Torsion effusion vapor pressure determinations of Os, Rh, Ru, W, Co, and Cr solid carbonyls

Article abstract of DOI:10.1016/j.jpcs.2004.10.001

Vaporization thermodynamics of low and high (>400 g/mol) molecular weight (MW) carbonyls is compared in this study. A gravimetric-torsion effusion method was used to measure vapor pressures of carbonyls such Os ₃(CO)₁₂, Rh₆(CO)₁₆, Ru ₃(CO)₁₂, Co₂(CO)₈, Cr(CO) ₆, and W(CO)₆ carbonyls. The vapor pressure thermodynamic data is used for many low and high temperature CVD applications. Disproportionation of solid carbonyls was found to be virtually independent of molecular weight of the species. Vaporization studies of Rh₆(CO) ₁₆ showed complete decomposition to nano-structured porous metallic Rh metal; the measured MW_{Rh6(CO)16(effusing gas)} is 27.75 g/mol was close to that of carbon monoxide as compared to MW _{Rh6(CO)16(solid)} of 1065.56 g/mol for the value of solid Rh ₆(CO)₁₆. However, Ru₃(CO)₁₂ and Co₂(CO)₈ show very complex behavior. The molecular weights of the vaporizing species have been estimated and the numbers of vaporizing species are proposed. For example, there was partial decomposition of Ru ₃(CO)₁₂ to (approx. 52%) metallic Ru and CO gas during vaporization as suggested by the difference in the molecular weight of the vaporizing species of 107.58 g/mol compared to the actual value of 639.33 g/mol. The dimer Co₂(CO)₈ partially disproportionates to tetramer Co₄(CO)₁₂, monomer Co(CO)₄, and CO gas. Whereas, the Os₃(CO)₁₂, Cr(CO)₆, and W(CO)₆ have shown virtually no disproportionation. The total vapor pressures of all the above-mentioned carbonyls, partial pressures of various species, average molecular weights of the effusing gases, equilibrium constants for the vaporization reactions, their enthalpies, entropies, and Gibbs energies have been determined.

Full text of DOI:10.1016/j.jpcs.2004.10.001

Journal of Physics and Chemistry of Solids 66 (2005) 241–245
www.elsevier.com/locate/jpcs
Torsion effusion vapor pressure determinations of Os, Rh, Ru, W, Co,
and Cr solid carbonyls*
a,
Dhanesh Chandra , K.H. Lau , Wen-Ming Chien , Michael Garner
b
a
a
*
aDepartment of Metallurgical and Materials Engineering, University of Nevada, Reno, Reno NV 89557, USA
bSRI International, Menlo Park, CA 94025, USA
Abstract
Vaporization thermodynamics of low and high (O400 g/mol) molecular weight (MW) carbonyls is compared in this study. A gravimetric-
torsion effusion method was used to measure vapor pressures of carbonyls such Os3(CO)12, Rh6(CO)16, Ru3(CO)12, Co2(CO)8, Cr(CO)6, and
W(CO)6 carbonyls. The vapor pressure thermodynamic data is used for many low and high temperature CVD applications.
Disproportionation of solid carbonyls was found to be virtually independent of molecular weight of the species. Vaporization studies of
Rh6(CO)16 showed complete decomposition to nano-structured porous metallic Rh metal; the measured MWRh6(CO)16(effusing
is
gas)
27.75 g/mol was close to that of carbon monoxide as compared to MWRh6(CO)16(solid) of 1065.56 g/mol for the value of solid Rh6(CO)16.
However, Ru3(CO)12 and Co2(CO)8 show very complex behavior. The molecular weights of the vaporizing species have been estimated and
the numbers of vaporizing species are proposed. For example, there was partial decomposition of Ru3(CO)12 to (approx. 52%) metallic Ru
and CO gas during vaporization as suggested by the difference in the molecular weight of the vaporizing species of 107.58 g/mol compared to
the actual value of 639.33 g/mol. The dimer Co2(CO)8 partially disproportionates to tetramer Co4(CO)12, monomer Co(CO)4, and CO gas.
Whereas, the Os3(CO)12, Cr(CO)6, and W(CO)6 have shown virtually no disproportionation. The total vapor pressures of all the above-
mentioned carbonyls, partial pressures of various species, average molecular weights of the effusing gases, equilibrium constants for the
vaporization reactions, their enthalpies, entropies, and Gibbs energies have been determined.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Vapor pressure of solid carbonyls; Torsion effusion; VCD; Os, Rh, Ru, W, Cu, Cr carbonyls
1. Introduction
Romberg [8] and Chandra et al. [10] reported vaporization
of Cr carbonyls. Hagihara et al. [11], Windsor and
Blanchard [7] reported early total vapor pressures of
Co2(CO)8, later Chandra et al. [12] reported details of
disproportionation of Co2(CO)8.
Metal carbonyls are used in chemical vapor deposition
(CVD) of metals and alloys, in the formation of composite
materials fabrication, and other technologies [1]. Carbonyl
CVD application applies to deposition of high-purity
metallic/alloy coatings. Chandra et al. [2,3] and Shorovsh-
rov [4,5], reported depositing metallic coating on fibers.
High temperature vapor pressure measurements on W(CO)6
were reported by Baev [6], Windsor and Blanchard [7],
Hieber and Romberg [8], Lander and Germer [9], and low
temperature by Garner and Chandra [2]. Hieber and
2. Experimental
A gravimetric-torsion effusion apparatus was used to
measure the vaporization behavior of the above-mentioned
carbonyls. A schematic of the apparatus is shown in Fig. 1.
This consists of an opposed dual-orifice Pt-30%Rh Knudsen
cell suspended on Pt-10%Ni torsion element ribbon
(58.6 cm long with a torque constant of 0.0947 cm/rad.).
Two orifice sizes were used in the cases where dispropor-
tionation behavior was suspected due variation of the
molecular weight of the effusing species. The ribbon was
attached to mirror assembly to take angular torque pressure
* Paper, PA-03, presented at the International IUPAC Conference on
High Temperature Materials Chemistry—XI, University of Tokyo, May
19–23, 2003, Tokyo, Japan.
* Corresponding author. Tel.: C1 775 784 4960; fax: C1 775 784 4316.
E-mail address: dchandra@unr.edu (D. Chandra).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.10.001

242
D. Chandra et al. / Journal of Physics and Chemistry of Solids 66 (2005) 241–245
Fig. 1. A schematic of the gravimetric torsion effusion system used to measure the vapor pressures of the carbonyls (left). Also shown is the mirror assembly
for suspension of the torsion cells (right).
measurements and suspended from an arm of the Cahn-
D100 (RH) electro-balance. A Quartz tube, 9 cm in
diameter was used to accommodate the Knudsen cell
assembly. A high vacuum system was interfaced with
the Knudsen cell assembly shown in Fig. 1. Vapor pressure
measurements were made on standards such as, KCl and
C10H8 to check the instrument accuracy; there was very
good agreement with the measured and literature values of
these standards [13].
Peq is the equilibrium pressure; P, the measured total pressure;
b,cellconfigurationconstant;C,theclausingfactor;andaisthe
area of the orifice. More details may be found in Ref. [14].
3. Results and discussions
3.1. Thermodynamic vaporization properties of rhodium,
ruthenium, and osmium carbonyls
The measurements were made using the following
equations; the total pressures of the effusing gases may be
obtained using the following equation:
Vapor pressure results of high molecular weight (MW, O
400 g/mol) carbonyls are present below. Vaporization of
Rh6(CO)16(s) showed complete disproportionation at rela-
tively low temperatures, ranging from 320 to 370 K.
Gravimetric data showed a vast discrepancy between the
molecular weight of pure Rh6(CO)16(s) (1065.6 g/mol) and
the effusing species; in this case, molecular weights of 27.1
(1.8) g/mol with 0.11 cm orifice cell, and 29.7 (3.6) g/mol
with 0.06 cm, were measured suggesting that the effusing
species is carbon monoxide. The total pressure measured
using cell no.1 (0.11 cm) and cell no. 2 (0.06 cm) was
obtained using second law method. The decomposition may
be represented as:
K2q
P
PT Z
(1)
n
iZ1ðaifidiÞ
where PT is the total pressure; K, torsion constant, q, the
measured angular deflection; fi, the force factor; ai, the area of
the orifice; di, the moment arm of effusion orifice; and i is the
numberofKnudsencellsrangingfromiZ1 to n [10,13], in our
case nZ2
P
ꢀ
ꢁ
n
2
W
iZ1ðaifidiÞ
P
M Z 2pRT
(2)
n
ðK2qÞ iZ1ðCaÞi
!
K2
Rh6ðCOÞ16ðsÞ Z 6RhðsÞ C16COðgÞ
(5)
n
X
M Z
miMiK1=2
(3)
iZ1
log P1 ðkPaÞ Z 11:6149ð0:063Þ K5467ð22Þ=T ðcell no: 1Þ
Mistheaveragemolecularweight;T, absolutetemperature;W,
thetotalrateofweightloss;R,thegasconstant;mi,massfraction
ofthespeciesi;andMi isthemolecularweightofthespecies.If
more than one molecular species effuses out due to molecular
disproportionation, then Eq. (2) is used. We did two sets of
experiments whenever disproportionation was suspected,
using 0.06 and 0.11 cm orifice diameter of the Knudsen cells
(6)
log P2 ðkPaÞ Z 12:1096ð0:087Þ K5494ð31Þ=T ðcell no: 2Þ
(7)
The equilibrium decomposition pressure, Peq, for Eq. (4) was
extrapolated by using Whitman–Motzfeldt plot (1/P vs
P
Ciai) to take in account the effect of orifice size and
temperature [13,14] giving the following equation
"
#
n
X
Peq Z P 1 Cb
ðCaÞi
(4)
log Peq Rh6ðCOÞ16ðsÞ ðkPaÞ Z 12:553 K5540=T
(8)
iZ1

D. Chandra et al. / Journal of Physics and Chemistry of Solids 66 (2005) 241–245
243
The vapor pressures obtained using the two cells and the
expressed as:
"
#
equilibrium pressures are plotted in Fig. 2A. The Gibbs
energy for Eq. (5) of decomposition of Rh6(CO)16(s) is given
as follows
1=2
Ru3ðCOÞ12ðgÞ
ð1KbÞM
p
Ru3ðCOÞ12 Z
(13)
1=2
ðCOÞðgÞ
ð1KbÞMR1=u23ðCOÞ12ðgÞC12bM
PT
DG+Rh6ðCOÞ16 ZKRT ln Kp Z1697:1K3:23T ðkJ=molÞ (9)
By substituting the above value of b in Eq. (13) we obtain
pRu3(CO)12(g)Z0.215PT and p(CO)(g)Z0.785PT. The solid–
gas equilibrium for direct vaporization of Ru3(CO)12 solid
to gas was also obtained for the following equation:
where, Kp is the equilibrium constant, and DH8ZC
1697.1 kJ/mol.
Vaporization of ruthenium carbonyl showed complex
behavior and may be represented as:
Ru3ðCOÞ12ðsÞZRu3ðCOÞ12ðgÞ
For which
(14)
Ru3ðCOÞ12ðsÞZð1KbÞRu3ðCOÞ12ðgÞC3bRuðsÞC12bCOðgÞ
(10)
Kp ZpRu3ðCOÞ12ðgasÞ Z0:215PT
DG+Ru3ðCOÞ12ðgÞ ZKRT lnKp Z103:2K0:214T ðkJ=molÞ
(15)
(16)
The vapor pressure equation is given by:
logPT ðkPaÞZ12:519ð0:03ÞK5392ð45Þ=T
(11)
The molecular weights of the effusing species were obtained
by the derived equation given below:
Concurrently, the decomposition of metallic Ru was also
occurring for which we write the following equation:
"
"
#
K2
n
Ru3ðCOÞ12ðsÞZ3RuðsÞC12COðgÞ
(17)
(18)
X
K1=2
MZ
miðMiÞ
12
12
iZ1
Kp ZðpCOÞ Zð0:785PTÞ
#
K2
½ð1KbÞMR1=u23ðCOÞ12ðgÞC12bM
ꢀ
ꢀ
1=2
ðCOÞðgÞ
and
Z
(12)
½ð1KbÞMRu3ðCOÞ12ðgÞC12bMðCOÞðgÞ
DG+ ZKRT lnKp Z1237:8K2:39T ðkJ=molÞ
(19)
By substituting the molecular weights of pure gases from
Eqs. (11) and (12), a general equation for bZ
K0.738 log MC2.049 was obtained, and for the measured
molecular weight, MZ92.022 g/mol, and bZ0.5923.
The partial pressure of ruthenium carbonyl may be
Thus, solid Ru3(CO)12 disproportionates to metallic Ru
and carbon monoxide near room temperature as shown in
Eq. (17). At PT 298 KZ4.86!10K7 kPa, and DG2+98K
549:4kJ=mol. The equilibrium (partial) pressures for
Z
Fig. 2. (A) Vapor pressures measured for Rh6(CO)16(s)Z6Rh(s)C16CO(g) obtained using Knudsen cells with orifice sizes 0.06 and 0.11 cm. Equilibrium
vapor pressure for this reaction was obtained using Whitman–Motzfeldt plot (see Ref. [3]). (B) Vapor pressures showing Ru3(CO)12(s)Z(1K
b)Ru3(CO)12(g)C3bRu(s)C12bCO(g) obtained using torsion effusion cells. The Peq of rhodium carbonyl (Rh6(CO)16) decomposition, and Os3(CO)12 are also
plotted in this figure for comparison of the high molecular weight carbonyls.

244
D. Chandra et al. / Journal of Physics and Chemistry of Solids 66 (2005) 241–245
Ru3(CO)12 vaporization relationship for solid–gas given
in Eq. (14) and the decomposition partial pressure, pCO
the following equilibrium equations:
,
Co2ðCOÞ8ðsÞZCo2ðCOÞ8ðgÞ DG+ ðkPaÞZ84;270K198T
(23)
given in Eq. (18) are plotted in Fig. 2B as comparison with
those of Rh6(CO)16.
The vaporization of solid to gas Os3(CO)12 was
determined and there was no disproportionation and
equation may be simply written as:
Co2ðCOÞ8ðsÞZ2CoðCOÞ4ðgÞ DG+ ðkPaÞ
Z168;614K405:46T
ð24Þ
ð25Þ
Os3ðCOÞ12ðsÞ Z Os3ðCOÞ12ðgÞ
(20)
(21)
(22)
Co2ðCOÞ8ðsÞZ1=2Co4ðCOÞ12ðgÞC2COðgÞ DG+ ðkPaÞ
Z210;723K492:82T
PTðkPaÞ Z 15:003ð0:05Þ K7101ð20Þ=T
DG+ðkJ=molÞ ZKRT ln Kp Z 134:2 K0:021T
Thus, based on our results it can be concluded that unstable
monomer decomposes to metallic cobalt and carbon
monoxide gas, and may be represented as Co2(CO)8(s)/
1/4Co4(CO)12(g)CCo(CO)4(g)CCO(g)/2Co(s)C8CO.
The vaporization thermodynamics of Cr(CO)6 was also
determined and there was no disproportionation and
equilibrium equation may be summarized as follows
[10,12]:
The plot for the equilibrium vapor pressure of Os3(CO)12 is
also shown in Fig. 2B.
3.2. Thermodynamic vaporization properties of cobalt,
chromium, and tungsten carbonyls
Vapor pressure results of low molecular weight (MW,
!400 g/mol) carbonyls are shown below. Several dispro-
portionation of this solid Co2(CO)8 carbonyl was observed.
After detailed analyses discussed in Ref. [10,12], we
summarize the results leading to the determination of
CrðCOÞ6ðsÞ Z CrðCOÞ6ðgÞ
(26)
The vapor pressure equation is given by:
log PT ðkPaÞ Z 9:628ð0:25Þ K3434ð68Þ=T
(27)
Fig. 3. A van’t Hoff summary plot of vaporization studies for Rh, Ru, Os, Co, W, and Cr carbonyls showing the various equilibrium vapor pressures of pure
compounds as well as disproportionated species.

D. Chandra et al. / Journal of Physics and Chemistry of Solids 66 (2005) 241–245
245
DG+CrðCOÞ6; s/g
molecular weight species in certain temperature range.
This trend applies to non-disproportionated species and may
not apply to the disproportionated or decomposed species.
Z 64; 888 K144:01T ðJ=molÞ; where DH2+69 K
Z 64:82ð1:30Þ ðkJ=molÞ
(28)
In the case of vaporization of solid W(CO)6 also there was
no disproportionation, and a simple equation may be written
as follows:
Acknowledgements
The authors thank National Science Foundation, NSF
grant No. 0132556, and Sputtering Materials Inc., Reno,
NV, for support of this program. We also acknowledge US
Bureau of Mines/Center for Pyrometallurgy Generic Center
of University of Missouri.
WðCOÞ6ðsÞ Z WðCOÞ6ðgÞ
The vapor pressure given by:
(29)
log PT ðkPaÞ Z 11:096ð0:11Þ K4060ð31Þ=T
(30)
DG+WðCOÞ ZKRT ln Kp
6
References
Z 77; 714 K173T; where DH2+77 K
[1] H.W. Pfeifer, Hybrid and Selected Metal Matrix Composites, Chapter
6, 1. Renton, Ed, AIAA, 12900 Avenues of Americas, NY, 1977.
[2] D. Chandra, J.D. Mote, P.K. Predecki, A feasibility study of Ni
carbonyl CVD coatings for graphite aluminum composites, J Report
US, U.S. MERADCOM, Fort Belvoir, 1982, USDOD Contract No.
DAAK-70-81-C-0035, 1982.
Z 77:74ð0:59Þ ðkJ=molÞ
(31)
In general high molecular weight carbonyls Rh6(CO)16
(MW: 1065.6 g/mol), Os3(CO)16 (MW: 906.7 g/mol), and
Ru3(CO)12 (MW: 639.3 g/mol) showed lower vapor press-
ure than the lower molecular weight carbonyls Co2(CO)8
(MW: 342 g/mol), W(CO)6 (MW: 330.3 g/mol), and
Cr(CO)6 (MW: 220.1 g/mol) in the temperature range of
312.5–400 K. Vapor pressures of the high and low
molecular weight species from our and other investigators
data are plotted in Fig. 3; these include disproportionated
and decomposed carbonyls.
[3] D. Chandra, J.D. Mote, P.K. Predecki, A feasibility study of zone
melting methods for the fabrication of Al-graphite to U.S.
MERADCOM, USDOD Contract No. DAAK-70—82-C-0140, 1985.
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(Metals Abstract (1976)).
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[7] L.M. Windsor, A.A. Blanchard, J. Am. Chem. Soc. 56 (1934) 823.
[8] W. Hieber, E. Romberg, Z. Anor. Chem. 22 (1935) 332.
[9] J.J. Lander, L.H. Germer, Tech. publication #2259, AIME Metals
Division, Metals Technology (London) 14, #6,71, PI, 1948.
[10] M.L. Garner, D. Chandra, K.H. Lau, J. Phase Equilibria 20 (6) (1999)
565–572.
4. Conclusions
Vaporization thermodynamic measurements, using
gravimetric torsion-Knudsen effusion method, of Os, W,
Cr, Rh, Ru, Co did not have any dependence on molecular
weight on disproportionation or decomposition. The
Rh6(CO)16, Ru3(CO)12, and Co2(CO)8 showed dispropor-
tionation or decomposition; equilibrium Gibbs energies for
vaporization were obtained and are being tabulated in a
database. The vapor pressures of the high molecular weight
carbonyls were generally lower than those of lower
[11] N. Hagihara, M. Kumada, R. Okawara, Handbook of Organometallic
Compounds, first ed., W.A. Benjuman Inc., New York, 1968. 856.
[12] M.L. Garner, D. Chandra, K.H. Lau, J. Phase Equilibria 16 (1) (1995)
24–29.
[13] J. Margrave, Characterization of High Temperature Vapors, Wiley,
New York, 1961.
[14] M. Garner, Vapor pressures and thermodynamics properties tungsten,
chromium, cobalt and rhodium carbonyls, MS Thesis, University of
Nevada, Reno, 1994.