S. Yu. Melchakova, L. F. Yamshchikova, V. A. Ivanov

Thermodynamic Properties of Alloys of Praseodymium with the Gallium–Indium Eutectic Melt
S. Yu. Melchakov^a, L. F. Yamshchikov^a, V. A. Ivanov^a, V. A. Volkovich^a,

S. P. Raspopin^a, and A. G. Osipenko^b
aUral Federal University, ul. Mira 19, Yekaterinburg, 620002, Russia

^bState Scientific Center “Research Institute for Atomic Reactors”,

Dimitrovgrad-10, Ul’yanovsk oblast, 433510, Russia
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)Cl_eut 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]: T_e = 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.

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 48-4-523–90 (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 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 mass-spectrometric 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

.(3)

Then the activity of α-praseodymium in indium melts in the range

T = 573 – 1073 K can be described by the equation

log a_α-Pr = 4.581 – 12598/T ± 0.019, (4)

S²_res = 0.010,

where S²_res is the residual variance.

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 praseodymium-saturated 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)

S²_res [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 E_i–T_i 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

. (6)

The activity of supercooled liquid praseodymium in these melts, calculated allowing for phase transitions [6], is described by the equation

log a_l-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 loga_Pr = 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-K_α 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 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.
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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).

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	, kJ/mol	, J/(mol∙K)	, kJ/mol	ΔT, K	Reference
Ga–Pr	303,7 293,1±2,7	116,4 110,4±3,2	210,60 204,79±0,32	675–975 663–973	[3] [5]
In–Pr	243,9 242,6±3,1 241,2±2,0	91,4 90,4±3,7 85,6±2,5	170,84 170,33±0,37 172,72±0,28	725–975 648–973 573–1073	[4] [5] [our study]
Ga–In–Pr	301,9±2,7	124,7±3,5	202,14±0,16	573–1073	[our study]