The system Hg-Tl to high pressures

THE

SYSTEM

P.

W.

HQ-Tl

RICHTER

TO

and

HIGH

C. W.

F. T.

PRESSURES* PISTORIUS

The r-Hg/Hg,Tl, eutectic temperature rises aith pressure P(kbar) according to the equation t(‘C) = -60.0 - 4.09P L 0.013iP”. The Hg,TlJ$-Tl eutectic temperature (0.9’C at atmospheric pressure) rises linearly with pressure with a slope of 3.6.5’C/kbar. So marked influence of pressure on the eutect ic Moderate extrapolation of these curves and the melting curves of Hg, compositions was observed. Hg,Tl, and Tl suggests that a f.c.c. high-pressure phase of Hg may appear above -65 kbar on the melting curve, and that a f.c.c. solid-solurlon series may stretch from Hg to beyond the Hg,Tl, composition. _ibove -90 kbar the Hg-Tl system can be expected to consist of a continuous f.c.c. soltd-solution series. ETUDE

DU

STSTEJIE

Hg-Tl

AUS

PRESSIOSS

ELEVEES

La temperature de l’eutectique z-Hg/Hg,Tl., augmente avec la pression P (kbars), suivant 1‘Cquation La temp&ature de l’eutectique Hg,Tl,/&Tl (0,9’C iL la pression t(Y) = -60,O Y- 4,09P A 0,0132P”. atmoaph&ique) augmente linkairement avec la pression, avec une pente de 3,6S’C/kbar. ducune influence nette de la presaion sur la composition des eutectiques n’a Qt6 observee. Si on estrapole de faton raisonnable ces courbes ainsi que les courbes de fusion de Hg, Hg,Tl, et Tl, on trouve qu’une phase c.f.c. de Hg & pression &levee peut apparaitre au-dessus de 65 kbar environ sur la courbe de fusion, et qu’une sbrie de solutions solides c.f.c. peuvent apparaitre depuis Hg jusqu’au-del& de la composition de Hg,Tl,. _iudessus de 90 kbars environ, on peut s’attendre & ce que le syst&me Hg-Tl consiste en une s&e continun de solutions solides c.f.c. DAY STSTEM

Hg-Tl BE1 HOHEX

DRUCKEN

Die eutekt.ische Temperatur van r-Hg/Hg5T1? nimmt mit dem Druck P(kbar) gemal der Gleichung (0,9% bei Atmost(‘C) = -60.0 - 4,09P + 0,0132P’ zu. Die eutektische Temperatur van Hg,Tl,/p-Tl phtirendruck) nimmt mit dem Druck linear zu; die Steigung der Geraden ist 3,65”C/kbar. Der EixAuD des Druckes auf die eutektische Zusammensetzung ist nur sehr klein. Eine Extrapolation dieser Kurven und der Schmelzkurven van Hg, Hg,Tl, und Tl deutet darauf bin, da0 oberhalb ~65 kbar auf der Schmelzkurve eine kubisch-fliichenzentrierte Hochdruckphase des Hg auftreten kdnnte und da13 sich eine kxbisch.fllchenzentrierte Legierungsreihe von Hg bis jenseits der Hg,Tl,- Zusammensetzung erstrecken kdnnte. Oberhalb -90 kbar kann man damit rechnen, da13 das System Hg-Tl aus einer kontinuierlichen Reihe kubisch-fltichenzentrierter Legienmgen besteht.

INTRODUCTION

The system Hg-Tl has attracted considerable interest , partly because the alloys are liquids at WC in the range 043 at. oA Tl. The system contains only one binary compound Hg,TI,, with a melting point of l-i.5’C. The x-Hg/Hg,Tl, eutectic is located at 8.5 to -6O’C, and t,he Hg,Tl,/$-Tl at. % Tl, -59’C eutectic at 40 at. % Tl, 0.6”C.(1) Solid Hg,Tl, is disordered f.c.c.,t2) and its lattice is capable of accommodating several percent of excess Hg or Tl. Thermod_ynamic(3) as well as liquid S-ray diffraction data(dT6) indicate that there is a more orderly arrangement in the liquid at the compound composition than at other compositions, and that the liquid retains some solid state characteristics with respect to short-range order. The maximum in t.he liquidus at the composition Hg,Tl, is broad and flat, with the liquidus line above 10°C from ~23 to ~35 at. % Tl. The high-pressure melting curves of Hg?“) Hg,Tl,,(*) and T1(g-lo) have been studied. By moderate estrapolation of the melting curve of Hg5Tl, it appeared that the melting curves of Hg and Hg5T12are tithin -10°C of each other from 25 kbar to at least 60 kbar, and possibly even beyond that.@) Furthermore, Hlel Received May 30, 1972. t Chemical Physics Group of the Kational Physical Sational Chemical Research Laboratories, P. 0. Box Pretoria, South Africa.

ACT_% METALLURGICA,

VOL.

21, APRIL

1973

and 395,

ment et al.“) concluded that the rhombohedral angle of the x-Hg unit cell (70.6”) would decrease with pressure along the melting curve, and suggested that the a-Hg structure continuously distorts toward f.c.c. with pressure. If this is the case, a point XTilleventually be reached, possibly via a discontinuous transition to a f.c.c. high-pressure phase of Hg, There the solidsolubility limits of f.c.c. Hg,Tl, will include Hg itself. This may result in the disappearance of the Hg/Hg5T12 eutectic point. The high-pressure pol_ymorph Tl III (above -9Okbar on the melting curve and above -37 kbar at P.5”C(g)) is also f.c.c.cll) Ultimately, therefore, it is possible that the system Hg-Tl at very high pressures may consist only of a continuous f.c.c. solid-solution series near the liquidus. Some inferential indications of such behaviour can be obtained from the effect of pressure on the eutectic temperatures in this system, and it xvas consequently decided to study these as functions of pressure. EXPERIMENTAL

Thallium rod with a stated purity of 99.999x, obtained from Koch-Light Laboratories, and triply distilled mercury, obtained from Johnson, Xatthey Bt Co., were .sveighed and thoroughly mised at room temperature in the two eutectic compositions. The samples obtained were stored under a nitrogen atmosphere. 391

ICTA

39’2

WETALLVRGICA.

generated in a piston-cylinder Pressures xere apparatus.(r”) Melting was observed by means of differential thermal analysis (DTX), using ChromelThe detailed esperimental Alumel thermocouples. procedure has been described before.(13) The samples were contained in stainless steel capsules, with no evidence of contamination. In order to prevent gross leakage, the pressure plate was first cooled to well below the eutectic points before pressure was applied to seal a capsule in situ. Heating-cooling rates were in the range 0.4-l .l”C/sec. Only melting temperatures are plotted. The eutectic melting curves obtained are believed to be correct x-ithin 2°C and ~1 kbar. Originally it was intended also to study the changes in the eutectic compositions brought about by pressure. This can, in principle, be done by completely melting the sample in every heat.ing-cooling cycle, and measuring the amount by which the beginning of freezing (i.e. the liquidus) exceeds the beginning of melting (i.e. the eutectic temperature). A comparison with the shapes of the intermediate liquidus curves then yields the effect of pressure on the eutectic composition, assuming one had started with the eutectic composition at atmospheric pressure. Even more accurate results could be obtained if runs are, in addition, made at compositions removed from the eutectic so as to yield information regarding the changes in the shapes of the liquidus curves with pressure. Unfortunately, however, we were unable to seal the capsules sufficiently well so as to preclude slight leakage of the samples after repeated complete heatingcooling cycles. Even the slightest leakage of the sample resulted in complete shorting of the thermocouples. It is suspected that the cause of leakage was minute amounts of liquid alloy trapped betxeen the capsule and its stopper, thereby preventing a complete Nevertheless, some isolated capsule-stopper seal. instances of complete heating-cooling cycles rrere obtained. In general, however, heating was stopped as soon as the sample started to melt, in order to prevent leakage.

VOL.

21.

2

?

1973

I 1

I ;

PilESSURE

FIG. 1. Effect

23

-

25

I

1

10

*5

KILOBARS

of pressure on the r_x-Hg/Hg,Tl, temperature.

eutectic

with a standard deviation of 0.8”C. The slope of the curve rises to 4.88 deg/kbar at 30 kbar. The highest pressure at which a complete meltingfreezing cycle was obtained without leakage was that of the cycle shown in Fig. 2. The pressure was 15.9 kbar, with an eutectic temperature of 9.2’C, whereas freezing started at 10.2% on the cooling cycle. The Hg5Tl,/$-Tl eutectic curve, shown in Fig. 3, is based on t1v-o separate consistent runs. The DTA signals obtained mere similar to, but somewhat sharper than, signals obtained for the a-HgiHg,Tl, eutectic. Our present atmospheric pressure eutectic point is 0.9 t O.S”C in good agreement with the literature.(l’ Within our present accuracy, no curvature on the eutectic line in the P-T plane could be observed, and

RESULTS

The x-Hg/Hg,Tl, eutectic curve, shown in Fig. 1, is based on several consistent and independent runs. Sharp and clear DTA signals (Fig. 2) were obtained over the pressure range studied. Our present atmospheric pressure eutectic point is -60 5 l°C in good agreement with the literature.“) The eutectic line has a slight but definite upwards (dsT/dP’ > 0) curvature in the P-T plane. The experimental points, lvith P in kbar, were fitted by t(“c) = -60.0

+ 4.09P + 0.0132P’

I

INCREASING

m

TIME

FIG. 2. Typical DT_A signal obtained at. 15.9 kbar for an original composition corresponding to the a-Hg/Hg,Tl, eutectic. Point d (9.YC) represents the eutectic temperature and point B the liquidus (102’C).

RICHTER

ASD

PISTORIUS

PRESSURE FIG. 3. Effect

the esperimental

THE

- KILOBAR

of pressure on the Hg,Tl,/B-Tl temperature.

points

SYSTEM

eutectic

were fitted by

t(“C) = 0.9 + 3.63P with a standard deviation of O.‘i’C. A complete melting-freezing cycle at 30.1 kbar resulted in almost exact agreement between the beginnings of melting and freezing.

Hg-Tl

TO

HIGH

393

PRESSURES

shifts lvith pressure towards a higher thallium content, the compositional change must be negligible. However, if the shift is in the opposite direction, even a change of 1 at. % would result only in -1’C difference between the liquidus and eutectic temperatures. Severtheless, even in this direction the shift cannot exceed this value. It can therefore be concluded that, at least in the pressure range studied, the assumptions of only a small effect of pressure on the eutectic compositions are justified. Positive curvature (i.e. d*T/dP” > 0) of the melting curve of a pure substance is an extremelv rare phenomenon. It is known for cerium, but in that case is almost certainly due to electronic changes. In the case of eutectic curves, however, positive curvature may be common, since even very slight changes in the eutectic composition with pressure could, in principle, give rise to this behaviour. The positive curvature observed for the c+Hg/Hg,Tl, eutectic curve is almost certainly due to such an effect. Figure 4 shows that the effect of pressure on the liquidus curves is one of a flattening-out of the maxima. At atmospheric pressure the x-Hg/Hg,T& eutectic is 74.5% below the melting point of Hg,Tl,, while at 40 kbar this difference has decreased to NX’C. Similarly, the Hg,Tl,/B-Tl eutectic at atmospheric pressure is 14°C below melting point of Hg5T12, while

DISCUSSION

It is now possible to construct a series of curves representing the liquidus lines of the Hg-Tl system at different pressures, using earlier data for the melting curves of Hg,(‘) Hg,Tl,(s) and Tl@-lO) in conjunction with the present results for the two eutectic curves. The assumption is made that the eutectic compositions do not change markedly lviiith pressure. The resulting curves are shown in Fig. 4. Since no experimental data are available at intervening compositions, the curvatures of the liquidus lines may change in a somem-hat different fashion to those indicated in the figure, but in essence this family of curves can be espetted to reproduce the high-pressure behaviour in this system. Several conclusions can immediately be drawn. The 1°C difference between the liquidus and eutectic temperatures of the r-Hg/Hg,Tl, composition at 15.9 kbar represent a change in the eutectic composition of only -0.03 at. %/kbar, but, of course, the direction of the change is not indicated. This effect is sufficiently small to be of littleimportance at present. In the case of the Hg5Tl,/&T1 eutectic, where essentially no difference was observed at 30 kbar between the liquidus and eutectic temperatures, it is obvious from Fig. 4 that if the eutectic composition

I 5CO

c

1

-‘OOo

”

”

I

I

I

10

20

30

’

“1

I8

40 At.

50 - %

1

I

I

I,

60

70

80

b

I 90

100

TO

FIG. 4. Liquidus lines in the system Hg-TI at various pressures. A: atmospheric pressure;“’ B: 10 kbar; C: 20 kbar; D: 30 kbar E: 40 kbar; F: Extrapolated to 50 kbar. Subsolidus relations are not shown, and the assumption (at least partly justified-see text) is made that the eutectic compositions do not change with pressure. The curvatures of the high-pressure lines (shown here as dashed curves) may be slightly in error.

ACT.1

394

at 40 kbar the difference

is only -4.X.

the Hg5TI,/$-Tl

xith

eutectic

at atmospheric

pressure

and

but due to the curvature the difference

of 302°C

beyond

-45

eutectic

kbar.

pressure

tively,

but above this pressure

these points are at ~ZO’C!,

olation

of melting

of extrapolation curvatures olation

curve

of Hg5Tl, at the same

of cr-Hg and the that

comparatively also small.

mainly

rapidly Further-

at 60 kbar

i.e. essentially

curves is hazardous, is

involved

would

kbar, ZWC.

It is here conceded

affect

x-Hg/

the extrap-

but the range

small,

and

the

Any errors in extrapthe

critical

pressure

obtained. It would therefore n-hat above the

z-Hg/HgjT1,

corresponding suggest,ing

at

may

to

maximum

of Hg,TI,

w-ill disappear,

pressures,

a continuous

stretch

This necessitates

f.c.c. high-pressure before.“)

higher

some-

corresponding

as well as the

to the melting series

at. % Tl.

the minimum

eutectic

that,

solid-solution ~40

appear that at a pressure

60 kbar

from

Hg itself

the appearance

to of a

phase of Hg, as has been predicted

Furthermore,

above -90

kbar Tl itself be-

comes f.c.c.,

and it can be expected

solid-solution

series above this pressure will include the

complete

range of compositions

that

the f.c.c.

in this system

from

Hg to Tl. Another,

somewhat

rhombohedral

less probable,

x-Hg transforms

phase via a higher-order

possibility

continuously

decreases

is that

to a f.c.c. high-pressure

transformation.

be the case if the cc-Hg rhombohedral lvith pressure

five-fold

greater

axis than perpendicular shape of the Hg-Tl

stifiness

to it.

How-

q-stern at high

woulcl be the same as in the case of a first-

order transition. ACKNOWLEDGEMEXTS

respec-

the difference

shows that

as the melting

eutectic.

pressures

moduli data(li-15’

are 21°C and

and 40 kbar,

of the melting eutectic

Simi-

with respect to the

after which it remains constant.

an approximately

ever, the eventual

decreases to disappear near -60

HgjTI,

suggest

curve of $Tl

curve of cr-Hg the differences

temperature

60’ is reached,

It should be noted that the elastic

of the melting

at atmospheric

extrapolation

1973

along the trigonal

will decrease

and the HgjT1.,/,3-Tl

and 369°C

exactly

?I,

respectively.

226°C

more,

to the melting

VOL.

40 kbar,

larly, for the X-Hg/Hg5Tl, melting

Considering

respect

of P-T1 itself, n-e find differences

MET_iLLCRGIC_i,

This would angle of 70.6”

until a value of

The

aut,hors

Pistorius fitting

x-ould like to thank

for witing

the data.

of t,his Institute

the computer

Thanks

apparatus

in good repair,

Martha

C.

used in

are due to Dr R. J. Murphy

for valuable

discussions.

and his staff and A. de Kleijn the manufacture

Jlrs

programs

J. Erasmus

and his staff kept the

and were responsible

of the furnace

parts.

for

Calculations

were carried out on the IBM 360/65H of the Sational Research

Institute

for Nathematical

Sciences.

REFERENCES 1. 11. HASSES and I<. ASDERKO, Co~wtitution of Bimry dllous. McGraw--Hill (19561. “. A. ~LKSDER, Z. Phys: Cheh. (Leipzig) A171, 45 (1935). 3. T. CLAIRE, R. CASTASET. H. TACHOIRE and 31. LAFITTE, BulZ. Sot. Chins. France 712 (1969). 4. F. QACERWALDand IV. TESKE, Z. ;112orq.C’hem. 210, ‘47 (1933). 5. F. SACERWALD and E. OSSWALD, Z. ilvorg. illlg. Chern_ 257, 19.5(194s). 6. R. E. SJL~LLZILSand B. R. T. FROST, dctcl Nel. 4. 611 (1956). 5. IV. KLEMEST, -1. ~J~Y.IFLU~ASancl G. C. KESSEDI., Phy;r. Rev. 131, 1 (1963). 8. P. W. RICHTER ancl C. W. F. T. PISTORK~, .?. lws-common Me~nZ.s.29, 215 (197”). 9. A. J~YAEMX~S, W. KLEMEXT, R. C. SEQTOS and G. C. KESXEDY, J. Phys. Chem. Solids. 24, 5 (1963). 10. P. S. ADLER and H. X.~GOLIS, Trm3. Jletall. Sot. d.1.M.E. 230, 1045 (1961). 11. G. J. PIERJL~ISI and C. E. WEIR, J. Rea. Snt. Bur. Sta&_ (L-S.) MA, 3% (1962). 12. G. C. KESSEDT and P. S. LAMORI, in Progress in Very High Pressure Research., eciiteclby F. P. BUSDY, W’. R. HIBBARD and H. 11. STROSC, p. 301. John Wiley (1961). 13. C. IV. F. T. PISTORICSancl J. B. CLARK, High Temp. High Press. 1, 561 (1969). 11. E. GR~XEISES ancl0. SCGELL, Ann. Phys. 19,38i (1934); Solid St. Phys. 7, “SA (19%). 15. \I-. H. ?hXLUILkSES, dcta &yet. 5, 19 (1952).