Thermodynamic Properties of Alloys of Praseodymium with
the Gallium–Indium Eutectic Melt
S. Yu. Melchakova, L. F. Yamshchikova, V. A. Ivanova, V. A. Volkovicha,
S. P. Raspopina, and A. G. Osipenkob
Ural Federal University, ul. Mira 19, Yekaterinburg, 620002, Russia
b
State Scientific Center “Research Institute for Atomic Reactors”,
Dimitrovgrad-10, Ul’yanovsk oblast, 433510, Russia
a
Abstract. Equilibrium potentials of Pr–In alloys containing 8.7–12.1 mol % Pr and
alloys of Pr with In–Ga eutectic saturated by praseodymium up to 6.71 mol % Pr
are measured by the emf method between 573–1073 K in the (Li–K–Cs)Cleut based
salt melt. Metallic praseodymium is used as the reference electrode when studying
the Pr–In system, and the two-phase praseodymium–indium alloy L + PrIn3 with
the known thermodynamic properties having no phase transitions in the studied
temperature range is used as the reference electrode when studying the
Pr–In–Ga system. Partial thermodynamic properties and activity of praseodymium
in alloys with indium and the Ga–In eutectic mixture are calculated.
Keywords: praseodymium, gallium, indium, alloys, activity
To develop short-closed fuel cycles of nuclear reactors, non-aqueous
methods of fuel reprocessing, including electrochemical ones, with the use of
thermal-resistant and radiation-resistant salt and metallic melts are developed. A
prerequisite of their development is the existence of reliable thermodynamic
information on the state of separated chemical elements in salt and metallic melts.
Promising metallic systems for developing the regeneration technology of
irradiated fuel are low-melting compositions based on Group IIIA elements.
Therefore, this study was aimed at investigating the thermodynamic properties of
Ga–In eutectic alloys saturated with praseodymium (fission element) in
temperature range T = 573–1073 K using the emf method.
Coordinates of the eutectic point in the Ga–In system is [1]: Te = 288.7–
289.0 K (15.7–15.9°C) with the indium content of 20.0–24.6 wt % (13.5– 16.5 mol
% In). There is no published data on the phase diagram and thermodynamics of the
Pr–Ga–In ternary system and thermodynamic properties. Only Pr–Ga and Pr–In
binary systems were investigated. The phase diagram of Pr–Ga alloys [2] is
characterized by the presence of Pr2Ga, Pr3Ga5, PrGa, PrGa2, and PrGa6 phases.
The last of them is formed according to peritectic reaction at T = 739 K. The
occurrence of PrGa4 phase, which incongruently melts at T = 897 K, is not reliably
confirmed. Intermetallic compounds PrGa6 (to 739 K) and PrGa2 (above 897 K)
should be in equilibrium with the saturated solution of praseodymium in liquid
gallium between 573–1073 K. The generalized phase diagram of Pr–In alloys [2] is
also characterized by the presence of five intermetallic compounds: Pr3In, Pr2In,
PrIn, Pr3In5, and PrIn3. The PrIn3 compound, which congruently melts at T = 1483
K, is in equilibrium with indium.
1
The thermodynamic properties of liquid alloys of praseodymium with
gallium and indium were previously studied by the emf method [3–5]. In this study
we measured the electrode potentials of Pr–Ga–In alloys relative to the reference
electrode (L + PrIn3) by the compensation method using an Autolab PGStat 302 N
potentiostat/galvanostat in the zero current mode.
The emf of the galvanic element
(–)L + PrIn3 |LiCl–KCl–CsCl+ PrCl3 |Pr–Ga–In(+)
(1)
was measured in a cell [6] made of stainless steel, which was connected to a
vacuum system and a system for purification of inert gas (Ar).
Initial metals and salts were of the following grade (wt. % of the host
substance, no smaller than): single-crystalline metallic gallium according to TU
(Technical Specifications) 48-4-35–84 (99.9999); metallic indium IN-000
according to GOST (State Standard) 10297–75 (99.999); metallic praseodymium,
ingot PrM-1 according to TU 48-1-215–72 (99.8); lithium chloride ROTH (≥99);
potassium chloride according to TU 6-09-3678–74; cesium chloride according to
TU 6-09-3778–82 (99.6); and praseodymium oxide PrO-L according to TU 48-4523–90 (99.9).
The starting salts and eutectic mixture of lithium, potassium, and cesium
chlorides were prepared according to procedure [7, 8]. The necessary PrCl3
concentration in the electrolyte was attained by chlorination of praseodymium
oxide in the melt of the LiCl–KCl–CsCl eutectic mixture by gaseous hydrogen
chloride [9]. The praseodymium concentration in the salt was determined by
chemical analysis [10]. Ready-to-use salts with Pr content of 2 wt. % Pr were
stored in an inert atmosphere. The alloys of the Ga–In eutectic composition with an
indium content of 21.4 wt % were prepared in inert atmosphere by fusing the
required amounts of individual gallium and indium at 150°C with the subsequent
prolonged (3–4 weeks) holding at this temperature.
The prepared Ga–In liquid alloy was introduced into preliminarily weighed
small crucibles made of beryllium oxide (2 cm3 in volume) using a dosing pipette,
the weight of liquid metal eutectic was determined, and required amounts of
praseodymium metal were added into the charge. Small crucibles (up to 20 at a
time) with components of the Pr–Ga–In alloys were transported into a container (a
crucible made of beryllium oxide to 300 cm3 in volume), tungsten conductors were
introduced into the crucibles, and crucibles were poured with the LiCl–KCl–CsCl
eutectic mixture containing praseodymium trichloride. Alloys with the
concentration <0.43 mol % Pr were prepared directly in experiments by the
cathodic deposition of praseodymium on the Ga–In alloy using praseodymium
enriched Pr–In alloys as the anode.
The preparation of the Ga–In eutectic alloys, charging the alloy and
electrolyte components into the crucibles, and final assembling of the prepared cell
were performed in an inert atmosphere of an MBraun Unilab 1200/780 glove box.
The cell was sealed and transported into a resistance furnace with an automated
temperature control, heated to T = 923–973 K, and held to homogenize alloys for
2
12 h. Electrode potentials of the alloys were measured relative to the L + PrIn3
alloy. The praseodymium concentration in the alloy corresponded to the two-phase
region (L is the saturated solution of Pr in In equilibrated with PrIn3 intermetallic
compound) and varied from 8.7 to12.1 mol. % Pr.
Alloy potentials were considered equilibrium at a fixed temperature if they
had no tendency to a monotonic shift and varied by no more than 0.1 – 0.5 mV for
1 h. In this case, potentials of the alloys of the same phase composition were
reproduced within ±0.1–0.2 mV. During one experiment, a temperature range of
573 – 1073 K was passed several times top-down and bottom-up with a step of
30 – 50 K. The cell temperature was held accurate to ± 1°K and monitored with
a K-type thermocouple placed into a sheath made of beryllium oxide, which was
immersed into the electrolyte melt.
It is known [11] that beryllium oxide belongs to most suitable construction
materials of the electrochemical cell contacting with the melts of alkali metal
chlorides and liquid low-melting metals including gallium, which contain rare
earth metals. This is also evidenced by our many years’ experience of work with
similar systems generalized in [6]. A microscopic analysis of cleavages of
beryllium oxide crucibles held for 60–100 h at T = 1300 K in contact with salt and
metal melts containing rare earth metals showed the absence of traces of the
chemical interaction of ceramics with liquid salts and metals.
Upon finishing the experiment, the cell was cooled and alloys were washed
with ice deionized water and analyzed for the praseodymium content by the massspectrometric method using an ELAN 9000 device.
To relate the potentials of a two-phase liquid-metal reference electrode (L +
PrIn3) of galvanic cell (1) to the potential of metal praseodymium, the emf of the
galvanic cell
(–)Pr | LiCl–KCl–CsCl + PrCl3 | L + PrIn3 (+),
(2)
was measured in an additional series of experiments.
Since α-Pr →β-Pr phase transition occurs at T = 1069 K, the
corresponding correction was introduced into values of emf measured at
temperatures above this point [12].
Dependence E = f (T) of cell (2) found experimentally between 573–1073 K
is satisfactorily approximated by the straight line equation
𝐸 [𝑉] = 0.8332 − 0.3030 × 10−3 𝑇 ± 1.9822√2.71 × 10−5 (
1
110
+
(𝑇−800.7)2
1470555
).(3)
Then the activity of α-praseodymium in indium melts in the range
T = 573 – 1073 K can be described by the equation
log aα-Pr = 4.581 – 12598/T ± 0.019,
S2res = 0.010,
where S2res is the residual variance.
3
(4)
These data agree with results of [4, 5], which were found for the narrower
temperature range.
The emf of galvanic cell (1) was recalculated relative to metal α-Pr using the
addition rule of electromotive forces. Our dependence E = f(T) for praseodymiumsaturated Ga–In melts is nonlinear in between 573–1073 K. It is satisfactorily
approximated by the second-order equation
E[V] = 1.1563 – 7.492 ×10-4 T + 2.184 × 10-7T2 ± 0.0030,
(5)
S2res [V2] = 10.48 × 10-6
and is plotted in Fig. 1.
The cause of nonlinearity is likely associated with the peritectic reaction that
occurs in gallium melts containing 8.7–12.1 mol. % Pr.
The table presents the partial thermodynamic characteristics of α-Pr in
praseodymium-saturated melts of gallium, indium, and the Ga–In eutectic melt.
Despite the nonlinearity of Eq. (5), the activity of α-praseodymium in
praseodymium-saturated Ga–In melts calculated by each pair of Ei–Ti
measurements of galvanic cell (1) in range T = 573 – 1073 K can be satisfactorily
approximated (see line 1 in Fig. 2) by the straight line equation
log 𝑎𝛼−Pr = 6.513 − 15770/𝑇 ± 1.968√2.025 × 10−2 (
1
286
+
(1/𝑇−0.00146)2
1.2305×10−5
). (6)
The activity of supercooled liquid praseodymium in these melts, calculated
allowing for phase transitions [6], is described by the equation
log al-Pr(Ga-In) = 6.967 – 16 297/T.
(7)
Reference data on the activity of α-Pr in liquid gallium [3, 5], indium [4, 5],
as well as individual gallium and the Ga–In eutectic melt according to our results
are compared in Fig. 2.
Our dependence of logaPr = f(T) for indium alloys L + PrIn3 agrees
satisfactorily with the literature data [4, 5] reported for the narrower temperature
range. It is seen that activities of α-Pr in the Ga–In eutectic alloys are closer to a
α-Pr in the Pr–Ga alloys than in the Pr–In alloys. This fact points to the preferential
interparticle interaction between praseodymium and gallium in the studied eutectic
alloy. The X-ray diffraction analysis (XRD) of the Pr–Ga–In alloys evidences the
same.
X-ray powder diffraction patterns of alloys with the electrolyte washed out
after the experiment were recorded using an X’PERT PRO diffractometer with the
Cu-Kα radiation. We performed the qualitative phase analysis and evaluated crystal
lattice parameters of presented phases. X-ray powder diffraction patterns were
decoded using the software of the diffractometer and the PDF-2 data file. XRD
results indicate that the only crystalline phase is the PrGa6 intermetallic
compound. No reflections of the PrIn3 phase were found, which is possibly
associated with the formation of the solid solution based on the PrIn3 compound,
which is indicated by an increase of lattice parameters of PrGa6. Reflections of
4
metallic gallium and indium were absent in X-ray powder diffraction patterns,
since the Ga–In alloy remained liquid at room temperature.
CONCLUSIONS
(i) Partial thermodynamic characteristics and activity of praseodymium in
the praseodymium-saturated eutectic Ga–In alloy are determined in a temperature
range of 573–1073 K.
(ii) Pr–Ga–In saturated alloys occupy the intermediate position between
praseodymium-saturated Pr–Ga and Pr–In alloys by the praseodymium activity,
which is apparently associated with the preferential interaction of praseodymium
with gallium in the Ga-In eutectic melt.
REFERENCES
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Moscow: Nauka, 1974.
2. ASM Binary Phase Diagrams. Software ASM International, USA, 1996.
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4, p. 1051.
4. Degtyar’ V.A., Bayanov A.P., Vnuchkova L.A., Serebrennikov, V.V. Zh. Fiz. Khim., 1971,
vol. 45, no. 7, p. 1816.
5. Kober, V.I., Nichkov, I.F., Raspopin, S.P., and Kuz’minykh, V.M., in Termodinamicheskie
svoistva metallicheskikh rasplavov: Materialy IV Vsesoyuznogo soveshchiya po termodinamike
metallicheskikh splavov (rasplavy). Chast’ 2 (Thermodynamic Properties of Metallic Melts:
Proc. IV All_Union Conf. on Thermodynamics of Metallic Alloys (Melts)), Alma-Ata: Nauka,
1989, vol. 2, p. 67.
6. Lebedev, V.A., Kober, V.I., and Yamshchikov, L.F., Termokhimiya splavov redkozemel’nykh
i aktinoidnykh elementov: Spravochnoe izdanie (Termokhimiya of Rare-Earth and Actinoid
Elements: Handbook), Chelyabinsk: Metallurgiya, 1989.
7. Shishkin, V.Yu. and Mityaev, V.S., Izv. Akad. Nauk SSSR, Neorg. Mater., 1982, vol. 18, no.
11, p. 1917.
8. Revzin, G.E., in Metody polucheniya khimicheskikh reaktivov i preparatov (Preparation
Methods of Chemical Reagents and Preparations), Moscow: IREA, 1967, no. 16, p. 124.
9. Volkovich, V.A., Medvedev, E.O., Vasin, B.D., and Danilov, D.A., Rasplavy, 2006,
no. 5, p. 24.
10. Schwartzenbach, G. and Flashka, G., Kompleksonometricheskoe titrovanie (Complexometric
Titration), Moscow: Khimiya, 1970.
11. Yatsenko, S.P. and Khayak, V.G., Kompozitsionnye pripoi na osnove legkoplavkikh splavov
(Composite Solders Based on Low-Melting Alloys), Yekaterinburg: Ural Otd. Ross. Akad.
Nauk, 1997.
12. Termicheskie konstanty veshchestv. Vypusk VIII. Chast’ 1. Tablitsy prinyatykh znachenii
(Thermal Constants of Substances. Vol. 1. Part 1. Tables of Accepted Values) Glushko, V.N.,
Ed., Moscow: Akad. Nauk SSSR, 1978.
5
Fig. 1. Temperature dependence of emf of praseodymium-saturated Ga–In melts.
Fig. 2. Activity of α-praseodymium in praseodymium-saturated binary alloys.
(1) In–Ga [our study], (2) Ga [5], (3) Ga [3], (4) In [5], (5) In [4], (6) In [our study]
(signs correspond to our study and lines correspond to the reference data).
6
Table - Partial thermodynamic functions of α-Pr in melts of gallium, indium, and
the Ga–In eutectics saturated with praseodymium at T = 800 K
System
Ga–Pr
In–Pr
Ga–In–Pr
H Pr ,
SPr ,
GPr ,
kJ/mol
J/(mol∙K)
kJ/mol
303,7
116,4
293,1±2,7
ΔT, K
Reference
210,60
675–975
[3]
110,4±3,2
204,79±0,32
663–973
[5]
243,9
91,4
170,84
725–975
[4]
242,6±3,1
90,4±3,7
170,33±0,37
648–973
[5]
241,2±2,0
85,6±2,5
172,72±0,28
573–1073
[our study]
301,9±2,7
124,7±3,5
202,14±0,16
573–1073
[our study]
7
