Standard enthalpies of formation of some 3d transition metal silicides by high temperature direct synthesis calorimetry

Journal of Alloys and Compounds 267 (1998) 128–135

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Standard enthalpies of formation of some 3d transition metal silicides by high temperature direct synthesis calorimetry S.V. Meschel, O.J. Kleppa* University of Chicago, The James Franck Institute, 5640 S. Ellis Ave, Chicago, IL, 60637, USA Received 26 August 1997

Abstract The standard enthalpies of formation of some 3d transition metal silicides have been measured by high temperature direct synthesis calorimetry at 147362 K. The following results, in kJ (mole of atoms)21 , are reported: ScSi: (282.362.1); TiSi: (272.661.9); Ti 5 Si 3 : (273.862.0); Ti 5 Si 4 : (278.562.1); V5 Si 3 : (259.062.0); V3 Si: (246.461.5); Cr 5 Si 3 : (233.661.0); Cr 3 Si: (227.261.1); CrSi: (234.261.6); Mn 5 Si 3 : (234.061.2); MnSi: (239.461.7); FeSi: (238.661.8); CoSi: (249.361.3); Co 2 Si: (237.962.0); CoSi 2 (234.961.1); Ni 2 Si (250.661.7); Ni 5 Si 2 : (245.161.4). The results are compared with some earlier values obtained by calorimetry or derived from EMF or vapor pressure measurements. They are also compared with predicted values from the semi-empirical model of Miedema and coworkers and with available enthalpies of formation of transition metal germanides and aluminides.  1998 Elsevier Science S.A. Keywords: Transition metal silicides; Enthalpy of formation; Calorimetry; Thermodynamics

1. Introduction Transition metal silicides formed by the 3d transition metals have been of considerable interest for some time. Their stability and resistance to oxidation make these alloys excellent candidates for the development of materials for high temperature structural application, for aerospace applications, for high temperature furnace construction and for high temperature coating [1,2]. Their relatively low electrical resistance has been utilized in microelectronics for developing integrated circuit technology [3,4]. Several transition metal silicides have also potential use in thermoelectric conversion processes. Several of these alloys exhibit superconductivity, for example V3 Si and CoSi 2 . V3 Si exhibits a particularly high transition temperature of 17.1 K [2,5]. Such compounds may be used in thermonuclear reactors, generators and power transmission processes. Some Cr and Ti silicides exhibit excellent resistance to oxidation; this makes them potential candidates for applications at very high temperatures. Among these compounds Cr 3 Si has recently been tested for its potential use as an oxidation resistant, high temperature material. The Tr 5 Si 3 type compounds, which represent *Corresponding author. Tel.: 001 773 7027284; fax: 001 773 7025863; e-mail: [email protected] 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00528-8

some of the highest melting alloys among the 3d silicides (among these e.g., Ti 5 Si 3 ), have received some attention due to their mechanical properties [1,2]. The thermodynamic properties of the 3d transition metal silicides are very important in considering their possible applications in electronic technology. Since the 1930s there has been a considerable research effort in studies of the thermochemistry of the transition metal silicides [1,2,6,7]. However, the distribution of this work has remained very uneven. The silicides of some of the transition metals have been studied very extensively, while the silicides of other metals have very scant information about their thermochemical properties. Moreover, the published values of the enthalpies of formation for some silicides show very wide differences; these differences clearly should be resolved. The literature in this field has been reviewed by Chart [6], by Schlesinger [1] and most recently by Chandrasekharaiah and Margrave [7]. Unfortunately, Schlesinger’s otherwise excellent review, does not list the uncertainties in the reported experimental values; this is a serious drawback. To illustrate the status of the information on the thermochemical properties of transition metal silicides, some citations from the quoted reviews are very descriptive: T. Chart (1972): ‘the absolute accuracy of thermo-

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chemical data for transition metal–silicon systems is not high’ M.E. Schlesinger (1990): ‘Eighteen years later this comment remains largely valid. The available data are in agreement enough to allow the generation of reliable thermodynamic tables for only nine of the 1001 transition metal silicide compounds.’ Some of the heats of formation of the transition metal silicides have been measured by several different experimental methods; however other values have been estimated only. Some of the enthalpies of formation reported in the published literature have been determined calorimetrically by room temperature reaction calorimetry [8,9], by combustion calorimetry [10–15], by acid solution calorimetry [13,16,17] and by high temperature solute– solvent drop calorimetry [18–21]. Other values have been derived from mass spectrometric measurements [22–28] or from EMF data [29–33]. The reliability associated with these methods has been critically assessed by Schlesinger [1] and by Chandrasekharaiah and Margrave [7]. To the best of our knowledge only one of the reported enthalpy values was measured by high temperature direct synthesis calorimetry [34]. In principle, this method provides the best chance to obtain a precise, reliable result when the reaction studied is complete. During recent years we have in this laboratory conducted systematic studies of the thermochemistry of transition metal and rare earth alloys with elements in the IIIB and IVB columns in the periodic table [35]. These investigations have included studies of borides, aluminides, silicides and germanides of transition metals and of the lanthanide elements. The earlier work by Topor and Kleppa was based on the use of the solute–solvent drop technique [18–21]. In the more recent studies of transition metal aluminides and stannides Meschel and Kleppa applied the direct synthesis method [36–39]. In the present communication we report new thermochemical values for some phases in the binary systems Sc–Si, Ti–Si, V–Si, Cr–Si, Mn–Si, Fe–Si, Co–Si and Ni–Si all obtained by direct synthesis calorimetry. Some of the phases among the 3d transition metal silicides were studied by Topor and Kleppa by high temperature solute– solvent drop calorimetry [18–21]. Most of these compounds, particularly the disilicides, were the lowest melting alloys in the considered phase diagrams. In the present study we focused our attention on the higher melting phases. Information regarding the binary phase diagrams and the structures of the considered phases is generally available in the literature [40–45]. However, we found that the melting points of several of the compounds which we studied are not known. We also found that the X-ray diffraction patterns of three of the alloys were not listed in the ASTM powder diffraction file.

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The published literature offers some values for the enthalpies of formation of the alloys which we studied. We will compare our results with these earlier data. We will also compare our values with predictions based on the semi-empirical model of Miedema and coworkers [46]. Our thermochemical measurements allow us to obtain a systematic picture of the enthalpies of formation of the 3d transition metal silicides in their dependence on the atomic number of the transition metal. We will also compare the thermochemical behavior of the transition metal silicides with that of the corresponding germanides and aluminides and with the rare earth silicides, germanides and aluminides. We are now extending this study to the 4d and 5d transition metal silicides.

2. Experimental and materials The experiments were carried out at 147362 K in a single unit differential microcalorimeter which has been described in an earlier communication from this laboratory [47]. All the experiments were performed under a protective atmosphere of Argon gas which was purified by passing it over Titanium chips at 9008C. A BN (boron nitride) crucible was used to contain the samples. All the materials were purchased from Johnson Matthey /Aesar, Ward Hill, MA; the purity and the particle size of the elements used are summarized in Table 1. The Fe, Co and Ni powders were freshly reduced under H 2 gas flow at 6008C prior to the preparation of the samples. 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 a subsequent set of experiments the reaction products were dropped into the calorimeter from room temperature to measure their heat contents. Between the two sets of experiments the samples were kept in a vacuum desiccator to prevent possible reaction with oxygen or moisture. Calibration of the calorimeter was achieved by dropping Table 1 Purity and particle size of the elements used in this study Element

Purity %

Particle size

Sc Ti V Cr Mn Fe Co Ni Si Ag

99.9 99.9 99.5 99.95 99.3 99.9 99.8 99.9 99.5 99.95

280 mesh (filed from ingot) 2200 mesh 2325 mesh 2325 mesh 2325 mesh 2325 mesh 2150 mesh 2150 mesh 2325 mesh 2325 mesh

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weighed 4-mm diameter pellets prepared from 2-mm OD high purity copper wire from room temperature into the calorimeter at 147362 K. The enthalpy of pure copper at this temperature, 46.465 kJ mole 21 of atoms, was obtained from Hultgren et al. [48]. The calibrations were reproducible to within 61.2%. 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 we have studied in this communication were fully reacted. We found no evidence for the presence of unreacted metal within the limits of detectability of the X-ray diffractometer. The Fe, Co and Ni silicide alloys were examined on a high sensitivity, computerized X-ray diffractometer. The phase diagram of the Sc–Si system shows one congruently melting phase, Sc 5 Si 3 and two peritectically melting phases, ScSi and Sc 3 Si 5 [40]. Sc 5 Si 3 was already studied in this laboratory by Topor and Kleppa [20]. The melting points of these compounds are not known. We prepared the higher melting peritectic phase, ScSi in the calorimeter. The X-ray diffraction pattern of ScSi matched well the pattern in the ASTM powder diffraction file. We observed only a very minor, barely detectable second phase, less than 2% of Sc 5 Si 3 . The phase diagram of the Ti–Si system shows two congruently melting phases, Ti 5 Si 3 (m.p. 21308C) and TiSi 2 (m.p. 15008C) [40]. In addition to these alloys there are some peritectically melting phases such as TiSi (m.p. 15708C) and Ti 5 Si 4 (m.p. 19208C). TiSi 2 was previously studied in this laboratory by Topor and Kleppa [19]. We prepared Ti 5 Si 3 , TiSi and Ti 5 Si 4 in the calorimeter. The X-ray diffraction patterns of all three compounds matched well the patterns in the ASTM powder diffraction file. Ti 5 Si 4 was present in the h.t. orthorhombic modification. We found Ti 5 Si 3 and Ti 5 Si 4 to be single phases, while the pattern of TiSi indicated a minor amount of a second phase, about 5% TiSi 2 . The phase diagram of the V–Si system shows three congruently melting compounds, V5 Si 3 (m.p. 20108C), V3 Si (m.p. 19258C) and VSi 2 (m.p. 16778C) [40]. The lowest melting of these compounds, VSi 2 , was already studied in this laboratory by Topor and Kleppa [19]. We made both V5 Si 3 and V3 Si in the calorimeter. The X-ray diffraction patterns of both alloys matched well the patterns in the ASTM powder diffraction file. The pattern of the V5 Si 3 phase showed that we obtained the high temperature, tetragonal modification (W5 Si 3 type). The pattern of V3 Si indicated a very minor amount of a second phase, less than 3% of V5 Si 3 . The phase diagram of the Cr–Si system shows three congruently melting compounds, Cr 5 Si 3 (m.p. 16808C), Cr 3 Si (m.p. 17708C) and CrSi 2 (m.p. 14908C) [40]. The phase diagram also indicates a peritectic CrSi phase (m.p.

14138C) [40]. The phase CrSi 2 was already studied in this laboratory by Topor and Kleppa [18]. We prepared Cr 5 Si 3 , Cr 3 Si and CrSi in the calorimeter. The X-ray diffraction pattern of Cr 3 Si showed excellent match of the pattern in the ASTM powder diffraction file. The patterns of Cr 5 Si 3 and CrSi were not listed in the ASTM powder diffraction file. We therefore generated these patterns from available unit cell parameters and the atomic coordinates given by Pearson [41]. Our pattern of CrSi matched well the generated pattern. Our pattern of Cr 5 Si 3 matched well the pattern of the low temperature, tetragonal modification of this phase (W5 Si 3 structure). We observed no detectable amount of a second phase in either pattern. The phase diagram of the Mn–Si system shows two congruently melting alloys, Mn 5 Si 3 (m.p. 13008C) and MnSi (m.p. 12768C) [40]. We prepared both compounds in the calorimeter. The X-ray diffraction pattern of Mn 5 Si 3 matched the major lines in the pattern in the ASTM powder diffraction file. However, numerous lines listed in the published pattern were absent in our experimental product. We found no other phases present (including Mn 5 Si 2 ). This suggests that our reaction product may be the h.t. modification rather than the l.t. form listed in the ASTM file. The X-ray diffraction pattern of MnSi is not listed in the ASTM powder diffraction file. We therefore generated it from available unit cell parameters and the atomic coordinates [41]. Our pattern matched well the generated pattern. However, we also observed a very minor amount of a second phase, approximately 3% of Mn 5 Si 3 . The phase diagram of the Fe–Si system shows one congruently melting alloy, FeSi (m.p. 14108C) [40]. We prepared this compound in the calorimeter. The X-ray diffraction pattern showed an excellent match with the pattern in the ASTM powder diffraction file. We observed no other phases present. The phase diagram of the Co–Si system shows three congruently melting compounds, Co 2 Si (m.p. 13348C), CoSi (m.p. 14608C) and CoSi 2 (m.p. 13268C) [40]. We prepared all three in the calorimeter. The X-ray diffraction pattern of Co 2 Si matched well the pattern of the low temperature, orthorhombic modification in the ASTM powder diffraction file. We observed a very minor amount of a second phase, approximately 3–5% CoSi. The X-ray diffraction pattern of CoSi showed an excellent match with the pattern in the ASTM powder diffraction file. The X-ray diffraction pattern of CoSi 2 matched well the pattern in the ASTM powder diffraction file. However, we also observed approximately 15% of CoSi. The phase diagram of the Ni–Si system shows three congruently melting alloys, Ni 5 Si 2 (m.p. 12428C), Ni 2 Si (m.p. 13068C) and NiSi (m.p. 9928C) [40]. The lowest melting compound, NiSi, was studied already in this laboratory by Topor and Kleppa [21]. We prepared Ni 5 Si 2 and Ni 2 Si in the calorimeter. The X-ray powder diffraction pattern of Ni 2 Si matched well the pattern of the delta,

S.V. Meschel, O. J. Kleppa / Journal of Alloys and Compounds 267 (1998) 128 – 135

orthorhombic modification in the ASTM powder diffraction file. The X-ray diffraction pattern of Ni 5 Si 2 matched well the pattern of the gamma phase in the ASTM powder diffraction file. The structure type is not available for this alloy. We observed no secondary phases present in either pattern.

3. Results and discussion The standard enthalpies of formation of the 3d transition metal silicides 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, 298K) 1 mSi(s, 298K) 5 TrSi m (s, 1473 K)

(1)

Here m represents the molar ratio Si / Tr, Tr is the considered 3d transition metal (Sc, Ti, V, Cr, Mn, Fe, Co and Ni) and s denotes solid. The reacted pellets were reused in a subsequent set of measurements to determine their heat contents: TrSi m (s, 298K) 5 TrSi m (s, 1473K)

(2)

The standard enthalpy of formation is given by: DH 0f 5 D(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 (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

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structure type. The heat effects associated with the reactions (1) and (2) are given in kJ (mole of atoms)21 as the averages of 5–7 consecutive measurements with the appropriate standard deviations. The last column shows the standard enthalpy of formation of the considered phases. The standard deviation given in the last column also reflects small contributions from the uncertainties in the calibrations. CrSi crumbled during the preparation of the pellets. In order not to risk contaminating the liner assembly and to insure good experimental precision, we used approximately 10 mole % of Ag powder as a binder. This method was discussed in detail in an earlier communication on transition metal borides by the present authors [49]. The results in Table 2 were corrected for the heat content of pure silver from the available data by Hultgren [48] (45.71 kJ (mole of atoms)21 ). Since CoSi 2 contained approximately 15% CoSi, we are reporting the enthalpy of formation of this alloy as indicative only. We are concerned that the lower melting, peritectic compound Ti 5 Si 4 yielded a more exothermic enthalpy of formation than the higher melting, congruent compound Ti 5 Si 3 . We cannot offer a full explanation for what seems to be contrary to expectation on the basis of the phase diagram. In Table 3 we compare our results with previous experimental values obtained by room temperature reaction calorimetry [8,9], by combustion calorimetry [10–15], by solution calorimetry [13,16,17], derived from EMF measurements [29–33], from mass spectrometry [22–28] and obtained by some other methods [50–52]. In general our results agree well with data from liquid Al solution calorimetry [17] and from room temperature reaction calorimetry [8,9]. It is worth noting that our experimental uncertainties usually are considerably smaller than for

Table 2 Standard enthalpies of formation of some 3d transition metal silicides by high temperature direct synthesis calorimetry. Data in kJ (mole of atoms)21 Compound

M.P. 8C

Structure type

DH(1)

DH(2)

DH 0f

ScSi TiSi Ti 5 Si 3 Ti 5 Si 4 V5 Si 3 V3 Si Cr 5 Si 3 Cr 3 Si CrSi Mn 5 Si 3 MnSi FeSi CoSi Co 2 Si CoSi 2 Ni 2 Si Ni 5 Si 2

?(p) 1570(p) 2130(c) 1920(p) 2010(c) 1925(c) 1680(c) 1770(c) 1413(p) 1300(c) 1276(c) 1410(c) 1460(c) 1334(c) 1326(c) 1306(c) 1242(c)

CrB FeB Mn 5 Si 3 ? W5 Si 3 Cr 3 Si W5 Si 3 Cr 3 Si FeSi Mn 5 Si 3 FeSi FeSi FeSi Co 2 Si CaF 2 Co 2 Si ?

249.861.7(7) 240.961.6(7) 242.461.0(7) 244.761.7(6) 225.961.4(7) 213.661.3(7) 10.460.3(6) 15.360.7(7) 12.860.8(6) 14.360.6(7) 20.760.5(5) 24.161.2(6) 218.261.0(7) 22.461.3(6) 24.860.6(7) 215.461.0(7) 210.460.4(5)

32.561.3(6) 31.761.1(6) 31.461.7(6) 33.861.3(6) 33.161.4(6) 32.860.8(6) 34.061.0(6) 32.660.9(6) 37.061.4(6) 38.361.0(5) 38.761.6(6) 34.561.4(5) 31.160.9(5) 35.561.5(5) 30.160.9(5) 35.261.4(6) 34.761.3(5)

282.362.1 272.661.9 273.862.0 278.562.1 259.062.0 246.461.5 233.661.0 227.261.1 234.261.6 234.061.2 239.461.7 238.661.8 249.361.3 237.962.0 234.961.1 a 250.661.7 245.161.4

c5congruent melting compound. p5peritectic melting compound. a Indicative value.

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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 Miedema and Coworkers [46]. Data in kJ (mole of atoms)21 Compound

DH 0f (exptl.) This study

DH 0f (exptl.) Literature

Method (Ref.)

DH 0f (pred.)

ScSi

282.362.1 272.661.9

Ti 5 Si 3

273.862.0

Ti 5 Si 4 V5 Si 3

278.562.1 259.062.0

V3 Si

246.461.5

Cr 5 Si 3

233.661.0

Cr 3 Si

227.261.1

CrSi

234.261.6

Mn 5 Si 3

234.061.2

MnSi

239.461.7

FeSi

238.661.8

CoSi

249.361.3

Co 2 Si

237.962.0

CoSi 2

234.961.1 d

Ni 2 Si Ni 5 Si 2

250.661.7 245.161.4

HCl Soln. Calor. [16] EMF (825–10458C) [29] Mass Spec. [22] Direct React. Cal. [51] Comb. Calor. [10] Reduction, 6508C [50] Mass Spec. [22] Comb. Calor. [10] Reduction by Na, 6508C [50] Direct React. Calor. [51] H.T. synthesis [34] Mass Spec. [22] Comb. Calor. [11] Comb. Calor. [15] EMF [30] Comb Calor. [11] Comb. Calor. [15] EMF (660–8608C) [30] Comb Calor. [12] EMF (973–1133 K) [31] Mass Spec. [23] EMF (700–8508C) [32] Mass Spec. [28] Comb. Calor. [12] Mass Spec. [23] EMF (973–1133 K) [31] Mass Spec. [28] EMF (700–8508C) [32] Comb. Calor. [12] EMF [31] EMF [32] Mass Spec. [28] Mass Spec. [23] Atomic Absorption [52] EMF (950–11508C) [33] HF Soln. Calor. [13] Mass Spec. [24] EMF (950–11508C) [33] Mass Spec. (1448–1787 K) [25] HF Soln. Calor. [13] Atomic Abs., 680–10008C [52] Mass Spec. [24] Mass Spec. [26] Al Soln. Calor. [17] Comb. Calor. [14] R.T. React. Calor. [8] Mass Spec. [27] R.T. React. Calor. [8] Mass Spec. [27] R.T. React. Calor. [8] Mass Spec. [27] R.T. React. Calor. [9] –

280

TiSi

2117.2623.8 a 287.162.1 271.565 265.160.4 282.066.3 278.765.0 278.165 276.966.3 276.662.5 272.560.8 272.461.9 275.965 250.2624.1 258.162.6 254.462.1 228.269.4 235.262.1 246.461.9 240.865.8 233.862.5 227.969 b 241.065.9 228.161.7 234.566.3 226.469 b 234.462.4 228.261.7 234.765.9 239.764.2 231.061.2 229.861.2 226.661.7 227.469 242.3611.7 234.261.9 225.167.9 230.562.5 239.861.0 237.063.7 255.6610.5 240.665.0 232.662.5 235.967.8 239.361.9 237.764.2 250.2 c 247.362.0 238.5 c 241.062.0 234.3 c 232.962.0 246.9 c –

a

Average error for five alloy measurements. Estimated error. c No error given; 64 kJ mole 21 of atoms, estimated by Chart [6]. d Indicative value. b

274

268

274 246

234

231

224

230

242

241

226

231 230 215 232 229

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most of the cited results. Our value for V3 Si is more exothermic, while the values for Cr 3 Si and Cr 5 Si 3 are somewhat less exothermic than the values obtained by combustion calorimetry. Our value for ScSi is significantly different from the value determined by HCl solution calorimetry by Golutvin [16]. However, our value for this compound (282.362.1 kJ (mole of atoms)21 ) compares well with the earlier value for Sc 5 Si 3 (289.964.3 kJ (mole of atoms)21 ) obtained by Topor and Kleppa by solute–solvent drop calorimetry [20]. The approximate melting point of ScSi is nearly 200 degrees lower than that of Sc 5 Si 3 , and ScSi melts peritectically. Hence it is reasonable to assume that the enthalpy of formation of ScSi may be somewhat less exothermic than Sc 5 Si 3 . Our value also compares well with the recent EMF value by Lukashenko et al. [29]. The last column in Table 3 shows the enthalpies of formation predicted on the basis of the semi-empirical model of Miedema and coworkers [46]. For ScSi, TiSi, Ti 5 Si 3 , Ti 5 Si 4 , Cr 5 Si 3 ,Cr 3 Si, CrSi and MnSi the agreement is reasonably good. For the V, Fe, Co and Ni alloys the predicted values are considerably less exothermic than the experimental values, while for Mn 5 Si 3 the predicted value is considerably more exothermic than our measured value. We compared our measured heat contents with values calculated according to the Neumann–Kopp rule. Usually,we observed reasonably good agreement. Only for the Co alloys was the difference outside our range of experimental error. In Fig. 1 we present a plot of the standard enthalpies of formation of the 3d silicides against the atomic number of the 3d transition metals from Sc to Ni. For Sc to Cr we plot the values for Tr 5 Si 3 , which are the most exothermic values; for Mn to Ni we plot the most exothermic values. Fig. 1 shows that the enthalpies of formation decrease steeply from Sc to Cr, show a minimum at Cr, and subsequently rise more slowly from Mn to Ni. The shape of the curve is roughly parabolic. We noticed a somewhat

Fig. 1. Standard enthalpies of formation of some 3d transition metal silicides. Data in kJ (mole of atoms)21 .

133

Fig. 2. Comparison of the standard enthalpies of formation of some 3d transition metal silicides with similar transition metal germanides. The molar composition of the silicides is given in Fig. 1.

similar correlation for the 3d transition metal aluminides studied by the present authors [37–39]. In Fig. 2 we compare our values for the 3d transition metal silicides with the enthalpies of formation of 3d transition metal germanides determined by Jung and Kleppa [53,54]. This figure shows that the standard enthalpies of formation of the 3d germanides are generally less exothermic than the values for the considered silicides. The only exception to this statement we find for Sc where we see a reversal. The shape of the curve is very similar for the silicides and the germanides. Both exhibit a fairly steep decrease from Sc to Cr, a minimum at Cr and a slow rise from Mn to Ni. In this respect the results for the transition metal silicides and germanides differ from our results for the lanthanide silicides and germanides where we found that the germanides always were more exothermic than the silicides [55]. This is of course consistent with the reversal which we found for Scandium silicides. We also noticed that for the lanthanide silicides the change in the enthalpy of formation from La to Lu is relatively small. We found that the enthalpy values become slightly more exothermic in a nearly linear fashion in that sequence. In Fig. 3 we compare our results for the 3d transition metal silicides with the standard enthalpies of formation of 3d aluminides previously studied by the present authors [35,37–39]. Qualitatively, the shapes of the curves are somewhat similar. However, the silicides have considerably more exothermic enthalpies of formation from Sc to Fe. The decrease from Sc to Cr is much steeper for the silicides. The minimum is in the same area, at Cr. However, from Mn to Ni the aluminides show a considerably steeper rise. For Co and Ni the aluminides have more exothermic enthalpies of formation than the silicides. The thermochemical behavior of the lanthanide aluminides are very different from the lanthanide silicides. The lanthanide silicides are much more exothermic than the

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[36,61]. The heats of formation of the 3d germanides were measured by Jung and Kleppa [53,54]. From carbides to silicides there is usually a substantial increase in the magnitude of the heat of formation and then we observe a decrease from Si to Sn. However, the heat of formation of TiC is somewhat more exothermic than the most exothermic silicide.

Acknowledgements

Fig. 3. Comparison of the standard enthalpies of formation of some 3d transition metal silicides with similar transition metal aluminides. The molar composition of the silicides is given in Fig. 1.

aluminides. However, in both groups the change in the heat of formation is very small from La to Lu. The enthalpies of formation of the lanthanide aluminides, with the exception of Eu and Yb, are for all practical purposes constant, 25262 kJ (mole of atoms)21 . The heats of formation for most of the lanthanide aluminides were determined by Colinet et al. [56] (La to Yb) and for Lu by Meschel and Kleppa [57]. In previous communications we reported that plots of the enthalpies of formation of compounds of the lanthanide elements with IVB elements in the periodic table exhibit a roughly parabolic relationship which has a minimum at Ge [55]. The enthalpies of formation of the 3d transition metals with elements in the IVB column of the periodic table show a significantly different behavior. Table 4 shows that the numerically largest enthalpy of formation is generally at Si. We cannot make a comparison with the appropriate compounds of Pb because their heats of formation generally are not available. The heat of formation of ScPb 3 was evaluated from their EMF data by Yamshchikov et al. [58]. We cited the heats of formation values for Mn 2 Sn from Lukashenko et al. [59], and for FeSn, CoSn and Ni 3 Sn from Predel and Vogelbein [60]. The enthalpies of formation of Sc 5 Sn 3 , Ti 6 Sn 5 , V3 Sn and of the 3d carbides were measured by the present authors

This investigation has been supported by the Department of Energy under Grant DE-FGO2-88ER4563, and has also benefited from the MRSEC facilities 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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Table 4 Comparison of the standard enthalpies of formation of some compounds of 3d transition metals with elements in the IVB Column in the periodic table. All data in kJ mole 21 of atoms Element

C

Si

Ge

Sn

Pb

Sc Ti V Cr Mn Fe Co Ni

245.5(Sc 2 C) 292.9(TiC) 240.3(V4 C 3 ) 214.1(Cr 7 C 3 ) 29.1(Mn 7 C 3 ) 4.7(Fe 3 C) 2.8(Co 3 C) 1.3(Ni 3 C)

289.9(Sc 5 Si 3 ) 273.8(Ti 5 Si 3 ) 259.0(V5 Si 3 ) 233.6(Cr 5 Si 3 ) 239.4(MnSi) 238.6(FeSi) 249.3(CoSi) 250.6(Ni 2 Si)

293.4(Sc 5 Ge 3 ) 270.8(Ti 5 Ge 3 ) 244.3(V5 Ge 3 ) 215.7(Cr 5 Ge 3 ) 218.2(Mn 5 Ge 3 ) 210.4(Fe 5 Ge 3 ) 219.6(Co 5 Ge 3 ) 231.1(Ni 5 Ge 3 )

270.2(Sc 5 Sn 3 ) 243.4(Ti 6 Sn 5 ) 221.7(V3 Sn) – 213.9(Mn 2 Sn) 28.0 (FeSn) 217.2(CoSn) 226.3(Ni 3 Sn)

219(ScPb 3 ) – – – – – – –

Carbides [61]; Silicides (present work); Sc 5 Si 3 [20]; Germanides [54]; Stannides [36,59,60]; ScPb 3 [58].

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