Lanthanide(III) halides: Thermodynamic properties and their correlation with crystal structure

Article abstract of DOI:10.1016/j.jallcom.2006.12.096

Temperatures and enthalpies of phase transitions of 17 lanthanide(III) halides determined experimentally are reported. Correlations were made between temperature of fusion of lanthanide(III) halides, on the one hand, and enthalpy of fusion, on the other, versus atomic number of lanthanide. According to this classification, the lanthanide(III) halides split into groups, as also do the corresponding crystal structures. A correlation between the crystal structure of lanthanide(III) halides and their respective entropy of fusion (or entropy of fusion + entropy of solid-solid phase transition) was inferred from the aforementioned features. Fusion in those halides with hexagonal, UCl₃-type and orthorhombic, PuBr₃-type, structures entails an entropy of fusion change (or entropy of fusion + entropy of solid-solid phase transition change) by 50 ± 4 J mol^-1 K^-1. The homologous entropy change within the group of halides having the rhomboedric, FeCl₃-type, structure, is smaller and equals 40 ± 4 J mol^-1 K^-1. Halides with monoclinic, AlCl₃-type, crystal structure constitute a third group associated to an even smaller entropy change upon fusion, only 31 ± 4 J mol^-1 K^-1. The halides with lower entropies of fusion also have a lower S_{1300 K} - S_{298 K} indicating either a higher degree of order in the liquid or a higher entropy in the solid at room temperatures.

Full text of DOI:10.1016/j.jallcom.2006.12.096

Journal of Alloys and Compounds 450 (2008) 167–174
Lanthanide(III) halides: Thermodynamic properties
and their correlation with crystal structure
a
b,∗
L. Rycerz , M. Gaune-Escard
a
Division of Chemical Metallurgy, Faculty of Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
b
Ecole Polytechnique, IUSTI CNRS UMR 6595, Technop oˆ le de Chateau-Gombert, 5 rue Enrico Fermi, 13453 Marseille Cedex 13, France
Received 13 November 2006; received in revised form 19 December 2006; accepted 20 December 2006
Available online 30 December 2006
Abstract
Temperatures and enthalpies of phase transitions of 17 lanthanide(III) halides determined experimentally are reported. Correlations were made
between temperature of fusion of lanthanide(III) halides, on the one hand, and enthalpy of fusion, on the other, versus atomic number of lanthanide.
According to this classification, the lanthanide(III) halides split into groups, as also do the corresponding crystal structures. A correlation between
the crystal structure of lanthanide(III) halides and their respective entropy of fusion (or entropy of fusion + entropy of solid–solid phase transition)
was inferred from the aforementioned features. Fusion in those halides with hexagonal, UCl
entails an entropy of fusion change (or entropy of fusion + entropy of solid–solid phase transition change) by 50 ± 4 J mol
entropy change within the group of halides having the rhomboedric, FeCl
-type, structure, is smaller and equals 40 ± 4 J mol
monoclinic, AlCl
3 3
-type and orthorhombic, PuBr -type, structures
−
1
−1
K
. The homologous
−
1
−1
3
K
. Halides with
−
1
−1
3
-type, crystal structure constitute a third group associated to an even smaller entropy change upon fusion, only 31 ± 4 J mol
K .
The halides with lower entropies of fusion also have a lower S1300 K − S298 K indicating either a higher degree of order in the liquid or a higher
entropy in the solid at room temperatures.
©
2007 Elsevier B.V. All rights reserved.
Keywords: Rare earth alloys and compounds; Crystal structure; Thermodynamic properties; Thermochemistry; Thermal analysis
1
. Introduction
2. Experimental
2
.1. Lanthanide halides preparation
Wide technological application of lanthanide halides requires
knowledge of their basic thermodynamic properties. However,
the thermodynamic data available in literature are scarce and it
is not uncommon that the values appearing in reference tables
come from estimation. In addition, experimental data in liter-
ature are very often contradictory. Thus, a multi-instrumental,
international program focussed on lanthanide halides and their
systems with alkali metal halides has been started several years
ago. The main purpose of this program is to contribute new
experimental data on unknown systems and, when relevant,
to assess the reliability of literature existing information. The
present work summarises results obtained on thermodynamic
properties of lanthanide(III) halides and correlates them with
crystal structure.
Taking into account fact that purity of compounds under investigation has a
major influence on data reliability, an enormous work has been done for the pur-
pose to obtain reliable methods for lanthanide halides synthesis, their chemical
analysis and appropriate materials selection. Lanthanide halides of high purity
(min 99.9%) were obtained as a result of above work. All the methods tested
and used for lanthanide(III) halides preparation were based on existing literature
information [1–3].
The lanthanide chlorides and bromides (LnX3) have been prepared from
oxides of 99.9% purity supplied by “Hydromet” Kowary, Poland (La2O3, CeO2,
Pr O ), by the Chemistry Department of the Lublin University (Nd O ) and
6
11
2
3
by Johnson Matthey (Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Er2O3, Tm2O3,
Yb2O3). The synthesis included the following steps:
-
dissolution of the oxide in hot concentrated hydrochloric (or hydrobromic)
acid,
-
-
crystallisation of the hydrate LnX3·6H2O,
partial dehydration of the LnX3·6H2O to LnX3·H2O,
∗
Corresponding author. Tel.: +33 4 91 10 68 87; fax: +33 4 91 11 74 39.
- final dehydration and distillation to form pure LnX3.
E-mail addresses: leszek.rycerz@pwr.wroc.pl (L. Rycerz),
Marcelle.Gaune-Escard@polytech.univ-mrs.fr (M. Gaune-Escard).
Details of the synthesis have been described elsewhere [4].
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.12.096

1
68
L. Rycerz, M. Gaune-Escard / Journal of Alloys and Compounds 450 (2008) 167–174
Another method of lanthanide chlorides and bromides preparation, which
wasusedandgivessatisfactoryresults, istheso-called“sinteringchlorination”or
sintering bromination”. Lanthanide oxide and ammonium chloride or bromide
Table 1
Temperatures and molar fusion enthalpies and entropies of lanthanide(III)
halides (in bold, authors results)
“
were mixed in a mole fraction Ln2O3:NH4X = 1:14, placed in quartz ampoule
and heated slowly under vacuum up to 600 K. After 3 h of reaction at 600 K,
the temperature was increased to 640 K and the residue of unreacted ammonium
halide was sublimed. Finally, the salt was melted and the ampoule with anhy-
drous LnX3 was transferred to a glove box. The crude lanthanide halide was
subsequently purified by distillation under reduced pressure in a manner similar
to that in the first method.
These methods could not be used for the synthesis of SmCl3, EuCl3 and
YbCl3 becauseofthedecompositiontendencyofthesecompounds. Thus, samar-
ium and europium chlorides were prepared by chlorination of the oxides with
thionyl chloride, SOCl2 [5]. A current of high purity argon saturated with SOCl2
vapour was flowed throughout the appropriate oxide (placed in a vertical quartz
reactor) at temperature 793–813 K for 24 h. This procedure, which revealed sat-
isfactory, was developed empirically, and no attempt was made to reduce the
duration of the chlorination cycle, although this may be possible.
LnCl3
Tfus (K)
ꢀfusHm
(
ꢀfusSm
−1 −1
)
Reference
kJ mol 1)
−
(J mol
K
LaCl3
1127 ± 1
1131
1128
1135
1127
55.7 ± 0.7
54.3
54.3
30.9
39.3
49.4 ± 0.7
48.1
48.1
27.2
34.9
[9]
[10]
[11]
[12]
[13]
[14]
1192
–
–
CeCl3
PrCl3
NdCl3
1086 ± 1
55.5 ± 0.6
53.5
51.1 ± 0.5
49.1
31.1
38.8
–
[9]
1
1
1
1
090
[15]
[16]
[13]
[17]
078 ± 3
33.5 ± 3.8
076
41.8
104
–
Ytterbium chloride, YbCl3, was obtained by long-time dehydration of
YbCl3·6H2O (about 150 h) at progressively increased temperatures, in atmo-
sphere of dry, gaseous hydrogen chloride.
Lanthanide triiodides were prepared by direct reaction of the lanthanide
metal with iodine vapour according to Druding and Corbett modified method
1061 ± 1
1059
52.1 ± 0.6
50.6
–
37.2
28.8
49.1 ± 0.6
47.8
–
35.3
27.6
[9]
[10]
[17]
[13]
[12]
1051 ± 1.2
1054
1043
[
6]. They were purified by a vacuum sublimation at temperature about 40 K
1032 ± 1
48.1 ± 0.5
50.2
46.6 ± 0.5
48.6
32.4
40.6
–
[9]
lower than melting temperature of LnI3 during 60–70 h. The details of this
synthesis have been described elsewhere [7]. The parameters of sublimation
were developed empirically and no attempt was made for their optimisation.
Because of their hygroscopicity all reaction products were handled in a glove
box and stored in sealed glass ampoules.
1
1
1
1
032
[10]
[16]
[13]
[18]
033 ± 4
33.5 ± 3.8
019
41.4
013
–
SmCl3
EuCl3
950 ± 2
47.6 ± 0.7
50.3 ± 0.7
[5]
9
9
9
50
50
55
46.0
–
48.4
–
[19]
[20]
[21]
2
.2. Measurements
–
–
The temperatures and enthalpies of phase transitions of LnX3 compounds
were measured with a Setaram DSC 121 differential scanning calorimeter. The
apparatus, its calibration and measurement procedure were described in detail
previously [7,8]. Samples of 300–500 mg were contained in quartz ampoules
894 ± 1
45.0 ± 0.6
50.3 ± 0.7
[22]
[23]
[24]
[20]
8
8
9
97 ± 1.5 51.0 ± 2.1
56.9 ± 2.3
96
01
–
–
–
–
(
about 6 mm diameter, 15 mm length) sealed under reduced pressure of argon.
The sidewalls of ampoules were ground in order to fit the cells snugly into the
heat flow detector of DSC. Experiments were conducted at heating and cooling
rates ranging between 5 and 1 K min
Another technique, high-temperature Calvet microcalorimetry, was used to
determine the melting temperature and enthalpy of LaCl3, CeCl3, TmCl3 and
TbBr3, which were out of the DSC experimental temperature range. The Calvet
calorimeter, its calibration and measurement technique were described in detail
in refs. [7–9].
GdCl3
TmCl3
LaBr3
873 ± 1
40.6 ± 0.6
46.5 ± 0.7
[9]
[15]
−1
875
40.5
46.3
.
1092 ± 1
094
35.6 ± 0.6
32.6 ± 0.6
[9]
[11]
1
37.6
34.4
1058 ± 1
54.2 ± 0.5
54.4
33.5
51.2 ± 0.5
51.3
31.5
[25]
[15]
[26]
1
1
061
062
NdBr3
TbBr3
LaI3
956 ± 1
45.5 ± 0.4
45.6
33.5
47.6 ± 0.4
47.7
35.1
[7,8]
[15]
[26]
3
. Results and discussion
9
9
55
55
3
.1. Temperatures and enthalpies of phase transitions of
1103 ± 1
37.4 ± 0.6
37.7
37.7
33.9 ± 0.5
34.2
34.2
[7,8,27]
[26]
[11]
lanthanide(III) halides
1
1
100
103
DSC cooling thermograms showed systematic undercooling
for all lanthanide(III) halides. Melting temperatures determined
from heating curves were from 5 K (PrCl3) up to 40 K (GdCl3)
larger that those indicated by cooling curves [9]. Thus, all tem-
peratures and enthalpies of phase transitions were determined
from heating curves. Experimental results are summarised in
Tables 1 and 2.
1047 ± 1
50.2 ± 0.8
56.1
33.5
47.9 ± 0.7
53.4
32.0
[7]
1
1
1
051
045
062
[15]
[26]
[28]
56.1
52.8
CeI3
1033 ± 1
033
1034
50.3 ± 0.5
51.9
51.0
48.7 ± 0.4
50.2
49.3
[7]
[15]
[28]
1
The older literature data concerning the lanthanide chlorides
are collected in Gmelin Handbook of Inorganic Chemistry [2].
They are characterised by significant discrepancies. The various
melting temperatures of PrCl3 (1038 K [32] and 1096 K [33]),
SmCl3 (938 K [34] and 1020 K [35]), GdCl3 (875 K [15] and
given as illustrative examples. The complete literature data sets
of melting temperatures can be found in the Gmelin Handbook
of Inorganic Chemistry [32] for lanthanide(III) bromides and
iodides.
9
43 K [36]) or TmCl3 (1085 ± 2 K [37] and 1139 K [33]) can be

L. Rycerz, M. Gaune-Escard / Journal of Alloys and Compounds 450 (2008) 167–174
169
Table 2
Temperatures and molar enthalpies and entropies of phase transitions in TbCl3, DyCl3, ErCl3 and NdI3 (in bold, authors results)
LnCl3
Ttrs (K)
ꢀtrsHm
(
ꢀtrsSm
(J mol K
Tfus (K)
ꢀfusHm
−1
ꢀfusSm
(J mol K
ꢀtrsSm + ꢀfusSm
−1
Reference
kJ mol 1)
−
1
−1
)
(kJ mol
)
1
−1
)
(J mol K
1
)
DyCl3
611 ± 2
1.4 ± 0.1
2.3 ± 0.2
919 ± 1
924
22.8 ± 0.2
25.5
24.8 ± 0.3
27.6
27.1 ± 0.5
27.6
[7,9]
[15]
[29]
[30]
–
–
–
–
–
–
–
–
–
929.4 ± 6.8
36.5 ± 7.3
39.3 ± 8.2
39.3 ± 8.2
928
–
–
–
ErCl3
TbCl3
1025 ± 2
5.3 ± 0.1
5.2 ± 0.1
1046 ± 1
31.1 ± 0.3
29.7 ± 0.3
34.9 ± 0.4
[9]
[15]
–
–
–
1049
32.6
31.0
31.0
790 ± 1
13.8 ± 0.2
14.2
23.1 ± 4.6
17.5 ± 0.3
18.2
29.1 ± 6.0
854 ± 1
855
856.9 ± 4.7
20.8 ± 0.4
19.4
31.6 ± 6.3
24.4 ± 0.4
22.7
36.9 ± 7.6
41.9 ± 0.7
40.9
66.0 ± 13.6
[31]
[15]
[29]
7
7
83
93.1 ± 4.7
NdI3
862 ± 1
13.3 ± 0.4
15.4 ± 0.5
1058 ± 1
36.0 ± 0.4
34.0 ± 0.4
49.4 ± 0.9
[7]
[15]
8
47
13.8
16.3
1060
41.4
39.1
55.4
As pointed out above, supercooling effect influenced the
temperatures of fusion determined from cooling DSC curves.
They were significantly lower than those obtained from heating
curves. The values of enthalpies of fusion and crystallisation
obtained from the heating and cooling curves were almost the
same, with exception of LaCl3. Several measurements per-
formed on this compound always gave the same result: the
enthalpy of crystallisation determined from the cooling curves
was larger than the enthalpy of fusion obtained from the heat-
ing curve. This, quite unrealistic result can be explained on the
basis of literature information. According to Savin [38], the
formation of a “chain structure” takes place during the melt-
ing of LaCl3. The destruction of this structure with increasing
temperature is connected with very high values of the heat capac-
reduced chlorine content with respect to the stoichiometric com-
position (EuCl2.97 instead of EuCl3). Also visual observations
confirmed chlorine release above molten samples. This ther-
mal decomposition of EuCl3 is likely to occur at temperatures
very close to melting. When EuCl3 decomposition takes place,
EuCl2 is formed and the sample under investigation turns into
a EuCl3–EuCl2 mixture. On the subsequent heating thermo-
gram, two thermal effects appear. They can be assessed to
the eutectic temperature T = 804 ± 1 K and liquidus tempera-
ture T = 887 ± 3 K of the EuCl3–EuCl2 system. The eutectic
temperature is in good agreement with that reported earlier by
Laptev et al. [39]. The same authors also concluded about ther-
mal decomposition of EuCl3 upon melting [40]. Therefore, this
is the single peak observed upon primary heating that corre-
sponds to real melting of EuCl3, since it was obtained from
a decomposition-free initial sample. This melting temperature
of EuCl3 determined from our experiments (894 ± 1 K) is in
a good agreement with the data of Kulagin and Laptev [23]
and Moriatry [24] (897 ± 1.5 and 896 K, respectively). Another
literature source [20] gives the value 901 K as the melting
temperature of EuCl3, but it was obtained by a questionable
extrapolation of experimental results on a partially dissociated
compound. It is also from the results obtained during pri-
mary heating runs that we determined the fusion enthalpy of
−
1
−1
). Taking into account this so-called
ity (up to 350 J mol
K
“after-fusion” effect, it is possible to explain the larger values
of ꢀcrystHm obtained from the cooling curves. During cooling,
there is no formation of a “chain structure” of LaCl3. It is likely
that a metastable liquid phase is formed instead which crys-
tallises directly to the solid phase. Thus, the value obtained for
ꢀcrystHm is the sum of ꢀfusHm and the enthalpy related to the
“after-fusion” effect.
Temperatures and enthalpies of melting of LaCl3, CeCl3,
PrCl3, NdCl3 and GdCl3 determined in this work are in a very
good agreement with Dworkin and Bredig results [10,15] that
were obtained by calorimetric method. Other literature data con-
cerning mainly fusion enthalpy differ significantly from our
results, but they were obtained by comparative DTA method,
which is less precise than calorimetric one. A good agreement
of temperature and fusion enthalpy with literature data [11,19]
was obtained for SmCl3 and TmCl3.
−
1
EuCl3, ꢀfusHm = 45.0 ± 0.7 kJ mol . This value is smaller by
about 10% than the previous semi-quantitative fusion enthalpy
obtained by DTA [23].
TbCl3 is the first compound in the lanthanide chloride series
that undergoes a solid–solid phase transition. There are numer-
ous, but at the same time contradictory literature data concerning
the structure of TbCl3. According to Forrester et al. [41],
TbCl3 has an orthorhombic PuBr3-type structure whereas Sped-
ding and Daane [42] reported monoclinic structure. Weigel
and Scherer [43] argue for the dimorphism of TbCl3 with
an orthorhombic structure at low temperatures and hexagonal,
UCl3-type structure at higher temperatures. According to the
newest literature data [44,45], the structure of TbCl3 depends on
the way it has been elaborated. Accordingly, either the hexago-
nal (UCl3-type) or orthorhombic (PuBr3-type) modification can
Analysis of several DSC runs performed on EuCl3 showed
an interesting phenomenon [22]. Upon primary heating, heating
thermograms show one endothermic effect at Tons = 894 ± 1 K
(
melting of EuCl3), while an additional effect appears at
Tons = 796 ± 2 K upon cooling. The thermograms from fur-
ther heating–cooling cycles evidenced existence of two thermal
effects on heating at 804 ± 1 and 887 ± 3 K, respectively. Chem-
ical analysis of EuCl3 samples after primary heating showed

1
70
L. Rycerz, M. Gaune-Escard / Journal of Alloys and Compounds 450 (2008) 167–174
be obtained. At temperature 640–670 K, the hexagonal modi-
fication transforms irreversibly into orthorhombic, PuBr3-type
structure, which successively undergoes reversible solid–solid
phase transition at 790 K to high temperature (orthorhom-
bic, trirutile-type) modification. When cooled below 640 K,
TbCl3 does not transform into hexagonal (UCl3-type) struc-
ture. A metastable phase (orthorhombic, PuBr3-type structure)
can be observed instead. The only possibility to obtain hexag-
onal, UCl3-type, structure of TbCl3 is when it is prepared at
temperatures below 640 K. Taking into account all above infor-
mation and fact that the last step of our TbCl3 synthesis was
distillation at temperature of about 100 K higher than melt-
ing temperature, one can assume that all measurements were
performed on orthorhombic, PuBr3-type modification of ter-
bium(III) chloride. Our experimental temperature of solid–solid
phase transition (orthorhombic, PuBr3-type → orthorhombic,
trirutile-type) 790 ± 1 K is in good agreement with Simon
and Urland data [45] and slightly higher than that given by
Dworkin and Bredig [15] or Mitra and Seifert [44] (783 and
Our results for lanthanide(III) iodides are presented in
Tables 1 and 2. The melting temperature of LaI3 (1047 ± 1 K) is
slightly less than given by Dworkin and Bredig [15] (1051 K),
and significantly lower than reported by Kubaschewski et al.
[28] (1062 K). The molar enthalpy of fusion is about 12% lower
than literature data [15,28]. A good agreement with literature
data [15,28] was found for CeI3. Our results obtained on NdI3
differ from those presented in literature [15]. Neodymium(III)
iodide undergoes the solid–solid phase transition from a low
temperature, orthorhombic, PuBr3-type structure to a high tem-
perature modification (hexagonal). According to Dworkin and
Bredig [15] this transition takes place at 847 K, whereas the
temperature at which we observed it is higher by 15 K. How-
ever, our experimental enthalpy change related to this transition
−
1
(13.3 ± 0.4 kJ mol ) is in good agreement with above liter-
ature data. Our temperature of fusion agrees very well with
literature, whereas our molar enthalpy is smaller than literature
−1
data [15] (1058 ± 1 and 1060 K, 36.0 ± 0.4 and 41.4 kJ mol
,
respectively).
7
84 K, respectively). Our molar enthalpy results for this tran-
sition, ꢀtrsHm, as well as temperature and enthalpy of fusion,
fusHm, agree well with Dworkin and Bredig data [15]. The
3.2. Lanthanide(III) chlorides: dependence of temperature
ꢀ
and enthalpy of fusion on atomic number of lanthanide
enthalpy data given by Goryushkin et al. [29] are signifi-
cantly higher, but they were obtained by a DTA comparative
method.
By a careful and systematic analysis of our experimental
melting temperature data on lanthanide(III) chlorides, it was
possible to establish a very interesting dependence of this tem-
perature on the atomic number of lanthanide. This correlation is
displayed in Fig. 1.
To the best of our knowledge, the solid–solid phase transition
in DyCl3 observed at 611 K, with an enthalpy change, ꢀtrsHm, of
−
1
about 1.4 ± 0.1 kJ mol , has not been reported in literature and
was found for the first time [7,9]. It is probably a transition from
the monoclinic, AlCl3-type, to the orthorhombic, PuBr3-type,
structure. However, it should be pointed out that information
concerning the crystal structure of DyCl3 is not clear, similar in
that to TbCl3. Templeton and Carter [46] claim that only a single
allotropic variety of DyCl3 exists, i.e. monoclinic, AlCl3-type,
whereas Bommer and Hohman [47] argue for two modifica-
tions: namely the monoclinic, AlCl3-type and the orthorhombic,
PuBr3-type. Weigel and Wishnewsky [48] found that mono-
clinic, AlCl3-type structure of DyCl3 transforms at 790 K under
high pressure into orthorhombic, PuBr3-type structure.
The melting temperatures of lanthanide chlorides vary
according to two different and marked linear regimes:
-
-
they decrease from lanthanum (Z = 57) up to terbium (Z = 65)
and then;
they increase starting from terbium up to lutetium (Z = 71).
This correlation was used to estimate the melting tempera-
ture of PmCl3. This temperature was estimated as 983 K (black
square in Fig. 1). The only available literature data on PmCl3
indicates 1010 K [2] for the melting temperature, value that
has not been used for the previous correlation. Taking into
account the aforementioned literature discrepancies for most
lanthanide chlorides, it is anticipated this estimation to be
reliable.
Melting temperature and enthalpy of ErCl3 agree very well
with Dworkin and Bredig data [15]. However, an extra thermal
effect, that we ascribed to a solid–solid phase transition, was
found in ErCl3 at the temperature 1025 ± 1 K. The origin of this
transition is actually unknown, as literature data [2,47] report
only one, monoclinic, AlCl3-type structure. It was suggested by
Seifert [49] this effect may result from the interaction of ErCl3
with quartz rather than from solid–solid phase transition.
Results obtained for lanthanide(III) bromides (LaBr3, NdBr3
and TbBr3) are collected in Table 1. The only thermal effect
observed on DSC curves is melting at the temperatures 1058 ± 1,
9
55 ± 1 and 1103 ± 1 K, respectively. These values are in
excellent agreement with literature data [11,15,26]. Melting
enthalpies of LaBr3 and NdBr3 are almost identical to those
obtained by Dworkin and Bredig [15], but are significantly larger
than those given by Wick and Block [26]. The fusion enthalpy
of TbBr3 is in excellent agreement with older literature data
Fig. 1. Dependence of melting temperature of lanthanide(III) chlorides on lan-
[
11,26].
thanide atomic number (Z): black square denotes PmCl3.

L. Rycerz, M. Gaune-Escard / Journal of Alloys and Compounds 450 (2008) 167–174
171
chlorides of UCl3-type crystal structure is equal to 49.4 ± 0.7,
5
4
1.1 ± 0.5, 49.1 ± 0.6, 46.6 ± 0.5, 50.3 ± 0.7, 50.3 ± 0.7 and
1
−
−1
6.5 ± 0.7 J mol
K for LaCl3, CeCl3, PrCl3, NdCl3, SmCl3,
EuCl3 and GdCl3, respectively. In the case of chlorides with
monoclinic, AlCl3-type structure this change, or sum of entropy
of solid–solid phase transition and melting (ꢀtrsSm + ꢀfusSm), is
2
7.1 ± 0.5 (DyCl3), 34.9 ± 0.4 (ErCl3) and 32.6 ± 0.6 (TmCl3)
−
1
−1
K . The change of entropy, ꢀtrsSm + ꢀ Sm, for
J mol
fus
TbCl3, that has PuBr3-type crystal structure at low temperature
−
1
−1
s, is about 41.9 ± 0.7 J mol
K .
Fig. 2. Dependence of molar enthalpy of melting of lanthanide(III) chlorides on
lanthanide atomic number (Z): black square denotes PmCl3.
This correlation between fusion entropy and structure of
lanthanide(III) chlorides was used to assess literature data con-
cerning the fusion enthalpy of YbCl3. According to Brewer
−
1
The dependence of molar melting enthalpy on the lanthanide
atomic number was also examined for all the LnCl3 lanthanide
chlorides. It is presented in Fig. 2.
[
[
51], this enthalpy is 37.7 kJ mol , whereas Laptev et al.
−
1
52] reported 63.6 kJ mol . The melting temperature in both
cases was the same (1148 K). Taking into account the fact
that this chloride has a monoclinic, AlCl3-type structure
As shown for melting temperatures, this correlation clearly
evidences that lanthanide chlorides fall into two groups: the first
one, from LaCl3 up to GdCl3, and the second one, from ter-
bium to lutetium, in which molar enthalpies of fusion linearly
decrease and increase, respectively, with increasing lanthanide
atomic number. From this, it was thus possible to estimate the
molar melting enthalpy of PmCl3, for which no data is available
[
3
2], one can expect that fusion entropy should be about
−
1
−1
K . The entropy determined from Laptev data
1 ± 4 J mol
−
1
−1
K
is noticeably larger (55.4 J mol
Brewer data (32.8 J mol
), whereas that based on
−1
K ) agrees very well with the theo-
−
1
retical value for chlorides with AlCl3-type structure. Thus, it is
evident that the fusion enthalpy reported on YbCl3 by Brewer
[51] is more reliable and should be preferred.
−
1
and the value ꢀfusHm = 48.0 kJ mol was obtained.
3
.3. Correlation between fusion entropy and crystal
3.4. Lanthanide(III) bromides: dependence of temperature
and enthalpy of fusion on atomic number of lanthanide
structure of lanthanide(III) chlorides
The partition of lanthanide(III) chlorides into two groups,
as shown above, agrees very well with their crystal structure.
Chlorides of La, Ce, Pr, Nd, Sm, Eu and Gd have the hexag-
onal, UCl3-type structure, whereas chlorides of Ho, Er, Tm,
Yb and Lu crystallise in monoclinic, AlCl3-type structure [2].
Dysprosium and terbium chlorides undergo a solid–solid phase
transition from low temperature to high temperature modifica-
tion. In the case of DyCl3 this transition is from monoclinic,
AlCl3-type, to orthorhombic, PuBr3-type, crystal structure. Ter-
bium(III) chloride transforms from orthorhombic, PuBr3-type
to orthorhombic, trirutile-type structure.
This correlation between the fusion parameters and lan-
thanide atomic number was of course an incentive to look for
similar dependencies in the family of lanthanide(III) bromides.
Our experimental data on LaBr3, NdBr3 and TbBr3 (Table 1),
together with those from literature for other bromides (Table 3)
were used in this analysis.
The dependence of temperature and enthalpy of fusion of
LnBr3 on atomic number (Z) of lanthanide is presented in
Figs. 3 and 4.
The melting temperatures of lanthanide bromides vary
according to two different and marked linear regimes:
In 1971, Dworkin and Bredig [15] evidenced the relation-
ship between crystal structure and entropy of fusion for a
few lanthanide(III) halides. The fusion entropy, ꢀfusSm, of
hexagonal, UCl3-type structure, lanthanide halides is about
- they decrease from lanthanum (Z = 57) up to neodymium
(Z = 60) and then;
−
1
−1
. However, the halides of monoclinic, AlCl3-
Table 3
5
0 ± 4 J mol
K
Temperatures, molar enthalpies and molar entropies of melting of lanthanide(III)
bromides, literature data
type structure have a significantly lower fusion entropy of about
3
and proposed a classification of trivalent chlorides of transition
metals into three groups. The first group involved the chlo-
rides of UCl3-type structure, with significantly larger fusion
−
1
−1
K . Tosi et al. [50] developed this observation
1 ± 4 J mol
LnBr3
Tfus (K)
ꢀfusHm
(
ꢀfusSm
(J mol K
Reference
kJ mol 1)
−
1
−1
)
CeBr3
PrBr3
1005
966
937
54.2
47.3
33.5
33.5
38.0
46.4
50.2
50.6
53.5
60.2
53.9
49.0
35.7
34.2
35.9
40.3
42.1
41.4
43.5
48.8
[15]
[15]
[26,53]
[11]
[15]
[26]
[15]
[53]
[11]
[11]
−
1
−1
) than the chlorides of
entropy (of about 50 ± 4 J mol
K
SmBr3
EuBr3
GdBr3
DyBr3
HoBr3
ErBr3
TmBr3
LuBr3
the second group, i.e. having monoclinic, AlCl3-type struc-
978
−
1
−1
K ). The third group consisted of only
ture (31 ± 4 J mol
1058
1152
1192
1223
1228
1233
one compound, TbCl3 chloride, with orthorhombic, PuBr3-
type structure. Fusion entropy of this chloride is intermediate
−
1
−1
between the first and second group (40.9 J mol
K ). Our
findings are in an excellent agreement with Tosi’s classifica-
tion. The change of entropy, ꢀfusSm, upon melting of lanthanide

1
72
L. Rycerz, M. Gaune-Escard / Journal of Alloys and Compounds 450 (2008) 167–174
PmBr3 fusion temperature decreases with Z, SmBr3 and EuBr3
have increasing fusion temperature.
A similar behaviour was observed for the evolution of fusion
enthalpy. Bromides with UCl3-type structure show decreasing
fusion enthalpy versus Z, whereas in the FeCl3-type bromide
group fusion enthalpy increases with the lanthanide atomic
number Z. Here, also the PuBr3-type bromides show a dual evo-
lution of the melting enthalpy versus Z: decrease for NdBr3, and
increase for SmBr3 and EuBr3. However, it should be pointed
out that both temperatures and enthalpies of fusion of SmBr3 and
EuBr3 are coming from estimation only. Due to the instability of
these bromides and their tendency to decomposition into SmBr2
and EuBr2 it is impossible to determine them experimentally.
Fig. 3. Fusion temperature of LnBr3 dependence on atomic number of lan-
thanide: open circles, literature data; black circles, our experimental data; black
square denotes PmBr3.
-
they increase starting from samarium up to lutetium
3.5. Correlation between fusion entropy and structure of
lanthanide(III) bromides
(Z = 71).
Tosi et al. [50] partition of lanthanide chlorides into three
groupsaccordingtofusionentropyandstructurecriteriadoesnot
fully match the lanthanide bromides. The fusion of UCl3-type
bromides(LaBr3, CeBr3 andPrBr3)correspondstolargeentropy
This correlation was used to estimate the melting temperature
of PmBr3 as 910 K (black square in Fig. 3).
The dependence of molar fusion enthalpy on lanthanide
atomic number divides lanthanide bromides into two groups.
However, the composition of these groups is different from that
observed for chlorides. Only four bromides belong to the group
with decreasing enthalpy (LaBr3, CeBr3, PrBr3 and NdBr3).
All others pertain to the second group, in which molar fusion
enthalpy increases with lanthanide atomic number.
These features observed for the LnBr3 are undoubtedly in
relation with their structure. All the lanthanide(III) bromides
can be divided into three groups, namely:
−
1
−1
(Table 1), similar in that
changes of about 50 ± 4 J mol
K
to the isostructural chlorides. However, for NdBr3 (PuBr3-type
−1
−1
K ) to
structure) this change is very close (47.6 ± 0.4 J mol
that of UCl3-type structure bromides, whereas in TbCl3 (also
PuBr3-type structure) the entropy change, ꢀtrsSm + ꢀfusSm,
−
1
−1
is significantly lower (41.9 ± 0.7 J mol
K ). The fusion
entropy of NdBr3 confirms Dworkin and Bredig conclusion [15],
that UCl3- and PuBr3-type lanthanide(III) halides melt with
−1
−1
. However,
molar entropy change of about 50 ± 4 J mol
K
the fusion entropy reported for SmBr3 and EuBr3, which have
-
-
-
bromides with hexagonal, UCl3-type structure (LaBr3, CeBr3
and PrBr3);
bromides with orthorhombic, PuBr3-type structure (NdBr3,
PmBr3, SmBr3 and EuBr3);
bromides with rhombohedric FeCl3-type structure (GdBr3,
TbBr3, DyBr3, HoBr3, ErBr3, TmBr3, YbBr3 and LuBr3) [3].
−1
−1
K
also PuBr3-type structure was only 34.2 and 35.9 J mol
Table 3). It is likely that these results are questionable because
(
of the high thermal instability of SmBr3 and EuBr3. The authors
of the present work have observed almost complete thermal
decomposition of EuBr3 at temperatures significantly lower than
melting temperature [7,8], thus invalidating these previous data.
Lanthanide(III) bromides having FeCl3-type structure melt with
The fusion temperature of UCl3-type structure bromides
−
1
−1
.
entropy change of about 40 ± 4 J mol
K
decreases with increasing lanthanide atomic number, whereas in
the FeCl3-type structure bromide group temperature increases
with lanthanide atomic number Z. However, the evolution of
fusion temperatures versus Z appeared more complex for lan-
thanide(III) bromides with PuBr3-type structure: NdBr3 and
3.6. Fusion temperature of lanthanide(III) iodides
dependence on atomic number of lanthanide
As for the lanthanide bromides LnBr3, the dependence of
fusion temperature on atomic number of lanthanide was inves-
tigated for the lanthanide iodides LnI3, using our experimental
results for LaI3, CeI3 and NdI3 (Tables 1 and 2) and literature
data for the other iodides (Table 4).
A similar dependence of fusion enthalpy could not be tested
due to the lack of data for many iodides. Fusion temperature
dependence on lanthanide atomic number is presented in Fig. 5,
showing the partition of iodides into two groups: the first one
with decreasing fusion temperature when lanthanide atomic
number increases (LaI3, CeI3 and PrI3) and the second one, with
increasing fusion temperature (iodides from NdI3 up to LuI3). In
the first group, iodides have a PuBr3-type crystal structure and in
Fig. 4. Molar enthalpy of fusion of lanthanide(III) bromides vs. atomic number
of lanthanide: open circles, literature data; black circles, our experimental data.
the second one, with exception of NdI , a FeCl -type structure
3
3

L. Rycerz, M. Gaune-Escard / Journal of Alloys and Compounds 450 (2008) 167–174
173
Table 4
Table 5
Temperatures, enthalpies and entropies of fusion of lanthanide(III) iodides,
literature data
Crystal structure and entropy of fusion of lanthanide(III) halides
LnX3
Crystal structure
ꢀfusSm
ꢀtrsSm + ꢀfusSm
LnBr3
Tfus (K)
ꢀfusHm
(
ꢀfusSm
(Jmol K
Reference
kJ mol 1)
−
1
−1
)
LaCl3
CeCl3
PrCl3
NdCl3
SmCl3
EuCl3
GdCl3
LaBr3
CeBr3
PrBr3
NdBr3
LaI3
UCl3
UCl3
UCl3
UCl3
UCl3
UCl3
UCl3
UCl3
49.4 ± 0.7
51.1 ± 0.5
49.1 ± 0.6
46.6 ± 0.5
50.3 ± 0.7
50.3 ± 0.7
46.5 ± 0.7
51.2 ± 0.5
51.9
49.4 ± 0.7
51.1 ± 0.5
49.1 ± 0.6
46.6 ± 0.5
50.3 ± 0.7
50.3 ± 0.7
46.5 ± 0.7
51.2 ± 0.5
51.9
PrI3
1011
1123
1150
1203
1228
1251
1267
1288
1294
1323
53.1
–
–
54.0
57.3
–
–
–
–
–
52.5
–
–
44.9
46.6
–
–
–
–
–
[15]
[42]
[11]
[15]
[15]
[42]
[42]
[42]
[42]
[42]
SmI3
EuI3
GdI3
TbI3
DyI3
HoI3
ErI3
UCl3
UCl3
48.9
48.9
TmI3
LuI3
PuBr3
PuBr3
PuBr3
PuBr3
PuBr3
PuBr3 (o-romb.)
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
AlCl3
47.6 ± 0.4
47.9 ± 0.7
48.7 ± 0.4
52.7
36.0 ± 0.4
24.4 ± 0.4
36.0
47.6 ± 0.4
47.9 ± 0.7
48.7 ± 0.4
52.7
49.4 ± 0.9
41.9 ± 0.7
36.0
CeI3
PrI3
NdI3
[
3,15]. Neodymium(III) iodide has also PuBr3-type structure,
TbCl3
GdBr3
TbBr3
DyBr3
HoBr3
ErBr3
TmBr3
LuBr3
GdI3
but belongs to the second group. This exception may be caused
by the solid–solid phase transition that takes place in this com-
pound, which according to literature data is from PuBr3-type to
hexagonal structure [54].
On the basis of our experimental data (Tables 1 and 2) and
literature data (Table 4) on fusion enthalpy of lanthanide(III)
iodides one can conclude that fusion of iodides with PuBr3-
type structure is related to a global entropy change, ꢀfusSm or
33.9 ± 0.5
33.9 ± 0.5
40.3
40.3
41.8
41.4
43.5
48.8
41.8
41.4
43.5
48.8
44.8
45.2
DyCl3
HoCl3
24.8 ± 0.3
27.1 ± 0.5
−
1
−1
(LaI3, CeI3, PrI3,
AlCl3
30.5
30.5
ꢀ
trsSm + ꢀfusSm, of about 50 ± 4 J mol
K
ErCl3
TmCl3
YbCl3
AlCl3
AlCl3
AlCl3
29.7 ± 0.3
32.6 ± 0.6
28.5
34.9 ± 0.4
32.6 ± 0.6
28.5
NdI3). The entropy change for iodides with FeCl3-type crystal
−
1
−1
.
structure, ꢀtrsSm + ꢀfusSm, is about 45 J mol
K
3
.7. Correlation between crystal structure and entropy of
fusion of lanthanide(III) halides—summary
halides consists of halides that have monoclinic, AlCl3-type
crystal structure and the entropy change on melting is about
−
1
−1
The correlation between lanthanide atomic number, crystal
structure of lanthanide halides and their fusion temperatures
and fusion enthalpies showed that fusion entropy (or the sum
of transition and fusion entropy) of lanthanide(III) chlorides,
bromides and iodides is related to their crystal structure. The
summary of these observations is presented in Table 5. All the
lanthanide(III) halides can be divided into three groups. The
first one corresponding to hexagonal, UCl3-type and orthorhom-
bic, PuBr3-type crystal structure in which melting is related to
31 ± 4 J mol
K .
Molar heat capacities were measured for all the lan-
thanide halides mentioned above both at the solid and the
liquid state [7,8]. They were used together with temper-
atures and enthalpies of phase transition in determination
of the thermodynamic functions of lanthanide halides [7,8].
One might expect that in the liquid state the absolute
entropy of the lanthanide halides would be similar since
the cation size and mass do not vary greatly. Fig. 6 shows
that the difference Sm(LnX3,l,1300 K) − Sm(LnX3,s,298.15 K)
−
1
−1
the entropy change of about 50 ± 4 J mol
K . The halides
of FeCl3-type crystal structure belong to the second group and
the entropy of fusion change (or sum of transition and fusion
−
1
−1
K . The third group of
entropy) is equal to 40 ± 4 J mol
Fig. 6. Dependence of Sm(LnX3,l,1300 K) − Sm(LnX3,s,298.15 K) on crystal
structure of lanthanide(III) halides: open circles, open squares and open trian-
gles, chlorides; bromides and iodides of UCl3- and PuBr3-type crystal structure;
open rhombus and black rhombus, bromides and iodides of FeCl3-type structure;
black circles and black squares, chlorides and bromides of AlCl3-type structure.
Fig. 5. Dependence of fusion temperature of LnI3 on lanthanide atomic number:
open circles, literature data; black circles, our experimental data.

1
74
L. Rycerz, M. Gaune-Escard / Journal of Alloys and Compounds 450 (2008) 167–174
is similar for the lanthanide(III) chlorides, bromides and
iodides with the same structure. This difference is about
[13] G.I. Novikov, A.K. Bayev, O.G. Polyachenok, Khim. Redk. Elem. Leningr.
Gos. Univ. (1964) 63.
[14] E.R. Harrison, J. Appl. Chem. 2 (1956) 601.
−
1
−1
2
16 ± 4 J mol
K
for halides with UCl3- and PuBr3-type
[
[
[
15] A.S. Dworkin, M.A. Bredig, High Temp. Sci. 3 (1) (1971) 81.
16] V.D. Savin, N.P. Mikhailova, Zh. Fiz. Khim. 55 (1981) 2237.
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[18] K. Cho, T. Kuroda, Denki Kagaku 40 (1972) 837.
[
19] L.B. Pankratz, Thermodynamic Properties of Halides, vol. 3, Bull. 674, US
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[
[
20] O.G. Polyachenok, G.I. Novikov, Russ. J. Inorg. Chem. 9 (4) (1964) 429.
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−
1
−1
K , respectively). This implies either a
and 190 ± 4 J mol
higher degree of order in the liquid for the lanthanide halides
of FeCl3- and AlCl3-type structure or a higher entropy for the
solids at room temperature due to differences in crystal struc-
ture. However, there are no many measured low temperature heat
capacity data available for lanthanide halides from which the
entropy at room temperature could be calculated. The thermo-
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literature, include estimates, which do not reflect the differences
in entropy discussed above. Therefore, as pointed out nearly 35
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[
[
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[
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[
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