Standard enthalpies of formation of ThAl2, ThSi2, ThGe2 and Th5Sn3, determined by high-temperature direct synthesis calorimetry

Article abstract of DOI:10.1016/S0925-8388(00)01218-4

The standard enthalpies of formation of ThAl₂, ThSi₂, ThGe₂ and Th₅Sn₃ have been determined by direct synthesis calorimetry at 1473±2 K. The following values of ΔH_f^o, in kJ(mol a

Full text of DOI:10.1016/S0925-8388(00)01218-4

Journal of Alloys and Compounds 313 (2000) 148–153
L
www.elsevier.com/locate/jallcom
Standard enthalpies of formation of ThAl2, ThSi2, ThGe2 and Th5Sn3,
determined by high-temperature direct synthesis calorimetry
Junwen Wanga,b, Qiti Guoa,c, O.J. Kleppaa ,
*
aThe James Franck Institute, The University of Chicago, 5640 S Ellis Avenue, Chicago, IL 60637, USA
bGuangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong Province 510640, People’s Republic of China
cInstitute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou Province 550002, People’s Republic of China
Received 18 August 2000; accepted 26 August 2000
Abstract
The standard enthalpies of formation of ThAl2, ThSi2, ThGe2 and Th5Sn3 have been determined by direct synthesis calorimetry at
147362 K. The following values of DHof , in kJ(mol atom)21, are reported: ThAl2, 246.862.1; ThSi2, 255.662.0, ThGe2, 272.261.4,
and Th5Sn3, 263.862.0. The results are compared with predicted values from the Miedema model and with an earlier calorimetric value
for ThSi2. For systematic reasons the results are also compared with earlier values of DHof for the equivalent Ce and U compounds.
2000 Elsevier Science B.V. All rights reserved.
Keywords: Actinide compounds; Enthalpy; Calorimetry
1. Introduction
were reported for ThCo5, ThNi5, ThRu2, ThRh3, ThPd3,
ThIr2 and ThPt3. It is a well-known fact that calorimetric
studies of actinide compounds are scarce in the literature.
Our recent review of the published literature in this field
revealed that there are only about 6 calorimetric studies
since 1970 [7,9–13]. However, going back about 45 years
we found a much larger number of calorimetric studies.
This search indicated that the research activity in this field
was quite active during 1950s, and even more active
during 1960s. Kubaschewski et al. included in their books
‘Metallurgical Thermochemistry’ (5th Edition) [14] and
‘Materials Thermochemistry’ (6th Edition) [15] the then
available calorimetric values of the standard enthalpy of
formation for some actinide compounds, such as, e.g. the
intermetallic compounds of Th and U and the borides,
aluminides, carbides, silicides, oxides, sulfides, selenides,
nitrides, phosphides, arsenides, fluorides, chlorides, bro-
mides, and iodides of these metals. It is worth noting that
there is a comprehensive review of the thermodynamic
properties of the actinide elements and their compounds
published by Chiotti et al. in 1981 [16]. This review
included almost all the calorimetric values known at that
time.
During the past two decades, this laboratory has carried
out systematic studies of the thermochemistry of inter-
metallic compounds and of some metal–nonmetal com-
pounds. Our work in this field has been summarized
recently in several review papers [1–6]. These reviews
show that in the past two decades we have determined the
standard enthalpies of formation for 273 intermetallic
compounds of early transition metals and lanthanide metals
with late transition metals and the noble metals [4]. We
have also studied a comparable number of compounds of
transition metals and lanthanide metals with elements from
Groups IIIB and IVB in the periodic table [5].
However, compounds between actinide elements and
other elements have not been extensively studied in this
laboratory. Until recently, we carried out only one study in
this field: An investigation of the thermochemistry of
intermetallic compounds of U with Ru, Rh and Pd. Values
of DHof were reported for URu3, URh3 and UPd3 by Jung
and Kleppa in 1991 [7]. During the past year we have
completed a study of the thermochemistry of alloys of Th
with elements from Group VIII [8]. Seven values of DHfo
In contrast to the relatively scarce calorimetric studies,
there exist much more published data on the Gibbs free
energy of formation for U and Th compounds in the
literature [17–61]. These data were mostly obtained by
*Corresponding author. Tel.: 11-773-702-7284; fax: 11-773-702-
5863.
E-mail address: meschel@control.uchicago.edu (O.J. Kleppa).
0925-8388/00/$ – see front matter
PII: S0925-8388(00)01218-4
2000 Elsevier Science B.V. All rights reserved.

J. Wang et al. / Journal of Alloys and Compounds 313 (2000) 148–153
149
EMF measurements, but some were also derived from
vapor pressure measurements or obtained by other meth-
ods.
estimate from the relative intensity of the ThO2 111 peak
(100% intensity) on the XRD patterns of the Th powder
indicated that this powder might contain as much as about
3–5% of ThO2. However, taking the X-ray penetration
depth in the very heavy Th metal into consideration, the
XRD patterns obtained may mostly reflect the average
composition of the surface layer and a limited depth of the
layer underneath the surface of the Th particles. For this
reason, we believe that the content of ThO2 in our starting
material of Th powder should be less than the rough
estimate of 3–5%. The starting material of Th powder was
examined again by XRD after the completion of the study.
A comparison was made between the XRD patterns
obtained before and after the study. It was hard to tell
whether there was an increase in the amount of ThO2 in
the Th powder.
In the present investigation we carried out calorimetric
experiments on six compounds of Th with Al, an element
from Group IIIB, and with Si, Ge and Sn, elements from
Group IVB (ThAl2, ThSi2, Th3Ge2, Th0.9Ge2, ThGe2, and
Th5Sn3). However, we found that our experiments were
not successful for two of the thorium–germanium com-
pounds. This was due to the formation of too large
percentages of other phases than the desired phase. As a
result, in this paper we report only on the standard
enthalpies of formation for the following four compounds:
ThAl2, ThSi2, ThGe2, and Th5Sn3.
2. Experimental details and starting materials
The experiments were all carried out by the direct
synthesis method, using a single-unit differential micro-
calorimeter, which has been described in detail in an
earlier communication [62]. This micro-calorimeter was
maintained at 147362 K throughout the whole research
project. All experiments were conducted in an inert
atmosphere of argon gas. In order to eliminate possible
traces of oxygen and nitrogen, this gas was purified by
passing it through a fused-silica tube filled with pure
titanium sponge (|3 mm particles) maintained at about
1173 K. The actual synthesis reactions took place in boron
nitride (BN) crucibles.
3. Experimental results
The standard enthalpy of formation of the compound
ThmXn (X stands for a second element) is obtained from
the difference between two sets of measurements. In the
first set the following reaction takes place in the calorime-
ter.
mTh (s, 298 K) 1 nX (s, 298 K) 5 ThmXn (s, 1473 K) (1)
The products of reaction (1) were reused in a subsequent
set of measurements to determine the corresponding heat
contents
Calibration of the calorimeter was achieved by dropping
pieces of 2 mm diameter high purity copper of known
mass from room temperature into the calorimeter at
147362 K. The enthalpy of pure copper at this tempera-
ture, 46 465 J mol21, was taken from Hultgren et al. [63].
The calibrations were reproducible within 61%.
Samples were prepared by mixing two component
powders that were accurately weighed according to the
appropriate stoichiometry. This mixture was then pressed
into 4 mm diameter pellets. The purity of the materials
used for the direct synthesis ranged from 99.5% for Si and
Al, to 99.8% for Th, to 99.995% for Ge and 99.999% for
Sn. The particle sizes were 2100 mesh for Th and Sn,
2150 mesh for Ge, and 2325 mesh for Al and Si. All
materials were purchased from Johnson Matthey, ALFA
ÆSAR Group (Ward Hill, MA). The germanium powders
used for the preparation of ThGe2 was purchased as 210
mesh particles. This powder was ground in an agate mortar
and then sifted through a 150 mesh sieve.
ThmXn (s, 298 K) 5 ThmXn (s, 1473 K)
From Eq. (1) and Eq. (2) we have
(2)
mTh (s, 298 K) 1 nX (s, 298 K) 5 ThmXn (s, 298 K)
The standard enthalpy of formation is given by
DHof (ThmXn) 5 DH(1) 2 DH(2)
(3)
where DH(1) and DH(2) are the enthalpy changes per
mole of atoms for Eq. (1) and Eq. (2), respectively.
Table 1 summarizes all the experimental results obtained
for the four compounds. The reported values of DH(1) and
(2) are averages of four to six individual determinations
with standard deviations of d1 and d2, respectively. If the
standard deviation for the calibration is d3, the overall
uncertainty in the reported standard enthalpy of formation
]
1
is calculated from d 5 d 2 1d 2 1d 2.
œ
2
3
After the measurements, all the reacted samples were
examined by XRD. The results of these examinations are
listed in Table 2. This table shows that Th5Sn3 does not
have a standard XRD pattern listed in the ASTM powder
diffraction file. Moreover, there are no standard XRD
patterns for other Th–Sn alloys. Using VersaTerm-Pro
software, purchased from Abelbeck Software Company
and based on the crystallographic data in the Pearson’s
Th is a radioactive element and its powder is apt to be
oxidized in air, especially in the presence of moisture. The
handling of the Th powder used for sample preparation
was exactly the same as in our earlier paper [8]. The Th
powder was examined by X-ray powder diffraction (XRD)
at the beginning of this study. The XRD showed that the
Th powder contained detectable amount of ThO2. A rough

150
J. Wang et al. / Journal of Alloys and Compounds 313 (2000) 148–153
Table 1
Observed heats of reaction, average heat contents at 1473 K, and calculated standard enthalpies of formation, in kJ (mol atom)21a
Compound
Melting
point (8C)
Structure
type
DH(1)
DH(2)
DHof
ThAl2
ThSi2
ThGe2
Th5Sn3
1520
1850
1900
1780
AlB2
215.460.9 (5)
223.661.5 (6)
232.060.7 (6)
232.460.6 (6)
31.361.8 (4)
31.961.2 (5)
40.260.8 (5)
31.561.4 (6)
246.862.1
255.662.0
272.261.4
263.862.0
ThSi2
ThGe2
Mn5Si3
a Numbers in parentheses indicate numbers of experiments averaged.
handbook of crystallographic data [64], the ideal diffrac-
tion patterns for Th5Sn3, ThSn2, ThSn3 and Th5Sn4 were
generated. With these XRD patterns and the standard XRD
patterns for pure Th and Sn, we established that our
Th5Sn3 samples did not contain any unreacted elements.
However, it might contain less than 10% of Th5Sn4 and
about 5% of ThO2.
desired phase will not be more than 10%, we feel confident
in the reported enthalpy values. We believe that the
formation of up to 10% of other phases during the direct
synthesis reactions will not introduce any significant errors
in our reported enthalpy values. This is because the heats
of formation of these second phases usually are roughly
comparable in magnitude to the heat of formation of the
desired phase.
All the other three compounds have standard XRD
patterns. From Table 2 we see that a small amount of ThO2
was found in all these samples. We believe that most of the
ThO2 found in the reacted samples had its origin in the Th
starting materials. However, we cannot completely exclude
the possibility that a part of the ThO2 was formed during
the sample preparation. The pre-existing ThO2 will not
participate in the reaction. Hence, it will not contribute to
the difference between the enthalpy changes of Eq. (1) and
Eq. (2). The probability that additional ThO2 was formed
during the direct synthesis reaction is minimal since the
reaction took place in a very good inert atmosphere. It
should be noted that the XRD pattern we obtained for our
ThGe2 sample did not match the standard XRD pattern for
ThGe2, nor the standard XRD patterns for any other
Th–Ge compounds. We also did not find any reflections for
the component elements (Th and Ge) in this sample.
Therefore, we believe that our reacted ThGe2 sample may
have a different structure from the one that was used to
provide the standard XRD file for this compound.
In addition to ThO2, a small amount of a second phase
was found in our reacted ThAl2 sample. In the same way
we found a small amount of an unknown phase in our
ThSi2 sample. This is because there were a few weak
unknown reflections in the XRD pattern of this sample.
Although it is impossible to make a quantitative estimate
of the percentage of a second phase in the reacted samples
solely based on the XRD patterns, a very rough estimate
may be good enough for our purpose. Once we are positive
that the total percentage of the phases other than the
4. Discussion
Fig. 1 shows plots of our experimental results for the
standard enthalpies of formation of the four compounds
ThAl2, ThSi2, ThGe2 and Th5Sn3. All values are given in
kJ(mol atom)21. Also plotted in this figure are predicted
values calculated from the semi-empirical model of
Miedema and co-workers [65]. We found an earlier
calorimetric value for the compound ThSi2, 258.065.6
kJ(mol atom)21, listed by Kubaschewski et al. in [15] in
1993. This value is plotted along with our own result in
Fig. 1. There are no earlier calorimetric values for any of
the other three compounds in the literature. From this
figure, we see that the Miedema model gives somewhat
more exothermic values than we observed for all the four
compounds. However, the earlier calorimetric value for
ThSi2 is in good agreement with our own value
(255.662.0 kJ(mol atom)21).
In Fig. 2 we compare our standard enthalpies of
formation for ThAl2, ThSi2, ThGe2 and Th5Sn3 with the
available calorimetric data for their equivalent Ce and U
compounds. The sources of the calorimetric values used
for the comparison are as follows: (1) In 1985 Colinet et
al. measured DHof for most of the REAl2 compounds.
Their value for CeAl2 was 252.2 kJ(mol atom)21 [66]. (2)
In 1993 Kubaschewski et al. listed values of DHof for UAl2
and USi2. They were 232.961.8 and 243.260.8 kJ(mol
Table 2
Summary of X-ray diffraction examination results
Compound
XRD examination result
ThAl2
ThSi2
ThGe2
Th5Sn3
No unreacted elements. ThAl2 1minor ThAl3 1ThO2 (|5%)
No unreacted elements. ThSi2 1three weak unknown peaks1ThO2 (|5%)
No unreacted elements. ThGe2 1ThO2 (|5%). (See the text for detail)
No ASTM file. No unreacted elements. Th5Sn3 1minor Th5Sn4 1ThO2
(|5%). (Based on the calculated XRD patterns for alloys in Th–Sn system)

J. Wang et al. / Journal of Alloys and Compounds 313 (2000) 148–153
151
Fig. 1. Standard enthalpies of formation for ThAl2, ThSi2, ThGe2 and Th5Sn3, compared with predicted values from the Miedema model [65], and with an
earlier value for ThSi2 given by Kubaschewski et al. [15].
atom)21 [15], respectively. (3) In 1995 Meschel and
Kleppa measured DHfo for CeSi2. Their value was
260.562.0 kJ(mol atom)21 [67]. (4) In 1982 Borzone et
al. measured DHof for Ce5Sn3. Their value was 273.2
kJ(mol atom)21 [68]. These values are all plotted in Fig. 2
for the comparison.
However, in order to consider the systematic variation in
magnitude of DHof for each of the above four families of
compounds we need more information. We found no
earlier calorimetric values in the literature for CeGe2,
UGe2 and U5Sn3. For CeGe2, we believe that its DHfo
calorimetric value of DHof for U3Sn2 listed by Rand and
Kubaschewski, 232.6 kJ(mol atom)21 [69]. This value
was also derived from vapor pressure data [70]. Since the
composition of the compound U5Sn3 (XU 50.625) is very
close to the composition of the compound of U3Sn2 (XU 5
0.600), we plotted the value of DHof (U3Sn2) in Fig. 2d.
From Fig. 2 it is apparent that in all the four cases
studied the magnitude of the standard enthalpy of forma-
tion decreases only moderately from the Ce compounds to
the Th compounds, and then very substantially from the Th
compounds to the U compounds.
should not be very different from the value for CeGe1.6
,
which was measured by Meschel and Kleppa in 1995 by
high-temperature direct synthesis calorimetry [67]. Their
value of DHof for this compound is 275.661.9 kJ(mol
atom)21. This value has been plotted in Fig. 2c as an
acceptable approximation for CeGe2. For UGe2 we found
a non-calorimetric value of DHof given by Rand and
Kubaschewski, 229.1 kJ(mol atom)21 [69]. This value
was derived from the vapor pressure data of Alcock and
Grieveson [70], and is plotted in Fig. 2c. For U5Sn3, we
found no earlier value. However, we found a non-
Acknowledgements
This work has been supported by NSF under Grant
DMR-9726699 and has also benefited from the general
facilities of the University of Chicago Materials Research
Science and Engineering Center which is supported by the
NSF under Grant DMR-9808595. We want to thank our
colleagues Doctor Ian Steele and Doctor Joseph Pluth for

152
J. Wang et al. / Journal of Alloys and Compounds 313 (2000) 148–153
Fig. 2. (a) Standard enthalpy of formation for ThAl2, compared with an earlier value for CeAl2 given by Colinet et al. [66] and with an earlier value for
UAl2 listed by Kubaschewski et al. [15]. (b) Standard enthalpy of formation for ThSi2, compared with an earlier value for CeSi2 given by Meschel and
Kleppa [67], and with an earlier value for USi2 listed by Kubaschewski et al. [15]. (c) Standard enthalpy of formation for ThGe2, compared with an earlier
value for CeGe1.6, determined by Meschel and Kleppa [67], and with a non-calorimetric value for UGe2 given by Rand and Kubaschewski [69]. (d)
Standard enthalpy of formation for Th5Sn3, compared with an earlier value for Ce5Sn3, determined by Borzone et al. [68], and with a non-calorimetric
value for U3Sn2 given by Rand and Kubaschewski [69].
perature Thermochemistry of Materials, TMS 2000, the 129th
Annual Meeting and Exhibition, March 12–16, Nashville, Tennes-
see, USA, 2000, To be published.
their help with the X-ray diffraction examination of our
samples. We are also indebted to Doctor Susan Meschel for
the many discussions conducted between us while we
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