Polymorphism and melting in the alkali nitrates to 40kb with some comments on the alkaline earth carbonates

J. F&s. G’hem.Sol&-&. Pergamon Press 1966. Vol. 27, pp. 1349-1363.

POLYMORPHISM NITRATES

Printed in Great Britain.

AND MELTING IN THE ALKALI

TO 40kb WITH

ON THE ALKALINE

SOME COMMENTS

EARTH CARBONATES*

ELIEZER RAPOPORTf Institute of Geophysics and Planetary Physics, University of California, Los Angeles

(Iiece&& 22 Jdy 1965) phase diagrams of the alkalinitrates were determined by differentialthermal analysis to 40 kb and 7OO’C. LiN03 shows no polymorphic transitions. The effect of pressure on the gradual transition in NaNOs can be expressed by the equation T = 275 +6.076 pO_038475 p* (Tin “C, pi in kb). The melting curves of LiNOs and NaNOs are very similar. The latter flattens considerably at higher pressure and cannot be &ted to the Simon equation. This might indicate an approach to a maximum in the melting curve similar to the one found in KNOs. The RbNOs IV-III and CsN03 II-I transirions were followed to 40 kb. The results are in very good agreement with Bridgman’s lower pressure data. The melting curves of CsNOs I, RbNOs III and KNOs VI rise steeply with pressure. Comparison is made between the alkali nitrates and the alkaline earth carbonates and a tentative phase diagram is proposed for GaCOs based on the analogous KNOs diagram and on available experimental data.

Abstract-The

P~~~o~HIc transformations at high pressure in the alkali nitrates were studied by BMDGMAN(~)by means of the volume discontinuity method to 12 kb and 200°C. Later Brid~an found new polymorphs in NaNOs at 52 kb,(s) in RbNOs above 17 kb@) and in CsNOs at 27.5 kb.3) JAMIESON and LAWSON@) made some prelimina~ and unsuccessful attempts to determine the crystal structure of the high pressure form of NaNOs. Some sim~arities in the phase diagrams of RbNOs, CsNOs and TlNOa were already pointed out in the early work of BRXDGMWQ~Recently RAPOPORT and KENNEDY@) reported the phase diagram of K_NOst which consists of seven polymorphs, to 40 kb. Rapoport and Kennedy suggested that the phase diagrams of KNOs above 15 kb, RbNOs above 2 kb and CsNOa are qualitatively similar, and that the corresponding phases are isostructural. *Publication #t454 Institute of Geophysics and Planetary Physics, University of California, Lo&Angeles. t Now at the Chemical Physics aroup of the National Ph&al and National Chen&al Research Laboratories, CSIR,

P.O. Box 395, Pretoria,

South Africa. 1349

The melting curves of KNOs I and KNOs V exhibit broad maxima(5*s) of the type hitherto known only in the elements cesium,W barium,@) tellurium@) and europium.(lOf The melting curves of the five alkali nitrates to 10 kb were recently

reported by OWBNS.@~-) There is a striking similarity in the melting curves of CsNOs, RbNOs above 2 kb and KNOs VI@) which again suggests that the corresponding solid phases are isostructural. The alkali nitrates are ~~stallograp~~lly very simiiar to the alkaline earth carbonates. The latter are naturally occurring minerals and are of much interest and importance to geologists. The calcitearagonite transition has been extensively studied by several investigators.@-1st The study of phase equilibria in these minerals is difficult, however, since some of the phase tr~sitions take pIace at very high temperatures, while transitions occurring at lower temperatures are extremely sluggish, and often show met~tabili~. Knowledge of the phase diagrams and the structural relationships in the group of the alkali. nitrates might broaden the understan~ng of phase equilibria in the a~aline earth carbonates.

1350

ELIEZER

RAPOPORT

EXPERIh%ENTAL PROCEDURE Analytical grade salts were used throughout this investigation. LiNOs and KNOs were Baker’s analyzed reagents of stated purity of 99.1% and 99.8% respectively. RbNOa and CsNOs were obtained from Koch-Light Laboratories Ltd. and had a stated purity of 99.9%. NaNOs was Mallinchrodt reagent. The salts were used without further purification, and were either melted into metallic capsules or dried at 120°C and then compacted into the capsules. Aluminum capsules pretreated with nitric acid in order to form an inert and protective oxide layer, were used. Nickel and stainless steel capsules were also used. The capsule design, incorporating a thermocouple well, was described previously by PIsToRIus.(rs) Pressures up to 40 kb were generated in a pistoncylinder apparatus previously described.(soJsr) Phase transitions were detected by means of the differential thermal analysis (DTA) technique.(sa~ss) Chromel-alumel thermocouples were used and temperatures were measured to a precision of + 2°C. No correction for the effect of pressure on thermal e.m.f. was made. Heating rates of 48”C/sec were used. Friction corrections were made in the usual mariner@@@@ by comparing the phase boundaries obtained on increasing and decreasing pressures. The double value of friction was typically 2.6 kb at 10 kb pressure, 4 kb at 30 kb and 5 kb at 40 kb. After allowing for friction the reported pressures are believed accurate to & 0.5 kb.

behavior in the melting of KNOs. The present experiments were carried out in stainless steel and in aluminum capsules with consistent results. The fusion curve of LiNOs is shown in Fig. 1.

NaNOs At room temperature NaNOs has the rhombohedral calcite structure. On heating NaNOs undergoes a gradual transition that starts around 180°C and is characterized by a marked weakening of the X-ray reflections to which only the oxygen atoms contribute.(ssps4) The transition terminates sharply at 275.5”C resulting in a complete disappearance of these X-ray reflections. The specific heat curve(s5) shows an anomalous increase which starts around 200°C and terminates in a ‘cusp’ (lambda point) at 275°C. Such anomalies in the specific heat are known in paramagnetic and ferromagnetic materials and in order-disorder transitions in alloys. The thermal expansion exhibits strong anisotropy,@-2s) being greatest along the hexagonal c-axis. There is an anomalous increase above 180°C. Both the thermal expansion coefficient along the c-axis and the volume expansion coefficient end in a cusp at 275” and then abruptly decrease back to their normal value at 280°C. In early investigations of the transition,(ssJ7) it was suggested that the transition is due to rotation of the nitrate ions about the hexagonal c-axis. The problem was later carefully re-examined by KF~TELAAR and STRrIK(ss) and by SIEGEL. The latter concluded that the nitrate ions are disordered rather than rotating, i.e. the nitrate group assumes EXPEFUMENTAL RESULTS AND DISCUSSION with equal probability both its original position and one rotated by 180” from it, and that the disThe alkali nitrates decompose about 70°C above their melting points. This decomposition will ordered nitrate has an aragonite, rather than calcite, type of coordination with respect to the neighborprobably be suppressed by pressure. Nevertheless, in all the experiments care was exercised not to ing sodium ions. The position of the sodium ion remains unchanged. Similar type of disorder was exceed the melting point by more than 30-40 deg. suggested by LANDER@~) for the high-temperature hexagonal forms of the alkaline earth carbonates LiNOs and is also known in KNOs I.(s1J2) LiNOs crystallizes in the rhombohedral calcite In the present experiments the beginning of the lattice. No solid-solid transformations are known transition on heating could be detected as an inin LiNOs up to its melting point and up to 40 kb. DTA signals obtained on melting of LiNOs were flection in the DTA curve. A second inflection marked the end point of the transition. These strong and somewhat broad. The freezing signals inflections, due apparently to changes in thermal were sharper and freezing points could be determined more precisely than the melting points. NO conductivity and specific heat, did not allow any supercooling behavior was observed. This was also precise determination of the transition point. It is known from zero pressure experiments that the the case in NaNOs. BABB et al.@) observed similar

POLYMORPHISM

200

AND

MELTING

IN

TO

NITRATES

3

I

30

PRESSURE FIG.

ALKALI

20

1

IO

THE

40 kb

1351

40

(KEAR)

1. The fusion curve of LiN03 to 40 kb.

beginning of the transition could not be determined accurately.(ss~s7) On cooling, however, small, consistent and very reproducible signals were obtained. These signals were taken for determinations of the transition temperatures. A portion of a typical record showing both the temperature and the DTA curve as recorded on a strip-chart of a two-pen recorder is reproduced in Fig. Z(a). For comparison similar records for the melting of NaNOs and for the IV-III transformation in RbNOs are reproduced in Fig. 2(b) and 2(c) respectively. The trajectory with pressure of the transition was determined to 40 kb. The transition has the same features at high pressure as described in previous investigations at zero pressure. It seems most probable that the transition line will meet Bridgman’s 52 kb transition in a triple point possibly around 55-65 kb and 450°C. This is well outside the range of the present experiments. Signals obtained on melting of NaNOs were large and somewhat broad similar to those observed in LiNOs. Experiments were performed in aluminum, nickel and stainless steel capsules. The experiments in stainless steel capsules showed some slight contamination and were disregarded. The melting curve of NaNOs at low pressure is in good agreement with that determined by OWENS. At higher pressures the melting curve

flattens considerably, and this might be an indication of an approach to a maximum. The phsse diagram of NaNOs is shown in Fig. 3. RbNOs and CsN03 At zero pressure four polymorphic forms of RbNOs are known between room temperature and the melting point.@%%) The zero pressure transition temperatures are : 164°C IV----+

219°C III -+

291°C II------+

310°C I----+

liquid. BRIDGMA#) followed the IV-III transition (II-III in Bridgman’s notation) to 6 kb. Recently OWENS determined the melting curve of RbNOs in the range 2-8 kb and presented a tentative phase diagram of RbNOa to 10 kb based on available high pressure determinations and room pressure thermodynamic and crystallographic data. In the present investigation the III-IV transfo~ation was followed to 40 kb. Sharp and strong DTA signals were obtained with no hysteresis upon cooling. The agreement with Bridgman’s results at low pressure(l) is very good. At higher pressures the phase boundary has a slight curvature. The phase diagram of RbNOs is shown in Fig, 4. The IV-V transition was observed by BRIDGMAN,(~) but the large experimental uncertainties did not allow an exact determination of

1352

ELIEZER

RAPOPORT INCREASING

TIME.

b

C 240O

FIG. 2. Record showing differential thermocouple response and sample temperature in a typical study of phase transformations (a) Transition in NaNOa. (b) Melting of NaNOs. (c) The IV-III transformation in RbNOs. Transition points and DTA signals are marked as arrows.

600 t

I

I

5

IO

I

I

15

20 PRESSURE

FIG.

I

25

I

30

1

35

I

40

POLYMORPHISM

AND

MELTING

IN

the boundary and the III-IV-V triple point. However, in the present investigation an abrupt and reversible change in the signal size of the III-IV transition was observed above 25 kb and 336°C. The size of the DTA signal is roughly proportional to the heat of the transition and the observed change was taken as an indirect indication of a change from the IV-III to the V-III boundary.

THE

ALKALI

NITRATES

TO

40

kb

1353

point occurs near 27.3 kb and 335°C. The phase diagram of CsNOs is presented in Fig. 5. GENERAL

DISCUSSION

The pressure dependence of the transition in NaNOs can be described by the equation T = 275+6+076p-0.038475~2

20 PRESSURE FIG.

IKEAR)

4. The phase diagram of RbN03 to 40 kb. The arrow marks the point where the change in signal size occurred.

The arrow in Fig. 4 indicates the place where the change in signal size occurred. The III-IV-V

obtained by fitting the experimental data to a polynomial by the method of least mean squares. T

1354

ELIEZER

RAPOPORT

PRESSURE(KBAR) FIG.

5.

The

phase diagram of CsNOs to 40 kb. The arrow point where the change in signal size occurred.

order transition with small latent heat and volume change. For a second-order transition the initial slope can be calculated by means of the Ehrenfest Relations(s5Js) AL% dTk = VAT,---

dp

AG

dT,,

AK

dp

Aa

-=-

where V, and TA are the volume and temperature (“K) at the X-point. AC,, Au and AK: are the change in specific heat, thermal expansion coefficient and compressib~ity, respectively. In the absence of compressibility data only the first of Ehrenfest’s relations could be tested. Pippard’s cylindrical approximation method@@ was used. C, values of h!h_Js~~~oKd~~) were plotted against the DCvalues obtained by KR&SSK(27) for corresponding temperatures and a straight line with a slope of 5.25 x 10s caljmole was obtained. This plot is shown in Fig. 6. The equation of the line is 4 VAT&a-t-cons t. C, = dTn From this the value of the initial slope ~~~~~~) calculated to be 1 I *7”C/kb as compared

was

marks

the

to the experimental value of the initial slope of 6.1 f @2”C/kb. The usefulness of such a plot is, however, made somewhat doubtful by the sensitivity of the values of cc and C, to the temperature. Considerable errors can be introduced if the temperature scales used in the determinations of u and C, differed by as little as O*Z”C, In practice we transformed the data of MusTA~oKI(~5) to yield a X-point at exactly 275*5”C, which is the temperature obtained by KRACEK.(27) For a first order transition the initial slope dT/dp is given by the Clausius-Clapeyron equation

dT dp-

_-

TAV AH

reporting specific heat measurements on NaN03, stated that the specific heat had a h-point but yet reported a latent heat for the transition of 944 Cal/mole. If we assume the difIerence between the volume at 275 -5°C and the volume obtained by extrapolation from expansion data below 200°C to yield AV, we obtain AV = 7.5 x 10-s cmslg, which in turn yields an initial slope of 8*8”C/kb. It wouid consequently seem that it is not at this stage possible to make a definite decision as to whether the NaNOs transition is thermodynamically first- or seeondorder. MusTAJoKI,(~~)

while

POLYMORPHISM

AND T&e

Solid LiNOs

NaNOs

(2)

IN

THE

ALKALI

1. Simon paTamet~s for the a&&

$1

&

c

&)

NITRATES

III

40

kb

Average deviation (“Cl

Source

0

526

15-O

2.15

32-2

6

Owens

527

10.76

2.76

29-7

2-5

Present work

0

527

10.87

2.76

29.9

2.8

Combined, present work and G. Owens

0

579

9.73

3.9

37.9

0.6

Owens

9.2

Present work

12.5

633

l-9

588

5.1

3.5

17.9

2.5

Owens

l-9

588

7,2

2-60

18.7

2-8

Present work

l-9

588

7.14

2-62

18.7

2-8

Combined, present work and Owens

The melting curves of the alkali nitrates were fitted to the Simon equation

1355

ligates

could not be fitted

KNOs VI

TO

0

NaNOs

RbNOs

MELTING

3-o

27.5

2-o

Rapoport Kennedy

and

The alkali nitrates as a part of the large crystallographic group of sofids of the ABOs type exhibit an array of crystal structures and it is instructive to attempt some comparisons. Crystallographic P-P* = data on the univalent nitrates were recently reviewed by WYCKOFF(37) and by MCLARRN.(33) by the method of least mean squares. The values Some of the pertinent data is collected in Table 3. of A and c are collected in TabIe 1. Comparison LiNOs and NaNOs have the rhombohedral calcite is also made with the values obtained by OWEHS@~) structure. NaNOs undergoes a transition to a disfor the melting curves below 10 kb. The agreement ordered calcite structure. The analogous transiis good except in the case of NaNOs. Although tion could apparently have occurred in LiNOa Owens could fit his data to the Simon equation but is probably intercepted by melting. The posover the restricted pressure range of his experi- sibility of this transition occurring in LiNOs at ments the curve flattens considerably at higher pres- high pressure cannot be ruled out on the basis of sure and there it cannot be fitted to the Simon the present experiments due to the difficulty in equation. In view of the fact that the fusion curve detecting such a transition. At room temperature and pressure KNOa of KNOs exhibits a maximum this might be taken as some indication of an approach to a maximum crystallizes with the orthorhombic aragonite strucin NaNOs that possibly occurs around 55-65 kb. ture. This structure is not known in the other The fusion curves of the alkali nitrates were also alkali nitrates but exists in the heavier alkaline earth carbonates. The 52 kb tr~ition in NaNOs might fitted to a quadratic equation. be to an aragonite phase. The structural data on T = T,,+ap+bp2 the KNOa polymorphs including high pressure KNOs III and KNOs IV were recently reviewed where T and To are in “C and p in kb. The by DAVIS and An~~s.(ssJs) Recently BROWNand coefficients are collected in Table 2. MCLAREN@*) reported that the structure of

+&I]

0.2 0

I

i 20

1

I I 1 40 60 a .105(da$‘)

1

I 80

I 100

FIG. 6. Specific heat Cp, plotted vs. the thermal expansion coefficient bl in the vicinity of the h-point in NaNOs (Pippard’s method).

Table 2. MeZting curves of the alkali nitrates. Least square equations

(Gkb)

T = TOf ap +bp2+

Average deviation

(“C4kbs)

-

Source

250

lS.l*

-0.364

4.6

Owens

255

16.79

- 0.295

3-18 x 10-3

3-o

Present work

2.53

16*90

- 0.959

3.15x10-3

3.0

Combined, present work and Owens

306

14.7

-

o-7

Owens

310%

12.6

-0.154

-

3.2

Present work

310

12.69

-0-W

-

3.0

Combined, present work and Owens

KNOs VI?

53.3

28.88

- 0,327

-

3.5

Rapoport Kennedy

RbN03

245

39.1

1.38

-

l-9

Owens

262

29.74

-0.466

-

3.4

Present work

261

30.01

- 0.483

-

3.0

Combined, present work and Owens

404

29.2

LiNOs

NaNOs

CsN03

III

0.313

---

* Owens data is expressed 7 For p > 12.5 kb.

in “C/katm.

and

Owens

______ _-.

--

__-

cp3

Salt

p(bars,

LiN03

1

PC

20”

Cell constants a0

=

Crystal type

Space group

5,747

a=4&QlX’

source

Rhombahedral {calcite)

Ref. 37

N&JO3II

1

23’

as = 6‘3247 dL= 470 16’

Rhombohedral (calcite)

Ref. 37

NaN03 I

1

310°

ao = 6.460 0: = 45” 31’

Rhombohedral (calcits-type)

Ref. 37

I(N03 f

1

33!?

ao = 7,542 OL= 42” 04’

Rhombohad~ &al&e-type)

Ref. 37

KN03 II

I

ffo = 9.1708 bo = 6425.9 co = 5.4175

Orthorhombic (aragonite)

Ref. 37

cro = 4.365 iL = 7ci0 56

Rhombohedrd (=QOa-type)

Ref. 37

O~horhombi~~?)

Refs, 32 and 47 Ref. 33

KN03 III

1

KN03 IV

4000

RbNOs I

1

RbNOs II

1

RbNOa III

1

RbN03

IV

1

CsN03

I

1 2

CsNUs II

120°

25”

?

Cubic

a = 7.31

Trigonal or tetragonal

Ref. 33

T$-Pa 3

Cubic

Ref. 37

a = 10.48 c = 7.45

P3112 or P3a 12

Hexagonal

Ref. 33

161’

a = 8.980

Ti--Pa3

Cubic

Ref. 37

-2.5”

a = lO”87

Hexago&

Ref. 33

1W” -25”

Q0 = g-74

E =

Ahost terti&y P3,fZ or P3,12

7.76

(compare RbNOz IV)

-__I___ RbNUs IV is‘ hexagonal and not ~~horhombic as stated in the ASTM Data File. C&Us II is also reported hexagonal.(s) The close similarity of the unit-cell constants of CsNOs II and RbNOs IV strongly suggests that CsNO3 II is isostructural with RbN03 IV, and consequently possesses the space group P3112 or p3zI2. RbNUs III and CsNUa I are cubic. No cubic polymo~hs are known among the lighter alkali nitrates. It is possible that one or more of the high pressure polymorphs found in KNOs by RAPOPORT and KYWNEDY@)may indeed be cubic. The type of struttural rearrangements taking place during the

-.“-

---

RbNUs IV-III and III-II transitions were discussed by BROm and ~~~~.~~~ COMPARISON WITH THE ALKALINE CARBONATES

EARTH

The carbonate ion is marphologically almost identical with the nitrate ion. Potassium and calcium occupy adjacent positions in the periodic table and thus the univalent potassium ion and the divalent calcium ion have the same electronic canfiguration. The two isoelectronic solids KNQs and CaCOs allow some interesting comparisons of their corresponding crystal structures and phase

1358

ELIEZER

RAPOPORT

500 -

0

FIG. 7.(a) ~xp~~~~~ly (see text). (b> Proposed

I 30

I 40

determined boundaries in tile C&03 pime diagram phase diagram for CaCOs. Solid lines represent experimentaily determined boundaries.

POLYMORPHISM

AND

MELTING

IN

THE

ALKALI

NITRATES

TO

40 kb

1359

diagrams. The same considerations also apply to conclusion that CaCOs II and CaCOa III were the pairs LiNOs and BeCOs, NaNOs and MgCOa, minor modifications of the calcite structure. JAMIESON@~) subjected aragonite to pressure of RbNOs and SrCOa, and CsNOa and BaCOa. Many of the structures that occur in the alkali 24.4 kb and showed that no transition to either nitrate group also appear in the carbonates of the CaCOs II or to CaCOs III occurred. Jamieson alkaline earths, and phase transformations in these presented density data and thermodynamic argutwo groups of solids have a number of features in ments that led to the conclusion that CaCOa II and common. Phase transformations in the carbonate CaCOa III are less dense than aragonite and theregroup occur at much higher temperatures and fore unstable with respect to aragonite. The transitions are thermopressures than in the nitrate group due to the CaCOs I-II and II-III stronger forces between the doubly charged ions. dynamically allowed but are metastable. Jamieson CaCOs occurs in nature in two forms. Calcite constructed a phase diagram of CaCOa and sug(rhombohedral) is the predominant form and is the gested that CaCOa II is a disordered-type calcite stable phase at one atmosphere. Aragonite (ortho- analogous to the high-temperature forms of rhombic) is a high-pressure form metastably NaNOa, SrCOa and BaCOa and tried to extraretained to atmospheric pressure. Its relatively rare polate the I-II line to 975°C at which some occurrence implies a limited stability field. reversible change in the heat content was reported Boundaries in the CaCOs phase diagram deter- by BoEKE.(~~) Recently DAVIS reported results of highmined by various investigators using different high-pressure devices are collected in Fig. 7(a). pressure X-ray diffraction on CaCOs II and Line A represents the calcite-aragonite trans- CaCOa III. Formation of CaCOa II from CaCOa I formation that was the subject of several in- involves a reduction in the hexagonal c-axis vestigations.(ls-l*) Of these the results of JAMIE- which is inconsistent with anion disorder. The latter involves expansion in the c-axis as in so~fl2)and of CLARK,(lQ)obtained in hydrostatic apparatus, may be considered the most reliable. the case of NaN0s.(2s~sa) The diffraction pattern The boundaries determined by MACDONALD~~) of CaCOs III could be indexed as orthoand by SIMMONS and BELLS are also consistent rhombic KNOs IV type and Davis suggested if one considers the pressure uncertainties in the that CaCOs II is isostructural with KNOa III thus ‘squeezer’ type apparatus. Line B was recently making a complete analogy with the KNOa phase determined by BELL and ENGLAND and re- diagram.@*@ Construction of the phase diagram of CaCOa is presents a rapid and reversible transition considered by these authors to be the calcite-aragonite transi- difficult due to the fragmentary evidence that is available and to the metastable behavior of aration that becomes rapid at high temperatures. Line E, also determined by Bell and England, gonite. The calcite-aragonite transformation is reconstructive and extremely slow. Rate studies represents results of quenching experiments. showed that 99% conAragonite was quenched below line E. Quench by DAVIS and HAMS products from the region between lines B and E version of aragonite to calcite requires times of the were invariably calcite in the form of fine powder@s) order of 1010 years at room temperature and lOa possibly indicating an as yet unobserved rapid years at 200°C. The aragonite-CaCOa II and transition with a large volume change. Bell and aragonite-CaCOs III transitions very probably England summarized : “There is a small probability have similar rate behavior. This explains why that this is an entirely new phase, but it cannot be Jamieson had to resort to indirect methods for the determination of the calcite-aragonite transition verified at present.” Lines C and D represent the CaCOa I-II and below 100°C and why no transition was observed CaCOs II-III transitions, respectively, that were in aragonite at 24.4 kb and 25°C. discovered by BRIDGMAN.(~~)These transitions The evidence presented here leads us to postucould be detected only when calcite was used as late that there is a complete analogy between the the initial material. The small volume change phase diagrams of KNOa and CaCOs and that each associated with the transitions (0*00135 and phase in the CaCOa diagram has its counterpart in 0*00956 cm3/g respectively) led Bridgman to the the KNOs diagram. For the purpose of comparison

1360

ELIEZER

RAPOPORT

the phase diagram of KNOs is reproduced in Fig. 8. Analogous phase diagrams in structurally and morphologically similar inorganic solids were demonstrated in the ~onium halides@@ and very recently in the pair NasSO* and NasCrO+.W A tentative phase diagram proposed for CaCOs is given in Fig. 7(b), constructed on the basis of the analogous KNOs diagram and the experimentally determined phase boundaries of CaCOs itself.

0 0

calcite in the form of fine powder indicating inversion.( 18~s) These considerations and the difference in slope between lines A and B suggest that lines A and B must intersect at a triple point somewhere around 14 kb and 700°C. This is proposed to be the CaCOs I-II-aragonite triple point. (2) Line E represents another stable boundary determined by means of the quenching technique by BELL and ENGLAND. Aragonite was

30

20

10

40

PRESSURE (KBAf?S) FIG. 8. The phase diagram of KNOI to 40 kb (after Rapoport and Kennedy,

Some impo~ant observations must be made: (1) line B is definitely a stable boundary and is not the calcite-aragonite boundary as was originally thought by BELL and ENGLAND@*) who determined it by means of DTA techniques since aragonite could not be quenched from the region between lines B and E. It is therefore suggested that line B is the calcite-CaCOs II stable boundary. The boundary between the corresponding KNOs I and III which are isostructural to calcite and CaCOa II, respectively, occupies the same relative position in the KNOs phase diagram. CaCOs 11 is known to transform back to calcite upon release of pressure and this is the reason why the quench products between lines B and E were

Ref. S).

the quench product in the region below line E. This boundary b.as a very peculiar shape. It starts off with a rather steep slope, then shows sharp curvature and ends with a straight line portion E’. Inspection of the corresponding KNOs diagram reveals that this is exactly the course taken by the KNOs III-IV and IV-VI boundaries. The straight line portion is the KNOa IV-VI boundary joining the former in a triple point. Since KNOs III and K.NOs IV are isostructural with CaCOs II and CaCOs III respectively,(*s) it is therefore proposed that the region below line E is the stability field of CaCOs III. It is further suggested by analogy with KNOs that line E also consists of two boundaries joining together in a

POLYMORPHISM

AND

MELTING

IN THE

triple point at around 25.5 kb and 800°C The straight-line portion E’ then represents a possible CaCOs III-IV boundary. CaCOs IV is as yet an unknown phase that probably occupies the aame position in the CaCOs diagram as that occupied by KNUs VI in the KNUs diagram. A tentative third boundary (CaCOa II-IV) joining at the C&X&II -III-IV triple point is indicated as the dotted line F in Fig, 7(b). (3) Density data on CaCOs

ALKALI

NITRATES

TO 40 kb

1361

III-II upon pressure rekzase is made impossible. The only transformation on releasing pressure that can then take place is the transformation from the orthorhombic CaCOs III phase to the orthorhombic tiagonite phase which is indeed thesmodynamically possible as Davis’s density data(@) show- This is a plausible explanation of the fact that the quench product below line E is aragonite. Similar experience is borne out in the experiments

Table 4. Density of CaCO3 polymorphs

Phase

Temperature (“C>

cecos II CaCOs III caco3

III

Aragonite

25” 25” 25O 209 0 2%

polymorphs used by JAMIESOW@~) and new data by DAYIS@@ are compared in Table 4. Davis’s data show that CaCOs III (density 3.17 gjcm3) is denser than aragonite (extrapolated density at 20 kb 3.04 g/ems) and therefore is the stable phase above 18 kb contrary to Jamieson’s assertion. A transition aragonite-CaCOs III is therefore thermodynamically possible at room temperature but might probably require geological times in order to take place. The density value of 2-77 g/cm3 for CaCOs II prohibits a transition aragonite-CaCOs II at room temperature. (4) The transformations CaCOs I-II and CaCOs II-III (lines C and D respectively in Fig. 7(a)) observed by BnrnoMA$ra) when calcite is pressed to 20 kb are rapid and reversible, CaCUa I-II is a metastabfe boundary lying in the aragonite field. Line D is probably a stable boundary though possibly displaced from its true position in the diagram. It is probable that CaCOa I and CaC03 II nucleation centers remaining in the CaCOa III phase facilitate the back tr~sfo~aiion CaCf& III-II. It is now proposed that when CaCOs is pressed to pressures above 24 kb and ‘cooked’ at around 800°C for periods of several hours, as was done by BELL and ENO~~~(1s) the CaCOa II nucleation centers are destroyed and the back transformation CaCOs

Density k/cm51 2.78 2.94 3.174 2-93 ::ZZ

Author Davis Jamieson Davis Jamieson &m&son Jamieson

of DARNELLand LIBBYW who successfully retained the high-pressure phases of InSb and InTe to room pressure only after ‘cooking’ for periods of several hours in order to destroy the nucleation seeds of the low-pressure phase. (5) Line C is tentatively drawn for the aragonite-CaCOa III boundary in Fig. 7(b). This transition is favored by Davis’s density data which also prohibit the ebonite-CaCOs II t~nsfo~ation in the vicinity of room temperature. Nothing can, however, be said about the actual value of the slope of line G. The corresponding boundary in the KNOs phase diagram is the KNOs II-IV boundary determined by BRIDGWW.(~)Line G will intersect line E at around 350” and 15 kb, This is the proposed aragonite-CaCOa II-III triple point. (5) A line II is drawn in Fig. 7(b) joining the aragoniteCaCOs II-III triple point with the aragoniteCaCOa I-II triple point. This is suggested as a tentative aragonite-CaCOs II boundary. The corresponding boundary with a negative slope indeed exists in the KNOa diagram as the KNOs II-III boundary. (7) A thermal arrest observed by Box.xst4s) at 975°C might be the analog of the transition in NaNOs. The effect of pressure on such a transformation is tentatively indicated by the upper dashed line K in Fig. 7(b).

1362

ELIEZER

This tentative diagram, though probably not yet complete, provides a plausible interpretation of the experiments of Bell and England and combines their data with Davis’s structural and density data for the CaCOs II and III polymorphs into a unified and consistent picture which is made even more attractive in view of its striking correspondence to the KNOs phase diagram. It is certainly more plausible than the previous diagram suggested by JAMIESON@~~on the basis of fragmentary data. Still unreconciled, however, are lines C and D in Fig. 7(a). Line C is a metastable boundary lying in the aragonite phase and therefore is excluded from a diagram in which onIy stable boundaries are drawn. Line D, apparently a stable boundary, is obviously displaced from its true position as CaCOs II cannot exist stably around room temperature. Lines C and D will evidently meet at a metastable triple point at around 15 kb and -5O”C, i.e. the same pressure as the aragonite-CaCOs II-III triple point. It is therefore suggested that lines C and D should be shifted higher in temperature by about 400°C. This will bring lines C and D roughly in coincidence with lines E and I-I respectively. The reason for this discrepancy is not yet clear. Acknowledgement-This research was carried out in Professor GEORGE C. KFZNNEDY’Shigh-pressure laboratory and the author wishes to acknowledge his gratitude to Professor Kennedy. The author would also like to thank Dr. CARL W. I?‘. T. PISTORIUS for reading the manuscript and for very interesting and valuable discussions and to MARTHA C. PISTORIUS who wrote the computer program and carried out the least square curve fitting. The author also acknowledges useful discussions with Professor R. C. Nrrwro~ and Dr. W. CEMENT. The apparatus was kept in good working order by T. J. THOMAS and LESTER FAUS. The author wishes to thank Dr. B. II. Owhis who supplied samples of NaN08 and CsN08. Partial financial support for this investigation is owing to our Contract Mineral Synthesis Onr 233(28).

REFERENCES 1. BRIDGMAN P. W., Proc. Am. Acad. Arts Sci. 51, 582 (1916). 2. BRIDG& P. W., ibid. 75, 1 (1945). 3. BRIDGMANP. W.. ibid. 72.46 (1937). 4. JAMISSONJ. C. arrd LAW&N A. W:, J. Appt. Phys. 33,776 (1962). E. and KENNEDY G. C., J. Phys. Ckem. 5. RAWPORT Solids 27, 93 (1966).

RAPOPORT 6. BABB S. E., CHANEY P. I?. and OWENS B. B., J. Chem. Phvs. 41.2210 (1964). 7. IO?S*INEDY 6. C., JAY&AN A. and NEWTON R. C., Pkys. Rew. 126, 3363 (1962). A., KLBMENT W. and KENNEDY G. C., 8. JAYAZ~AMAN P&s. Rev. .Lett. 10,387(1963). 9. KLEMENT W., COHEN L. H. and KENNEDY G. C., J. Phys. Chem. Solids 27, 171 (1966). 10. JAYARAMANA., Phys. Rev. 135, Al056 (1964). 11. OWENS B. B., J. Chem. Phys. 42,2259 (1965). 12. JAMIESONJ. C., J. Chem. Phys. 21, 1385 (1953). 13. MACDONALDG. J. F., Am. Miner. 41,744 (1956). 14. CLARK S. P., Am. Miner. 42, 564 (1957). 139, 15. SIMMONS G. and BELL P. M., Siience N.Y. 1197 (1963). 16. CRAWFORD W. A. and FYFE W. S., Science N.Y. 144,157o (1964). 17. SCLAR C. B., CARRI~~N L. C. and SCHWARTZC. M., High Presswe ~eas~r~e~t (editor8 GIARDINI A. A. and LLOM E. C.). Butte~o~hs, Washington (1963). 18. BELL P. M. and ENGLAND J. L., Annual Report of the Director of the Geophysical Laboratory, Carnegie Institution, Washington, D.C. (19631964) p. 176. 19. PISTORIUS C. W. F. T., J. Phys. Chem. Sotids 26, 1543 (1965). 20. KENNED; G.’ C. and NEWTON R. C., Solids under Pressure (editors PAUL W. and WARSZHAUER D. M.). McGraw-Hill, New York (1963). 21. KENNEDY G. C. and LAMORX P. N., Progress in Very High Pressure Research (editors BLINDYF. P., HIBBARD W. R. and STRONGH. M.). John Wiley, New York (1961). A., KLEMENTW., NE--TON R. C. and 22. JAYARAIM~+N KENNEDY G. C., J. Phqs. Ckem. Solids 24, 7 (1963). 23. KRACEKF. C., POSNJAKE. and HENDRICKSS. B., J. Am. Chem. Sot. 53, 3339 (1931). 24. SIEGELL. A., J. Chem. Phys. 17,1146 (1949). Sci. Fennicae Ser. 25. MUSTAJOKI A., Ann. Acad. A VI 5 (1957). 26. KANTOLA M. a’nd VILHO~N E., Ann. Aead. Sci. Fennicae Ser. A VI 54 (1960). KRACEKF. C., J. Am. Chkm. Sic. 53,2609 (1931). it:: AUSTIN J. B. and PIERCEA. H. H., J. Am. Chem. Sot. 55, 661 (1933). 29. K~ELAAR J. A. A. and STRIJKB., Reef. Tras. Chim. Pays-Bas Be&. 64, 174 (1945). 30. LANDERJ. J., .I. Chem. Phys. 17, 892 (1949). 31. K~CXK F. C., BARTHT. F. W. and KSANDAC. J., Phys. Rev. 40, 1034 (1932). 32. DAVIS B. L. and ADAMS L. H., J. Phys. Chem. Solids 24, 787 (1963). 33. MCLAREN A. C., Rev, Pure Appl. Chem. 12, 54 (1962). 34. BROW R. N. and MCLARENA. C., Acta Crystallogr. 15,974 (1962). 35. GUGGENHEI~M E. A., Thermodynamics p. 320-333. North Holland (1957).

POLYMORPHISM

AND

MELTING

IN

36. PIPPARD A. B., Elements of Classical Thermodynamics Chap. 9. Cambridge University Press (1957). 37. WYCKOFF R. W. G., Crystal structures Vol. 2. Interscience (1964). 38. DAVIS B. L. and ADAMS L. H., Z. Kristallogr. 117, 399 (1962). 39. BELL P. M. (oral communication). 40. BRIDGMANP. W., Am. J. Sci. 237.7 (1939). 41. JAMIESON J. C., J. Geol. 65, 334 (i95?). 42. See Ref. 41 for discussion of this transition.

THE

ALKALI

NITRATES

TO

40 kb

1363

43. DAVIS B. L., Science N. Y. 145,489 (1964). 44. DAVIS B. L. and ADAMS L. H., J. Geophys. Res. 70, 433 (1965). 45. STEVENSONR., J. Chem. Phys. 34, 1757 (1961). 46. PISTORIUS C. W. F. T., J. Chem. Phys. 43, 2895 (1965). 47. JAMIESONJ. C., Z. Kristallogr. 107, 65 (1956). 48. DARNELL A. J. and LIBBY W. F., Science N. Y. 139, 1301 (1963); Phys. Rev. 135, A1453 (1964), also DARNELL A. J., YENCHA A. J. and LIBBY W. F., Science N.Y. 141, 713 (1963).