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Electrical conductivity measurements for immiscible In–Se–Te alloys
Journal of Alloys and Compounds 288 (1999) 151–154
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Electrical conductivity measurements for immiscible In–Se–Te alloys Yu. Plevachuk* Institute of Applied Physics, Ivan Franko State University, 49 General Chuprynka Str., 290044 Lviv, Ukraine Received 16 November 1998
Abstract The electrical conductivity of liquid In 0.80 (Se x Te 12x ) 0.20 alloys has been measured by a contact method in the temperature range from the monotectic temperatures up to about 1200 K under pressures of argon gas (up to 50 MPa). It was revealed that the ternary In 0.80 (Se x Te 12x ) 0.20 system may be considered as a set of quasibinary In–(Se / Te) alloys of almost critical concentrations. It was found that variation of Se–Te ratio at a constant content of In changes the properties of coexisting liquids and affects the phase separation temperature T c which decreases in a nonlinear manner from 949 K for In 0.80 Se 0.20 to 817 K for In 0.80 Te 0.20 . A brief comparison analysis of our results with available data for binary and ternary immiscible alloys has been performed. 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Critical point; Electrical transport; Miscibility gap; Phase diagrams; Ternary alloys
1. Introduction In contrast to dielectric solutions with a miscibility gap binary and ternary liquid conducting alloys are relatively less studied in the region of a limited mutual component solubility [1]. In the case of metal–chalcogen systems the situation is due to the difficulties of high-temperature experimentation, the absence of a generally accepted measuring technique and, subsequently, is caused by the peculiarities of each experimental method employed. It should be noted that due to the high vapor pressure the metal–chalcogen systems can hardly be investigated by high-temperature solution calorimetry, and their thermodynamics is poorly known. Nevertheless, owing to a renewed scientific and practical interest similar many-component systems become the subject of intensive studies in order to understand the complex processes occurring during cooling through a miscibility gap [2,3]. In this paper we report results of precise electrical conductivity s (T ) measurements for the liquid In–Se–Te alloys in the concentration–temperature range of the immiscible gap. These investigations have been carried out *Corresponding author. Tel.: 1380-322-353475; fax: 1380-322631565. E-mail address: [email protected] (Y. Plevachuk)
in the frame of a general program examining the region of liquid–liquid equilibrium in the metal–chalcogen systems [4,5].
2. Experimental details The electrical conductivity measurements have been carried out by a contact method in accordance with the four-point scheme. The experimental details and equipment have been described elsewhere [6]. Weighed (to within 610 24 g) amounts of In (99.999% metallic purity), Se and Te (99.999%) were melted in evacuated and sealed quartz ampules at 12 Pa. Each of the samples was filled into the measuring cell directly inside a high-pressure vessel. Thus, the sample composition was accurate to within 60.02 at.%. Measuring cells, manufactured from boron nitride ceramic in the form of vertical cylindrical containers, with an operating cavity height of 60 mm and a diameter of 3.5 mm, were used for this purpose. Seven graphite electrodes, two for current (at the top and the bottom) and five for potential measurements, were placed in the wall of the container along its vertical axis. A high-temperature heater with three independently controlled heating elements, together with the heat capacity and thermoconductivity of a copper container-‘thermostat’ enables us to create a
0925-8388 / 99 / $ – see front matter 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00125-5
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Y. Plevachuk / Journal of Alloys and Compounds 288 (1999) 151 – 154
temperature field homogeneous within 60.3 K in the range of absolute temperatures #1200 K. Temperature gradients along the cell were additionally controlled to within 0.1 K by a five-point Pt / Pt–Rh differential thermocouple. Each measuring cell was initially heated to a maximum temperature of #1200 K and maintained until the resistivities between the five intermediate potential electrodes coincided. The homogeneous alloy then was cooled at a rate of 8–10 K h -1 corresponding to quasi-equilibrium conditions. The experiments were performed under ambient pressures of argon gas (up to 50 MPa) to avoid intensive melt evaporation and provide constant chemical composition of the samples. Nevertheless, each alloy was weighed after the end of a measurement series and compared with the sum of the masses of the charged components. The differences detected were less that 0.4% of the total mass. The error of the electrical conductivity determination did not exceed 1.5% [6].
The electrical conductivity measurements were carried out for ternary In 0.80 (Se x Te 12x ) 0.20 alloys where x50.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9. The conductivity s (T ) data for an alloy with x50.5 are shown in Fig. 1. The same electrical conductivity was observed also for other investigated alloys. As can be seen, cooling a sample of a completely mixed liquid from 1200 K is accompanied by a negligible change in their electronic properties. Moreover, the absolute s (T ) values, in the temperature region above a consolute point, are determined mainly by the initial sample concentration. Upon reaching the phase separation temperature, T s (x), gravity separated the melt into two phases, and a horizontal boundary surface, the so-called meniscus, between the liquid layers appeared. Further cooling was accompanied by a continuous component
redistribution. The denser layer was enriched by In and at the expense of the less dense upper liquid. The latter, in turn, was enriched by Se and Te. The general pattern of s (T ) values agrees with those observed earlier in the Tl 0.80 (Se x Te 12x ) 0.20 system [7]. For identical temperature reductions, different changes for each phase form the different temperature dependences of electronic properties of coexisting liquids (a metal–nonmetal transition takes place). This transition manifested itself in our experiments as the beginning of a s (T ) divergence at the T s (x) point into two branches: sL (T ) describes the electrical conductivity of the lower liquid, while sU (T ) describes the electrical conductivity of the upper liquid. As can be seen from Fig. 2, the phase separation temperature T s (x) depends strongly on the Se–Te ratio. In comparison to the binary In 0.80 Se 0.20 alloy (T s 594961.8 K [4]) the phase separation temperature of the In 0.80 Se 0.18 Te 0.2 melt was 93761.8 K and decreases further upon continued Te substitution to 81761.7 K for the liquid In 0.8 Te 0.2 alloy [5] (Table 1). The monotectic line temperatures T M (x) decreases from 79461.7 K for the In 0.80 Se 0.2 alloy to 69161.5 K for the In 0.80 Te 0.20 alloy. The concentration dependencies of the phase separation and the monotectic line temperatures on the Se–Te ratio for the investigated In–Se–Te alloys are plotted in Fig. 3. The lack of experimental data for similar ternary alloys as well as the absence of an adequate theory forced us to realize only a brief comparative analysis of our results with those for the immiscible In–Se and In–Te binaries. To our knowledge, this concentration region for the In–Se system has not been studied previously [4]. The possible cause of observed discrepancy in the critical temperature values determined earlier by different experimental methods for the In–Te system [8,9] has been explained in [5]. Taking into account that the critical concentration for both binaries consists of approximately 80 at.% In and 20 at.% chalcogen, we assumed that the measured T s (x) values are due to the critical point temperature during the transition from
Fig. 1. Temperature dependence of the electrical conductivity for the liquid In 0.80 Se 0.10 Te 0.10 alloy.
Fig. 2. Temperature dependence of the electrical conductivity for liquid In 0.80 (Se x Te 12x ) 0.20 alloys, x50, 0.8, 0.5, 0.2, 1.
3. Results and discussion
Y. Plevachuk / Journal of Alloys and Compounds 288 (1999) 151 – 154
153
Table 1 Experimental data of the phase separation and monotectic line temperatures for the In–Se–Te system Composition
Phase separation temperature, T s (K)
Monotectic line temperature, T M (K)
In 0.80 Se 0.20 In 0.80 Se 0.18 Te 0.02 In 0.80 Se 0.16 Te 0.04 In 0.80 Se 0.14 Te 0.06 In 0.80 Se 0.12 Te 0.08 In 0.80 Se 0.12 Te 0.08 In 0.80 Se 0.08 Te 0.12 In 0.80 Se 0.06 Te 0.14 In 0.80 Se 0.04 Te 0.16 In 0.80 Se 0.02 Te 0.18 In 0.80 Te 0.20
949 937 927 914 905 893 880 865 850 836 817
794 774 755 743 735 730 716 710 703 693 691
one binary system to the other, due to the partial substitution of Te for Se. By this means, the data in the Fig. 3 may be considered as those for a set of quasibinary In–(Se / Te) alloys of almost critical concentration. Moreover, the measured s (T ) dependences (see Figs. 1, 2) show the same behavior as for the binary alloys [4,5]. The revealed T s (x) behaviour does not show a linear dependency on the alloy composition. At the same time, contrary to earlier investigated ternary Ga–Hg–In [10], Cd–Ga–Hg [11] and In–Tl–Te [12] liquid alloys, a gradual temperature decrease upon Te substitution is observed. A qualitative explanation, based on a simple mean-field model describing the liquid–liquid coexistence for binary metal alloys [13,14] is proposed. In accordance with this model, the critical point parameters are mainly determined by the difference in the valences of the alloy components. For the isovalent components the difference in specific atom volumes or ‘effective’ ion-core radii dominates. A functional dependency of the critical concentration value x c on these parameters is relative weak, while the critical temperature varies approximately as (r a2 2 r b2 )2 , where r a and r b are the ‘effective’ ion-core radii of the components. Their absolute values are single fitting parameters in the corresponding pseudopotential expres-
sions, being used for the theoretical modeling. In our case, Te and Se atoms have the same configuration of outer electron shells, but different nuclear charges and atomic radii, which increase with increases in the atomic number, Z. Thermodynamic considerations alloys with demixing tendency also support the statement that the difference between the atomic radii of the components is of influence. The critical demixing temperature increases with increasing difference of the radii [15]. This may be considered as one of the reasons for the observed T s (x) dependence.
4. Conclusion We have found from the electrical conductivity measurements for liquid In 0.80 (Se x Te 12x ) 0.20 alloys that variations of the Se–Te ratio, for a constant content of In, changes the properties of coexisting liquids and variously affects the phase separation temperature, T s (x). The partial chalcogen substitution leads to a decrease of T s (x). Similar behavior has been noticed earlier for the Tl 0.80 (Se x Te 12x ) 0.20 system. At the same time, the lines of the T s (x) points for the immiscible alloys with a predominant metal content, as in In–Tl–Te, and the similar intermetallic Ga–Hg–In and Cd–Ga–Hg, have clearly defined maximums. Experimental studies of other ternary systems with a miscibility gap are in progress now and the obtained results will be presented soon.
References
Fig. 3. The phase separation temperatures of liquid In 0.80 (Se x Te 12x ) 0.20 alloys for different Se–Te ratios.
[1] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, 2nd ed, ASM, Materials Park, OH, 1990. [2] L. Ratke, S. Diefenbach, Mater. Sci. Eng. R15 (1995) 263. [3] J. Alkemper, L. Ratke, Z. Metallkd. 85 (1994) 365. [4] V. Didoukh, Yu. Plevachuk, B. Sokolovskii, J. Phase Equilibria 17 (5) (1996) 414. [5] Yu. Plevachuk, V. Didoukh, B. Sokolovskii, in: K.E. Spear (Ed.), High Temperature Materials Chemistry IX, The Electrochemical Society Inc, Pennington, NJ, 1997, p. 203.
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[6] B. Sokolovskii, V. Sklyarchuk, V. Didoukh, Yu. Plevachuk, High Temp. Mater. Sci. 34 (1995) 275. [7] V. Didoukh, B. Sokolovskii, Yu. Plevachuk, J. Phys. Condensed Matter 9 (1997) 3343. [8] J. Vollmann, V. Didoukh, J. Non-Cryst. Solids 205–207 (1996) 408. [9] A.R. Regel, V.M. Glazov, S.G. Kim, Fiz. Tekh. Poluprov. 20 (1986) 1353, in Russian.
[10] G.H.M. Gubbels, Z. Metallkd. 81 (1990) 202. [11] G.H.M. Gubbels, Z. Metallkd. 85 (1994) 148. [12] Yu. Plevachuk, V. Didoukh, B. Sokolovskii, J. Alloys Comp. 274 (1998) 206. [13] D. Stroud, Phys. Rev. B. 7 (10) (1973) 4405. [14] D. Stroud, Phys. Rev. B. 8 (4) (1973) 1308. [15] B. Predel, J. Phase Equilibria 18 (4) (1997) 327.
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Electrical conductivity measurements for immiscible In–Se–Te alloys Yu. Plevachuk* Institute of Applied Physics, Ivan Franko State University, 49 General Chuprynka Str., 290044 Lviv, Ukraine Received 16 November 1998
Abstract The electrical conductivity of liquid In 0.80 (Se x Te 12x ) 0.20 alloys has been measured by a contact method in the temperature range from the monotectic temperatures up to about 1200 K under pressures of argon gas (up to 50 MPa). It was revealed that the ternary In 0.80 (Se x Te 12x ) 0.20 system may be considered as a set of quasibinary In–(Se / Te) alloys of almost critical concentrations. It was found that variation of Se–Te ratio at a constant content of In changes the properties of coexisting liquids and affects the phase separation temperature T c which decreases in a nonlinear manner from 949 K for In 0.80 Se 0.20 to 817 K for In 0.80 Te 0.20 . A brief comparison analysis of our results with available data for binary and ternary immiscible alloys has been performed. 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Critical point; Electrical transport; Miscibility gap; Phase diagrams; Ternary alloys
1. Introduction In contrast to dielectric solutions with a miscibility gap binary and ternary liquid conducting alloys are relatively less studied in the region of a limited mutual component solubility [1]. In the case of metal–chalcogen systems the situation is due to the difficulties of high-temperature experimentation, the absence of a generally accepted measuring technique and, subsequently, is caused by the peculiarities of each experimental method employed. It should be noted that due to the high vapor pressure the metal–chalcogen systems can hardly be investigated by high-temperature solution calorimetry, and their thermodynamics is poorly known. Nevertheless, owing to a renewed scientific and practical interest similar many-component systems become the subject of intensive studies in order to understand the complex processes occurring during cooling through a miscibility gap [2,3]. In this paper we report results of precise electrical conductivity s (T ) measurements for the liquid In–Se–Te alloys in the concentration–temperature range of the immiscible gap. These investigations have been carried out *Corresponding author. Tel.: 1380-322-353475; fax: 1380-322631565. E-mail address: [email protected] (Y. Plevachuk)
in the frame of a general program examining the region of liquid–liquid equilibrium in the metal–chalcogen systems [4,5].
2. Experimental details The electrical conductivity measurements have been carried out by a contact method in accordance with the four-point scheme. The experimental details and equipment have been described elsewhere [6]. Weighed (to within 610 24 g) amounts of In (99.999% metallic purity), Se and Te (99.999%) were melted in evacuated and sealed quartz ampules at 12 Pa. Each of the samples was filled into the measuring cell directly inside a high-pressure vessel. Thus, the sample composition was accurate to within 60.02 at.%. Measuring cells, manufactured from boron nitride ceramic in the form of vertical cylindrical containers, with an operating cavity height of 60 mm and a diameter of 3.5 mm, were used for this purpose. Seven graphite electrodes, two for current (at the top and the bottom) and five for potential measurements, were placed in the wall of the container along its vertical axis. A high-temperature heater with three independently controlled heating elements, together with the heat capacity and thermoconductivity of a copper container-‘thermostat’ enables us to create a
0925-8388 / 99 / $ – see front matter 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00125-5
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Y. Plevachuk / Journal of Alloys and Compounds 288 (1999) 151 – 154
temperature field homogeneous within 60.3 K in the range of absolute temperatures #1200 K. Temperature gradients along the cell were additionally controlled to within 0.1 K by a five-point Pt / Pt–Rh differential thermocouple. Each measuring cell was initially heated to a maximum temperature of #1200 K and maintained until the resistivities between the five intermediate potential electrodes coincided. The homogeneous alloy then was cooled at a rate of 8–10 K h -1 corresponding to quasi-equilibrium conditions. The experiments were performed under ambient pressures of argon gas (up to 50 MPa) to avoid intensive melt evaporation and provide constant chemical composition of the samples. Nevertheless, each alloy was weighed after the end of a measurement series and compared with the sum of the masses of the charged components. The differences detected were less that 0.4% of the total mass. The error of the electrical conductivity determination did not exceed 1.5% [6].
The electrical conductivity measurements were carried out for ternary In 0.80 (Se x Te 12x ) 0.20 alloys where x50.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9. The conductivity s (T ) data for an alloy with x50.5 are shown in Fig. 1. The same electrical conductivity was observed also for other investigated alloys. As can be seen, cooling a sample of a completely mixed liquid from 1200 K is accompanied by a negligible change in their electronic properties. Moreover, the absolute s (T ) values, in the temperature region above a consolute point, are determined mainly by the initial sample concentration. Upon reaching the phase separation temperature, T s (x), gravity separated the melt into two phases, and a horizontal boundary surface, the so-called meniscus, between the liquid layers appeared. Further cooling was accompanied by a continuous component
redistribution. The denser layer was enriched by In and at the expense of the less dense upper liquid. The latter, in turn, was enriched by Se and Te. The general pattern of s (T ) values agrees with those observed earlier in the Tl 0.80 (Se x Te 12x ) 0.20 system [7]. For identical temperature reductions, different changes for each phase form the different temperature dependences of electronic properties of coexisting liquids (a metal–nonmetal transition takes place). This transition manifested itself in our experiments as the beginning of a s (T ) divergence at the T s (x) point into two branches: sL (T ) describes the electrical conductivity of the lower liquid, while sU (T ) describes the electrical conductivity of the upper liquid. As can be seen from Fig. 2, the phase separation temperature T s (x) depends strongly on the Se–Te ratio. In comparison to the binary In 0.80 Se 0.20 alloy (T s 594961.8 K [4]) the phase separation temperature of the In 0.80 Se 0.18 Te 0.2 melt was 93761.8 K and decreases further upon continued Te substitution to 81761.7 K for the liquid In 0.8 Te 0.2 alloy [5] (Table 1). The monotectic line temperatures T M (x) decreases from 79461.7 K for the In 0.80 Se 0.2 alloy to 69161.5 K for the In 0.80 Te 0.20 alloy. The concentration dependencies of the phase separation and the monotectic line temperatures on the Se–Te ratio for the investigated In–Se–Te alloys are plotted in Fig. 3. The lack of experimental data for similar ternary alloys as well as the absence of an adequate theory forced us to realize only a brief comparative analysis of our results with those for the immiscible In–Se and In–Te binaries. To our knowledge, this concentration region for the In–Se system has not been studied previously [4]. The possible cause of observed discrepancy in the critical temperature values determined earlier by different experimental methods for the In–Te system [8,9] has been explained in [5]. Taking into account that the critical concentration for both binaries consists of approximately 80 at.% In and 20 at.% chalcogen, we assumed that the measured T s (x) values are due to the critical point temperature during the transition from
Fig. 1. Temperature dependence of the electrical conductivity for the liquid In 0.80 Se 0.10 Te 0.10 alloy.
Fig. 2. Temperature dependence of the electrical conductivity for liquid In 0.80 (Se x Te 12x ) 0.20 alloys, x50, 0.8, 0.5, 0.2, 1.
3. Results and discussion
Y. Plevachuk / Journal of Alloys and Compounds 288 (1999) 151 – 154
153
Table 1 Experimental data of the phase separation and monotectic line temperatures for the In–Se–Te system Composition
Phase separation temperature, T s (K)
Monotectic line temperature, T M (K)
In 0.80 Se 0.20 In 0.80 Se 0.18 Te 0.02 In 0.80 Se 0.16 Te 0.04 In 0.80 Se 0.14 Te 0.06 In 0.80 Se 0.12 Te 0.08 In 0.80 Se 0.12 Te 0.08 In 0.80 Se 0.08 Te 0.12 In 0.80 Se 0.06 Te 0.14 In 0.80 Se 0.04 Te 0.16 In 0.80 Se 0.02 Te 0.18 In 0.80 Te 0.20
949 937 927 914 905 893 880 865 850 836 817
794 774 755 743 735 730 716 710 703 693 691
one binary system to the other, due to the partial substitution of Te for Se. By this means, the data in the Fig. 3 may be considered as those for a set of quasibinary In–(Se / Te) alloys of almost critical concentration. Moreover, the measured s (T ) dependences (see Figs. 1, 2) show the same behavior as for the binary alloys [4,5]. The revealed T s (x) behaviour does not show a linear dependency on the alloy composition. At the same time, contrary to earlier investigated ternary Ga–Hg–In [10], Cd–Ga–Hg [11] and In–Tl–Te [12] liquid alloys, a gradual temperature decrease upon Te substitution is observed. A qualitative explanation, based on a simple mean-field model describing the liquid–liquid coexistence for binary metal alloys [13,14] is proposed. In accordance with this model, the critical point parameters are mainly determined by the difference in the valences of the alloy components. For the isovalent components the difference in specific atom volumes or ‘effective’ ion-core radii dominates. A functional dependency of the critical concentration value x c on these parameters is relative weak, while the critical temperature varies approximately as (r a2 2 r b2 )2 , where r a and r b are the ‘effective’ ion-core radii of the components. Their absolute values are single fitting parameters in the corresponding pseudopotential expres-
sions, being used for the theoretical modeling. In our case, Te and Se atoms have the same configuration of outer electron shells, but different nuclear charges and atomic radii, which increase with increases in the atomic number, Z. Thermodynamic considerations alloys with demixing tendency also support the statement that the difference between the atomic radii of the components is of influence. The critical demixing temperature increases with increasing difference of the radii [15]. This may be considered as one of the reasons for the observed T s (x) dependence.
4. Conclusion We have found from the electrical conductivity measurements for liquid In 0.80 (Se x Te 12x ) 0.20 alloys that variations of the Se–Te ratio, for a constant content of In, changes the properties of coexisting liquids and variously affects the phase separation temperature, T s (x). The partial chalcogen substitution leads to a decrease of T s (x). Similar behavior has been noticed earlier for the Tl 0.80 (Se x Te 12x ) 0.20 system. At the same time, the lines of the T s (x) points for the immiscible alloys with a predominant metal content, as in In–Tl–Te, and the similar intermetallic Ga–Hg–In and Cd–Ga–Hg, have clearly defined maximums. Experimental studies of other ternary systems with a miscibility gap are in progress now and the obtained results will be presented soon.
References
Fig. 3. The phase separation temperatures of liquid In 0.80 (Se x Te 12x ) 0.20 alloys for different Se–Te ratios.
[1] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, 2nd ed, ASM, Materials Park, OH, 1990. [2] L. Ratke, S. Diefenbach, Mater. Sci. Eng. R15 (1995) 263. [3] J. Alkemper, L. Ratke, Z. Metallkd. 85 (1994) 365. [4] V. Didoukh, Yu. Plevachuk, B. Sokolovskii, J. Phase Equilibria 17 (5) (1996) 414. [5] Yu. Plevachuk, V. Didoukh, B. Sokolovskii, in: K.E. Spear (Ed.), High Temperature Materials Chemistry IX, The Electrochemical Society Inc, Pennington, NJ, 1997, p. 203.
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[6] B. Sokolovskii, V. Sklyarchuk, V. Didoukh, Yu. Plevachuk, High Temp. Mater. Sci. 34 (1995) 275. [7] V. Didoukh, B. Sokolovskii, Yu. Plevachuk, J. Phys. Condensed Matter 9 (1997) 3343. [8] J. Vollmann, V. Didoukh, J. Non-Cryst. Solids 205–207 (1996) 408. [9] A.R. Regel, V.M. Glazov, S.G. Kim, Fiz. Tekh. Poluprov. 20 (1986) 1353, in Russian.
[10] G.H.M. Gubbels, Z. Metallkd. 81 (1990) 202. [11] G.H.M. Gubbels, Z. Metallkd. 85 (1994) 148. [12] Yu. Plevachuk, V. Didoukh, B. Sokolovskii, J. Alloys Comp. 274 (1998) 206. [13] D. Stroud, Phys. Rev. B. 7 (10) (1973) 4405. [14] D. Stroud, Phys. Rev. B. 8 (4) (1973) 1308. [15] B. Predel, J. Phase Equilibria 18 (4) (1997) 327.