6692
J. Phys. Chem. B 1997, 101, 6692-6694
Experimental Evidence for a Demixing Phenomenon in the Liquid State around the Binary Compound CoSi2 J. Rogez* Centre de Thermodynamique et de Microcalorime´ trie du CNRS, 26 rue du 141e` me R.I.A., F-13331 Marseille Cedex 3, France
P. Knauth EDIFIS (UMR 6518 CNRS), Faculte´ des Sciences de Marseille-Saint Je´ roˆ me, Case 511, F-13397 Marseille Cedex 20, France ReceiVed: February 20, 1997; In Final Form: May 21, 1997X
For the first time, a demixing phenomenon is observed in a liquid binary metal-silicon system. Combined results from high-temperature differential thermal analysis and adiabatic calorimetry allow a determination of the enthalpy of phase transformation and the heat capacities in the solid and liquid states; furthermore, the equilibrium phase diagram around the ordered CoSi2 compound is corrected. The maximum of the liquidus curve lies only 8 K above the invariant transformation of the crystalline CoSi2 into two immiscible liquids.
I. Introduction Demixing phenomena in the liquid state above a definite compound are very seldom observed. One of the rare examples is the BiCl3-Bi system where demixing occurs above the BiCl compound.1 Previous experiments with CoSi2 showed the presence of two different eutectics in the intergranular boundaries of massive samples prepared by fusion solidification. This finding is incompatible with elementary thermodynamics. The Gibbs phase rule2 states that the variance V, i.e. the number of independent thermodynamic variables, like temperature, pressure, or chemical potentials, that can be modified in a system with C components without modifying the number of phases P, can be calculated according to the following relation:
V)C-P+2
(1)
In binary systems (C ) 2), V equals (4 - P). This means that together with the gas phase, a maximum of three condensed phases can be at equilibrium. In this case, all thermodynamic parameters, temperature, pressure, and chemical compositions, are fixed: this is a so-called invariant equilibrium. An example is the invariant equilibrium of an eutectic melt with two solid phases at constant temperature and pressure. The experimental observation of two different eutectics in the same solidified sample is a clear violation of the Gibbs phase rule and suggests a nonequilibrium state of matter. On the other hand, the standard equilibrium diagram of the Co-Si system3 shows the existence of three compounds Co2Si, CoSi, and CoSi2 stable up to their congruent melting points. The melting temperature of CoSi2 (1599 K) is quite close to the temperature of the eutectic equilibrium on the cobalt-rich side (1583 K). The temperature of the silicon-rich eutectic equilibrium is much lower (1532 K). Some experimental points are, however, reported in the diagram at the same temperature in the cobalt-rich domain. This is also a violation of elementary thermodynamics, which shows that equilibrium was not attained in these experiments. In an older investigation of Lewkonja,4 X
Abstract published in AdVance ACS Abstracts, August 1, 1997.
S1089-5647(97)00633-0 CCC: $14.00
a peritectic equilibrium of CoSi2 is reported, i.e. decomposition into CoSi at melting. Altogether, these experimental findings seem puzzling enough to start an in-depth investigation of the phase relations around the CoSi2 compound. To get reliable conclusions, we did very careful experiments by high-temperature differential thermal analysis (DTA) and adiabatic calorimetry working as close as possible to thermal and chemical equilibria with previous in situ preparation and annealing of the samples. II. Experimental Section The multicell DTA apparatus5 permits work up to 1800 K under controlled oxidizing or reducing atmosphere inside the cell. The cell contains two or three crucibles, each with a volume of 0.45 cm3. In this work, alumina crucibles (Degussa AL23) are used. The apparatus was built according to the following requirements: (i) low thermal resistance between the heating source and the crucibles; (ii) high thermal resistance between sample crucibles and reference crucible; (iii) low to moderate rates of heating (0.1-5 K‚min-1). In DTA experiments, the temperature of the sample and reference increase following a linear program, and the difference of temperatures (DT), which is recorded, should be zero. In practice, a small temperature difference is observed within an error margin because of a dissimilarity between thermocouples and parasitic emf due to various origins. During an invariant phase transformation, the temperature (T) of the sample remains constant and the temperature difference (DT) increases linearly. The corresponding temperature of the transformation can be determined either as a plateau on the simultaneously recorded temperature curve of the sample (T ) f(t)) or from the departure of the differential thermogram (DT ) f(t)) from the base line (DT ) 0). After the end of the transformation, the temperature difference decreases following Newton’s law. It should be noted that in DTA experiments the intensity of the signal increases with increasing rates of heating.6 The sensitivity of our DTA apparatus is high enough to allow the use of small heating rates that permit an approach to thermal equilibrium. Thus, invariant phase equilibria with temperatures in close proximity can be distinguished. Furthermore, chemical and structural equilibria are also promoted. © 1997 American Chemical Society
Demixing Phenomenon in a Liquid Binary System
Figure 1. Differential thermogram of a (Co,Si) sample at 68 at. % silicon. Point A corresponds to incongruent melting of the CoSi2 compound, and point B corresponds to the liquidus line.
Temperature calibration is performed using the melting point of gold (1337.3 K).7 The samples are prepared from the pure elements (>99.9%) by melting in a levitation furnace under pure argon in the composition range 60.3 < xSi/at. % < 73.0 (uncertainty 0.1 at. %). Before each experiment, the materials are remelted and annealed in situ in the crucibles under pure argon to attain equilibrium conditions. The high-temperature adiabatic calorimeter is a new version of the apparatus described previously.8 Heat capacity ((2.5%) and enthalpies of phase transitions ((1%) can be measured up to 1800 K in reducing or inert atmosphere. The metallic parts of the main furnace and the adiabatic shields are made from molybdenum. Temperature and energy calibrations are performed using pure iron.9 Initial preparation of CoSi2 in boron nitride or thin alumina crucibles was unsuccessful, because the crucibles were destroyed during solidification, probably by infiltration of the melt into microfissures. Finally, about 0.5 mol of CoSi2 was prepared and annealed in a crucible of pure R-alumina sintered at high temperature. The concentration uncertainty was (0.3 at. %. A nickel cover was placed on the crucible to getter silicon vapor and prevent too rapid corrosion of PtRh10%-Pt thermocouples. All investigations were performed under inert argon atmosphere with heating rates (0.1-5 K‚min-1) comparable to those of the DTA experiments.
J. Phys. Chem. B, Vol. 101, No. 34, 1997 6693
Figure 2. Equilibrium phase diagram around the CoSi2 composition.
Figure 3. Differential thermogram of a (Co,Si) sample at 63 at. % silicon. Point A corresponds to melting of the cobalt-rich eutectic, point B corresponds to incongruent melting of the CoSi2 compound, and point C corresponds to the liquidus line.
III. Results A. Differential Thermal Analysis. Nonequilibrium behavior was observed in samples not previously annealed or for heating rates higher than 0.5 K‚min-1. Especially in the cobaltrich domain, the temperature corresponding to the silicon-rich eutectic was found, confirming the erroneous results reported in the phase diagram.3 This artifact is thus related to a too high rate of heating. With rates of heating of less than 0.5 K‚min-1, three domains of composition can be distinguished: (i) Above 66.6 at. % silicon, the thermograms are similar to Figure 1. Point A corresponds to invariant equilibrium at 1587 K (open circles in Figure 2). Point B is the end of biphased equilibrium (inverse triangles in Figure 2). The silicon-rich eutectic equilibrium is observed separately at 1523 K (not reported in Figure 1). (ii) In the range between 62.0 and 66.6 at. % silicon, the cobalt-rich eutectic equilibrium is observed at 1577 K (point A in Figure 3). Point C corresponds to the liquidus line (inverse triangles in Figure 2). The additional thermal effect observed between points A and C corresponds to a plateau on the direct temperature curve. It is characteristic of the invariant equilibrium at 1587 K.
Figure 4. Adiabatic thermogram T(t) for the CoSi2 compound: [A,B] (solid f liquid1 + liquid2) invariant transformation; [B,C] (liquid1 + liquid2) monovariant equilibrium. Point C corresponds to the liquidus line.
(iii) For less than 62.0 at. % silicon, the thermograms present two well-separated thermal effects, one corresponding to the cobalt-rich eutectic equilibrium at 1577 K and the other to the liquidus line at a distinctly higher temperature (Figure 2). B. Adiabatic Calorimetry. The enthalpy of fusion of the compound CoSi2 is measured by adiabatic calorimetry. Heat capacities in the solid and liquid states are also determined. Furthermore, these experiments confirm the DTA results, especially the existence of a constant temperature plateau (between points A and B in Figure 4) followed by a monovariant biphased equilibrium (temperature changes between B and C in Figure 4) before complete mixing into an unique homogeneous liquid. The total enthalpy (66.3 ( 0.7 kJ‚mol-1) is the sum of the enthalpy of the invariant transformation at 1587 K (42.8 ( 0.5 kJ‚mol-1) and the enthalpy of mixing in the liquid.
6694 J. Phys. Chem. B, Vol. 101, No. 34, 1997
Rogez and Knauth
Figure 5. Heat capacity of CoSi2 in the solid and liquid states.
It is in good agreement with previous results (69.8 kJ‚mol-1).10 The heat capacity of the compound is reported versus temperature in Figure 5, showing clearly the slight increase of heat capacity starting 300 K below the phase transformation. IV. Discussion The eutectic temperatures measured in this work are slightly lower than the literature data,3 i.e. 1583 K for the CoSi-liqCoSi2 equilibrium and 1532 K for the CoSi2-liq-Si equilibrium. The invariant equilibrium at 1587 K, only 10 K above the cobaltrich eutectic temperature, is reported here for the first time. This invariant equilibrium can be interpreted as an incongruent melting of CoSi2 into two immiscible liquids. The composition of the cobalt-rich liquid is near 63 at. % silicon and the siliconrich liquid near 69 at. % silicon (Figure 2). In this range, the maximum of the liquidus lies only 8 K above the incongruent melting temperature of the compound. The outlined unmixing phenomenon is probably the origin of the two eutectics found in CoSi2 samples prepared by rapid solidification from the melt. One may ask if the origin of the observed thermal effect cannot be a structural phase transition by analogy with iron and nickel disilicides, which present solid state transitions at respectively 1243 and 1254 K, the respective “melting points” being 1490 and 1266 K. However, this hypothesis can be excluded for two reasons: (i) The enthalpy associated with the invariant is too large to be due to a structural phase transition. It would be about twice the enthalpy of melting. (ii) The thermogram T ) f(t) (Figure 4) shows clearly that the second thermal effect does not present a constant temperature plateau, characteristic of an invariant transformation, like the first one. It is interesting to compare the phase diagram of different MSi2 compounds of the VIIIth transition metal group (M ) Fe, Co, Ni). “Iron disilicide” is a silicon-rich nonstoichiometric compound in its high-temperature R-phase (FeSi2+x); nickel disilicide melts incongruently giving a nickel-enriched liquid (xSi ) 0.59). The cobalt disilicide melts into two liquids. Its behavior is thus intermediate between the peritectic decomposition of the nickel compound into a nickel-excess liquid and the iron-deficiency in the nonstoichiometric solid iron compound.
Figure 6. Excess Gibbs free energy for the binary systems Ni-Si (b), Co-Si, (9), and Fe-Si (2).
Assessment of thermodynamic data for the three metalsilicon systems3,11,12 permits one to calculate the relative stability of the liquid phases (Figure 6). One notices that in the case of the Ni-Si system,11 where the published data are most extensive, the Gibbs free energy of mixing curve presents two points of inflection around the disilicide composition, due to a configurational entropy effect. A demixing phenomenon in the liquid state should thus also be observable in the Ni-Si binary system, although experimental evidence to date has not shown this. Clearly, the occurrence of two immiscible liquid phases in the Co-Si system should be confirmed by an experimental determination of the local environment in the liquid state, for instance by neutron diffraction. However, given the restricted temperature domain where demixing can be observed, this is certainly a big challenge. References and Notes (1) Yosim, S. J.; Darnell, A. J.; Gehman, W. G.; Mayer, S. W. J. Phys. Chem. 1959, 63, 231. (2) Lupis, C. H. P. Chemical Thermodynamics of Materials; Prentice Hall: Engelwood Cliffs: NJ, 1983; p 68. (3) Ishida, K.; Nishizawa, T.; Schlesinger, M. E. J. Phase Equilib. 1991, 12, 578. (4) Lewkonja, K. Z. Anorg. Chem., 1908, 59, 293. (5) Ganteaume, M.; Rogez, J. ReV. Sci. Instrum., in preparation. See also: Bennour, F.; Rogez, J.; Mathieu, J. C. J. Am. Ceram. Soc. 1996, 79 (10), 2752-2754. (6) Hemminger, G.; Ho¨hne, G. Calorimetry, Fundamentals and Practice; VCH-D: Weinheim, 1984; p 203. (7) Thomas, H. P. Metrologia 1990, 27, 3. (8) Rogez, J.; LeCoze, J. ReV. Phys. Appl. 1980, 15, 341. (9) Rogez, J. Doctoral Thesis, University of Grenoble, 1979. (10) Pankratz, L. B.; Stuve, J. M.; Gokcen, N. A. Thermodynamic Data for Mineral Technology; Bulletin 677; Bureau of Mines: Washington, DC, 1984; p 315. (11) Nash, P.; Nash, A. Bull. Alloy Phase Diagrams 1987, 8, 6. (12) Hultgren, R.; Desai, P. D.; Hawkins, D. T.; Gleiser, M.; Kelley, K. K.; Wagman, D.D. Selected Values of the Thermodynamic Properties of Binary Mixtures; American Society for Metals: Metal Parks, OH, 1973; p 878.
