Thermal Analysis and Phase Equilibria of the Molten System Na 3 AlF 6 – NdF 3 – Nd 2 O 3

Phase diagrams of the systems Na 3 AlF 6 – NdF 3 and (Na 3 AlF 6 – NdF 3 ) eut – Nd 2 O 3 were experimentally determined by thermal analysis up to 60 mol % of NdF 3 , and 45 mol % of Nd 2 O 3 , respectively. The Na 3 AlF 6 – NdF 3 system was found to be a simple eutectic system with the eutectic point with the following approximate coordinates: 49 mol % NdF 3 , and 905°C. This eutectic composition was then used for the thermal analysis of the (Na 3 AlF 6 – NdF 3 ) eut – Nd 2 O 3 system. The coordinates of the eutectic point of the (Na 3 AlF 6 – NdF 3 ) eut – Nd 2 O 3 system were found to be at approximately 46 mol % Nd 2 O 3 and 733°C. After the thermal analysis, the X – ray diffraction analysis of the solidi�ed samples of both systems was performed. The XRD analysis of the Na 3 AlF 6 – NdF 3 system has shown the formation of two new compounds; NaNdF 4 and NdOF. The formation of NdOF is probably the product of the high temperature hydrolysis between the moisture in the atmosphere and NdF 3 . The XRD analysis of the solidi�ed samples of the (Na 3 AlF 6 – NdF 3 ) eut – Nd 2 O 3 system has shown the formation of the following new compounds; NaNdF 4 , NdOF, NdAlO 3, and NaF.


Introduction
Molten salts provide a unique working environment.They offer an ionic, nonaqueous habitat wherein oxide and other ores may be dissolved.Molten salts also provide a relatively wide electrochemical window where the potentials between which one can operate without decomposing the solvent may have been even several volts.
Molten salt systems are however widely used not only for industrial electrolysis and treatment of metals (like production of aluminium, magnesium, alkaline earth metals, baths for various alloy heat treatment, carburizing, nitrocarburizing, and silicifying of steel), but they can be found in such industrial applications like a special type of nuclear molten salts reactors, concentrated solar heat exchangers and energy storage media, and in high temperature batteries and fuel cells [1][2][3][4][5][6].
The rare earth elements (REE) constitute the biggest coherent group of chemically similar elements in the periodic table.Though generally unfamiliar, REE are vital for many industrial applications.The speci city, exibility, and modality of REE have given them a high level of technological, economic, and environmental signi cance.The varied nuclear, chemical, physical, metallurgical, and catalytic, properties of the REE have led to a dramatic increase in scale and variety of advanced industrial applications of REE over the past few decades.This has given REE a geopolitical status of strategically very important materials, especially when taking into account the critical dominance of China (China covers a 90% of global mine production of REE), complex and interlinked production chains (e.g.due to a presence of the radioactive minerals in many REE mine deposits), and fragile global supply chains due to the unstable geopolitics [7][8][9][10].
Neodymium -iron -boron (NdFeB) permanent magnet production is today a key industrial Nd application, considered the main driver of the growth of the REE market in the last 20 years [10].The excellent strength -to -weight ratio of Nd -based magnets has been overriding other competitor materials in terms of density of the magnetic ux.The niche of the industrial use of the Nd -based magnets ranges from miniature hard drivers in laptops, headphones, and other communication systems to green electric technologies like electric cars and wind turbines.
Today neodymium is industrially produced by high temperature electrolysis of Nd 2 O 3 dissolved in molten LiF -NdF 3 electrolyte (the molar LiF/NdF 3 ratio of the initial electrolyte is approximately 0.9).The concentration of the oxide raw material in the molten uoride electrolyte is around 2 wt.% (0.96 mol %) and the operational temperature is between 1050°C and 1100°C [1,7] One of the obvious disadvantages of the current industrial electrolysis of neodymium is the very low solubility of Nd 2 O 3 in the molten electrolyte.The low solubility of the oxide in the molten electrolyte in uences not only the performance of the cell, and the oxide feeding operation, however, it is also responsible for the occurrence of the anode effects with the correspondent emission of the very powerful greenhouse per uorocarbons gases.The current e ciency of the process is also relatively low, 50% -80%.Because of the high market demand for primary neodymium, an enhancement of the process is rather required, which would imply higher oxide row material solubility, higher current and energy e ciency, fewer losses of electrolyte, more e cient process control, and the reduction of the emission of per uorocarbon gases [11].
The solubility of the neodymium oxide raw material in molten uorides and a thorough comprehension of the related phase equilibria and dissolution dynamics are both of theoretical and practical signi cance for the design of the proper electrolyte compositions and optimal process control and operation.In contrast to comprehensive works done in the eld of the cryolite -alumina electrolyte systems [3,12], a small has been done in the eld of rare earth oxide -uoride electrolytes.Moreover, a discrepancy is there in the results from various studies in this eld.Due to this, the availability of reliable data is rather problematic.
The present paper deals with phase equilibria of the molten Na 3 AlF 6 -NdF 3 -Nd 2 O 3 system.The phase diagrams of the systems Na 3 AlF 6 -NdF 3 and (Na 3 AlF 6 -NdF 3 ) eut -Nd 2 O 3 were experimentally determined by thermal analysis and the solidi ed samples after that analysis were then analysed by Xray diffraction.
Berul and Voskresenskaya [13] were the rst to investigate the solubility of Nd 2 O 3 in molten cryolite (Na 3 AlF 6 ).They found that Nd 2 O 3 in molten cryolite forms NdF 3 during the dissolution process.
Abbasalizadeh et al. [14] studied the uorination effect of the addition of Na 3 AlF 6 into the molten LiF -Nd 2 O 3 bath.The authors found (XRD analysis of solidi ed samples), contrary to the ndings from the previous work, that neodymium from neodymium oxide reacts with cryolite and gets substituted in place of aluminium in cryolite to form Na 1.5 Nd 1.5 F 6 compound.Both above mentioned works are the only works in open literature related to the behaviour of Nd 2 O 3 in molten cryolite systems.To our knowledge, there is yet no literature data related to the investigation of phase diagrams of the systems studied in the present work (Na 3 AlF 6 -NdF 3 and (Na 3 AlF 6 -NdF 3 ) eut -Nd 2 O 3 ).
A comprehensive review of the works related to the dissolution of different REE oxides in different molten uoride systems done after the work of Berul and Voskresenskaya can be found in the work of Guo et al. [15].The important fact related to the solubility of different REE oxides in molten uoride melts is con rmed by several authors [17] -the increased solubility of REE oxides (e.g.Y 2 O 3 , CeO 2 , and Nd 2 O 3 ) with increasing concentration of REE uoride (e.g.YF 3 , CeF 3 and NdF 3 ) in both binary and ternary uoride systems.
The only phase diagram between Na 3 AlF 6 and any REE oxide was reported by Ambrová et al. [19].These authors used thermal analysis and reported the phase diagram of the Na 3 AlF 6 -La 2 O 3 system.The present system is to be the simple eutectic system with the following coordinates of the eutectic point; 11.5 mol % La 2 O 3 and 934°C.

Experimental
The phase equilibria of the investigated system were determined by a thermal analysis method.All samples were prepared in a glove box under an inert atmosphere (Ar, 99.999 %, SIAD, Slovakia).The powdered samples of 12 g were homogenized in the inert atmosphere and then transferred in a platinum crucible into the preheated (ca 90°C) electric resistance furnace (Fig. 1) under a dried argon atmosphere (99.996 %, SIAD, Slovakia).One platinum crucible holds the sample, the other one, the reference material (high purity powders of Al 2 O 3 ).The temperature of the furnace was controlled by a Pt10Rh/Pt thermocouple, inserted in the reference material.For the preparation of the samples, the following chemicals were used: NaF (99.9 %, Merck, Germany), AlF 3 (sublimated, dried at 300 °C, min.99.0 %, Slovalco, Slovakia), NdF 3 (99.9% Chempur, Germany), Nd 2 O 3 (99.9% Chempur, Germany).The furnace was rstly heated at the heating rate of 7 °C.min−1 to a temperature approximately 50 °C above the melting point of the melt.The samples were then kept at this temperature for approximately 50 min.The temperature of primary crystallization and the other heat effects were recorded at the cooling rate of 1.5 °C.min - .A computerized measuring device, developed at the Institute of Inorganic Chemistry SAS, Slovakia, was used for the temperature control of the furnace and data acquisition.The temperature of the sample was controlled by a Pt10Rh/Pt thermocouple calibrated using the melting points of the following pure chemicals; NaCl, NaF, and Li 2 CO 3 .The accuracy of the temperature measurement of the measured thermocouples was found to be ±2 °C.
All the samples were after the thermal analysis measurements taken for the X -ray diffraction (XRD) analysis.XRD patterns were measured using an Empyrean PANalytical diffractometer with Cu Kα1,2 radiation in Bragg -Brentano geometry and a b -lter (Ni The results of the thermal analysis of the system Na 3 AlF 6 -NdF 3 are presented in Tab. 1 and the phase diagram of this system is shown in Fig. 2. The investigated system was found to be a simple eutectic one.The NdF 3 -rich side of the phase diagram was investigated only up to 60 mol % of NdF 3 .The coordinates of the eutectic point were found to be in the frame of this range at approximately 49 mol % of NdF 3 and 905 °C. Tab. 1. Temperatures of primary (t p ) and solidus (t s ) crystallizations in the molten system Na 3 AlF 6 -NdF 3 .
x/ mol % NdF Based on the freezing point depression theory, the number of new species originating from a solute (in our case NdF 3 ) added to a molten solvent (Na 3 AlF 6 ) can be determined for low concentrations of solute from the following simpli ed equation [20].
T f (K) is the melting point of the pure solvent (Na 3 AlF 6 ), H f (J.mol -1 ) is the enthalpy of fusion of solvent (Na 3 AlF 6 ) and k st is the so -called Stortenbeker factor which in fact represents a number of new particles formed/introduced when NdF 3 is added and dissolved in molten Na 3 AlF 6 .ΔT (K) is the difference between the actual temperature of primary crystallization of the mixture and the melting point of pure solvent (T f ), x NdF3 is the NdF 3 molar fraction in the system, and R is the gas constant.
Since the enthalpy related to the melting of pure Na 3 AlF 6 is known (H f (Na 3 AlF 6 ) = 106.7 kJ.mol -1 [21]), theoretical curves representing the introduction of 1, 2 and 3 new species based on Eqn. ( 1) can be calculated.Figure 3 shows the results of that analysis where can be seen how many new species were formed when NdF 3 was added to molten Na 3 AlF 6 .We can conclude based on the freezing point depression analysis that the addition of NdF 3 into the molten Na 3 AlF 6 introduced one new species.We can only speculate about the form and the structure of that new species forms in this molten system, but the XRD analysis of the solidi ed samples (done ex-post after the thermal analysis) (Fig. 4) indicates the formation of [NdF 4 ] 1-complex anion.
Figure 4 shows the complete XRD analysis of the solidi ed samples of the system Na 3 AlF 6 -NdF 3 as a function of NdF 3 concentration (mol %).A1 pattern in this gure represents the XRD pattern of the sample with pure Na 3 AlF 6 , while the patterns A2 -A9 represent the samples with increasing concentration of NdF 3 .Besides the signals of the original compounds Na 3 AlF 6 and NdF 3 , the XRD analysis shows the formation of two new compounds; NaAlF 4 and NdOF.NaAlF 4 is a product of the interaction between Na 3 AlF 6 and NdF 3 , while the NdOF is likely the product of the high temperature hydrolysis between the moisture in the atmosphere and NdF 3 [22].Since, there is no NdOF signal at the low and zero concentrations of NdF 3 , the appearance of oxygen content compound in the XRD analysis must be interrelated with the presence of NdF 3 in the investigated samples.
The maximum intensity of the NaNdF 4 signals is in the concentration range of 2.5 -10 mol % of NdF 3 .In the concentration range between 10 and 50 mol %, the intensity of these peaks is decreasing and completely disappears at the maximal concentration of NdF 3 at 60 mol %.
The rst XRD signals of NdOF appear at the concentration of NdF 3 at 5 mol % and the maximum was reached at the concentration of NdF 3 around between 10 -20 mol % and then continually decreases.
To elucidate the possible chemical reaction to form NaNdF 4 , we have to take into account the well known thermal dissociation of molten cryolite systems with the formation of NaAlF 4 vapours according to the following reaction scheme [1,3].
It means that in any cryolitic system in a molten state, the reaction system loses NaAlF 4 in the form of vapours and the molten system becomes less acidic (an increase of the cryolite NaF/AlF 3 ratio, CR).The nal NaF concentration in the system due to reaction (2) increases and free NaF may react with NdF 3 according to the following reaction scheme.The phase diagram of this system seems to be more complex than the simple eutectic system of Na 3 AlF 6 -NdF 3 .The (Na 3 AlF 6 -NdF 3 ) eut -rich side of the phase diagram contents, besides the eutectic horizontal line, also two other horizontal lines.One line is located at the temperature range between 860 °C and 869 °C and in the concentration range between 0 and 3 mol % of Nd 2 O 3 , another horizontal line is located at the temperature range between 817 °C and 825 °C and in the concentration range between 3 mol % and 30 mol % of Nd 2 O 3 .These two experimentally determined lines then probably constitute 4 different elds between the liquidus and eutectic borders in this, (Na 3 AlF 6 -NdF 3 ) eut -rich, side of the phase diagram.
Tab. 2. Temperatures of the primary crystallization (t p ) and other heat effects (t 2, t 3 ) on the cooling curves in the molten system (Na 3 AlF 6 -NdF 3 ) eut -Nd 2 O 3 . x In the case of the formation/consumption of NaF, it is the thermal dissociation of Na 3 AlF 6 (2) [1,3] and the reactions ( 4) and (6); in the case of the formation of NdOF, it may be the reaction ( 5) and the pyrohydrolysis of NdF 3 [22].

Conclusions
The phase equilibria of Na 3 AlF 6 -NdF 3 and (Na 3 AlF 6 -NdF 3 ) eut -Nd 2 O 3 systems have been analysed by means of thermal analysis and for the rst time, to our knowledge, presented.Both investigated mixtures seem to form a simple eutectic system.The coordinates of the eutectic point in the case of the Na 3 AlF 6 -NdF 3 system were found to be at approximately 49 mol % of NdF 3 and 905°C.The coordinates Figures The

NaF + NdF 3 3
AlF 6 -NdF 3 ) eut -Nd 2 O 3 The results of the thermal analysis of the system (Na 3 AlF 6 -NdF 3 ) eut -Nd 2 O 3 are presented in Tab. 2 and the phase diagram of this system is shown in Fig. 5.The composition of the left side of the phase diagram (binary eutectic point of Na 3 AlF 6 -NdF 3 ) was set, as a result of the previous part of this work, as 49 mol % of NdF 3 .The (Na 3 AlF 6 -NdF 3 ) eut -rich side of the phase diagram was investigated only up to 45 mol % of Nd 2 O 3 .The coordinates of the eutectic point were found to be in the frame of this range at approximately 45 mol % Nd 2 O 3 and 733 °C.

Figure 6 4 )
Figure 6 shows an XRD analysis of the solidi ed samples of the system (Na 3 AlF 6 -NdF 3 ) eut -Nd 2 O 3 as a function of Nd 2 O 3 concentration (mol %).The B1 pattern in this gure represents the XRD pattern of the sample without Nd 2 O 3 , while the patterns B2 -B8 represent the samples with increasing concentration of Nd 2 O 3 .The sample without the addition of Nd 2 O 3 contains, as in the case of the previous Na 3 AlF 6 -NdF 3 system, only the signals of the following compounds: Na 3 AlF 6 , NdF 3 , NaNdF 4 , and NdOF.NaNdF 3 isa product of the reaction between NaF and NdF 3 according to the reaction scheme (3).The formation of NdOF is like in the previous system a product of the pyro -hydrolysis between NdF 3 and the moisture in the atmosphere[22].The samples with the higher concentrations of Nd 2 O 3 (B2 -B8) contain further, besides the above mentioned compounds, the signals of NdAlO 3 and NaF.

Figure 2 Phase
Figure 2

Figure 5 Phase
Figure 5 Phase analysis was performed with Oxford Cryosystem Crystallographica Search -Match 2.1 ).A solid -state PIXcel detector was used to record XRD patterns.The measurements were carried out at room temperature, with 2θ varying from 10° to 90°.