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Standard enthalpies of formation of some 3d transition metal gallides by high temperature direct synthesis calorimetry
Journal of Alloys and Compounds 290 (1999) 150–156
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Standard enthalpies of formation of some 3d transition metal gallides by high temperature direct synthesis calorimetry S.V. Meschel, O.J. Kleppa* James Franck Institute, The University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA Received 30 March 1999
Abstract The standard enthalpies of formation of some 3d transition metal gallides have been measured by high temperature direct synthesis calorimetry at 137362 K. The following results in kJ / mol of atoms are reported: Sc 5 Ga 3 (259.462.0); Ti 2 Ga (239.061.4); V2 Ga 5 (216.661.7); Cr 3 Ga (211.861.8); FeGa 3 (224.862.1); CoGa 3 (229.462.2) and NiGa (244.961.9). The results are compared with some earlier values obtained by solution calorimetry or derived from EMF measurements. They are also compared with predicted values from the semi-empirical model of de Boer et al. and with available enthalpies of formation for 3d transition metal aluminides and germanides. 1999 Elsevier Science S.A. All rights reserved.
1. Introduction In binary alloys formed by a transition metal and a polyvalent non-transition metal such as Ga, relatively high enthalpies of formation may be anticipated. Thermochemical studies of such alloys are directed toward a better understanding of their chemical bonding. In spite of the low melting point of Ga, the binary transition metal– gallium systems have several intermetallic compounds which are stable at high temperature [1]. Among the 3d transition metal gallides the system V–Ga is of particular interest; the phase V3 Ga belongs to the A15 type superconductors which have high superconducting transition temperatures and may be of future technological use [2]. The alloys of Ti with Ga and In are also of technical interest, since they may be used for soldering oxide materials [3]. During recent years we have conducted in this laboratory systematic studies of the thermochemistry of transition metal and lanthanide elements with VB and IIIB elements in the periodic table by high temperature calorimetric methods [4]. These investigations began with the work of Topor and Kleppa on some borides and silicides of group IIIA metals and was followed by the work of Jung and Kleppa on the corresponding germanides [4]. More recent*Corresponding author. Tel.: 11-773-702-7284; fax: 11-773-7025863. E-mail address: [email protected] (O.J. Kleppa)
ly the present authors studied the thermochemistry of 3d, 4d and 5d silicides, stannides and aluminides by direct synthesis calorimetry [5–11]. As Ga is a neighbor of Al in the periodic table, one might expect the 3d transition metal gallides to exhibit thermochemical behavior somewhat similar to 3d transition metal aluminides. We searched the literature for thermochemical information on 3d transition metal gallides and found that relatively few systems have been studied. This is particularly true for alloys in the solid state. However, the gallides of Fe, Co and Ni were studied by Predel and co-workers [12,13], by Martosudirjo and Pratt [14], by Jacobi et al. [16] and by Henig et al. [17], all by calorimetric techniques. The heat conductivity of V3 Ga was studied in some detail [2], since it exhibits superconducting properties. Since the enthalpies of formation provide valuable information on the thermochemical stability and bonding character of the compounds, a systematic study of transition metal gallides is of some interest. We have already demonstrated, in a previous study of PdGa and Pd 2 Ga, that our calorimetric technique in working on gallides is fully compatible with the results of measurements obtained by other methods [18]. In the present communication we report new thermochemical data for some phases in the binary systems Sc–Ga, Ti–Ga, V–Ga, Cr–Ga, Fe–Ga, Co–Ga and Ni–Ga all obtained by high temperature direct synthesis calorimetry. Information regarding the binary phase diagrams and the
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S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
structures of most of the considered phases is generally available in the literature [1,19–21]. However, we found that the melting points of three of the compounds which we studied were not known [1]. We also found that the X-ray diffraction patterns of several of the alloys were not listed in the ASTM powder diffraction file. The published literature offers values for the enthalpies of formation of some of the alloys which we studied, obtained by solution calorimetry [12–14,16,17], and by the EMF method [15,21–23]. There are also studies of the Fe–Ga, Co–Ga and Ni–Ga systems in the liquid state by Haddad et al. [24,25]. We will only compare our results with the data for the solid state systems.We will also compare our values with predictions based on the semiempirical model of deBoer et al. [26]. Our thermochemical measurements allow us to obtain a systematic picture of the enthalpies of formation of the 3d gallides in their dependence on the atomic number of the transition metal. We will also compare the thermochemical behavior of the 3d gallides with that of the corresponding aluminides and germanides. We are now extending this study to the gallides of the 4d and 5d transition metals and to the lanthanide gallides.
2. Experimental and materials The experiments were carried out at 137362 K in a single unit differential microcalorimeter which has been described in an earlier communication from this laboratory [27]. All the experiments were performed under a protective atmosphere of argon gas, which was purified by passing over titanium chips at 9008C. A boron nitride (BN) crucible was used to contain the samples. The materials were purchased from Johnson Matthey / Aesar (Ward Hill, MA, USA); Sc and Ga were in ingot form. The purities and particle sizes of the samples are summarized in Table 1. The Fe, Co and Ni powders were freshly reduced under a flow of hydrogen gas at 6008C prior to the preparation of the samples; this insures that the samples are as free of surface oxidation as possible. Ga melts at 298C. We noticed in preliminary experiments that it is possible to file Ga to a powdered sample as long as its temperature remains under Table 1 Purities and particle sizes of the elements used in this study Elements
Purity
Particle size
Sc Ti V Cr Fe Co Ni Ga
99.9 99.9 99.5 99.95 99.9 99.8 99.9 99.999
Filed from ingot 2200 mesh 2325 mesh 2325 mesh 2325 mesh 1.6 mm 2.2–30 mm Filed from ingot
151
room temperature. We were able to accomplish this by keeping the ingots in the refrigerator and filing them only for short periods of time. We consider this technique to be a significant improvement over the crushing of Ga into small chips as we did in our previous communication [18]. The two components were carefully mixed in the appropriate molar ratio, pressed into 4-mm pellets and dropped into the calorimeter from room temperature. In the preparation of the gallide pellets the pressure applied by the hydraulic press is critical. Since this process generates some heat, the pressure must be kept to a minimum; if not, the Ga may be squeezed out of the sample. For each alloy we empirically determined the most suitable pressure. Fortunately, the process may be monitored very effectively by checking the weight of the pressed pellets and compare it with the weight of the unpressed mixture.We described the basic technique in a previous communication [18]. However, in the present paper some improvements were made. For example, we are now able to produce finely powdered Ga, which makes the samples more homogeneous. In a subsequent set of experiments the reaction products were dropped into the calorimeter to measure their heat content. Between the two sets of experiments the samples were kept in a vacuum desiccator to prevent reaction with oxygen or moisture. Calibration of the calorimeter was achieved by dropping weighed segments of high purity, 2 mm O.D., Cu wire from room temperature into the calorimeter at 137362 K. The enthalpy of pure copper at 1373 K, 43.184 kJ / mol of atoms, was obtained from Hultgren et al. [28]. The calibrations were reproducible to within 61.4%. The reacted samples were examined by X-ray powder diffraction to assess their structures and to ascertain the absence of unreacted metals. The results of these analyses were conclusive; hence we did not feel the need to check the samples further by X-ray microprobe analysis. All the alloys studied in this communication were fully reacted; we saw no evidence of unreacted metals within the limits of detectability of the X-ray diffractometer. For alloys where the X-ray diffraction patterns were not listed in the ASTM powder diffraction file we generated the patterns using the available unit cell parameters and atomic coordinates in Pearson’s compilation of crystallographic data [29]. The Fe, Co and Ni alloys were examined on a high sensitivity, computerized X-ray diffractometer. The phase diagram of the Sc–Ga system shows two congruently melting phases, namely ScGa 2 (m.p. 11408C) and Sc 5 Ga 3 (m.p. 14308C), and also some peritectically melting phases such as ScGa 3 (m.p. 10308C) [1]. We attempted to prepare these three compounds in the calorimeter. The X-ray diffraction pattern of Sc 5 Ga 3 matched well the pattern in the ASTM powder diffraction file; however, we noticed two small unidentified peaks (less than 5%). We checked for the presence of ScGa 3 , ScGa, Sc 2 Ga 3 and found none. The phase diagram indi-
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cates the existence of Sc 5 Ga 4 . The pattern of this compound is not listed in the ASTM powder diffraction file. The peaks could be attributed to a minor second phase of Sc 5 Ga 4 ; however, since this structure is not listed in Pearson’s compilation of crystallographic data, we were not able to generate its X-ray diffraction pattern for positive identification [29]. The pattern of our sample of ScGa 2 showed a mixed phase of the two crystalline modifications of ScGa 2 and a significant amount of ScGa 3 (more than 30%). The preparation of ScGa 3 was not successful. This compound wet the BN crucible and we were not able to remove it for structure identification. The Ti–Ga phase diagram shows one congruent phase, Ti 2 Ga [1,19]. The exact melting point is not provided. We prepared Ti 2 Ga in the calorimeter. The X-ray diffraction pattern of Ti 2 Ga is not listed in the ASTM powder diffraction file; we therefore generated it from available unit cell parameters and atomic coordinates [29]. The X-ray diffraction pattern matched the generated pattern closely. However, we noticed a minor second phase, less than 2% Ti 3 Ga. The V–Ga phase diagram shows several peritectically melting phases, namely V3 Ga (m.p. 13008C), V6 Ga 5 (m.p. 9958C) and V2 Ga 5 (m.p. 10508C) [1,20]. We attempted to prepare all three compounds in the calorimeter. The X-ray diffraction pattern of V3 Ga showed a mixed phase, predominantly V6 Ga 5 . The pattern of V6 Ga 5 also showed a mixture of three compounds, V2 Ga 5 , V6 Ga 5 and V6 Ga 7 . The pattern of V2 Ga 5 was the only one which showed a nearly single phase. Its pattern also matched well the pattern in the ASTM powder diffraction file. We noticed only one unidentified peak, a reflection of about 3–4% of second phase. The Cr–Ga phase diagram shows several peritectically melting phases, notably Cr 3 Ga and Cr 5 Ga 6 (m.p. 7608C) [1]. The exact melting point of Cr 3 Ga was not provided. We prepared this compound in the calorimeter. The X-ray diffraction pattern of this phase matched well the pattern of the cubic l.t. modification in the ASTM powder diffraction file. We found no evidence for the presence of the monoclinic modification or of any other Cr–Ga phases. We were not able to prepare fully reacted compounds in the Mn–Ga system. The Fe–Ga phase diagram shows numerous peritectically melting phases [1,21]. We prepared FeGa 3 (m.p. 8248C) in the calorimeter. The X-ray diffraction pattern showed an excellent match with the pattern in the ASTM powder diffraction file. We found no other detectable phases. The Co–Ga phase diagram shows the existence of one peritectically melting compound with a sharply defined composition, CoGa 3 (m.p. 8558C), and also the alloy CoGa which exists over a wide range of homogeneity [1]. We attempted to prepare both compounds in the calorimeter. The X-ray diffraction pattern of CoGa was not listed in the ASTM powder diffraction file; we therefore generated it from available unit cell parameters and atomic coordinates
[29]. Our diffraction pattern of CoGa showed a mixed phase of CoGa and CoGa 3 . However, the pattern of CoGa 3 matched well the pattern in the ASTM powder diffraction file. We found no other phases present. The Ni–Ga phase diagram shows one congruently melting phase, namely NiGa (m.p. 12208C) and numerous peritectically melting compounds [1]. We prepared NiGa in the calorimeter. The X-ray diffraction pattern is not listed in the ASTM powder diffraction file; we therefore generated it from available unit cell parameters and atomic coordinates [29]. Our pattern matched well the generated pattern. We noticed no other detectable phases.
3. Discussion The standard enthalpies of formation of the 3d transition metal gallides determined in this study were obtained as the difference between the results of two sets of measurements. In the first set the following reaction takes place in the calorimeter: Tr(s, 298 K) 1 mGa(s, 298 K) 5 TrGa m (s, or 1, 1373 K) (1) Here m represents the molar ratio of Ga:Tr, while Tr represents Sc, Ti, V, Cr, Fe, Co, Ni, s denotes solid and 1 denotes liquid. The reacted pellets were reused in a subsequent set of measurements to determine their heat contents: TrGa m (s, 298 K) 5 TrGa m (s, or 1, 1373 K)
(2)
The standard enthalpy of formation is given by: DH 0f 5 DH(1) 2 DH(2)
(3)
where DH(1) and DH(2) are the enthalpy changes per mole of atoms in the compound associated with reactions in Eqs. (1 and 2). The experimental results are summarized in Table 2. The second column shows the melting points of the phases (where known), while the third column indicates the structure. The heat effects associated with the reactions in Eqs. (1 and 2) are given in kilojoules per mol of atoms as the averages of 6–7 consecutive measurements with the appropriate standard deviations.The last column shows the standard enthalpy of formation of the considered phases. The standard deviation in the last column also reflects the small contributions from the uncertainties in the calibrations. In Table 3 we compare our results with previous experimental values obtained by solution calorimetry [12– 14,16,17] or derived from EMF measurements [15,22,23]. We list the reference states for the considered metal and for Ga in a separate column. Our results agree best with the calorimetric measurements where the temperature and the reference states are similar. For example, the enthalpy
S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
153
Table 2 Summary of the measured standard enthalpies of formation of some 3d transition metal gallides a Compound
m.p. (8C)
Structure
DH(1)
DH(2)
DH 0f
Sc 5 Ga 3 Ti 2 Ga V2 Ga 5 Cr 3 Ga FeGa 3 CoGa 3 NiGa
1430 (c) |1440 1015–1085 (p) |1620 824 855 1220
Mn 5 Si 3 Ni 2 In Mn 2 Hg 5 Cr 3 Si CoGa 3 CoGa 3 CsCl
229.661.5(6) 28.961.1(7) 23.961.3(7) 25.460.7(6) 34.761.8(7) 26.261.4(9) 212.461.7(6)
29.861.3(6) 30.160.8(7) 40.561.1(7) 37.261.7(6) 59.561.1(6) 55.661.7(5) 32.560.8(6)
259.462.0 239.061.4 216.661.7 211.861.8 224.862.1 229.462.2 244.961.9
a
Data in kJ / mol of atoms.
of formation of CoGa 3 by solution calorimetry is given as 233.0 kJ / mol of atoms for solid Co and liquid Ga by Henig et al. [17]. The heat of fusion of Ga at the melting point is 5.6 kJ / mol of atoms [28]. If we make the usual assumption that the heat of fusion of the pure metal does not vary significantly with temperature, we can adjust the heat of formation of CoGa 3 reported by Henig et al. for the contribution of the heat of fusion. That will make the value 4.2 kJ / mol of atoms less exothermic, i.e. 228.8 kJ / mol of atoms for Ga(s) at 1100 K. The agreement with our value at 298 K is quite good. Our value for the enthalpy of formation of FeGa 3 is somewhat more exothermic than the value by Predel et al. [12]. Our results for both V2 Ga 5 and Cr 3 Ga are significantly more exothermic than the values derived from EMF reported by Goncharuk and Lukashenko [22] and by Eremenko et al. [23], respectively.
Our value for the enthalpy of formation of NiGa agrees well with three of the reported values [13,15,16]. The last column in Table 3 shows the enthalpies of formation predicted on the basis of the semi-empirical model of deBoer et al. [26]. Our experimental values agree quite well with their predictions from Sc to Cr; however, the experimental values are considerably more exothermic than the predicted ones from Fe to Ni. In Fig. 1 we present a plot of the standard enthalpies of formation of the 3d gallides against the atomic number of the transition metal elements from Sc to Ni. We have no value for the Mn–Ga system. We recognize that the molar compositions of the 3d gallides are all quite different from one another. The molar compositions are indicated in Fig. 1. Therefore it is difficult to assess in full detail the systematic changes with respect to the atomic numbers;
Table 3 Comparison of the measured standard enthalpies of formation with some experimental data in the literature and with predicted values from the semi-empirical model of deBoer et al. [26] a Compound
DH 0f (experimental)
Ref. state;
Method
DH 0f
This work
Literature
Tr; Ga
[Ref.]
(predicted)
Sc 5 Ga 3 Ti 2 Ga V2 Ga 5
259.462.0 239.061.4 216.661.7
2 2 214.961.6
2 2 s,l
259 244 218
Cr 3 Ga
211.861.8
24.661.4
s,l
FeGa 3
224.862.1
22163
s,s
CoGa 3
229.462.2
233.0
s,l
24563
s,s
238.260.7
s,s
24662
s,l
247.9
s,l
248.162.1
s,s
2 2 EMF (700–900 K) [23] EMF (700–900 K) [22] Soln. Calor. (Room temp.; Br–HBr) [12] Soln. Calor. (Ga, 1100 K) [17] Soln. Calor. (Room temp.; Br–HBr) [12] Soln. Calor. (Sn, 773–832 K) [14] Soln.Calor. (Sn,1023 K) [16] EMF (873–1100 K) [15] Soln. Calor. (Sn, 1093 K) [13]
NiGa
a
244.961.9
Data in kJ / mol of atoms.
212 210
218
237
154
S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
Fig. 1. Standard enthalpies of formation of some 3d transition metal gallides; data in kJ / mol of atoms.
however, we can show some indicative trends. Fig. 1 shows that the enthalpies of formation decrease steeply from Sc to Cr, show a minimum at Cr and subsequently rise from Fe to Ni. The shape of the curve is roughly parabolic. We previously noticed similar correlations for the 3d transition metal aluminides, silicides and stannides [5–11]. In Fig. 2 we compare our values for the 3d transition
metal gallides with the heats of formation of 3d aluminides as determined by Meschel and Kleppa and by Jung et al. [6–8,30]. This figure shows that for the Sc–Ga system the heat of formation of the gallide is significantly more exothermic than for the Sc–Al system. However, for Ti and Fe gallides and aluminides the values are very similar. From V to Ni the heats of formation of the gallides are slightly less exothermic than the aluminides. The shapes of
Fig. 2. Comparison of the standard enthalpies of formation of some 3d transition metal gallides with similar transition metal aluminides. The molar composition of the gallides is given in Fig. 1.
S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
155
Fig. 3. Comparison of the standard enthalpies of formation of some 3d transition metal gallides with similar transition metal germanides. The molar composition of the gallides is given in Fig. 1.
the curves are very similar for both families of systems. Both exhibit a steep decrease from Sc to Cr, a minimum at Cr and a rise from Cr to Ni. In Fig. 3 we compare our values for the 3d transition metal gallides with the heats of formation of the 3d germanides as determined by Jung and Kleppa [31,32]. Since Ge and Ga occupy the same positions within the columns VB and IIIB in the periodic table, the comparison is of some interest. This figure shows that from Sc to Mn the germanides have the more exothermic values, while from Fe to Ni the gallides are more exothermic.The shapes of the curves are again very similar, both roughly parabolic. In our study of the 3d, 4d and 5d transition metal aluminides we found no correlation between the observed enthalpies of formation and the difference in the electronegativities of the two component elements in the alloys [6–8]. The same situation holds for the 3d gallides. For example the electronegativity difference between metal and gallium is the same for Fe, Co and Ni, while the enthalpies of formation differ by nearly a factor of two. This suggests that the chemical bonding in the Tr–Ga compounds may be covalent rather than ionic as was noted for aluminides by Colinet [33] and by Pasturel et al. [34]. We also attempted to compare the melting points with the enthalpies of formation and found no correlation. For example the melting point of Cr 3 Ga is the highest among the 3d gallides while its enthalpy of formation is numerically the lowest. However, it is difficult to assess this effect completely since several of the melting points are not known exactly. We observed the same lack of correla-
tion between the heats of formation and the melting points for the 3d aluminides [6–8]. Gellatt et al. provided a detailed theoretical analysis of the important factors which contribute to the enthalpies of formation of transition metal–polyvalent metal systems [35]. Further work along similar lines has been carried out by Pasturel et al. [34]. Pasturel’s theory on 3d transition metal aluminides predicts a roughly parabolic dependence of the enthalpies of formation on the number of d electrons of the metal, with the weakest bonding in the middle of the series. This is also consistent with our results for the 3d gallides.
Acknowledgements This investigation has been supported by the Department of Energy under Grant DE-FGO2-88ER4563, and has also benefited from the MRSEC facilites at the University of Chicago. We are indebted to Dr. Joseph Pluth for his help with generating the X-ray diffraction patterns from the reported unit cell parameters and the atomic coordinates
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Standard enthalpies of formation of some 3d transition metal gallides by high temperature direct synthesis calorimetry S.V. Meschel, O.J. Kleppa* James Franck Institute, The University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA Received 30 March 1999
Abstract The standard enthalpies of formation of some 3d transition metal gallides have been measured by high temperature direct synthesis calorimetry at 137362 K. The following results in kJ / mol of atoms are reported: Sc 5 Ga 3 (259.462.0); Ti 2 Ga (239.061.4); V2 Ga 5 (216.661.7); Cr 3 Ga (211.861.8); FeGa 3 (224.862.1); CoGa 3 (229.462.2) and NiGa (244.961.9). The results are compared with some earlier values obtained by solution calorimetry or derived from EMF measurements. They are also compared with predicted values from the semi-empirical model of de Boer et al. and with available enthalpies of formation for 3d transition metal aluminides and germanides. 1999 Elsevier Science S.A. All rights reserved.
1. Introduction In binary alloys formed by a transition metal and a polyvalent non-transition metal such as Ga, relatively high enthalpies of formation may be anticipated. Thermochemical studies of such alloys are directed toward a better understanding of their chemical bonding. In spite of the low melting point of Ga, the binary transition metal– gallium systems have several intermetallic compounds which are stable at high temperature [1]. Among the 3d transition metal gallides the system V–Ga is of particular interest; the phase V3 Ga belongs to the A15 type superconductors which have high superconducting transition temperatures and may be of future technological use [2]. The alloys of Ti with Ga and In are also of technical interest, since they may be used for soldering oxide materials [3]. During recent years we have conducted in this laboratory systematic studies of the thermochemistry of transition metal and lanthanide elements with VB and IIIB elements in the periodic table by high temperature calorimetric methods [4]. These investigations began with the work of Topor and Kleppa on some borides and silicides of group IIIA metals and was followed by the work of Jung and Kleppa on the corresponding germanides [4]. More recent*Corresponding author. Tel.: 11-773-702-7284; fax: 11-773-7025863. E-mail address: [email protected] (O.J. Kleppa)
ly the present authors studied the thermochemistry of 3d, 4d and 5d silicides, stannides and aluminides by direct synthesis calorimetry [5–11]. As Ga is a neighbor of Al in the periodic table, one might expect the 3d transition metal gallides to exhibit thermochemical behavior somewhat similar to 3d transition metal aluminides. We searched the literature for thermochemical information on 3d transition metal gallides and found that relatively few systems have been studied. This is particularly true for alloys in the solid state. However, the gallides of Fe, Co and Ni were studied by Predel and co-workers [12,13], by Martosudirjo and Pratt [14], by Jacobi et al. [16] and by Henig et al. [17], all by calorimetric techniques. The heat conductivity of V3 Ga was studied in some detail [2], since it exhibits superconducting properties. Since the enthalpies of formation provide valuable information on the thermochemical stability and bonding character of the compounds, a systematic study of transition metal gallides is of some interest. We have already demonstrated, in a previous study of PdGa and Pd 2 Ga, that our calorimetric technique in working on gallides is fully compatible with the results of measurements obtained by other methods [18]. In the present communication we report new thermochemical data for some phases in the binary systems Sc–Ga, Ti–Ga, V–Ga, Cr–Ga, Fe–Ga, Co–Ga and Ni–Ga all obtained by high temperature direct synthesis calorimetry. Information regarding the binary phase diagrams and the
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S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
structures of most of the considered phases is generally available in the literature [1,19–21]. However, we found that the melting points of three of the compounds which we studied were not known [1]. We also found that the X-ray diffraction patterns of several of the alloys were not listed in the ASTM powder diffraction file. The published literature offers values for the enthalpies of formation of some of the alloys which we studied, obtained by solution calorimetry [12–14,16,17], and by the EMF method [15,21–23]. There are also studies of the Fe–Ga, Co–Ga and Ni–Ga systems in the liquid state by Haddad et al. [24,25]. We will only compare our results with the data for the solid state systems.We will also compare our values with predictions based on the semiempirical model of deBoer et al. [26]. Our thermochemical measurements allow us to obtain a systematic picture of the enthalpies of formation of the 3d gallides in their dependence on the atomic number of the transition metal. We will also compare the thermochemical behavior of the 3d gallides with that of the corresponding aluminides and germanides. We are now extending this study to the gallides of the 4d and 5d transition metals and to the lanthanide gallides.
2. Experimental and materials The experiments were carried out at 137362 K in a single unit differential microcalorimeter which has been described in an earlier communication from this laboratory [27]. All the experiments were performed under a protective atmosphere of argon gas, which was purified by passing over titanium chips at 9008C. A boron nitride (BN) crucible was used to contain the samples. The materials were purchased from Johnson Matthey / Aesar (Ward Hill, MA, USA); Sc and Ga were in ingot form. The purities and particle sizes of the samples are summarized in Table 1. The Fe, Co and Ni powders were freshly reduced under a flow of hydrogen gas at 6008C prior to the preparation of the samples; this insures that the samples are as free of surface oxidation as possible. Ga melts at 298C. We noticed in preliminary experiments that it is possible to file Ga to a powdered sample as long as its temperature remains under Table 1 Purities and particle sizes of the elements used in this study Elements
Purity
Particle size
Sc Ti V Cr Fe Co Ni Ga
99.9 99.9 99.5 99.95 99.9 99.8 99.9 99.999
Filed from ingot 2200 mesh 2325 mesh 2325 mesh 2325 mesh 1.6 mm 2.2–30 mm Filed from ingot
151
room temperature. We were able to accomplish this by keeping the ingots in the refrigerator and filing them only for short periods of time. We consider this technique to be a significant improvement over the crushing of Ga into small chips as we did in our previous communication [18]. The two components were carefully mixed in the appropriate molar ratio, pressed into 4-mm pellets and dropped into the calorimeter from room temperature. In the preparation of the gallide pellets the pressure applied by the hydraulic press is critical. Since this process generates some heat, the pressure must be kept to a minimum; if not, the Ga may be squeezed out of the sample. For each alloy we empirically determined the most suitable pressure. Fortunately, the process may be monitored very effectively by checking the weight of the pressed pellets and compare it with the weight of the unpressed mixture.We described the basic technique in a previous communication [18]. However, in the present paper some improvements were made. For example, we are now able to produce finely powdered Ga, which makes the samples more homogeneous. In a subsequent set of experiments the reaction products were dropped into the calorimeter to measure their heat content. Between the two sets of experiments the samples were kept in a vacuum desiccator to prevent reaction with oxygen or moisture. Calibration of the calorimeter was achieved by dropping weighed segments of high purity, 2 mm O.D., Cu wire from room temperature into the calorimeter at 137362 K. The enthalpy of pure copper at 1373 K, 43.184 kJ / mol of atoms, was obtained from Hultgren et al. [28]. The calibrations were reproducible to within 61.4%. The reacted samples were examined by X-ray powder diffraction to assess their structures and to ascertain the absence of unreacted metals. The results of these analyses were conclusive; hence we did not feel the need to check the samples further by X-ray microprobe analysis. All the alloys studied in this communication were fully reacted; we saw no evidence of unreacted metals within the limits of detectability of the X-ray diffractometer. For alloys where the X-ray diffraction patterns were not listed in the ASTM powder diffraction file we generated the patterns using the available unit cell parameters and atomic coordinates in Pearson’s compilation of crystallographic data [29]. The Fe, Co and Ni alloys were examined on a high sensitivity, computerized X-ray diffractometer. The phase diagram of the Sc–Ga system shows two congruently melting phases, namely ScGa 2 (m.p. 11408C) and Sc 5 Ga 3 (m.p. 14308C), and also some peritectically melting phases such as ScGa 3 (m.p. 10308C) [1]. We attempted to prepare these three compounds in the calorimeter. The X-ray diffraction pattern of Sc 5 Ga 3 matched well the pattern in the ASTM powder diffraction file; however, we noticed two small unidentified peaks (less than 5%). We checked for the presence of ScGa 3 , ScGa, Sc 2 Ga 3 and found none. The phase diagram indi-
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cates the existence of Sc 5 Ga 4 . The pattern of this compound is not listed in the ASTM powder diffraction file. The peaks could be attributed to a minor second phase of Sc 5 Ga 4 ; however, since this structure is not listed in Pearson’s compilation of crystallographic data, we were not able to generate its X-ray diffraction pattern for positive identification [29]. The pattern of our sample of ScGa 2 showed a mixed phase of the two crystalline modifications of ScGa 2 and a significant amount of ScGa 3 (more than 30%). The preparation of ScGa 3 was not successful. This compound wet the BN crucible and we were not able to remove it for structure identification. The Ti–Ga phase diagram shows one congruent phase, Ti 2 Ga [1,19]. The exact melting point is not provided. We prepared Ti 2 Ga in the calorimeter. The X-ray diffraction pattern of Ti 2 Ga is not listed in the ASTM powder diffraction file; we therefore generated it from available unit cell parameters and atomic coordinates [29]. The X-ray diffraction pattern matched the generated pattern closely. However, we noticed a minor second phase, less than 2% Ti 3 Ga. The V–Ga phase diagram shows several peritectically melting phases, namely V3 Ga (m.p. 13008C), V6 Ga 5 (m.p. 9958C) and V2 Ga 5 (m.p. 10508C) [1,20]. We attempted to prepare all three compounds in the calorimeter. The X-ray diffraction pattern of V3 Ga showed a mixed phase, predominantly V6 Ga 5 . The pattern of V6 Ga 5 also showed a mixture of three compounds, V2 Ga 5 , V6 Ga 5 and V6 Ga 7 . The pattern of V2 Ga 5 was the only one which showed a nearly single phase. Its pattern also matched well the pattern in the ASTM powder diffraction file. We noticed only one unidentified peak, a reflection of about 3–4% of second phase. The Cr–Ga phase diagram shows several peritectically melting phases, notably Cr 3 Ga and Cr 5 Ga 6 (m.p. 7608C) [1]. The exact melting point of Cr 3 Ga was not provided. We prepared this compound in the calorimeter. The X-ray diffraction pattern of this phase matched well the pattern of the cubic l.t. modification in the ASTM powder diffraction file. We found no evidence for the presence of the monoclinic modification or of any other Cr–Ga phases. We were not able to prepare fully reacted compounds in the Mn–Ga system. The Fe–Ga phase diagram shows numerous peritectically melting phases [1,21]. We prepared FeGa 3 (m.p. 8248C) in the calorimeter. The X-ray diffraction pattern showed an excellent match with the pattern in the ASTM powder diffraction file. We found no other detectable phases. The Co–Ga phase diagram shows the existence of one peritectically melting compound with a sharply defined composition, CoGa 3 (m.p. 8558C), and also the alloy CoGa which exists over a wide range of homogeneity [1]. We attempted to prepare both compounds in the calorimeter. The X-ray diffraction pattern of CoGa was not listed in the ASTM powder diffraction file; we therefore generated it from available unit cell parameters and atomic coordinates
[29]. Our diffraction pattern of CoGa showed a mixed phase of CoGa and CoGa 3 . However, the pattern of CoGa 3 matched well the pattern in the ASTM powder diffraction file. We found no other phases present. The Ni–Ga phase diagram shows one congruently melting phase, namely NiGa (m.p. 12208C) and numerous peritectically melting compounds [1]. We prepared NiGa in the calorimeter. The X-ray diffraction pattern is not listed in the ASTM powder diffraction file; we therefore generated it from available unit cell parameters and atomic coordinates [29]. Our pattern matched well the generated pattern. We noticed no other detectable phases.
3. Discussion The standard enthalpies of formation of the 3d transition metal gallides determined in this study were obtained as the difference between the results of two sets of measurements. In the first set the following reaction takes place in the calorimeter: Tr(s, 298 K) 1 mGa(s, 298 K) 5 TrGa m (s, or 1, 1373 K) (1) Here m represents the molar ratio of Ga:Tr, while Tr represents Sc, Ti, V, Cr, Fe, Co, Ni, s denotes solid and 1 denotes liquid. The reacted pellets were reused in a subsequent set of measurements to determine their heat contents: TrGa m (s, 298 K) 5 TrGa m (s, or 1, 1373 K)
(2)
The standard enthalpy of formation is given by: DH 0f 5 DH(1) 2 DH(2)
(3)
where DH(1) and DH(2) are the enthalpy changes per mole of atoms in the compound associated with reactions in Eqs. (1 and 2). The experimental results are summarized in Table 2. The second column shows the melting points of the phases (where known), while the third column indicates the structure. The heat effects associated with the reactions in Eqs. (1 and 2) are given in kilojoules per mol of atoms as the averages of 6–7 consecutive measurements with the appropriate standard deviations.The last column shows the standard enthalpy of formation of the considered phases. The standard deviation in the last column also reflects the small contributions from the uncertainties in the calibrations. In Table 3 we compare our results with previous experimental values obtained by solution calorimetry [12– 14,16,17] or derived from EMF measurements [15,22,23]. We list the reference states for the considered metal and for Ga in a separate column. Our results agree best with the calorimetric measurements where the temperature and the reference states are similar. For example, the enthalpy
S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
153
Table 2 Summary of the measured standard enthalpies of formation of some 3d transition metal gallides a Compound
m.p. (8C)
Structure
DH(1)
DH(2)
DH 0f
Sc 5 Ga 3 Ti 2 Ga V2 Ga 5 Cr 3 Ga FeGa 3 CoGa 3 NiGa
1430 (c) |1440 1015–1085 (p) |1620 824 855 1220
Mn 5 Si 3 Ni 2 In Mn 2 Hg 5 Cr 3 Si CoGa 3 CoGa 3 CsCl
229.661.5(6) 28.961.1(7) 23.961.3(7) 25.460.7(6) 34.761.8(7) 26.261.4(9) 212.461.7(6)
29.861.3(6) 30.160.8(7) 40.561.1(7) 37.261.7(6) 59.561.1(6) 55.661.7(5) 32.560.8(6)
259.462.0 239.061.4 216.661.7 211.861.8 224.862.1 229.462.2 244.961.9
a
Data in kJ / mol of atoms.
of formation of CoGa 3 by solution calorimetry is given as 233.0 kJ / mol of atoms for solid Co and liquid Ga by Henig et al. [17]. The heat of fusion of Ga at the melting point is 5.6 kJ / mol of atoms [28]. If we make the usual assumption that the heat of fusion of the pure metal does not vary significantly with temperature, we can adjust the heat of formation of CoGa 3 reported by Henig et al. for the contribution of the heat of fusion. That will make the value 4.2 kJ / mol of atoms less exothermic, i.e. 228.8 kJ / mol of atoms for Ga(s) at 1100 K. The agreement with our value at 298 K is quite good. Our value for the enthalpy of formation of FeGa 3 is somewhat more exothermic than the value by Predel et al. [12]. Our results for both V2 Ga 5 and Cr 3 Ga are significantly more exothermic than the values derived from EMF reported by Goncharuk and Lukashenko [22] and by Eremenko et al. [23], respectively.
Our value for the enthalpy of formation of NiGa agrees well with three of the reported values [13,15,16]. The last column in Table 3 shows the enthalpies of formation predicted on the basis of the semi-empirical model of deBoer et al. [26]. Our experimental values agree quite well with their predictions from Sc to Cr; however, the experimental values are considerably more exothermic than the predicted ones from Fe to Ni. In Fig. 1 we present a plot of the standard enthalpies of formation of the 3d gallides against the atomic number of the transition metal elements from Sc to Ni. We have no value for the Mn–Ga system. We recognize that the molar compositions of the 3d gallides are all quite different from one another. The molar compositions are indicated in Fig. 1. Therefore it is difficult to assess in full detail the systematic changes with respect to the atomic numbers;
Table 3 Comparison of the measured standard enthalpies of formation with some experimental data in the literature and with predicted values from the semi-empirical model of deBoer et al. [26] a Compound
DH 0f (experimental)
Ref. state;
Method
DH 0f
This work
Literature
Tr; Ga
[Ref.]
(predicted)
Sc 5 Ga 3 Ti 2 Ga V2 Ga 5
259.462.0 239.061.4 216.661.7
2 2 214.961.6
2 2 s,l
259 244 218
Cr 3 Ga
211.861.8
24.661.4
s,l
FeGa 3
224.862.1
22163
s,s
CoGa 3
229.462.2
233.0
s,l
24563
s,s
238.260.7
s,s
24662
s,l
247.9
s,l
248.162.1
s,s
2 2 EMF (700–900 K) [23] EMF (700–900 K) [22] Soln. Calor. (Room temp.; Br–HBr) [12] Soln. Calor. (Ga, 1100 K) [17] Soln. Calor. (Room temp.; Br–HBr) [12] Soln. Calor. (Sn, 773–832 K) [14] Soln.Calor. (Sn,1023 K) [16] EMF (873–1100 K) [15] Soln. Calor. (Sn, 1093 K) [13]
NiGa
a
244.961.9
Data in kJ / mol of atoms.
212 210
218
237
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S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
Fig. 1. Standard enthalpies of formation of some 3d transition metal gallides; data in kJ / mol of atoms.
however, we can show some indicative trends. Fig. 1 shows that the enthalpies of formation decrease steeply from Sc to Cr, show a minimum at Cr and subsequently rise from Fe to Ni. The shape of the curve is roughly parabolic. We previously noticed similar correlations for the 3d transition metal aluminides, silicides and stannides [5–11]. In Fig. 2 we compare our values for the 3d transition
metal gallides with the heats of formation of 3d aluminides as determined by Meschel and Kleppa and by Jung et al. [6–8,30]. This figure shows that for the Sc–Ga system the heat of formation of the gallide is significantly more exothermic than for the Sc–Al system. However, for Ti and Fe gallides and aluminides the values are very similar. From V to Ni the heats of formation of the gallides are slightly less exothermic than the aluminides. The shapes of
Fig. 2. Comparison of the standard enthalpies of formation of some 3d transition metal gallides with similar transition metal aluminides. The molar composition of the gallides is given in Fig. 1.
S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 290 (1999) 150 – 156
155
Fig. 3. Comparison of the standard enthalpies of formation of some 3d transition metal gallides with similar transition metal germanides. The molar composition of the gallides is given in Fig. 1.
the curves are very similar for both families of systems. Both exhibit a steep decrease from Sc to Cr, a minimum at Cr and a rise from Cr to Ni. In Fig. 3 we compare our values for the 3d transition metal gallides with the heats of formation of the 3d germanides as determined by Jung and Kleppa [31,32]. Since Ge and Ga occupy the same positions within the columns VB and IIIB in the periodic table, the comparison is of some interest. This figure shows that from Sc to Mn the germanides have the more exothermic values, while from Fe to Ni the gallides are more exothermic.The shapes of the curves are again very similar, both roughly parabolic. In our study of the 3d, 4d and 5d transition metal aluminides we found no correlation between the observed enthalpies of formation and the difference in the electronegativities of the two component elements in the alloys [6–8]. The same situation holds for the 3d gallides. For example the electronegativity difference between metal and gallium is the same for Fe, Co and Ni, while the enthalpies of formation differ by nearly a factor of two. This suggests that the chemical bonding in the Tr–Ga compounds may be covalent rather than ionic as was noted for aluminides by Colinet [33] and by Pasturel et al. [34]. We also attempted to compare the melting points with the enthalpies of formation and found no correlation. For example the melting point of Cr 3 Ga is the highest among the 3d gallides while its enthalpy of formation is numerically the lowest. However, it is difficult to assess this effect completely since several of the melting points are not known exactly. We observed the same lack of correla-
tion between the heats of formation and the melting points for the 3d aluminides [6–8]. Gellatt et al. provided a detailed theoretical analysis of the important factors which contribute to the enthalpies of formation of transition metal–polyvalent metal systems [35]. Further work along similar lines has been carried out by Pasturel et al. [34]. Pasturel’s theory on 3d transition metal aluminides predicts a roughly parabolic dependence of the enthalpies of formation on the number of d electrons of the metal, with the weakest bonding in the middle of the series. This is also consistent with our results for the 3d gallides.
Acknowledgements This investigation has been supported by the Department of Energy under Grant DE-FGO2-88ER4563, and has also benefited from the MRSEC facilites at the University of Chicago. We are indebted to Dr. Joseph Pluth for his help with generating the X-ray diffraction patterns from the reported unit cell parameters and the atomic coordinates
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