Thermodynamic properties of vanadium silicides. I. Standard molar enthalpy of formationΔfHmoof vanadium disilicide (VSi2) at the temperature 298.15 K

J. Chem. Thermodynamics 1999, 31, 1385–1395 Article No. jcht.1999.0579 Available online at http://www.idealibrary.com on

Thermodynamic properties of vanadium silicides. I. Standard molar enthalpy of formation 1f Hmo of vanadium disilicide (VSi2 ) at the temperature 298.15 K P. A. G. O’Harea Physical and Chemical Properties Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-0001, U.S.A.

K. Watling, and G. A. Hope Griffith University, Nathan, Queensland, Australia

The massic energy of combustion of vanadium disilicide in fluorine according to the reaction VSi2 (cr) + (13/2)F2 (g) = VF5 (g) + 2SiF4 (g) has been measured calorimetrically. On the basis of the derived molar enthalpy of reaction and the standard molar enthalpies o of VF (g) and SiF (g), 1 H o (VSi , cr, 298.15 K) = −(133.3 ± of formation 1f Hm 5 4 f m 2 −1 2.0) kJ · mol at T = 298.15 K and p = 0.1 MPa. This result is compared with other o (VSi ) from the literature. The complete conventional thermodynamic values of 1f Hm 2 properties of VSi2 are also tabulated between the temperatures 298.15 K and 1950 K. c 1999 Academic Press

KEYWORDS: vanadium silicide; fluorine calorimetry; enthalpy of formation; thermodynamic properties; combustion calorimetry

1. Introduction According to the phase diagram,(1) there are four distinct solid compounds in (vanadium + silicon); namely, V3 Si, V5 Si3 , V6 Si5 , and VSi2 . Only V3 Si has a significant range of composition, and V6 Si5 appears to be stable only at elevated temperatures. Vanadium is one of the more prominent transition metals used in solid-state equipment. When deposited from the gas phase on a heated silicon substrate in a semiconductor device, vanadium forms a uniform film of the hexagonal silicide VSi2 .(2) Molybdenum disilicidebased composites that incorporate transition metal silicides such as VSi2 perform well in a To whom correspondence should be addressed (E-mail: [email protected]).

0021–9614/99/111385 + 11 $30.00/0

c 1999 Academic Press

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P. A. G. O’Hare, K. Watling and G. A. Hope

oxidizing environments at temperatures in excess of 1500 K.(3) Gaseous vanadium deposited on Si-layered substrates yields VSi2 coatings that are homogeneous and form excellent wires and electrodes in semiconductor devices.(4) In each process just described, the formation of VSi2 is of central significance. Accordingly, information on the thermodynamics of this substance can give valuable insight into the mechanisms of the industrial operations involved. These might require, for example, an analysis of the reaction of V(g) with Si(cr) to form VSi2 . Other practical illustrations would include correlations like that of Andrews and Phillips(5) between the standard molar enthalpies of formation 1f Hmo and the barrier heights of interfaces between transition-metal silicides and silicon. Thermodynamic quantities could lead to an explanation of other observations such as that of Tu et al.(6) who, in a study of interactions of vanadium films with Si and oxidized Si substrates, found that, on Si, only VSi2 formed at temperatures between 900 K and 1300 K, while on the oxide, other vanadium silicides, V3 Si and V3 Si3 , were observed at temperatures above 1100 K. We know of six investigations that lead, directly or indirectly, to results for 1f Hmo (VSi2 , 298.15 K). One(7) was based upon the technique of high-temperature Knudsen-cell mass spectrometry, four on calorimetric studies,(8–11) and one on e.m.f. measurements.(12) According to Smith’s review,(1) these values have a significant spread. This paper will describe the determination of one of the basic thermodynamic properties of one of the vanadium silicides, 1f Hmo (VSi2 , cr, 298.15 K), by means of fluorine bomb calorimetry. Subsequent publications will describe similar work on V3 Si and V5 Si3 .

2. Experimental A description has been given elsewhere(13) of the calorimetric system used in this laboratory for fluorine bomb investigations. Calibrations were based on the combustion of benzoic acid (NIST-SRM-39j) whose certified massic energy of combustion in oxygen under prescribed conditions is −(26434 ± 3) J · g−1 . A series of eight calibration experiments gave a mean value and standard deviation of the mean of (13926.6 ± 1.2) J · K−1 for the energy equivalent of the calorimetric system, ε(calor). Vanadium disilicide was synthesized at Griffith University by arc-melting together vanadium and silicon in a copper crucible. Vanadium turnings (Aldrich, catalog no. 26, 292-7)† were stated to have a mass fraction w(V) = 0.997; silicon powder (Koch-Light, catalog no. 92210) had w(Si) = 0.99999. Both elements were used as received. Impurity contents, mass fractions 3 · 10−3 and 1 · 10−5 by difference, refer, in all probability, to trace metals only. Fusion was carried out under an argon atmosphere by means of a Hobart arc welder. Specimens were melted three times for periods of 20 s to 40 s. The initial mixture consisted of 3.9495 g of V and 4.3555 g of Si, and the mass of the final button of silicide was 8.3024 g. Thus, the mass loss during the synthesis was 0.0026 g which, it is believed, was †Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment are necessarily the best available for the purpose.

Thermodynamic properties of vanadium silicides

1387

probably caused by the sublimation of oxide impurity (V2 O5 ), Si, or a combination of both. If it is assumed that only the oxide sublimed, then the composition of the final button would correspond to n(Si)/n(V) ≈ 2.001; the second assumption implies n(Si)/n(V) ≈ 1.999. In either case, or for a combination of both scenarios, the final composition of the silicide is very close to VSi2 , in agreement with the conclusions of Storms and Myers(7) that VSi2 has a negligible range of homogeneity. X-ray diffraction patterns of portions of the crushed button were in agreement with earlier literature,(14) but a small peak was observed where a weak line occurs in the pattern for Si. This is not believed to be indicative of any significant mass of uncombined Si in the preparation, primarily because the peak was very sharp, and other, stronger, reflections characteristic of Si were absent. Chemical analyses were performed (LECO, St Joseph, MI) for C, H, O, and N, and highresolution glow discharge mass spectrometric analyses (Shiva Technologies, Inc., Cicero, NY) for trace metals. Thermochemically significant contaminant levels were as follows (106 · w): C, (194 ± 31); H, (35 ± 2); O, (853 ± 40); N, (90 ± 12); Al, (22 ± 5); Cr, (240 ± 20); Fe, (960 ± 100); Ni, (170 ± 20); Cu, (36 ± 7); Nb, (80 ± 10); Mo, (200 ± 20); and W, (170 ± 20). Preliminary experiments (that were also used to prefluorinate the internal surfaces of the bomb) indicated that, although VSi2 reacted upon contact with F2 at ambient temperature, it did not lead to the vigorous combustion desirable for successful calorimetric measurements. Therefore, tungsten metal and rhombohedral sulfur were employed as auxiliary materials; the sulfur to ignite the silicide and tungsten, and the latter, as it burned to WF6 , to promote the fluorination of the VSi2 . Because of the sensitivity of VSi2 to F2 it was necessary to use a two-compartment reaction vessel(15) as part of the calorimetric system (its energy equivalent was given earlier). The tank of the apparatus was filled to a pressure of 1 MPa with distilled F2 that had been passed through a column of NaF to remove possible traces of HF. A 10 g nickel crucible that had been exposed to hot F2 to build a protective coating of NiF2 was placed on the lid of the combustion bomb and supported a saucer-shaped tungsten foil (thickness, 0.025 mm; mass, ≈0.8 g, Schwarzkopf Devlopment, Holliston, MA) that contained the specimen of VSi2 , upon which a few mg of S had been sprinkled. Our choice of the mass of VSi2 to be reacted was dictated by the desire to have only gaseous VF5 produced in the combustion, that is by the vapor pressure of VF5 (l), 29 kPa at T = 298.2 K,(16) the approximate final temperature of the experiments. In preparation for an experiment, the bomb containing the crucible and sample was closed, sealed, and connected to the tank. It is our practice to perform such operations, including the weighing of the W, VSi2 , and S, in a glovebox with a high-purity recirculating nitrogen atmosphere. The assembled reaction vessel was removed from the glovebox and transferred to the calorimeter. Calorimetric measurements followed standard procedures; at the end of the initial drift period, the valve of the tank was opened remotely, F2 flowed into the bomb, and the combustion began. Temperature was measured with a quartz-crystal thermometer (HewlettPackard, model 2804-A). At the conclusion of the after-rating period, the vessel was removed from the calorimeter

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P. A. G. O’Hare, K. Watling and G. A. Hope

and attached to a manifold where the combustion gases were condensed into a Monel trap cooled with liquid nitrogen to T ≈ 80 K, at which temperature F2 is in the liquid state, and the fluorides are solids. Excess fluorine was removed by pumping, and the remaining gases were warmed and examined at ambient temperature by F.t.i.r. spectroscopy. Only absorption peaks that could be attributed to VF5 , SiF4 , WF6 , and SF6 were observed in the spectrum. Accordingly, the tungsten was assumed to have formed WF6 , the sulfur SF6 , and the silicide VF5 and SiF4 , according to the following reaction: VSi2 (cr) + (13/2)F2 (g) = VF5 (g) + 2SiF4 (g).

(1)

A small bead (<0.7 mg) of glassy yellow substance invariably remained in the crucible after each experiment. In the absence of quantities large enough to be assayed, the residue was assumed to be VF3 (which is colored yellow) because, in other experiments with vanadium-containing substances,(17, 18) traces of VF3 were always found as a combustion byproduct.

3. Results Detailed calorimetric results for six experiments are given in table 1. Masses of substances (±0.03 mg) reacted are denoted by m; 1θc (±0.0001 K) is the corrected temperature rise, calculated as recommended by Hubbard;(19) 1U (cont) is the correction for the heat absorbed by such items as the crucible, F2 , and the combustion gases, none of which is normally considered as contents of the empty bomb to which ε(calor) is referred; 1U (gas) accounts for the interactions of the gases in the initial and final states of the experiment; and 1U (blank) and its calculation have been explained elsewhere.(20) Contributions to the overall energy change of the experiment from combustion of tungsten and sulfur were calculated as: 1U (W) = m(W) · 1c u o (W) = m(W) · 9374.4 J · g−1 and 1U (S) = m(S)·1c u o (S) = m(S) · 37956 J · g−1 , where −9374.4 J · g−1 and −37956 J · g−1 are, respectively, the standard massic energies of combustion of W(21) and S(22) in F2 . Corrections for VF3 residues were based upon the unpublished results of Johnson(23) who determined 1c u o = −2250.2 J · g−1 for the reaction: VF3 (cr) + F2 (g) = VF5 (g),

(2)

at T = 298.15 K. In experiment no 5, this correction also included an allowance for a fragment of unburned W discovered during the postcombustion examination of the interior of the bomb. The massic energy of combustion of the sample of VSi2 is denoted by 1c u. Computer modeling studies at Griffith University indicated that oxygen impurity should be combined in the sample as SiO2 , nitrogen as VN, and that carbon should be in the elemental state. Details of impurity corrections based on this scenario are given in table 2. Individual impurities are denoted by A, and their assumed states of combination by AB. Standard molar enthalpies of formation of AB are given, along with 1c u o (AB), the standard massic energy of combustion in F2 to the most stable fluorides of A and B. It was assumed that Al and Cu were uncombined because there appear to be no values for 1f Hmo s of silicides of these elements, even if they exist. Mass fractions of impurities w(A) are listed along with the mass fractions w(AB) of the corresponding compounds. Individual corrections were calculated by multiplying w(AB) by {1c u(VSi2 , sample) −1c u o (AB)},

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TABLE 1. Massic energy of combustion of VSi2 (cr) in fluorine; T = 298.15 K, p o = 101.325 kPa Expt no

1

2

m(VSi2 )/g

0.12362

0.12688

0.12171

0.11973

0.12008

0.11889

m(W)/g

0.75812

0.79395

0.88410

0.81258

0.75579

0.84184

m(S)/g

0.00258

0.00589

0.00289

0.00254

0.00260

0.00270

1θc /K

0.89135

0.93430

0.97137

0.91507

0.87991

0.93173

1U (cont)/J

−6.1

−6.4

−6.7

−6.3

−6.1

−6.4

1U (gas)/J

0.6

0.6

0.6

0.6

0.6

0.6

1U (blank)/J

1.1

1.1

1.1

1.1

1.1

1.1

hε(calor)i(−1θc )

3

4

5

6

−12413.5

−13011.6

−13527.9

−12743.8

−12254.2

−12975.8

1U (W)/J

7106.9

7442.8

8287.9

7617.5

7085.1

7891.7

1U (S)/J

97.9

223.6

109.7

96.4

98.7

102.5

1U (res)/J

−0.3

−0.8

−0.3

−13.7

12.9

−25.0

−42173

−42171

−42195

−42163

−42154

−42151

1c u/(J · g−1 )

h1c ui = −(42168 ± 7) J · g−1a Impurity correction = −(85 ± 3) J · g−1b 1c u o (VSi2 , cr, 298.15 K) = −(42253 ± 14) J · g−1c a The uncertainty is the standard deviation of the mean of the individual values of 1 u. b Impurity corc rection documented in table 2. Uncertainty corresponds to twice the standard deviation of the mean. c Uncertainty corresponds to twice the standard deviation of the mean.

where 1c u(VSi2 , sample) = −42168 J · g−1 is given in table 1. In all instances, uncertainties in this table correspond to twice the standard deviation of the mean. As shown in table 1, addition of the impurity correction to the massic energy of combustion of the sample gives the standard massic energy of combustion 1c u o (VSi2 ). We should point out that the combined uncertainties from the spread of the results, and the impurity and “blank” corrections dwarf all others. Multiplication of 1c u o (VSi2 ) by the molar mass of VSi2 , 107.1125 g · mol−1 , gives the standard molar energy of combustion 1c Umo = −(4525.8 ± 1.5) kJ · mol−1 , referred to reaction (1). On the basis of equation (1), 1ν g RT = −8.7 kJ · mol−1 and, thus, the standard molar enthalpy of combustion 1c Hmo for reaction (1) is −(4534.5 ± 1.5) kJ · mol−1 . This latter value, combined with 1f Hmo (VF5 , g, 298.15 K) = −(1436.2 ± 0.8) kJ · mol−1(24) and 2 · 1f Hmo (SiF4 , g, 298.15 K) = −2 · (1615.8 ± 0.8) kJ · mol−1(25) yields 1f Hmo (VSi2 , cr, 298.15 K) = −(133.3 ± 2.0) kJ · mol−1 , where the uncertainties in the 1f Hmo and 1c Hmo have been combined in quadrature to obtain the final uncertainty of ±2.0 kJ · mol−1 .

4. Discussion Previous Gorelkin

determinations of 1f Hmo (VSi2 ) have and Mikhailikov,(8) Gorelkin et al.,(9)

been described by Storms and Myers,(7) Golutvin and Kozlovskaya,(10) Topor and

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P. A. G. O’Hare, K. Watling and G. A. Hope TABLE 2. Thermochemical corrections for impurities in VSi2 a,b o (AB) 1f Hm kJ · mol−1

1c u o (AB) J · g−1

−42168 − 1c u o (AB) J · g−1

w(A) 10−6

w(AB) 10−6

1U (corr) J · g−1

A

AB

C

C

−77847

35319

194

194

7±2

O

SiO2

−910.9

−11732

−30436

853

1604

−49 ± 4

N

VN

−218.1

−18689

−23479

90

417

−10 ± 2

Al

Al

−55842

13674

22

22

0.3 ± 0.1

Cr

CrSi2

−96

−39623

−2545

240

499

−1.3 ± 0.2

Fe

FeSi

−73.6

−30091

−12077

960

1443

−17.4 ± 1.7

Ni

NiSi

−86.2

−25143

−17025

170

251

−4.3 ± 0.2

Cu

Cu

−8501

−33667

36

36

−1.2 ± 0.2

Nb

NbSi2

−117

−32971

−9197

80

128

−1.2 ± 0.1

Mo

MoSi2

−137.1

−30824

−11344

200

317

−3.6 ± 0.1

W

WSi2

−79

−20520

−21648

170

222

−4.8 ± 0.3

Total: −(85 ± 3) J · g−1 a Standard molar enthalpies of formation for SiO , CF (g), VN(cr), and CrSi were taken from Gurvich et al.;(26) 2 4 2 for FeSi, NiSi, and NbSi2 (by analogy with TaSi2 ) from Wagman et al.;(27) for the solid fluorides from Hubbard et al.;(28) and for SiF4 (g) from Johnson.(25) All molar masses were based on the standard atomic weights (1995). b The difference between the massic energies of combustion of the VSi2 sample and the compound AB is given by {−42168− 1c u o (AB)}.

Kleppa,(11) and Yeremenko et al.(12) Review articles by Chart,(29) Freund and Spear,(30) Schlesinger,(31) and Smith(1) have discussed most of these studies. Golutvin and Kozlovskaya(10) were the first to report 1f Hmo (VSi2 ). They described oxygen bomb calorimetric measurements of the energy of combustion of a vanadium silicide whose composition was given as VSi2.344 . This material was synthesized from singlecrystal Si and vanadium {w(V) = 0.951}. There are many shortcomings in this work. Since it has, in the meantime, been firmly established(7) that the range of homogeneity of the disilicide departs from n(Si)/n(V) = 2 by very little (<0.01), a composition with n(Si)/n(V) = 2.344 should, logically, have contained excess Si, as there are no vanadium silicides with n(Si)/n(V) > 2. It was reported(10) that the uptake of O2 during the calorimetric measurements was about 4 per cent greater than that expected on the basis of the analyses of the silicide. Nevertheless, the average of these amounts was taken as the mass of O2 that had reacted, on which the energy of combustion was based. The authors state that complete oxidation was not attained in any of the experiments with VSi2 ; the combustion yield spanned the extremes of 82 per cent to 45 per cent. Corresponding energies of combustion, which themselves exhibited considerable scatter, were extrapolated to “100 per cent combustion”, to yield 1c Hmo = −(2602 ± 84) kJ · mol−1 for the reaction: VSi2.344 (cr) + 3.594O2 (g) = (1/2)V2 O5 (cr) + 2.344SiO2 (am).

(3)

The form of SiO2 is assumed to be amorphous by analogy with Golutvin’s earlier studies(32) of the combustion of Si and titanium silicides in O2 . Although the 1f Hmo (SiO2 , am)

Thermodynamic properties of vanadium silicides

1391

= −(916.3 ± 4.2) kJ · mol−1 used by Golutvin and Kozlovskaya to derive 1f Hmo (VSi2.344 ) differs significantly from the −903.5 kJ · mol−1 recommended in the NBS tables,(27) their value for 1f Hmo (V2 O5 , cr), −(1548 ± 4) kJ · mol−1 , determined as part of the study of vanadium silicides, is close to −1550.6 kJ · mol−1 , the NBS selection.(27) Even with the large associated uncertainty, Golutvin and Kozlovskaya’s result for 1f Hmo , −(314 ± 88) kJ· mol−1 , is clearly unreasonable. In a brief report, Gorelkin and Mikhailikov(8) gave 1f Hmo (VSi2 ) = −(125 ± 13) kJ· mol−1 , based on a high-temperature calorimetric study of the reduction of V2 O5 by Si, initiated by means of an ignition mixture of BaO2 and Al. A critical assessment of this investigation is not possible because insufficient experimental details are given. A group(9) from the same Institute later described measurements of the enthalpy of synthesis of VSi2 in a bomb calorimeter. Pelleted stoichiometric mixtures of V and Si were ignited by means of metallothermic charges of known energies of reaction. Products were identified by X-ray and chemical analyses. Gorelkin et al.(9) determined 1f Hmo (VSi2 ) = −(151 ± 25) kJ · mol−1 . Topor and Kleppa(11) reported the most recent calorimetric study of VSi2 . They measured the enthalpies of formation of a liquid (palladium + silicon + vanadium) alloy at T = (1400 ± 2) K from the elements and from (vanadium disilicide + palladium) in two series of experiments. The reactions were as follows: Pd0.85 Si0.10 V0.05 (l, 1400 K) = 0.85Pd(s, 298.15 K) + 0.05VSi2 (s, 298.15 K), 1f Hmo = −(295.6 ± 3.6) kJ · mol−1 ; 0.85Pd(s, 298.15 K) + 0.10Si(s, 298.15 K) + 0.05V(s, 298.15 K) = Pd0.85 Si0.10 V0.05 (l, 1400 K ), 1f Hmo

= (183.2 ± 4.8) kJ · mol

−1

(4)

(5)

.

1f Hmo

Combination of for reactions (4) and (5), according to Topor and Kleppa, leads to 1f Hmo (VSi2 , 298.15 K) = −(112.4 ± 6.0) kJ · mol−1 . However, the composite of equations (4) and (5) actually corresponds to the formation of 0.05VSi2 : 0.05V(cr) + 0.10Si(cr) = 0.05VSi2 (cr).

(6)

Thus, in equations (4) and (5), the stoichiometric coefficients, but not the thermochemical quantities, should be multiplied by 20. Yeremenko et al.(12) derived the molar Gibbs free energies, enthalpies, and entropies of formation of a number of silicides, among them VSi2 , from measurements of e.m.f. in a high-temperature cell. For the formation reaction: V(cr) + 2Si(cr) = VSi2 (cr),

(7)

they gave the following expression: 1f G om /(kJ · mol−1 ) = −(124.52 ± 5.15) + (0.012 ± 0.005) · (T /K),

(8)

for the temperature region between 933 K and 1133 K, with hT i = 1033 K. Conversion of 1f Hmo (T = 1033 K) = −(124.52 ± 5.15) kJ · mol−1 to T 0 = 298.15 K by means of the following values of 1TT 0 Hmo : 17.99 kJ · mol−1 for Si;(26) 20.17 kJ · mol−1 for V;(26) and

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P. A. G. O’Hare, K. Watling and G. A. Hope TABLE 3. Summary of experimental results for o (VSi ) at T = 298.15 K 1f Hm 2 Source

o /(kJ · mol−1 ) 1f Hm

Golutvin and Kozlovskaya(10)

−314 ± 88

Gorelkin and Mikhailikov(8)

−125 ± 13

Gorelkin et al.(9)

−151 ± 25

Topor and Kleppa(11)

−112 ± 6

Yeremenko et al.(12)

−125 ± 6 (second law) −120 ± 6 (third law)

Storms and Myers(7)

−119 ± 3

This research

−133 ± 2

55.59 kJ · mol−1 for VSi2 (33) (table 4) leads to 1f Hmo (VSi2 , cr, 298.15 K) = −(125.1 ± o − 1T H o /T ) of: 5.8) kJ · mol−1 . A third-law calculation with values of 8om (T ) = (10T Sm T0 m −1 −1 (26) −1 −1 and 31.03 J · K−1 · 97.16 J · K · mol for VSi2 (table 4); 42.6 J · K · mol for V; mol−1(26) for Si, yields 1f Hmo (VSi2 , cr, 298.15 K) = −(119.8 ± 6) kJ · mol−1 . Combined Knudsen effusion and mass spectrometric experiments reported by Storms and Myers(7) yielded the value: 1f G om (VSi2 ) = −106.0 kJ · mol−1 at T = 1650 K. When this result is combined with values of 8om (T ):125.37 J · K−1 · mol−1 for VSi2 (table 4); 52.9 J · K−1 · mol−1 for V;(26) and 40.1 J · K−1 · mol−1 for Si,(26) one obtains 1f Hmo (VSi2 , cr, 298.15 K) = −118.8 kJ · mol−1 . An uncertainty of ±2.5 kJ · mol−1 was quoted by Storms and Myers. In the most recent review of the thermodynamic properties of vanadium silicides, Schlesinger(31) recommended 1f Hmo (VSi2 ) = −121 kJ · mol−1 , without any listed uncertainty. A critical assessment by Freund and Spear(30) of (vanadium + silicon + oxygen) concluded, from consideration of the reactions: 11VSi2 (cr) + 14VO(cr) = 7SiO2 (vit) + 5V5 Si3 (cr), VSi2 (cr) + 2VO(cr) = SiO2 (vit) + V3 Si(cr),

(9) (10)

that 1f Hmo (VSi2 , cr, 298.15 K) = −(121 ± 13) kJ · mol−1 . This analysis was based on the results of Yeremenko et al.(12) for (silicon + vanadium) and, as such, may not be entirely definitive because Smith’s(1) review shows large differences between the Yeremenko et al. and other results for the 1f Hmo s of V5 Si3 and V3 Si. A list of the previous values of 1f Hmo (VSi2 ) is given in table 3. Clearly, the present result agrees with those of the Gorelkin group,(8, 9) but is distinctly more negative than the others(7, 11, 12) with the exception of the determination by Golutvin and Kozlovskaya.(10) A value of 1f Hmo (VSi2 ) ≈ −121 kJ · mol−1 would require 1c u o (VSi2 ) = −42368 J· −1 g , about 115 J · g−1 more negative than the result given in table 1. An adjustment of this magnitude would imply, for example, that 1θc was in error by 0.27 per cent or approximately 0.0044 K, well outside the measurement error of 0.0001 K, or that 1U (blank)

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exceeded significantly the Joule-Thomson cooling effect for the expansion of F2 into the evacuated bomb, both of which are unreasonable. If the results of the modeling study on which the disposition of the impurities was chosen were ignored and, instead, a combination of impurities selected to yield the most negative thermochemical correction possible, −117 J · g−1 (composed of contributions of −68 J · g−1 from V2 O5 ; −6 J · g−1 from VC; −10 J · g−1 from VN; and −33 J · g−1 from the trace metals), the effect would be to change 1f Hmo (VSi2 ) only to −129.8 kJ · mol−1 . We believe that the result reported here for 1f Hmo , based as it is on careful measurements of the massic energy of combustion of a thoroughly characterized specimen of VSi2 , is reliable. It does not, however, support any of the previous determinations of 1f Hmo (VSi2 ) in particular. REFERENCES 1. Smith, J. F. Bull. Alloy Phase Diagrams 1981, 2, 42–48. 2. Tanigawa, A.; Nagasawa, E.; Okabayashi, H. Jpn Kokai Tokkyo Koho JP 62 40, 364 (see Chem. Abstr. 1987, 107, 31899g). 3. Petrovic, J. J.; Honnell, R. E.; Gibbs, W. S. U. S. Patent 4970179 (see Chem. Abstr. 1991, 114, 29027x). 4. Tanigawa, A.; Nagasawa, E.; Okabayashi, H. Jpn Kokai Tokkyo JP 61 61170030 (see Chem. Abstr. 1987, 106, 147925w). 5. Andrews, J. M.; Phillips, J. C. Phys. Rev. Lett. 1975, 35, 56–59. 6. Tu, K. N.; Ziegler, J. F.; Kircher, C. J. Appl. Phys. Lett. 1973, 23, 493–495. 7. Storms, E. K.; Myers, C. E. High Temp. Sci. 1985, 20, 87–96. 8. Gorelkin, O. S.; Mikhailikov, S. V. Zh. Fiz. Khim. 1971, 45, 2682–2683. 9. Gorelkin, O. S.; Dubrovin, A. S.; Kolesnikova, O. D.; Chirkov, N. A. Zh. Fiz. Khim. 1972, 46, 754–755. 10. Golutvin, Yu. M.; Kozlovskaya, T. M. Zh. Fiz. Khim. 1960, 34, 2350–2354. 11. Topor, L.; Kleppa, O. L. Metall. Trans. 1986, 17A, 1217–1221. 12. Yeremenko, V. N.; Lukashenko, G. M.; Sidorko, V. R. Rev. Int. Htes. Temp. et Refract. 1975, 12, 237–240. 13. O’Hare, P. A. G.; Hope, G. A. J. Chem. Thermodynamics 1992, 24, 639–647. 14. Wallbaum, H. J. Z. Metallkde 1941, 33, 378–381. 15. Nuttall, R. L.; Wise, S.; Hubbard, W. N. Rev. Sci. Instrum. 1961, 32, 1402–1403. 16. Trevorrow, L. E.; Fischer, J.; Steunenberg, R. K. J. Am. Chem. Soc. 1957, 79, 5167–5168. 17. Lewis, B. M.; O’Hare, P. A. G.; Mukdeeprom, P.; Edwards, J. G. J. Chem. Thermodynamics 1987, 19, 1325–1331. 18. O’Hare, P. A. G. to be published. 19. Hubbard, W. N. Experimental Thermochemistry. Rossini, F. D.: editor. Interscience: New York. 1956, 94. 20. O’Hare, P. A. G. J. Chem. Thermodynamics 1985, 17, 349–354. 21. O’Hare, P. A. G.; Tomaszkiewicz, I.; Beck, C. M. II; Seifert, H.-J. J. Chem. Thermodynamics 1999, 31, 303–322. 22. O’Hare, P. A. G.; Susman, S.; Volin, K. J.; Rowland, S. C. J. Chem. Thermodynamics 1992, 24, 1009–1017. 23. Johnson, G. K. (Argonne National Laboratory), unpublished result. 24. Johnson, G. K.; Hubbard, W. N. J. Chem. Thermodynamics 1974, 6, 59–63. 25. Johnson, G. K. J. Chem. Thermodynamics 1986, 18, 801–802. 26. Gurvich, L. V.; Iorish, V. S.; Chekhovskoi, D. V.; Yungman, V. S. IVTANTHERMO—A Thermodynamic Database and Software System for the Personal Computer. NIST Special Database 5. 1993.

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27. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data 1982, 11, Suppl. No. 2. 28. Hubbard, W. N.; Johnson, G. K.; Leonidov, V. Ya. Experimental Chemical Thermodynamics: Vol. 1. Combustion Calorimetry, Chap. 12. Sunner, S.; M˚ansson, M.: editors. Pergamon: New York. 1979. 29. Chart, T. G. High Temp.–High Press 1973, 5, 241–252. 30. Freund, P. F.; Spear, K. E. J. Less-Common Met. 1978, 60, 185–193. 31. Schlesinger, M. E. Chem. Rev. 1990, 90, 607–628. 32. Golutvin, Yu. M. Zh. Fiz. Khim. 1956, 30, 2251–2259. 33. Kalishevich, G. I.; Gel’d, P. V.; Putintsev, Yu. V. Teplofiz. Vys. Temp. 1968, 6, 1033–1006. 34. Kalishevich, G. I.; Gel’d, P. V.; Krentsis, R. P. Zh. Fiz. Khim. 1968, 42, 1288–1289. (Received 19 March 1999; in final form 19 July 1999)

SP(2)-08

Appendix: thermodynamic properties of VSi2 The conventional thermodynamic properties of VSi2 as a function of temperature appear not to have been published in a readily accessible form, even though the information and equations required to calculate them are available from the literature. Smith(1) has accepted the results of Kalishevich et al.,(34) and given the following exTABLE 4. Thermodynamic properties of VSi2 ( p o = 0.1 MPa, T 0 = 298.15 K) o 10T Sm

o 1TT 0 Hm

T /K

C p,m J · K−1 · mol−1

J · K−1 · mol−1

8om (T )a J · K−1 · mol−1

kJ · mol−1

o 1f Hm kJ · mol−1

1f G om kJ · mol−1

298.15 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 1950

64.848 64.967 69.675 72.660 74.967 76.953 78.769 80.485 82.140 83.755 85.342 86.909 88.463 90.007 91.542 93.072 94.596 96.117 96.876

59.100 59.501 78.902 94.789 108.247 119.955 130.351 139.728 148.294 156.199 163.555 170.448 176.945 183.101 188.959 194.555 199.918 205.073 207.580

59.100 59.101 61.714 66.789 72.605 78.551 84.388 90.024 95.429 100.599 105.542 110.273 114.805 119.155 123.336 127.362 131.245 134.996 136.825

0.000 0.120 6.875 14.000 21.385 28.983 36.770 44.733 52.865 61.160 69.615 78.228 86.997 95.920 104.998 114.228 123.612 133.147 137.972

−133.300 −133.299 −133.302 −133.383 −133.484 −133.585 −133.680 −133.766 −133.847 −133.925 −134.006 −134.094 −134.195 −134.315 −134.460 −235.017 −234.802 −234.561 −234.433

−131.156 −131.143 −130.428 −129.700 −128.954 −128.191 −127.414 −126.626 −125.828 −125.022 −124.210 −123.390 −122.563 −121.728 −120.884 −119.436 −112.643 −105.863 −102.478

a 8o (T ) = 1T S o − 1T H o /T . m 0 m T0 m

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pression, valid to T = 1950 K, for the molar heat capacity of VSi2 : C op,m /(J · K−1 · mol−1 ) = 67.8 + 0.015 · (T /K) − 6.6 · 105 · (T /K)−2 .

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o , along A value of 59.1 J · K−1 · mol−1 was reported(34) for the entropy increment 1T0 Sm with 1TT 0 Hmo = −10.47 kJ · mol−1 , where, in this instance, T 0 → 0. The above results, combined with 1f Hmo (298.15 K) reported in the present study, lead to the thermodynamic properties given for VSi2 in table 4.