on the p-t-x phase diagram of the mn-te system bound... · on the p-t-x phase diagram of the mn-te...

R 700 Philips Res. Repts 24,284-298,1969

ON THE P-T-x PHASE DIAGRAMOF THE Mn-Te SYSTEM

by J. van den BOOMGAARD

Abstract

The sublimation behaviour of solid MnTe has been investigated. Atelevated temperatures the gaseous phase in equilibrium with solidMnTe consists of Mn atoms and Te2 molecules. The maximum meltingpoint of MnTe has been found to be > 1243 °C. An estimated P-Tdiagram of the system is given and the T-x projection of the existenceregion of the solid is discussed.

1. Introduetion

Johnston and Sestrich 1) and Delvis and Lewis 2) found 1167 oe as themelting point of MnTe; Miller 3) gives the value of 1170 °C. The periteetictemperature of MnTe2 has been reported to be 735°C 3) and 742 oe 4) whileit may be concluded from the work of Kunitomi 5) that this temperature isabout 700 °C.Manganese telluride shows a sharp solid-state phase-transition point at

1039 ± 4°C ("NiAs" structure +t "NaCl" structure) 1). The existence regionof solid MnTe is probably small. At 500°C Furberg 6) found the compositionMnTeo.s5±o.o5 for the Mn-rich boundary and MnTe1.05±O.OSfor the Te-richboundary. According to Johansen 7) at 700 °C these boundaries were situatedat MnTe1.oo2 and MnTe1.o13 respectively.In the following sections the phase equilibria including solid MnTe are

discussed to find out among other things whether single crystals of MnTe canbe grown from the vapour phase below 1039°C.

2. Experimental

The following sublimation experiments were carried out.(a) A mixture of 65 at. % Mn and 35 at. % Te was heated for 18 hours at1170 °C in an evacuated silica tube to obtain solid Mn in equilibrium withsolid MnTe. Then the material was powdered and about 10 grammes were putinto a silica tube of about 40 cm length, which was evacuated and sealed off.The end of the tube containing the mixture of MnTe + Mn was heated at1027 oe. The temperature of the other end was much lower. A very slowsublimation took place. The first condensate (part II) precipitated at a regionof intermediate temperature. After some time the condensate precipitated atthe coldest end of the tube (part 1). After two hours the tube was removed

ON THE P-T-x PHASE DIAGRAM OF THE Mn-Te SYSTEM 285

from the furnace and the condensate was chemically analyzed. The results aregiven in table 1.

TABLE I

28·268·2436·50

gramat. Mn.::..____ - (chem. analysis)gramat. Te

I11

total

total weight of thesublimed material (mg)part

42·716·036·6

(b) Ten grammes of MnTe were heated for 3 hours at 1050 °C in a silica tubein which vacuum was maintained by pumping. Because the tube was protrudingfrom the furnace there was a température gradient in it. After cooling to roomtemperature the sublimate was divided into three parts and analyzed. Theresidue was also weighed and analyzed (M2 in table 11).(c) A similar experiment was done in a wider tube at 1180 °C for 1 hour *).The solid MnTe did not melt. The sublimate was divided into four parts andanalyzed. The residue was weighed and analyzed (M2a in table II).

A series of melting experiments were carried out in a stream of forming gas.The furnace contained three temperature zones. In the high-temperature zonethe melting of MnTe could be observed visually by means of a known tech-nique 8). The following was observed:(1) Powdered MnTe showed a sharp melting point at 1169°C, giving twomelts. Above 1210 oe only one liquid phase was observed. After solidifyingand reheating frem 1150 °C melting started at 1177 oe (one liquid + solid)and was completed at 1180 °C. X-ray analysis showed that the material con-sisted of MnTe and a very small amount of MnTe2'(2) Powdered MnTe was slowly heated (6°C/min) and kept at 1169 oe for1 hour. The MnTe remained solid. However, at 1171 oe it melted completely,giving two liquids. X-ray analysis, after quenching, showed that the surface ofthe solidified lump consisted for the greater part of MnTe2' The bulk consistedof MnTe.(3) Powdered MnTe was heated and showed two phases between 1189 oe and1236 oe, probably a solid floating on a liquid; above 1236 °C, however, onlyone liquid phase existed. After cooling down to 1100 oe and reheating, themelting trajectory was from 1188oe to 1243oe. X-ray analysis after solidi-fication gave the same result as mentioned under (2).*) MnTe was put in an alumina crucible within the tube because Mu attackes silica seriously

at temperatures above 1100 oe.

TABLE II

Sublimation experiments. The parts I, II and III are condensation regions with different temperatures; in experimentM2' TI < Tu < TIll; in experiment M2a, TI < TIIA < TuB < TIJI

chemical analysissubl. time' quantity at.%experiments material temp. (h) (grammes) result of X-ray diffraction quantity("C)

(mg)Mn Te

M2 starting material (MI) - 10'040 pure MnTe - 50 50M2' part I (at TI) 1050 3 - - 17·6 28·7 71·3M2' part n (at Tu) 1050 3 - MnTe + very small quantity - 50'4 49·6

of a second phaseM2' part III (at Tm) 1050 3 - - 3-9 12-8 87-2sublimation residue 1050 3 9'440 pure MnTe - 50 50

M2a starting material (M 1) - - 10'002 pure MnTe - 50 50M2a, part I (at TI) II80 1 - - 47·5 11-3 88'7M2a, part nA (at TUA) lI80 1 - - 137 83'5 16'5M2a, part nB (at TuB) 1180 1 - MnTe + very small quantity - - -

of a second phaseM2a, part III (at Tm) 1180 1 - - 32-4 49'3 50'7sublimation residue 1180 1 7'050 RI pure MnTe - 50'1 49'9

tv000\

:-<gR'::>t,;oois:

~


(4) Powdered MnTe was heated and showed a coexisting solid phase and liquidphase between 1172 oe and 1175 oe, a solid phase at 1176 oe, a solid phase anda liquid phase at 1182 oe, and a liquid phase at 1195 oe.(5) Pre-melted material was taken as the starting material. At 1173 oe it gavea solid and a liquid with the solid floating on the liquid. At 1238 oe it wasentirely molten. Upon cooling, the liquid solidified from the outside to theinside and some liquid was pressed outward.(6) Powdered MnTe produced a solid and a liquid at 1167 oe and two liquidsabove 1172 oe. At 1178 oe the experiment was stopped, the two melts stillbeing present.The following melting experiments were carried out whilst a vacuum of about

10-4 Torr was maintained by pumping.(7) Powdered MnTe transformed from solid to single-phase liquid between1200 and 1220 oe.(8) A premelted lump of MnTe showed a melting trajectory from 1180 oe to1200 oe, the solid floated on the liquid. After heating for some minutes at1243 oe, a solidification trajectory from 1160 oe to 1145 oe was observed oncooling. In this case, however, the liquid was on the top of the solid. The finalsolid showed a hole in the middle and contained 50·8 ± 0·1 at.% Mn (chem.anal.). X-ray analysis revealed a trace of MnO. The evaporation rate wassmall compared with powdered specimens.(9) A premelted lump of MnTe showed a rather sharp melting point at 1207 oeon heating and a rather sharp solidification point at 1187 oe on cooling.Again evaporation was much less than from powdered MnTe.

Finally the partial tellurium equilibrium vapour pressurePTe2' at the bound-ary of the existence region of solid MnTe was determined by observation ofthe melting points using a known technique 8). The above-mentioned furnacewith three temperature zones was used. The low-temperature zone served tofix the Te2 pressure and the high-temperature zone to determine the meltingpoints of MnTe. The results are given in part I of table lIJ.

3. Discussion of the results

3.1. The composition of the vapourphaseThe vapour phase coexisting with solid MnTe may consist of elementary

manganese and tellurium and of molecules MnxTey. For the pressures underconsideration (see for instance table Ill) elementary manganese will be presentas atoms and elementary tellurium as Te2 molecules. Representing the sublima-tion of MnTe by the following chemical equations

1 ·y-xMn'Te, +t - Mnx Te, g +-- Tez g

x 2x(I)

P - KP (x-)!)/2MnxTcy - TC2 (1)

288 J., van den BOOMGAARD

TABLE III

Part J. Melting points

temperature of Te melting point of MnTeseries

(Oe ± 2°C) (OK) (1000;oK) CC) (OK) (1000;oK

1 570 834 1·186 1151± 3 1424 0'7021 627 900 1·111 1119± 3 1382 0·7241 696 969 1·032 1041± 3 1314 0'7692 778 1051 0·951 868± 6 1141 0·8772 749 1022 0·978 982± 3 1255 0·7972 650 923 1·083 1076± 3 1349 0·7423 517 790 1·266 1166± 2 1439 0·695

2 482 755 1·342 1168± 2 1441 0·694

and(ll)

and as the chemical potentialof solid MnTe may be considered as a constantbecause the existence region of solid MnTe is small, it can easily be derived that

and(2)

in which K and Kp are constants.Equations (1) and (2) say that if PMnxTey is not negligible and larger than

PMn or PTe2' sublimation should increase for x> y with decreasing PTC2 orincreasing PMn whilst for x < y the opposite should be the case. However,according to experiment (a) the sublimation rate under the saturation pres-sure of pure Mn at the chosen temperature of 1027 °C is very low, i.e. muchsmaller than 18·25 mgfh (see table I), whilst at a lower PMn the sublimationrate at 1050 oe is much higher, i.e. 200 mgfh under the conditions of exper-iment M2 (table 11).This difference cannot be explained by the small temper-ature difference of 23°C. Consequently the number of Mn,Te)!molecules inthe vapour phase with x > y is negligible. On the other hand the data of tableIn show that for PTe2' ~ 0·468 Torr no observable sublimation takes place atall at about 1170 °C. From this it follows in the first place that molecules with

ON THEP-T-x PHASE DIAGRAMOF THE Mn-TeSYSTEM

TABLE III

289

Part Il. Partial vapour pressures

0·49 -2·48 3·1 1.86.10-3 3·1 0·49 10·94 -3·00 8·7 5.38.10-4 8·7 0·94 11·42 -4·05 26·3 1.74.10-5 26·3 1·42 11·90 -6·55 79·5 3.16.10-8 79·5 1·90 1 *) ,

1·73 -4·70 53·8 1·7 .10-6 53·8 1·73 1 *)1·11 -3·41 12·9 1.07.10-4 12·9 1·11 10·02 -2·32 1·05 4.68.10-3 1·05 0·02 1

0·33 -2·30 0·468 7.33.10-3 0·475 -0·32 0.99 {s?me sublima-ItlOntook place

*) The shape ofthe crystals, growing from the melt in these two experiments differed markedlyfrom that in the other experiments. This may be connected with the solid-state phasetransition at 1039 °C.

IIogPTe2 lOlogs, PTe2 PMn r, lOlogr,inTorr) (PinTorr) (PinTorr) (PinTorr) (PinTorr) (PinTorr)

remarks

x < y are also practically absent in the vapour phase and secondly that theconcentration of Mn,Tel' with x = y also must be negligible, because in thatcase the sublimation rate can never be suppressed entirely.Therefore it may be concluded that the vapour phase, coexisting with solid

MnTe, only consists of Mn atoms and Te2 molecules. This conclusion can beconfirmed in the following way. The value of Kp can be calculated with theaid of thermodynamic standard tables *). For the range between 1000 and1600 "K it holds that **)

(3)

For 1027 oe this gives lOlogKp = -8·4. The value of K, at 1027 oe can also

*) The thermodynamic data used in this paper are taken from:(a) Handbook of chemistry and physics, 40th edition;(b) O. Kubachewski et al., Metallurgical thermodynamics, Pergamon Press, 3d edition;(c) K. K. Kelly, J. Am. chem. Soc. 61, 213, 1939.The saturation pressure data were obtained from A. M. Nesmayenov, Vapour pressureof the chemical elements, Elsevier's Publishing Company, Amsterdam, London, NewYork. I

**) Because MnTe at 1039 "C (1312 OK) has a transition from the "NiAs" structure to the"NaCF' structure the lOlog Kp vs lIT curve cannot be a straightIine. Generally. theLJ.H of such a transformation is negligible compared with the LJ.H of reactionMn'I'e , =<± Mn, + t Tez and the effect of this transformation on the slope of the mentioned curve will be vanishingly small.

290 J. van den BOOMGAARD

be calculated with the aid of the experimental data in table I. If a steady stateis reached during the sublimation and the sublimation rate is not too high thesublimate will have the same composition as the vapour in equilibrium withsolid MnTe + Mn at 1027 oe. Because the starting material was heated beforein an evacuated sealed silica tube at 1190 oe, it will take some time beforethis steady state is reached. This is why the :first condensate, which is pre-dominantly condensed in the region of part IJ (see table I), contains more Te.Therefore K,must be calculated from part I in table I, which also may containa small excess of Te from the :firstcondensate. With gramat. Mnfgramat. Te =42·7 it follows for the vapour (consisting of Mn and Te2 in equilibrium withso)id MnTe and solid Mn) that

Because solid MnTe under the experimental conditions is in equilibrium withsolid Mn, which contains only a small amount of Te, we may substitute forPMn the saturation pressure of pure Mn at 1027 oe, and with eq. (2) oneobtains:

lOlogs, ~ -7,6,

which is in fair agreement with the value of -8,4 calculated from thermo-dynamic data.

3.2. The sublimation curve of solid MnTe

According to Gibbs' phase rule for any given temperature there exists anumber of compositions of the vapour, which are in equilibrium with solidMnTe. Among these compositions there may be one which equals the com-position of the solid. In that composition MnTe sublimes without dispropor-tionation. The equilibria MnTe, +t vapour with the condition Xs = xg arerepresented by a curve in the P-T-x phase diagram. This curve is called thesublimation curve. Not all binary compounds which dissociate totally in thevapour phase have such a curve. In the case of compounds which dissociatecompletely in the vapour phase it can be shown that such a sublimation curveonly occurs if, at a given temperature, the total pressure of the vapour in equi-librium with the solid compound has a minimum value within the existenceregion of that compound. The condition (oPr/bx)r = 0 leads to

PMn mln = 2 PTe2 mln

and with eq. (2) toPMnmln

PTe2 mln

Prmln

= 21/3 s,2/3,= 2-2/3 K//3,= 2-2/3.3 K//J,

and with eq. (3)


15577lOlogPt min = - -'-- + 6,7,

T·(4)

with P, in atm.Tbis minimum is situated only within the existence region of solid MnTe at

a certain temperature if at this temperature PMn min is smaller than PMn at theMn-rich boundary of the existence region, and if PTe2 min is smaller thanPTe2 at the Te-rich boundary at the same temperature.It is evident tbat the first condition is fulfilled because PMn at the Mn-rich

boundary is practically equal to the saturation pressure of pure Mn. Tbe secondcondition is also fulfilled, as can easily be checked witb the data of table. Ill.Tbese facts bave two consequences wbicb are of interest. First, MnTe can besublimed witbout disproportionation of the solid phase, which opens possibili-ties of growing undisturbed MnTe single crystals below the solid-pbase-transi-tion temperature (1039 "C) (see, however, sec. 3.4). Second, MnTe, contam-inated with a second pbase, can be made single-pb ase at a given temperatureby beating it at this temperature in vacuum and pumping away tbe vapour *).Tbe existence of the curve of minimum total pressure is confirmed by theexperiments (b) en (c) (see table II) in whicb the composition of tbe residueis practically the same as the composition before sublimation took place.

3.3. The partial P-T-x phase diagram of the Mn-Te system

With the aid of the experimental results given in sec. 2 and tbe results ofsec. 3 it is possible to draw the greater part of the P-T projections of tbe tbree-pbase curves and some two-phase curves of tbe Mn-Te system and a tentativemodel of tbe T-x projection of the existence region of solid MnTe in equilibriumwitb the vapour phase. For the P-T projection it is sufficient to know one ofthe partial equilibrium pressures because the other one and the total pressurecan be calculated with the aid of K; (eq. (3». In figs 1 and 2 the lOlogPt vs1000lT and the lOlogPTe2 vs WOOIT curves are given with the pressures ex-pressed in Torr. Tbe greater part oftbe three-pbase line Mn'Te, :<±L:<±G is ob-tained from the calculated data in table III part IJ. The saturation pressuresof pure Mn and Te as a function of temperature:

5983lOlogPT (0) = - __ + 7·95

e2 Tand

14000lOlogPMn (0) =-T + 9·2

(P, in Torr) are taken from standard tables.

*) This single-phase MnTe may disproportionate at lower temperature if the compositionat the point of minimum total pressure at the first temperature lies outside the existenceregion at the lower temperature (see sec. 3.3),


2~--r---.---.---.-~~~~--~~~~.---~1010911

(P in Torr) 1----+---+-=-f-1:.-:-:-__(-----:-+=---'=-__:~~::_I_

r

h~:.,jC=~~."...,:-l--_J.--_I_---l ---- Calculated--x- Measured--_ - Estimated- - - - - - Direction not known

1·0 1-1_ IDOO(oW

')

T

Fig. 1. lO!og P, vs lOOO/T projection of the Mn-Te system.

0·7 0·9

-

Between 1000oe and 738 oe (the periteetic of MnTe2) the partial Te2 pres-sure at the Te-rich boundary of MnTe approaches that ofthe saturation vapourpressure of pure Te. The same holds for P, because PMn is negligible in thisregion. At the Mn-rich boundary, below 1145oe the partial Mn pressureequals the saturation pressure of pure Mn and so does P, because here theTe2' pressure is negligibly small. The pressure of tellurium along this boundaryis given to a good approximation by PTe2= (Kp/PMn (0))2. Also the subli-


mation curve (x, =Xg) is drawn in figs 1 and 2. According to experiment (3) themaximum melting point of MnTe is ;;;::::1243 (± 3°C). This maximum meltingpoint must lie above the curve Xs = xg (fig. 1) which is tangent to the curveMnTes ~ L ~ G in N (fig. 3), and it must certainly lie below the horizontalline through the experimental point 1°logP, = -0·32, 1000jT = 0·69 oK-I.Consequently the upper limit of the maximum melting temperature is given bythe intersection point of these two curves, which is at about 1290 °C. As MnTeis totally dissociated in the vapour phase the maximum melting temperaturemust be substantially lower than 1290 °C. The value of 1243± 3 "C thereforecannot be very far from the true value. The end point of the sublimation curveis estimated to be 1207 "C (see experiment (9». The starting point of a possiblecurve of minimum total pressure in the L ~ G equilibrium is estimated to be1187 "C (see experiment (9». It is evident from fig. 2 that there are two regionsin the Mn'Te, ~ L ~ G curve in which PTe2 changes very rapidly at only asmall change of temperature. This may correspond to a relatively large changein the composition of the solid along this three-phase curve at a constant tem-perature. Together with the results of experiments (1)-(9) this gives rise to thetentative T-x projection in the neighbourhood of the compound MnTe pre-sented in fig. 3. Because the native defects in solid MnTe and the equilibriumconstants relating the equilibrium vapour pressure with these defects are notknown, it is impossible at the moment to derive the exact shape and positionof the existence region of solid MnTe in equilibrium with the vapour phase.From the experiments (1), (2) and (3) it follows that MnTe heated to tem-peratures ;;;::::1170 "C always contains MnTe2 after cooling to room temper-ature. Because it may be concluded from experiments (1)-(9) that the com-position of the melt in these experiments is situated at the Mn-rich side of theexistence region of solid MnTe above 1170 oe, the whole existence region ofsolid MnTe at higher temperatures must be situated at the Te-rich side of thisregion at low temperatures. This is in accordance with the results of Seuter 9)who found that samples of MnTe brought into equilibrium with a mixtureof MnTe +Mn at 1100oe contained precipitates of MnTe2 after cooling toroom temperature. The results of Furberg 6), indicating that at 500 "C theexact stoichiometrie composition lies within the existence region of solid MnTe,and those of Johansen 7) indicating that at 700°C this existence region issituated at the Te side of the exact stoichiometrie composition, support thisconclusion. In fig. 3 there are two shoulders in the solidus curve, one to theright and one to the left of the maximum melting point corresponding to thesteep parts in the Mn'Te, ~ L ~ G curve of fig. 2, mentioned above. A verysmall region of immiscibility occurs just above the shoulder at the Mn-richside according to experiments (2) and (6), corresponding to the rather sharp"melting point" found in these experiments. This possibly explains the meltingpoints at about 1170 "C found in the literature 1.2.3). The sublimation curve


1~Or-----~~-------..--~~ __=

r~E::;:"""':::::::'ï~::.v

-----1----- __1I11

.1[1111I

\ MnTesr:;MnTe2s!:;G

\y: I, 1,I"-At

(x=O)Mnl._..J'-----~.l.;_tI,;_---~------L-.Te(x=1)Xp

-- ..... X

Fig. 3. Tentative model of the T-x projection of the Mn-Te system in the neighbourhood ofthe compound MnTe on a large x-scale.

(xs = Xc) in fig. 3 turns to the Te-rich side, especially at higher temperature.This will now be discussed in more detail.The results of the experiments (1) to (6) can be correlated with the fact that

the evaporation rate of powdered MnTe is slowed down both by the presenceof inert gas and by the formation of large crystals, giving rise to a smallereffective surface. A powder of composition xp (point A in fig. 3) heated in astream of inert gas of 1 atmosphere, will give off a vapour containing moreTe than Mn. Therefore at increasing temperature the composition of the solidwill shift (curve I) into the direction of the sublimation curve (point C at T2).

At higher temperatures the composition of the solid should follow the subli-mation curve. However, due to the fact that this curve bends to higher x valuesat higher temperatures (see below) and due to recrystallization, a deviationfrom this curve takes place to the manganese side of the existence region, cul-minating in a small trajectory DE (~1167 °C -+-~ 1172 "C) followed by atwo-melt trajectory EF (~1172 °C-+- ~1210 "C) and resulting finally in onemelt (the deviation increases due to the reduction of the surface area of thecondensed phase during melting). The trajectory DE may be short (experiment

ON THE P-T-x PHASE DIAGRAM OF THE Mn-Te SYSTEM

(6» or almost zero (an apparently sharp melting point, first part of experiment(1) and experiment (2». Point D may even be situated at the right side of Rinfig. 3 on the solidus, so that the melting trajectory (solid ze liquid) starts athigher temperatures (experiments (3) and (4». If the path followed by solidMnTe lies very close to the solidus at the Mn .side of the existence region attemperatures above 1170'oC, then it may cross this solidus (melt partly) andre-enter the existence region again at a higher temperature (resolidification)depending on the rate of heating, as is the case in experiment (4).During heating in a stream of inert gas a pre-me1ted lump ,of mate rial of

composition xp will change only little in composition, due to its small surface,and will approximately follow path Il (fig. 3) and melt at higher temperatures,e.g. somewhere between 1170 oe (the quadruple point O.P.Q.R.) and themaximum melting point (experiment (2) second half, and experiment (3)).If, on the other hand, the evaporation is rapid, for instance when a powder

is heated in vacuum, the sublimation curve is followed more closely and thematerial will rpelt in the neighbourhood of the intersection point of solidusand sublimation curve, i.e. the maximum sublimation point (N in fig. 3). Thispoint will be situated at about 1207 °C according to the experiments (7) and(9).Because a melt in vacuum will evaporate in a reasonable time the composi-

tion will shift towards the curve of minimum total pressure of the vapour inequilibrium with the melt if any. From experiment (9) it may be concluded thatsuch a curve exists. If a solid at a temperature and of a composition given by N(fig. 3) melts, the composition of the equilibrium melt is given by the point onthe liquidus at the same temperature. Due to evaporation this composition willshift at constant temperature to the curve Xl = Xg, the curve of minimumtotal pressure in the liquid. On slow cooling it will follow this curve until thiscurve intersects the liquidus (point H in fig. 3) where solidification sets in. Ifthe difference in composition between H and the composition of the equilibriumsolid is not too great and the amount of MnTe is not too large, the temperaturewill remain practically constant until the whole mass has solidified. Accordingto experiment (9) this point H may be situated at 1187 °C. If the liquid isheated to 1243 °C in vacuum (experiment (8» it will follow the curve of mini-mum total pressure in the liquid to point Z in fig. 3. Rapid cooling shows asolidification trajectory VW in which W may indicate the eutectic temperatureof L ~ Mn, ~ G ~ MnTe; This means that this temperature is ~ 1145 °C.From the experimental results it follows that the densities of Lj,Lz and MnTehave the sequence dL1> dMnTes < dLz• Therefore, because the maximummelting point is situated in the region of Lj, the sequence of the maxima ofsolidus, liquidus and vapour curve *) must be the sequence given in fig. 3 10).

") The maximum of the vapour curve ·is situated outside fig. 3.

295


The bending of the sublimation curve at higher temperatures to the Te-richside provides a good explanation of the results of the' sublimation experimentsgiven in table II.In experiment M2a Mn'Te, is heated up to 1170 oe. At low temperatures Te2

evaporates from the source and condenses in the colder parts (l) of the subli-mation tube. At higher temperatures the substance tries to follow the sublimationcurve (xs = xg). As a consequence more Mn evaporates than Te2' Because Mnis far less volatile than Te2' it condenses not only in part I but also in otherparts of the tube (UA). After being heated for some time at 1180oe the sublim-ing solid has reached the composition given by the sublimation curve at thattemperature and sublimes as MnTe which, due to the fact that it is less volatilethan Mn, condenses in still hotter parts of the tube (lIB and III). If the tubeis cooled down after some time the solid again tries to follow the sublimationcurve, releasing more Te2 than Mn, which condenses in all three regions.Because the first condensate in region III is nearly pure MnTe, the averagecomposition will shift to a somewhat higher Te content (of course, the Te con-tent has also shifted to a higher value during cooling in region I and IIA, buthere we have no reference composition). In experiment M2 the sublimationtemperature was 1050 oe. At that temperature the sublimation curve has notyet bent so far to a higher Te content as under the conditions of experimentM2a. As the vapour pressures at this temperature are smaller, the supersatura-tion at lower temperatures will also be smaller, and especially the less volatilesubstance will condense in a lower temperature region. Thus, at about 470 oe,Te will condense in region I. At higher temperatures Mn will condense inregion I and 11and, at 1050 oe, MnTe will condense predominantly in region 11and only for a small part in region Ill. During cooling again more Te than Mnwill condense in all three regions.Thus especially between 1050oe and 1180 oe the sublimation curve bends

strongly towards higher x-values, more than between room temperature and1050 oe. This is indicated in fig. 3.The width of the miscibility gap in the liquid at 1170 oe is very small. From

the experiments in sec. 2.2 it may be concluded that the whole miscibility gapmust lie between x = 0·492 and the composition of the solid in the quadruplepoint O.P.Q.R., which cannot deviate much from x = t. The height of themiscibility gap is ~ 40 oe.3.4. The sublimation of solid MnTe in a closed system

If heated at Tl in vacuum in a sealed tube, after degassing, in the presenceof a cold spot, the final composition of solid MnTe is given by the sublimationcurve at Tl and with this composition the solid sublimes as has been pointedout before.If, however, the lower temperature T2 is high, such that no Mn or Te can


condense, the situation is quite different and what will actually happen-dependson the composition of the starting material.

If the starting material has the composition Xl> obtained by degassing beforeat Tl and pumping away the released vapour, the composition of the conden-sate at T2 will generally deviate from Xl if the sublimation line is curved. Asa consequence, when the condensation proceeds the vapour changes its com-position, becoming richer in one of the components (in this case Te2), changingin its turn the composition of the MnTe source at Tl and-thus slowing downthe sublimation until it is practically stopped after some time.If first MnTe is degassed at T2 and then placed in the Tl zone, the solid

tries to reach the composition Xl' increasing the pressure of one of the com-ponents (in this case Mn) in the vapour phase. Therefore the sublimatiori mayalready be hampered before any condensation takes place, the more so as thecurvature of the sublimation line is more pronounced. In sec. 3 it has beenshown that the sublimation curve bends rather strongly with increasing tem-perature towards higher x-values in the region between 1050 and 1160 oe.Therefore we may expect very little or no sublimation of MnTe in a smallevacuated and sealed tube, with a temperature difference over the tube of about100 oe, Tl being about 1150 oe and T2 being about 1050 oe.Some experiments were carried out in a quartz tube of 125 mm long and

20 mm wide, containing about 10grammes of MnTe in different temperaturegradients. The degassing temperatures were Tl>T2 and lower than T2• In allthe experiments the amount of sublimed material was extremely small.This is a confirmation of the form of the sublimation curve in the temperature

region between 1050 and 1160 oe. It is also in accordance with the low valueof Kp and the fact that MnTe is completely dissociated in the vapour phase inMn and Te2'

4. Conclusions(1) The maximum melting point of MnTe in equilibrium with its vapour is

> 1240 oe, probably 1243± 3 oe.(2) MnTe is completely dissociated into Mn and Te2 in the vapour phase, at

least at elevated temperatures.(3) Mn'Te, has a curve of minimum total pressure, i.e. a sublimation curve.(4) At higher temperatures the existence region of solid MnTe shifts to higher

x-values,(5) There is an indication of an extremely small miscibility gap in the liquid

at the Mn side of the maximum melting point of MnTe.

AcknowledgementThe author is indebted to Dr W. Albers for fruitful discussions and to Mr

Knaape for the chemical analyses. Eindhoven, February 1969

·298 J. van den BOOMGAARD

REFERENCES1) W. D. Johnston and D. C. Sestrich, J. inorg. nucl. Chem. 19, 229, 1968.2) .R. T. Delvis and B. Lewis, J. Phys. Chem. Sol. 24, 549-556, 1963.3) R. C. Miller, Science and engineering of thermoelectricity, Survey of known thermo-

electric materials, Interscience Publ., New York-London, 1961.4) L. D. Dudkin and V. J. Var d an ick, Sov. Phys. solid State 2, 1381-1389, 1961.5) N. Kunitomi et al., J. de Physique 25, 568-574, 1964.6) S. Furberg, Acta chem. Scand. 7, 693-694, 1953.7) H. A. Johansen, J. inorg. nucl. Chem. 6, 344-346, 1958.8) J. van den Boomgaard and K. Schol, Philips Res. Repts 12, 127-140, 1959.9) A. M. J. H. Seuter, Private communication.

10) F. E. C. Scheffer, Heterogene evenwichten in unaire en binaire stelsels, Waltman, Delft,p.212.

on the p-t-x phase diagram of the mn-te system bound... · on the p-t-x phase diagram of the mn-te...

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