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Abstract. Phase equilibria in the silver bromide^cadmium dibromide (AgBr^CdBr2) system have been investigated by differential scanning calorimetry and x-ray ...
High Temperatures ^ High Pressures, 2002, volume 34, pages 349 ^ 353

DOI:10.1068/htjr030

Phase equilibrium diagram of the system silver bromide ^ cadmium dibromide Alina Wojakowska, Agata Go¨rniak

Department of Inorganic Chemistry, Wroc•aw Medical University, ul. Szewska 38, PL 50139 Wroc•aw, Poland; fax: +48 71 442277; email: [email protected]

Andrzej Wojakowski

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Oko¨lna 2, 50950 Wroc•aw 2, Poland Received 29 May 2001

Abstract. Phase equilibria in the silver bromide ^ cadmium dibromide (AgBr ^ CdBr2) system have been investigated by differential scanning calorimetry and x-ray diffraction. The phase diagram for the system has been constructed. The most significant feature of the AgBr ^ CdBr2 phase diagram at high temperatures is the occurrence of solid solution areas based on either AgBr or CdBr2. The solid solution based on AgBr extends to 40 mol% CdBr2 where it decomposes peritectically at 442 8C into the solid solution based on CdBr2 (4.5 mol% AgBr) and the molten salt solution (62 mol% AgBr). The solid solubility is negligible at room temperature. No intermediate compound has been found in the system.

1 Introduction The first data on the solid ^ liquid phase diagram for the AgBr ^ CdBr2 system were reported by Zakharchenko (1951). Only temperatures of primary crystallisation were given for the twelve investigated mixtures. The kinds of respective crystallising phases were not assigned. No invariance was indicated. In the region of composition from about 56.5 to 80.0 mol% AgBr, no change of temperature of the liquidus curve was observed. The relevant value of 449 8C was interpreted in terms of the decomposition of an intermediate compound, the formula of which was not proposed. Nothing about the formation of solid solutions was mentioned. On the other hand, several properties of AgBr have been measured as a function of temperature, and many of these properties have been studied on doped samples as a function of Cd2‡ concentration. The properties measured included among others electric conductivity (Teltow 1949; Hanlon 1960), electric conductivity at high pressures (Kurnick 1952), diffusion coefficients (Scho«ne et al 1951; Tannhauser 1958), the thermal expansion coefficient (Zieten 1956), thermoelectric power (Patrick and Lawson 1954; Christy et al 1959), and specific heat (Vomhof and No«lting 1975; Lin and Schmalzried 1976). With few exceptions, cadmium-doped silver bromide needed for those experiments contained not more than 1 mol% cadmium bromide and it was assumed to be a homogeneous phase in the appropriate range of temperatures. Nevertheless, a limit of the solid solubility of CdBr2 in AgBr has not been established. In order to clarify the phase equilibria existing in the silver bromide ^ cadmium dibromide condensed system, in this work we present the phase equilibrium diagram for the whole range of compositions of the system, based on our DSC and x-ray diffraction experiments. 2 Experimental 2.1 Preparation of salt mixtures Silver bromide was precipitated from a dilute aqueous solution of silver nitrate with 1 M solution of potassium bromide. The precipitate was carefully washed, dried at 50 8C

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A Wojakowska, A Go¨rniak, A Wojakowski

in vacuum and then heated to melting in an electric furnace. The product was stored in the dark after solidification. Anhydrous cadmium bromide was obtained from CdBr2  4H2 O. Ammonium bromide was used as a water-removing carrier. First, a mixture of cadmium bromide tetrahydrate and ammonium bromide was heated under vacuum to about 200 8C. A single charge consisted of about 15 g of each salt. Next, the heating was carried out in the air to evaporate most of the water and to advance sublimation of the ammonium bromide escaping with traces of water. After the white fumes had disappeared (at about 450 8C ), the salt was heated slowly (3 8C minÿ1 ) up to melting. The molten anhydrous cadmium bromide was then distilled under vacuum at 610 8C in silica tubes. Mixtures for DSC and x-ray diffraction experiments were prepared from appropriate quantities of silver bromide and cadmium dibromide by melting them in silica ampoules sealed under vacuum. Particular precautions were taken in the preparation of pure AgBr and of mixtures containing AgBr under red light. 2.2 DSC measurements Phase transition studies were made with a Mettler Toledo DSC 25 measuring cell with improved ceramic heat flow sensor, TC15 TA Controller and STAR 4.0 software. Samples were prepared by weighing appropriate quantities of components on a Mettler Toledo AT 261 balance (0:01 mg) and putting them directly in small silica ampoules with flattened bottoms used for DSC measurements. Next, the ampoules were sealed under vacuum and heated to melting so that the mixtures became homogeneous. The total weight of a typical sample was 20 ^ 30 mg. We also measured a few samples with a weight ten times greater. About 70 samples were prepared. Heating and cooling cycles for all the samples were performed at a rate of 5 8C minÿ1 between room temperature and the liquidus temperature. Other rates: 0.1, 0.5, 1, 2, and 20 8C minÿ1 were also used when needed. 2.3 x-Ray diffraction experiments x-Ray powder diffraction patterns were obtained at room temperature on a DRON diffractometer with CuKa radiation. Two series of diffractograms were taken: one for samples melted and cooled slowly to room temperature, and the second for samples that were annealed for a week at 380 8C after being melted and then quenched with liquid nitrogen. 2.4 Conductivity measurements A few runs of conductivity measurements as a function of temperature (Wojakowska and Go¨rniak 2001) were performed by AC techniques (Wojakowska et al 1998) to check some phase transitions in the system under investigation. 3 Results and discussion The phase equilibrium diagram obtained in this work is shown in figure 1. Melting points of the components AgBr and CdBr2 were found to be 420.4 and 567.9 8C, respectively. One invariant temperature at 442  2 8C, corresponding to the triple phase equilibrium: CdBr2-based solid solution (a phase) ^ AgBr-based solid solution (b phase) ^ liquid solution (L), has been found. Compositions of singular points corresponding to the peritectic transition are: 4:5  0:3 mol% AgBr, 60:0  0:2 mol% AgBr, and 61:9  0:2 mol% AgBr, respectively. Below the peritectic invariance, the solid solubility in the system falls while the temperature is decreasing and finally becomes negligible. The border of the CdBr2 -based solid solution was determined only approximately because the respective thermal events were hardly detectable by means of DSC curves. However, the part above the invariance has been confirmed in the course of electric

AgBr ^ CdBr2 phase equilibrium diagram

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Figure 1. Phase diagram of the AgBr ^ CdBr2 system; thermal events registered by DSC: * (this work), conductivity measurements: ~ (Wojakowska and Go¨rniak 2001) and & (Hanlon 1960).

conductivity measurements (Wojakowska and Go¨rniak 2001) by visible changes of the electric conductivity caused by the appearance or disappearance of the liquid phase on heating or on cooling, respectively, eg at 546 8C for 2 mol% AgBr (figure 1). The maximal solid solubility of AgBr in CdBr2 was estimated to be about 4.5 mol%, although the thermal events corresponding to the peritectic reaction at 442 8C disappear near 10 mol% AgBr and the respective side of Tamman triangle cuts the peritectic line close to this composition. Nevertheless, a marked change of the specific conductivity at the peritectic temperature yields a clear indication that even at the total composition of 5 mol% AgBr, the peritectic reaction takes place. Additionally, the x-ray powder diffraction pattern for the sample containing 5 mol% AgBr quenched from 380 8C (figure 2) shows the presence of a slight amount of the second phase (AgBr-based solid solution) which can be deduced from a broader CdBr2 peak (2y  328). The latter effect is caused by the superposition of the neighbouring line of AgBr (2y  318), the most intensive in the AgBr spectrum.

Intensity (arbitrary units)

AgBr 0:92AgBr ‡ 0:08CdBr2 0:60AgBr ‡ 0:40CdBr2 0:05AgBr ‡ 0:95CdBr2

CdBr2 20

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Figure 2. x-Ray powder diffraction patterns for samples annealed at 380 8C and quenched with liquid nitrogen.

A Wojakowska, A Go¨rniak, A Wojakowski

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The AgBr-based solid solution embodies a far broader composition range, extending up to 40 mol% CdBr2 at the peritectic temperature. The peritectic point is only 2 mol% away. Moving to the AgBr-rich side, we have solidus and liquidus lines close to each other. Two separate peaks were only observed on cooling curves taken with a rate of 1 C8 minÿ1 or less (figure 3a). Past the composition of 89 mol% AgBr, the solidus and liquidus lines are clearly distinct. They markedly fall down to the melting point of the pure silver bromide. The CdBr2 -rich border of the AgBr-based solid solution (figure 1) has been carefully determined from both heating and cooling DSC curves. Near the peritectic decomposition, two distinct thermal effects were observed on cooling curves 92:7% AgBr ‡ 7:3% CdBr2

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Figure 3. Examples of DSC curves: (a) separation of the liquidus and solidus peaks on cooling; (b) two distinct thermal effects observed on coolingöperitectic reaction and decomposition of the AgBr-based solid solution; (c) formation of the AgBr-based solid solution observed on fast heating.

AgBr ^ CdBr2 phase equilibrium diagram

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(figure 3b) for samples containing as little as a few tenths mol% AgBr more than 60 mol% AgBr, which is the composition that was established for the peritectic transition. At lower temperatures, heating experiments, including those performed at a rate of 20 or even 50 8C minÿ1 , in most cases gave better evidence for the solid state transition. An example of this kind of DSC curve is shown in figure 3c. The border between the AgBrbased solid solution (b phase) and the two-phase region (a ‡ b phases) approaches pure AgBr under 100 8C (figure 1). For mixtures rich in AgBr, the course of the above line is in agreement with limits of solubility of CdBr2 in AgBr given by Teltow (1949) and Hanlon (1960) on the basis of conductivity measurements. Heat capacities measured by Vomhof and No«lting (1975) for 2, 4, and 8 mol% CdBr2 give values somewhat higher, but still quite similar. The x-ray powder diffraction pattern for the sample of composition 92.1 mol% AgBr, which was annealed for a week at 380 8C and then quenched with liquid nitrogen, showed a slight change in lattice constants, compared with the corresponding pure AgBr sample, owing to formation of a solid solution based on AgBr (figure 2). No evidence of a second phase was found. On the other hand, the x-ray diffraction pattern for the sample of composition 60.0 mol% AgBr (figure 2), prepared in the same way, indicates a two-phase region consisting of two limiting solid solutions, one based on CdBr2 and the other based on AgBr. x-Ray diffraction patterns for mixtures of compositions 20.3 and 80.0 mol% AgBr (which were melted and then cooled slowly to room temperature) show only peaks of AgBr and CdBr2. In conclusion, we can say that the phase equilibrium diagram of the AgBr ^ CdBr2 system is of the peritectic type with limiting solid solutions on both sides. No intermediate compound has been found in the system. For mixtures containing more than 62 mol% AgBr, a thermal effect apparent at a nearly constant temperature up to about 89 mol% AgBr and actually corresponding to a double transition was assigned not to an invariance as suggested Zakharchenko (1951) but to liquidus and solidus lines lying very close to each other. With this type of liquid ^ solid equilibria, positive deviations from ideal solution behaviour may be expected for molten salt solutions. This has been reported by Kundys (1981) who examined the thermodynamic properties of the AgBr ^ CdBr2 molten system and found positive enthalpies of mixing for solutions rich in AgBr, ie in the region of compositions where a broad area of AgBr-based solid solution occurs close to the liquidus curve. There is a striking resemblance between the phase equilibrium diagram for the system AgCl ^ CdCl2 (Blachnik and Alberts 1982) and that given in this work for the system AgBr ^ CdBr2 . This may result from the similarity of properties such as the type of structure or the melting points of the respective components of both systems. References Blachnik R, Alberts J E, 1982 Z. Anorg. Allg. Chem. 489 161 ^ 172 Christy R W, Fukushima E, Li H T, 1959 J. Chem. Phys. 30 136 ^ 138 Hanlon J E, 1960 J. Chem. Phys. 32 1492 ^ 1500 Kundys E, 1981 Pol. J. Chem. 55 2485 ^ 2488 Kurnick S J, 1952 J. Chem. Phys. 20 218 ^ 228 Lin P L, Schmalzried H, 1976 Z. Phys. Chem., Neue Folge 99 161 ^ 170 Patrick L, Lawson A W, 1954 J. Chem. Phys. 22 1492 ^ 1495 Scho«ne E, Stasiw O, Teltow J, 1951 Z. Phys. Chem. 197 145 ^ 160 Tannhauser D S, 1958 J. Phys. Chem. Solids 5 224 ^ 235 Teltow J, 1949 Ann. Phys. (6) (Leipzig) 5 63 ^ 70, 71 ^ 88 Vomhof H G, No«lting J, 1975 Ber. Ges. Phys. Chem. 79 991 ^ 996 Wojakowska A, Go¨rniak A, 2001, in XXVII Journe¨es des Equilibres entre Phases Eds R M MarinAyral, M C Record (Montpelier, France 2, Universite¨ Montpellier II) pp 155 ^ 158 Wojakowska A, Plinska S, Josiak J, Kundys E, 1998 High Temp. ^ High Press. 30 113 ^ 118 Zakharchenko G A, 1951 Zh. Obshch. Khim. 21 453 ^ 456 Zieten W, 1956 Z. Phys. 145 125 ^ 130

ß 2002 a Pion publication printed in Great Britain