, two internal modes (lines 1 and 2) and one lattice mode (line 6) underwent mode softening at pressures greater than about 300kbar. In addition to mode softening, several new Raman bands appeared at 350kbar. Nevertheless, some Raman modes of SrC03 I probably persisted up to 390kbar. So, the SrC03 I + SrC03 11 transition may take place around 350kbar at room temperature. During decompression,not all of the new Raman bands of SrCOgH disappearedwhenthe pressurewas released below 350kbar. Some of the new bands of SrC03 II persistedto around 160kbar. The frequency for two of these latter bands did not follow the u–P trajectory observedduring compression,and formed what couldlook likea hysteresisloop between160and

C.-C. LIN and L.-G. LIU 1220

1200 1180 1160 1140

/

/.”

1120 1 1 1 1

1100(

1 1

1080-

1120 I100 1080 ,Ofjo

~

o

100

200

300

Pressure,kb

linto ’’’’’’”’’’”’’’’’’’’’’”” “’’’’’’’’’”~ 400 300 100 200 Presaure,kb

Fig. l. Pressuredependence of Ramanfrequencies of SrCO~underquasi-hydrostatic conditionand atroom temperature: (a)and(b)internalmodes,(c)latticemodes.Both(a)and (c)wereobtainedfromnaturalstrontianitecrystalsand(b) from syntheticSrC03powders.The opensymbolsrepresentthe data obtainedduringcompression,whilethe solidsymbols representthedataobtainedduringdecompression. Thecurvesandlinesareusedforguidanceonly.Linel’”in(a)wasobtained froma crystalthatwassubjectedto non-hydrostaticity.

350kbar in the v–P plane (seelines l–l’ and 2–2’in Fig. 1). To the best of our knowledge,this phenomenon has not been observed in any previous studies and it was confirmed in both samples used in the present study (see Fig. l(a) and (b)). It may not be attributed to the hydrostaticityof the samplesbecause earlier studies of other materials conducted in our laboratory and elsewhere did not reveal the same phenomenon (e.g. Zr02 by Perry et al. [17] and BaC03 by Lin and Liu [7]). The hysteresis loop seemsto occur also in the lattice mode of line 6’. On the basisof the modeassignmentslistedin Table 1,it is noticed that the mode softening and the loop-like pressure dependen% involve only those modes having vibrations parallel to the x–y plane (i.e. the ab plane of the unit cell if the Pmcn group is selected) or the plane of the C03 group. The cause of this phenomenon is unclear unless the detailed crystal structure of the high-pressurephase is known. It is likely, however,that the SrC03 I + SrC03 H transition dependssignificantlyon the displacementof C03 groups along a direction parallel to the plane of the

C03 triangle or on the rotation of the C03 group around an axis that is perpendicularto C03 triangle. The displacement or rotation of C03 groups might undergo a spatial hindrance and repulsion between C03 groups. The onset of mode softening thus indicates that the energy barrier has been overcome. Since further moving to the most stable position needs additional energy, the change of frequency with pressure is gradual not abrupt. When all atoms haveattained their equilibriumpositions in the SrC03 II structure, a discontinuity in the pressure dependence of frequencyappears. Thus the onset of mode softening may involve the presence of a metastable state. Although the plane of the C03 group in SrC03 I tilts slightlyfrom the ab plane of the unit cell (N 2.5°) [18],the displacementand/or rotation of C03 groups may reflect on the change of lattice parameters if the crystal system remained unchanged. The lattice parameters for both phases of SrC03 and their changes after transformation are listed in Table 3. After normalizing,it is obviousthat the change in the

Phasetransitionsin strontianite (a)

981

(b) 7’

6425 kbar

/7

243

353

2’ 2“

1

243

90

0.001

100

200

300

400

500

-L 600

600

1

1

I

700

800

900

Raman sbift, cm-1

1

1

1000 1100 1200 1300

Raman shift, cm-1

Fig. 2. SelectedRaman spectra of SrC03 as a functionof pressureat room temperature:(a) latticemodes,(b) internal modes. Thesespectra werecollectedduringcompression.Numbersbesidetbe Raman bands correspondto those used in Fig. 1.The full scalein (b) is twicethat in (a).

caxis,whichisnearlyperpendicularto the planeof the C03, is trivial. If the three axes of SrC03 II correspond directly to those of SrC03 I, the volume contraction in SrC03 I during compressionis mainly along the direction of the b axis.Thea axisof SrC03 I is less compressiblethan the b axis. This may partly explain the mode softening of mode Bzg (M-C03

translation (x), line 6 in Fig. l(c)), though the directions of both a and b axes in SrC03 II need not be the same as those in SrC03 I. Theseinferencesare consistent with the previous speculation, i.e. that the SrC03 I -+ SrC03 H transition is related to the displacementof C03 groupsalong a directionparallel to the plane of the C03 triangle or the rotation of C03

Table 2. AmbientRaman frequenciesand the valuesof the constantsderivedfrom (v = V.+ aF’+ b~ + c~) for the various modesof botb SrC03 I and SrC03 H at room temperature Phase

Line no. 7 6 3

SrC03 I

2 1

SrC03 II

5’ 4’ 2“ 11/ 1/!/ 1II

Ambient 148 183 248 263 701 711 1073 1072~ 1079 1446 1546

Vo

a

b (X 10-4)

c (x 10-’)

Rt

151.6 181.6 250.9

0.205 0.586 0.368

4.0 –7.3

–5.81

0.998 0.9999

0-324 0-352 0-224

699

0.123

0.998

0-160

0.9999 0.997

0-378 0-352

1073.4 0.269 1071.4$ 0.293

215.4 256.1 717.9$ 1086.1$ 1100.75 1085.7~

0.255 0.294 0.158 0.217 0.252 0.220

tR =coefficientof regression. $Data colleetedfrom the syntheticSrC03 powders. $Basedon both compressionand decompressiondata.

–0.93 –2.6

–5.63 –4.3

0.996 0.986 0.995 0.999 0.996 0.993

Pressurerange (kbar)

350-425 350-425 160-425 160-402 140-420 210-372

982

C.-C. LIN and L.-G. LIU

Table3.Latticeparameters(,&)ofthecarbonatesat ambient [3].The samplequenchedfrom high pressureand high conditionsand theirchangeaftertransformation(M= Sr, temperature yielded a Raman spectrum possessing Ba,andPb)t the same frequencies (in particular the 1086cm-’ Phase band) as those of the recovered sample shown in Normalized~ Fig. 1 at zero pressure. Thus, it is inferred that the Carbonates MC03 I MC03 11 axis change (7.) present high-pressurephase of SrC03 formed above a 5.1074 10.151A –0.62 350kbar at room temperature should be the same as b SrC03 8.4144 16.17,& –3.91 the one revealed in our earlier studies at high presc 6.029A 6.045~ –0.27 sures and high temperatures. Line 1* in Fig. l(a) a 5.1954 lo.395aA –0.05 b PbC03 8.4364 16.39.$ –2.86 should be the remnant band of SrC03 II. c 6.152A 6.18A 0.46 It is also noteworthy that the loop-like pressure 5.314A 10.927a~ –2.81 dependence of frequency cannot be observed if the : –4.14 BaC03 8.904i$ 17.07A samples have not been subjected to more than c 6.430A 6.373A –0.89 350kbar at room temperature. The Raman frequentParameters u, b, and c of MC03 I are based on Pmcn. Data citedfromLinand Liu [3],JCPDS 5-378,5-417,and ciesof samplessubjectedto below350kbar followthe 5-418. exact compressionpaths during decompression.This ~For a and b axes, the normalized axis change is may simplybe explainedby the fact that SrC03 H has 100[2u0– a’]/2a0,but it is IOO[aO – a’] for c axis, where aOand a’ are the parameters of MC03 I and MC03 H, not been formed below 350kbar. respectively. Line l’” in Fig. l(a) has only been observedon one occasion where the hydrostaticity of the crystal was groups around an axis that is perpendicular to the lost before 150kbar. It should be one of the Raman C03 triangle. bands ofSrC0311 and isorientation-relatedbecauseit Similarargumentsrelatingphase transition to rota- was observedin some crystals but not in the others. tion of a planar anion group and mode softeninghave Other internal modesof SrC03 II (not shown)for this been reported for the pressure-inducedferroelectric non-hydrostatic crystal also appeared at w240kbar, transition in NaN03 [19, 20] and the orthorhom- indicating that the transition was promoted by the bic-+ disordered-calcite transition in BaC03 and presence of shear stress. The same effect of nonSrC03 at high temperatures [4]. The rotation of hydrostaticity(or shear stress)has also been observed anion groups in these examplesleads to a change in on the I + 11transition of BaC03 [3]. space group. Line 1“for syntheticSrC03 occurred at N265kbar On the basis of the above discussion,it is suggested (Fig. l(b)), which is significantlylower than that for that the displacement of C03 groups parallel to the the corresponding Raman mode in natural strontiaC03 plane (or the ab plane in the unit cell of SrC03 I) nite. The natural strontianite used in this work conmay be the most crucialeventin the SrC03 I + SrC03 tains 14mole0/0of Ca. Therefore the results in the II transition. Althoughthe possiblecontribution from syntheticsamplesuggestthat the presenceof Ca may rotation of the C03 groups may not be totally have significantlyinhibited the SrC03 I+ SrC03 II excluded, it seems trivial or may never occur. Of transition. On the other hand, the reverse transition course, either displacement or rotation of the C03 was hardly affectedby the presenceof Ca. groups will inevitably accompany the movement of 3.1.4. Pressure dependence of Raman spectrum of cations and thus a change in the cation polyhedra. SrC03 11. Nine Raman bands of SrC03 11 were Likewise, the pressure dependencies of Raman observed at pressures above 350kbar. However, frequenciesduring decompressionmay also be related owingto the small range of pressure(350= 470kbar) to the movement of C03 groups along the opposite spanned, the pressure dependence for most of these route, beginningat N 250kbar, stoppingat N 160kbar bands cannot be reliably determined except for lines and forminga hysteresisloop. l“, l’”, and 2“, because the latter lines persisted to All Raman bands correspondingto SrC03 H dis- lower pressures. The data of regression for these appeared below about 160kbar exceptlines 1’and 2’ modes of SrC03 II are also given in Table 2. As of the natural strontianite. The latter may correspond would be expected,all Raman modes of SrC03 H are to line 2 of SrC03 I with a slight shift towards high less compressible than the corresponding modes of frequencyafter compression.The line marked 1*for SrC03 I. It is interestingto note that the frequencyfor the natural strontianite in Fig. l(a) may correspond line 1“of the natural strontianite extrapolated to zero either to line 1’or line 1“for SrC03 II. The frequency pressureis identical to that (1086cm-’) for line 1* of of line 1*at zero pressure is 1086cm-1. In an earlier SrC03 II retained at zero pressure. study, we have subjected natural strontianite under Like BaC03 H [7], SrC03 II is also kinetically non-hydrostaticconditionsat ~220kbar and w1OOO”C unstable under ambient conditions.The retrogression

983

Phase transitions in strontianite

structure. Therefore the structure of cerussite may not be of aragonite, or at least it is not a perfect Pmna (or Pmcn (62)) in space group. Durman et al. [22]suggestedthat cerussitehas a distorted aragonite 3.2. PbC03 lattice witha spacegroupPna21—a non-centric space 3.2.1. Ambient Raman spectrum of cerussite group. The assignmentsof Raman bands for cerussite (PbC031). The ambient Raman spectrum of cerus- in Table 1 are based on the aragonite lattice, and site in the range 100–1600cm-’ gives 14 bands: 131 thereforemay be erroneous. However,it is likelythat (w), 150(m), 176(m),219(m),245(w,sh),673(w),682 the fivebands of 131,150,176,219,and 245cm-’ are (m),696(w),839(m), 1054(vs), 1063(m, sh), 1372(s), the lattice modes, whilethe other nine bands are the 1418(w,sh),and 1474(s).Theyare in goodagreement internal modes. If this discrimination is correct, a with those reported in literature [8, 9]. Bands at 673 greater number of internal modes may indicate and 696cm–’ were not always observed.Thus these that the symmetry of cerussite is lower than that of two bands may be polarized, i.e. orientation-related. aragonite. 3.2.2. Pressure dependence of Raman spectra of The structure of cerrusite is, in general,considered to be of an aragonite type [21].However,this may be cerussite. Figure 3 shows the Raman bands of questionable if one compares the Raman spectrum cerussite as a function of pressure. Selected Raman with those of other aragonite-typecarbonates.Table 1 spectraat variouspressuresand room temperature are listed the corresponding Raman bands of cerussite displayed in Fig. 4. Six additional bands appeared and the other three aragonite-type carbonates. It is during compression,three of them (lines 3, 7, and 9) found that eight bands (673,682,688,696,912, 1372, may be the internal modes and the other three (lines 1418,and 1474cm-’) in cerussitecannot be fittedwell 12, 16,and 17)may be the lattice modes.The possible with those vibration modes in the aragonite-type vibration modes for these additional bands are given rate of SrC03 II depends on the residual stressof the sample as revealed in our earlier study without a pressuremedium [3].

1200

1

I

I

I

1

1

!

(a)

I

I

(b)

400

t

----* -*--* -*-----*-- * -*.*

‘4

11’● ●

i

-

800-

750 -

700

,,o~ o

50

100 150 200 250 300 350 Pressure, kb

400

100I

o

‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ I 50 100 150 200 250 300 350 400 Pressure, kb

Fig. 3. Pressure dependenceof Raman frequenciesof PbC03 under quasi-hydrostaticcondition and at room temperature: (a) internal modes, (b) lattice modes. The open symbolsrepresent the data obtained during compression,while the solid symbolsrepresentthe data obtained duringdecompression.The curvesand linesare used for guidanceonly.

C.-C. LIN and L.-G. LIU I

I

I

I

I

I

(a)

1,

3’

(b)

1’ 1“

17’ h 14,13’ I 1, 395,,+

2’

395,,

301

11

2’

213

1

1

14

/

3 I 12

6

4

0.001 i.. . 1 100 200 300 400 500 600 700 800 900 Raman shift, cm-1

)

950

I

!,

I

1000 1050 1100 1150 1200

Raman shift, cm -1

Fig. 4. SelectedRaman spectra of PbC03 as a functionof pressure at room temperature. The spectra werecollectedduring compression.Numbers besidethe Raman bands correspondto those usedin Fig. 3. The asteriskdenotesthe band of H20(,). The full scalein (a) is one third that in (b).

inside the parentheses in Table 1. However, these assignments are tentative, especially those for the internal modes. Except for lines 1, 3, and 4, the Raman frequency for all other bands increasedwith increasingpressure. A phase change,whichis discussedin the next section, takes placeat*170 kbar. Line 7 splitinto lines7and 9 above N1OO kbar. Below170kbar, lines4 and 6 can be fitted wellby polynomialsof degreeof 3. The pressure dependencefor other Raman bands can be fitted by either a linear or a quadratic regression.The constants and coefficientsof regressionare givenin Table4. The very much lower R value for line 9 is due to its frequency being nearly pressure independent. Like strontianite, the pressure dependenceof Raman frequenciesin the lattice modes is far larger than that in the internal modes. This indicates that the volume compressionis contributed mainly by contraction in the cation polyhedra. 3.2.3. Phase transformation incerussite. The phase transition of cerussite at room temwrature is somewhat similar to that of strontianite. For the lattice modes,a discontinuityin the v–P plane was observed to occur at *170kbar for all the bands exceptlines 10 and 16which disappeared before 170kbar. For inter-

nal modes, the changeis more complex.Line 1can be fitted well with a linear regression below 325kbar, then it shows mode softening and finally displays a discontinuouschange at ~350kbar. The other internal modescan bedividedinto two groups:lines3and 4 may be attributed to the out-of-planebendingof C03, whilelines6, 7,and 9 may be attributed to the in-plane bending of C03. The transition pressure at room temperature is vague for these bending modes. Lines 6 and 7 disappeared at ~190kbar. Line 9 is the only band that acts like those lattice modes and displays a discernible discontinuity in the v–P plane at ~210kbar. Line 2’whichappeared at ~170kbar and line 5’ at ~210kbar are the two likely bands for PbC03 II. These results suggest that the PbC03 I + PbC03 11transition starts at -170kbar and may finishat ~210kbar at room temperature. When pressure was further increased, more new bands were observed, i.e. lines l“, l’”, 8’, 11’, 14’, 18’,and 19’.This isnot Iikelyto be due to another new phase becausethese bands did not appear at the same pressure.The absenceof these Raman bands at lower presures may be attributed to their weak intensity or crystal orientation. The in situ Raman spectrum of PbC03 H obtained in our earlier study at ~220kbar

Phase transitions in strontianite

985

Table 4. AmbientRaman frequenciesand valuesof constants derivedfrom (v= VO+ aP + b~ + c~) for the variousmodes of both PbC03 I and PbC03 II at room temperature Phase

Line no

Ambient

17

PbC03 I

16 15 14 13 12 10 9 7 6 4 3 1

PbC03 11

19’ 18’ 17’ 15’ 14’ 13’ 12’ 11’ 8’ 9’ 7’ 5’ 4’ 3’ 2’ 1’ 1# 1/!)

131 150 176 219 673 682 696 839 1054 1063 1372 1418 1474

b (X10-3)

VO

a

61.6 122.1 131 149.4 175.1 199.1 220 687.9 642.6

0.712 0.501 0.724 0.673 0.630 0.911 0.987 0.0002 0.584

681.3

0.0782

2.32

837.3 912.4 1053.9

–0.00919 –0.570 0.241

–1.83 1.44

125.1 128.1 132.8 162.4 194.9 252.1 263.2 265.lt 668.3t 674.4 692.3 714.1 822.5 885.7 1049,3 1058.6t 1066.4t 1087.6t

0.113 0.226 0.295 0.308 0.301 0.0781 0.279 0.424 0.126 0.0631 0.131 0.196 –0.0707 –0.203 0.142 0.193 0.209 0.208

c (x 10-6)

112-172 73-131 0-172 0-172 0-172 25-172 0-152 112-213 73-192

–8.22

0.999

0-192

5.51

0.992 0.997 0.999

0-172 73-172 0-325

0.797 0.992 0.998 0.996 0.997 0.997 0.996 0.968 0.867 0.981 0.997 0.993 0.985 0.982 0.993 0.986 0.985 0.981

300-395 341-368 172-395 172–395 213-395 172-395 172-325 395-310 213-395 213-395 213-395 213-395 172–395 172–395 192-388 358-251 168-395 310-395

–1.51 –1.37 –1.13

–0.103 0.179

Pressurerange (kbar)

0.996 0.998 0.9997 0.9998 0.996 0.9998 0.999 0.022 0.999

–1,43 –1.05

0.511

R

tBased on both compressionand decompressiondata. after heating at w 1000”C under non-hydrostatic conditions [3]is the same as that observedat ~220kbar

and room temperature in the present study. Therefore it is proposed that the high-pressurephase of PbC03 formed at above 170kbar and room temperature in the present study is the same phase as that formed at high pressuresand quenchedfrom w1OOO”C. The mode softening of line 1 at pressures above 325kbar onlyindicatesthe changeof PbC03 II from a metastable state to a stable high-pressureform. The PbC03 I -+ PbC0311transition islikelyto involvethe spatial hindrance of the C03 group with a mechanism similar to that in SrC03 described in Section 3.1.3. Althoughthe modesofteningin the presentsamplesof carbonate was attributed to the spatial effecton the displacementof the C03 group, it need not be directly correlated with the MC03 I + MC03 H phase transition. On the other hand, the fact that no mode softening was observed in BaC03 suggests that mode softening in these carbonates may also be related to the cation size.A larger cation (such as Ba2+)givesa

larger and open cation polyhedronand thus it has less spatial effect on the displacement of the C03 group under compressionif the crystal structure for all these carbonates is the same. UnlikeSrC03, a hysteresis1oopwasnot observedin the bendingmodes of PbC03. Instead, another behavior of the pressuredependencieswas revealedby lines 3’and 7’duringdecompression(Fig. 3(a)).The change in frequencyfor these two modes during decompression did not followthe compressiontrajectory; rather they showeda slowchangewithpressure.From Fig. 3, we cannot be sure whether part of PbC03 II retained after the pressurewastotally released,though it can be preservedas shownin an earliernon-hydrostaticstudy [3].However,similar to SrC03 II and BaC03 II, the high-pressurepolymorphof PbC03 is also kinetically unstable under ambient conditions. 3.2.4. Pressure dependence of Raman spectrum of PbC03 II. Above 300kbar, 18 Raman bands were observed for PbC03 II. The variations of Raman frequencieswith pressure can be described by either

986

C.-C. LIN and L.-G. LIU

a linear or a quadratic regression. Table 4 lists the constants of the regressions.Measurementsfor some bands (e.g. l’”, 8’, 11’, 18’, and 19’)showed lower reliabilities due to the small number of data being collected. Except for line 9’, all Raman modes of PbC03 11showed smaller dv/dP values than those of the corresponding bands in PbC03 I, indicating that the high-pressurepolymorphis lesscompressible than cerussite.In comparisonwithbothBaC0311 and SrC03 II, more of the observed Raman bands of PbC03 II are orientation-dependent. 3.3. Features and implications of the MC03 I+ MC03 II transformation Ithas been reported that SrC03 II, BaC03 II, and PbC03 11all have the same crystal structure [3].The lattice parameters for all these phases and their correspondinglow-pressurephases are listed in Table 3, in which the changes in axis after transition are also given.If the a, b, and c axesof the unit cellof MC03 I correspond to those of MC03 H, then the changesin axis indicate that the contraction during compression is mainly contributed by the shorteningof the b axis. Except for the c axis of PbC03, all axes of these carbonates become shorter after the formation of MC03 11.The abnormal behavior of PbC03 may be related to the fact that the structure of cerussiteis not of true aragonite type. It wasproposedearlier that the SrC03 I + SrC03 II transition at room temperature may involve the displacement of the C03 group along a direction parallel to the plane of the C03 triangle or the ab plane of the aragonite-typeunit cell. Although mode softeningin the VImode ofPbC03 I (line1in Fig. 3(a)) isnot directlyrelatedto the 1/11transition, it manifests the role of the displacementof the C03 group. Therefore the mechanism of the PbC03 I+ PbC03 II transition at room temperature may be identical to that of SrC03. Comparingthe hysteresisloopsin both Figs l(a) and 3(a), the frequencydifferencebetween the compressionand decompressionbranches of the 1oopat the samepressureis smallerfor PbC03, whilst mode softening was not observed in BaC03. These results seemto support the argument that mode softening for carbonates during compression may be related to the differencesin cation size because the spatial hindranceof themovementof the C03 groupis trivial for a more open lattice or cation polyhedron. The smaller the cation radius, the more prominent is mode softening. Therefore the MC03 I --+MC03 H phase transition for the three aragonite-typecarbonates at room temperature may operate with the same mechanism. On the basisof the above resultsand discussion,the

MC03 I -+ MC03 II transition in the aragonite-type carbonates have the followingfeatures: (1) it can be promoted by shear stress, and therefore may be displacive,(2)the transition involvesthe displacementof the C03 group along a direction parallel to the plane of the C03 triangle or the ab plane of the aragonite lattice, and (3) it is related to the cation size. The mode softening can only be found in smaller-cation carbonates. MC03 H (M = Ba, Sr, and Pb) wasreported to form at and above 40kbar at WIOOODC (the low pressure was limited by the equipment) by Lin and Liu [3]. Thus, it would be expected that these high-pressure polymorphs may also be formed below 40kbar at N1OOO”C. The same high-pressurepolymorphs were formed in the pressure range 80- 350kbar at room temperature in the present and earlier studies[3].This indicatesthat the transition boundary for MC03 I++ MC03 II has a negativeClapeyron slope though the transition mechanismmay not necessarilybe the same throughout the whole range of temperature. The transition pressures for SrC03, BaC03, and PbC03 at room temperature was plotted against the cation size in Fig. 5. Under quasi-hydrostatic conditions and at room temperature, the transition pressure was found to increase steeply with increasing cation size. Extrapolating the curve in Fig. 5 to 1.18~ (radius of Ca2+(XI))predicts that the CaC03

1200I

200 ‘1

~~ 1.2

1.3

1.4

1.5

Cationradius,A Fig. 5. Transitionpressurefor MC03 I -+MC03 11as a functionof cationradius.The transition pressures are those under quasi-hydrostatic compression and at room temperature.

987

Phase transitions in strontianite + CaCOg(SrC03 II-type) transition may take place only at pressures greater than 1000kbar at room temperature. If the MC03 I -+ MC03 H transition is displacive, however, the transition pressure may drop significantly in the presence of a shear stress

that the transition boundaries for MC03 I - MC03 H have a negative Clapeyron slope in a P–T diagram. It was also found that the presenceof Ca in strontianite can significantly suppress the MC03 I + [23].In a shock experiment,Vizgirdaand Ahrens [24] MC03 II transition under compression, but the claimedthat a denserphase of CaC03 (calciteVI)was reverse transition is hardly affected by Ca during obtained at 55w 76kbar. The so-calledcalciteVI has decompression.The recovered BaC03 II, SrC03 H, an estimatedzero-pressuredensity(3.2g/cm3)whichis and PbC03 H are all kineticallyunstable at ambient in close proximity to that (3.15g.cm3) of the post- conditions. The transition pressures for MC03 I + aragonite CaC03 predicted in our earlier study [3]. MC03 H at room temperature are correlated to the However,the crystal structure and the temperatureof cation sizeof thesecarbonates. Byextrapolation it was the formationfor calciteVI werenot reported.Thus it estimatedthat the post-aragonitetransition in CaC03 remains to be confirmedwhether the so-calledcalcite may take place at quasi-hydrostaticpressuresexceedVI is the post-aragoniteCaC03. ing 1000kbar at room temperature. (aragonite)

REFERENCES 4. CONCLUSIONS The post-aragonite phase transitions of strontianite

(SrC03 1) and cerussite (PbC03 1) under quasihydrostatic pressure and room temperature have beeninvestigatedin the presentstudy.ByusingRaman spectroscopy it was found that strontianite transformed to the high-pressurepolymorph (SrC03 II) at N350kbar, whilecerussitetransformedto PbC03 II at w170kbar. Some Raman modes for both SrC03 I and PbC03 I showedmode softening.The mode softening was attributed to the spatial hindrance on the movementof the C03 groups.Futherrnore,it mayalso be related to the cation size,i.e. the mode softeningis more prominent for smaller-cation carbonates. The spatial hindrance is also considered to be the cause of a hysteresis loop in the pressure dependence of vibrational frequency. On the basis of the observedmode softeningand a comparison of the lattice parameters, it is suggested that the MC03 I + MC03 H transition (M= Sr, Ba, and Pb) at room temperature for thesearagonite-type carbonates may operate with the same mechanism— the transition involves the displacement of C03 groups along a direction parallel to the plane of the C03 triangle or the ab plane of the aragonite lattice. However,understandingtransitionmechanismentails a knowledge of the crystal structure of these high-pressurepolymorphs. Of course, the possible contribution of the rotation of C03 groups cannot be totally excluded although it seems trivial or may never occur. The MC03 I + MC03 H transition at room temperature can be promoted by shear stress, and hence may be displacive.Combining the resuks of the present and earlier studies [3]indicates

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10. Yamamoto A., Shiro Y, and Murata H., Bull. Chem. Soc. Jpn 47,265 (1974).

11. Frech R., WangE. C. and BatesJ. B, Spec~rochim.Acla 36A, 915 (1980). 12. Chang L. L. Y., J. Geol. 73, 346(1965). 13. Kraft S., Knittle E. and WilliamsQ., J. Geophys. Res.

96B, 17997(1991). 14. Gillet P., BiellmanC., Reynard B. and McMillan P., Phys. Chem. Minerals 20, 1(1993). 15. ChopelasA., Phys. Chem. Minerals 17, 149 (1990). 16. Gillet P., Fiquet G., MalezieuxJ. M. and Geiger C. A., Eur. J. Minerals 4,651 (1992). 17. Perry C. H., Lu F., Liu D. W. and AlzyabB., J. Raman Specrrosc. 21,577 (1990). 18. De VilliersJ. P. R., Am. Mineral. 56,758 (1971). 19. Barnett J. D., Pack J. and Hall H. T., Trans. Am. Crysta[logr. Assoc. 5, 113 (1969). 20. Lettieri T. R., Brody E. M. and Bassett W. A., Solid State Commun. 26,235 (1978). 21. SpeerJ. A., Carbonates: Mineralogy and Chemistry, ed.

R. J. Reeder. Reviewsin Mineralogy.Vol. 11.MineralogicalSocietyofAmerica,WashingtonDC, 1983.p. 145, 22. Durman R., Jayasooriya U. A. and Kettle S. F. A., J. Chem. Soc., Chem.Commun.916(1985). 23, KrivenW. M., in Proc. Ini. Corf on Solid+ Solid Phase Transformations, edsH. 1.Aaronson,D. E. Laughlin,R. F. Sekerka and C. M. Wayman, The Metallurgical Societyof AIME, Warrendale, PA, 1982,p. 1507. 24. VizgirdaJ. and AhrensT, J., J. Geophys. Res. B87,4747 (1982).