3.1 Na3AlF6 – NdF3
The results of the thermal analysis of the system Na3AlF6 – NdF3 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 NdF3 – rich side of the phase diagram was investigated only up to 60 mol % of NdF3. The coordinates of the eutectic point were found to be in the frame of this range at approximately 49 mol % of NdF3 and 905 °C.
Tab. 1. Temperatures of primary (tp) and solidus (ts) crystallizations in the molten system Na3AlF6 – NdF3.
|
x/ mol % NdF3
|
tp/ °C
|
ts/ °C
|
x/ mol % NdF3
|
tp/ °C
|
ts/ °C
|
0
|
1011
|
|
20
|
977
|
907
|
0.5
|
1011
|
|
22.5
|
968
|
903
|
1
|
1009
|
|
25
|
963
|
904
|
2
|
1009
|
|
27.5
|
957
|
906
|
2.5
|
1006
|
886
|
30
|
951
|
905
|
4
|
1005
|
|
32.5
|
946
|
905
|
5
|
1003
|
880
|
35
|
938
|
905
|
7.5
|
1001
|
896
|
37.5
|
932
|
907
|
10
|
997
|
897
|
40
|
923
|
905
|
12.5
|
991
|
899.5
|
45
|
909
|
907
|
15
|
985
|
904
|
50
|
910
|
907.5
|
17.5
|
983
|
906
|
60
|
|
906
|
Based on the freezing point depression theory, the number of new species originating from a solute (in our case NdF3) added to a molten solvent (Na3AlF6) can be determined for low concentrations of solute from the following simplified equation [20].
Tf (K) is the melting point of the pure solvent (Na3AlF6), Hf (J.mol-1) is the enthalpy of fusion of solvent (Na3AlF6) and kst is the so – called Stortenbeker factor which in fact represents a number of new particles formed/introduced when NdF3 is added and dissolved in molten Na3AlF6. ΔT (K) is the difference between the actual temperature of primary crystallization of the mixture and the melting point of pure solvent (Tf), xNdF3 is the NdF3 molar fraction in the system, and R is the gas constant.
Since the enthalpy related to the melting of pure Na3AlF6 is known (Hf(Na3AlF6) = 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 NdF3 was added to molten Na3AlF6. We can conclude based on the freezing point depression analysis that the addition of NdF3 into the molten Na3AlF6 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 solidified samples (done ex–post after the thermal analysis) (Fig. 4) indicates the formation of [NdF4]1- complex anion.
Figure 4 shows the complete XRD analysis of the solidified samples of the system Na3AlF6 – NdF3 as a function of NdF3 concentration (mol %). A1 pattern in this figure represents the XRD pattern of the sample with pure Na3AlF6, while the patterns A2 – A9 represent the samples with increasing concentration of NdF3. Besides the signals of the original compounds Na3AlF6 and NdF3, the XRD analysis shows the formation of two new compounds; NaAlF4 and NdOF. NaAlF4 is a product of the interaction between Na3AlF6 and NdF3, while the NdOF is likely the product of the high temperature hydrolysis between the moisture in the atmosphere and NdF3 [22]. Since, there is no NdOF signal at the low and zero concentrations of NdF3, the appearance of oxygen content compound in the XRD analysis must be interrelated with the presence of NdF3 in the investigated samples.
The maximum intensity of the NaNdF4 signals is in the concentration range of 2.5 – 10 mol % of NdF3. In the concentration range between 10 and 50 mol %, the intensity of these peaks is decreasing and completely disappears at the maximal concentration of NdF3 at 60 mol %.
The first XRD signals of NdOF appear at the concentration of NdF3 at 5 mol % and the maximum was reached at the concentration of NdF3 around between 10 – 20 mol % and then continually decreases.
To elucidate the possible chemical reaction to form NaNdF4, we have to take into account the well known thermal dissociation of molten cryolite systems with the formation of NaAlF4 vapours according to the following reaction scheme [1, 3].
Na3AlF6 = 2NaF + NaAlF4
|
(2)
|
It means that in any cryolitic system in a molten state, the reaction system loses NaAlF4 in the form of vapours and the molten system becomes less acidic (an increase of the cryolite NaF/AlF3 ratio, CR). The final NaF concentration in the system due to reaction (2) increases and free NaF may react with NdF3 according to the following reaction scheme.
3.2 (Na3AlF6 – NdF3)eut – Nd2O3
The results of the thermal analysis of the system (Na3AlF6 – NdF3)eut – Nd2O3 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 Na3AlF6 – NdF3) was set, as a result of the previous part of this work, as 49 mol % of NdF3. The (Na3AlF6 – NdF3)eut – rich side of the phase diagram was investigated only up to 45 mol % of Nd2O3. The coordinates of the eutectic point were found to be in the frame of this range at approximately 45 mol % Nd2O3 and 733 °C. The phase diagram of this system seems to be more complex than the simple eutectic system of Na3AlF6 – NdF3. The (Na3AlF6 – NdF3)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 Nd2O3, 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 Nd2O3. These two experimentally determined lines then probably constitute 4 different fields between the liquidus and eutectic borders in this, (Na3AlF6 – NdF3)eut – rich, side of the phase diagram.
Tab. 2. Temperatures of the primary crystallization (tp) and other heat effects (t2, t3) on the cooling curves in the molten system (Na3AlF6 – NdF3)eut – Nd2O3.
|
x/ mol % Nd2O3
|
tp/ °C
|
t2/ °C
|
t3/ °C
|
x/ mol % Nd2O3
|
tp/ °C
|
t2/ °C
|
t3/ °C
|
0
|
905
|
|
|
7.5
|
897
|
|
|
0.5
|
900
|
860
|
|
9
|
888
|
817
|
|
1
|
900
|
|
|
10
|
892
|
|
|
1
|
897
|
865
|
|
15
|
875
|
826
|
722
|
1.5
|
896
|
866
|
|
20
|
876
|
827
|
|
2
|
896
|
866
|
|
25
|
868
|
827
|
|
2.5
|
898
|
868
|
|
30
|
825
|
|
|
3
|
897
|
869
|
|
35
|
811
|
728
|
|
3.5
|
895
|
869
|
|
40
|
768
|
|
|
4
|
896
|
|
|
45
|
743
|
735
|
|
5
|
897
|
|
|
|
|
|
|
Figure 6 shows an XRD analysis of the solidified samples of the system (Na3AlF6 – NdF3)eut – Nd2O3 as a function of Nd2O3 concentration (mol %). The B1 pattern in this figure represents the XRD pattern of the sample without Nd2O3, while the patterns B2 – B8 represent the samples with increasing concentration of Nd2O3. The sample without the addition of Nd2O3 contains, as in the case of the previous Na3AlF6 – NdF3 system, only the signals of the following compounds: Na3AlF6, NdF3, NaNdF4, and NdOF. NaNdF3 is a product of the reaction between NaF and NdF3 according to the reaction scheme (3). The formation of NdOF is like in the previous system a product of the pyro – hydrolysis between NdF3 and the moisture in the atmosphere [22]. The samples with the higher concentrations of Nd2O3 (B2 – B8) contain further, besides the above mentioned compounds, the signals of NdAlO3 and NaF.
If we compare the evolution of the signals of NaNdF4 in both systems, one can see that in the case of the Na3AlF6 – NdF3 system the intensity of the NaNdF4 signals slowly increases upon the addition of NdF3 with the maximum intensity in the concentration range somewhere around 5 mol % NdF3, and then disappears at higher concentration of NdF3. This is contrary to the system with Nd2O3 where the relatively intense and constant signals of NaNdF4 appear in all samples. These findings indicate that NaNdF4 is in this system formed also by other reaction(s) than by only reaction (3). The occurrence of the relatively constant and intense peaks of NaNdF4 can be explained by the reaction between Na3AlF6 and Nd2O3 according to the following reaction scheme.
Na3AlF6 + Nd2O3 = NaNdF4 + NdAlO3 + 2NaF
|
(4)
|
This reaction can also explain the presence and the evolution of the intensity of the XRD peaks of NdAlO3 in the system with Nd2O3, where the intensity of the signals of the NdAlO3 patterns linearly increases with the concentration Nd2O3 (Fig. 6). The reaction (5) can, on the other hand, explain the complete disappearance of the NdF3 peaks upon the addition of Nd2O3, as well as a “non–linear” evolution of the intensity of XRD peaks of NdOF in the (Na3AlF6 – NdF3)eut – Nd2O3 system.
The “non – linear” evolution here means that the intensity of the signals of the NdOF patterns in Fig 6 firstly (in the concentration range between 0 and 5 mol % Nd2O3) increases, then (in the concentration range 5 – 30 mol % Nd2O3), decreases, and, then again, continually increases in the highest concentrations of Nd2O3 (30 – 40 mol %). A similar (“non – linear”) evolution of the intensity of peaks can be seen also in the case of NaF. This phenomenon can be explained by the existence of parallel competitive reactions in the formation of NaF and NdOF.
In the case of the formation/consumption of NaF, it is the thermal dissociation of Na3AlF6 (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 pyro–hydrolysis of NdF3 [22].