LiCl (M/s Sigma Aldrich, anhydrous, 99 wt.%), KCl (M/s Alfa Aesar, anhydrous, 99 wt.%), AgCl (M/s Alfa Aesar, anhydrous, 99.99 wt.%), AlCl₃ (M/s Alfa Aesar, anhydrous, 99.985 wt.%), anhydrous SmCl₃ (M/s Alfa Aesar, 99.99 wt.%) and Sm₂O₃ (M/s Indian Rare Earths, 99.9 wt.%) were used in the present study. Quartz distilled HNO₃ (M/s Merck, India) was used in the chemical analysis. De-ionized water (18Mω) was used for the preparation of samples.

All chemicals were handled inside a glove box in Ar ambient at 20 to 60 mm of water column pressure. Oxygen and moisture impurity levels of the glove box were maintained below 10 ppm. A rotary vacuum pump (ED-15, M/s Hindhivac Private Ltd., Bangalore, India) was used to dry LiCl and KCl eutectic mixture (mole ratio of LiCl:KCl = 58.8:41.2, m.p. 626 K) at 425 K for 120 h. The vacuum dried LiCl-KCl mixture (300 g) was loaded in alumina crucible and placed inside a chlorination vessel, which was made of quartz bottom cell attached with a teflon top flange. The salt was melted in chlorine gas atmosphere. After melting, chlorine gas was bubbled through the melt for 0.5 h at 773 K. Subsequently, Ar gas was bubbled through the melt for about 1 h to remove excess chlorine. Finally, the salt was cooled under Ar gas flow and stored inside a glove box.

LiCl-KCl-1wt.% AgCl was prepared by grinding the required amount of AgCl in a mortar pestle, thoroughly mixing with purified LiCl-KCl salt inside the Ar box and chlorinating at 773 K following a similar method as described above for blank LiCl-KCl salt.

Both AlCl₃ (S) and Sm₂O₃ (S) were dried inside the glove box to remove any moisture present in these before performing the conversion experiments. Weight loss profile of Sm₂O₃ was studied with thermo gravimetric analysis (TGA/SDTA 851; M/s. Mettler Toledo Instrument Co. Ltd, Switzerland) in the temperature range from room temperature (300 K) to 1273 K.

The conversion reaction was carried out inside the argon glove box pit, which was attached with a split type furnace at the bottom of the glove box with a provision of cooling by recirculating chilled water. The schematic diagram of experimental set up for conversion reaction is given in supplementary data (see supporting Fig. S1). LiCl-KCl eutectic mixture was loaded in alumina crucible, which was kept inside a secondary iron crucible in the pit. After melting the salt, pre-dried Sm₂O₃ and AlCl₃ were added to the melt at 773 K under continuous stirring with an in-house fabricated tantalum stirrer at 400 rpm. The experimental temperature of 773 K in the present study was selected as per the standard operating temperature of similar pyrochemical process above the melting point (626 K) of LiCl-KCl eutectic melt [23].

After conversion reactions, the salts retrieved at the end of the above process were dissolved in H₂O and filterd using Whatman-42 paper. Chloride salts such as LiCl, KCl, SmCl₃ and AlCl₃ were dissolved in water while oxides were insoluble in nature. The filtrate (sample F-1) was stabilized with dilute HNO₃ (0.01 M) to avoid hydrolysis and was analysed for Sm and Al. The water insoluble part (containing Sm₂O₃ and Al₂O₃) was dissolved in 8 M HNO₃ and filtered). In this, only Sm₂O₃ was dissolved while Al₂O₃ was insoluble. The dissolved portion (sample F-2) was analysed for Sm and the amount of unreacted Sm₂O₃ was calculated. Standard SmCl₃ solution was prepared by dissolving commercially procured SmCl₃ (Alfa Aesar, 99.99 wt.%) in water and adjusting the solution with 0.01 M HNO₃.

Sample F-1 and F-2 were analysed for quantitative estimation of Sm with a spectrophotometer (AvaSpec-2048, Netherlands) in the range of 220–480 nm. Fluorescence spectra were recorded with Edinburgh spectrofluorimeter (model FLS920) for qualitative assessment of Sm. In addition, F-1 and F-2 were quantitatively analysed for Sm and Al by ICP-AES (M/s. Ametek GmBH, Spectro Arcos, Germany) which had polychromator with recirculation purging facility and hermetically sealed CCD detector.

The reaction product was analysed with XRD (M/s. GNR Explorer, Italy, Cu-K_α1, l ~1.5406 Å). Samples were prepared and loaded on zero diffraction holder and covered with Scotch tape to avoid the effect of atmospheric oxygen and moisture during diffraction measurement.

After the conversion reaction, electrochemical studies were carried out with the reaction product in a tapered and recrystallized alumina crucible (OD~ 44 mm, L~100 mm) which in turn was loaded inside a cylindrical vessel made of SS 304 L seamless pipe (ID~54 mm, L~500 mm) attached with CF-63 flange. Tungsten (W) and molybdenum (Mo) wire were used as working electrodes during electrochemical analysis (voltammetry). A high density (>1.8 g cm⁻³) graphite rod (D~8 mm, L~50 mm) was attached to a SS wire and was used as counter electrode. The reference electrode was made of pyrex tube (ID~4 mm, OD~6 mm, and ~500 mm). Bottom portion of the tube was shaped like a bulb of thin membrane. LiCl-KCl-1wt.% AgCl along with Ag wire as loaded into the pyrex tube. The potentials reported in this work were measured with respect to this Ag/AgCl reference electrode.

The electrochemical cell was degassed at 673 K for 4 h under vacuum along with counter electrode and alumina crucible to remove any adsorbed moisture and gaseous impurities. It was loaded with electrolyte and reference electrode inside the inert glove box. After loading the cell, it was taken out of the glove box and was kept in a furnace at 773 K under Ar atmosphere. The temperature of the melt was measured with a chromel-alumel thermocouple (K-type) sheathed in an alumina tube by immersing it in the electrolyte. Voltammograms were recorded with Iviumstat. xre potentiostat (M/s Ivium Technologies, Netherlands).

TGA plots for commercially procured Sm₂O₃ powder in the temperature range of 323–1273 K is shown in Fig. 1. The profile reveals three regions of mass loss at 513 K, 633 K and 873 K. The first mass loss is due to the adhered water molecule and second and third mass losses are due to the presence of hydroxide and carbonate impurities. To purify the commercial Sm₂O₃, 10.0 g (per batch) of Sm₂O₃ is heated in two steps. In the first step, heating is done at 520 K for 15 h and in the second step, heating is carried out at 935 K for 15 h. After drying, an average mass loss of ~ 1.0 g per batch is observed for Sm₂O₃. In addition, 8.0 g (per batch) of AlCl₃ is heated inside the glove box at 350 K for 20 h in Ar atmosphere. As a result, an average mass loss of ~ 2.0 g per batch is recorded for AlCl₃.

As discussed in the experimental section, the conversion reaction is carried out inside the argon glove box pit at 773 K. The reaction of Sm₂O₃ and AlCl₃ is written as

The feasibility of the above chlorination reaction is verified by calculating the Gibbs free energy of the reaction (ΔG^θ) at 773 K using the reaction module of Fact Sage 6.4 software [35]. The value of ΔG^θ is found to be −403.35 kJ which confirms that the reaction is thermodynamically favourable at 773K.

According to the Eq. (1), SmCl₃ and Al₂O₃ are the reaction products. Thus, LiCl-KCl melt contains Sm₂O₃ + AlCl₃ before equilibration, Sm₂O₃ + SmCl₃ + Al₂O₃ + AlCl₃ during equilibration and SmCl₃ + Al₂O₃ after equilibration. Among these products, SmCl₃ and AlCl₃ are soluble electroactive species in LiCl-KCl melt. Cyclic voltammograms (CV) and squarewave voltammograms (SWV) help in determining the redox viz. Sm³⁺ and Al³⁺ species present in the electrolyte [36]. In the present study, CV and SWV are recorded in equilibration product of LiCl-KCl-Sm₂O₃-AlCl₃ at 773 K to identify the presence of Sm³⁺ and Al³⁺ ions in the electrolyte. The schematic of the electrochemical cell assembly is given in supplementary data (see supporting Fig. S2).

CVs are recorded using W (for converted SmCl₃), Mo (for commercial SmCl₃) as working electrode, graphite as counter electrode and Ag/AgCl as reference electrode. The line marked (a) in Fig. 2 shows the CV recorded for blank LiCl-KCl eutectic melt in the potential range +1.5 V to −2.45 V. The cathodic limit is due to reduction of Li⁺ to Li metal at −2.35 V. The anodic limit of the melt is due to the oxidation of Cl⁻ ion to Cl₂ gas at +1.15 V. The line marked (b) in Fig. 2 shows the representational behaviour of Sm₂O₃ in LiCl-KCl melt in the present work. CV recorded for LiCl-KCl-Sm₂O₃ melt (4.3 × 10⁻⁵ mol cm⁻³) in the potential range of −2.45 V to + 0.5 V shows peaks only for Li⁺/Li reduction and Li/Li⁺ oxidation, which are similar to blank LiCl-KCl salt. No characteristic current peaks for Sm³⁺ ions are observed in this voltammogram and this is ascribed to the absence of electrochemically active species in LiCl-KCl melt. As the chlorination process uses AlCl₃, the redox behaviour for Al³⁺ ions are studied in LiCl-KCl electrolyte. The line marked (c) in Fig. 2 shows the CV recorded in LiCl-KCl-AlCl₃ melt (5.5 × 10⁻⁴ mol cm⁻³), which exhibits a cathodic peak at −1.02 V and an anodic peak at −0.80 V corresponding to Al³⁺/Al redox couple. To study the redox behaviour, commercially available SmCl₃ is added to the LiCl-KCl melt at 773 K, stirred for 2 h and subsequently CVs are recorded. In Fig. 3 line marked (a) shows the CVs recorded in LiCl-KCl-SmCl₃ (7.9 × 10⁻⁵ mol cm⁻³) salt at 773 K on inert (Mo) electrode. A pair of peaks (A/A′) is observed in the voltammogram around −0.8 V in addition to the peaks related to Li⁺/Li reduction and Li/Li⁺ oxidation. The cathodic peak at −0.80 V and corresponding anodic peak at −0.65 V are due to the reduction and oxidation of Sm³⁺ ions in the LiCl-KCl electrolyte. In order to carry out a detail study on the conversion reaction, different amount of dried Sm₂O₃ is equilibrated with AlCl₃ at 773 K. The reaction products of the equilibrated LiCl-KCl-SmCl₃ melt are given in Table 1,–3.

The concentrations of Sm are determined with UV-Vis spectrophotometry analyses and are discussed in detail in the latter section. At this point, a comparative CV study is carried out with 3.46 g of Sm₂O₃ equilibrated with 5.31 g of AlCl₃ in 100 g of LiCl-KCl for 6 h at 773 K. Thus, the CV recorded in equilibrated LiCl-KCl-SmCl₃ (3.08 × 10⁻⁴ mol cm⁻³) melt is shown with line marked (b) in Fig. 3. A pair of peaks (A₁/A′₁) is observed in the voltammogram around −0.80 V and the cathodic peak at −0.78 V represents the Sm³⁺/Sm²⁺ reduction in the melt similar to LiCl-KCl-SmCl₃ (commercial) electrolyte. This confirms the conversion of Sm₂O₃ to SmCl₃ in LiCl-KCl melt. Interestingly, the absence of cathodic peak at −0.9 V corresponding to Al³⁺/Al⁰ indicates complete consumption of AlCl₃ used for the chlorination of Sm₂O₃.

CVs are recorded on LiCl-KCl-SmCl₃ (commercial, 6.5 × 10⁻⁵ mol cm⁻³) melt in the potential range 0 to −1.2 V at different scan rates ranging from 75 to 800 mV s⁻¹ (see Fig. 4a). It is observed that the peak potentials do not change with the scan rates. The shape of the voltammograms resemble closely to reversible soluble-soluble system in the melt. Fig. 4b shows the CVs recorded in equilibrated LiCl-KCl-SmCl₃ (~3.08 × 10⁻⁴ mol cm⁻³) melt at different scan rates from 50 to 500 mV s⁻¹ at 773 K. The number of electrons transferred in the reduction process is calculated using the equation

where E_p is cathodic peak potential (V), E_p/2 is cathodic half peak potential (V), R is the universal gas constant (8.314 J K⁻¹ mol⁻¹), T is the absolute temperature (K) and F is the Faraday constant (96485 C mol⁻¹).

Details of the peak analyses are given in Table 4 and it is seen that the difference between cathodic peak potential E_p (V) and cathodic half peak potential E_p/2 (V) remains almost same for each scan rate.

The value of n has been estimated using Eq. (2) and listed in the Table 4. The number of electron transfer for LiCl-KCl-SmCl₃ (commercial) is found to be 1.08 ± 0.03, which points to the fact that Sm³⁺ reduction is a one electron transfer process and the reduction is expressed as

It is not feasible to study the Sm²⁺/Sm redox system in this electrochemical window as the reduction potential of Sm²⁺/Sm is more negative than Li⁺/Li [37]. Similar observations have been reported by Cordoba and Castrillejo et al. [38,39].

The number of electron transfer for equilibrated LiCl-KCl-SmCl₃ is calculated as 1.02 ± 0.02 which indicates that Sm³⁺ is reduced to Sm²⁺. Identical number of electron transfer is calculated for the commercially procured SmCl₃ (6.5 × 10⁻⁵ mol cm⁻³). The cathodic peak current of CV is related to the potential scan rate according to the Randles-Sevcik equation [40]:

Where, I_p is the cathodic peak current (in Amp), A is the electrode area (cm²), C₀ is the solute concentration (mol cm⁻³), D is the diffusion coefficient (cm² s⁻¹) and ν is the potential sweep rate (V s⁻¹).

In Fig. 5a, the I_p is plotted against ν for LiCl-KCl-SmCl₃ (commercially procured SmCl₃, C₀~6.5 × 10⁻⁵ mol cm⁻³) and for equilibrated LiCl-KCl-SmCl₃ (C₀~3.08 × 10⁻⁴ mol cm⁻³) melt. The linear dependency of I_p on ν suggests that the reduction is reversible and diffusion controlled. The values of D are calculated for LiCl-KCl-SmCl₃ (commercially procured SmCl₃, C₀~6.5 × 10⁻⁵ mol cm⁻³) and for equilibrated LiCl-KCl-SmCl₃ (C₀~3.08 × 10⁻⁴ mol cm⁻³) melt from the slope of the I_p vs ν plot (Fig. 5a) using Eq. (3) at 773 K. The values are found to be 1.23 × 10⁻⁵ cm² s⁻¹ (a) and 1.19 × 10⁻⁵ cm² s⁻¹ (b), respectively. So the D value for Sm⁺³ ions of the equilibrated SmCl₃ is very close to that of LiCl-KCl-SmCl₃ (commercial) system and is also in good agreement with the report of Castrillejo et al. (~1.04 × 10⁻⁵ cm² s⁻¹) [39] and Cordoba et al. (~1.30 × 10⁻⁵ cm² s⁻¹) [38].

The plot of cathodic and anodic peak potentials ( Epc and EpA) vs log ν in Fig. 5b shows that the peak potentials values do not change with increasing scan rate. From the above plots (Fig. 4 and 5) and discussions, it is evident that the reduction of Sm³⁺ to Sm²⁺ is a reversible process controlled by the diffusion of the Sm³⁺ ions in the solution. The relative rate of electron transfer is greater than that of mass transfer and Nernstian equilibrium is established at the electrode surface immediately upon any change in applied potential.

Equilibrated LiCl-KCl-SmCl₃ is compared with LiCl-KCl-SmCl₃ (commercial) by SWV method. SW voltammograms are recorded for LiCl-KCl-SmCl₃ (commercial, 6.5 × 10⁻⁵ mol cm⁻³) as well as in equilibrated LiCl-KCl-SmCl₃ (~3.08 × 10⁻⁴ mol cm⁻³) in the potential range −0.2 to −1.2 V at amplitude of 10 mV with varying pulse frequencies at 773 K. In Fig. 6, the representative SWV recorded at 12 Hz for LiCl-KCl-SmCl₃ (commercial, 6.5 × 10⁻⁵ mol cm⁻³, marked a) shows only one cathodic peak at −0.66 V, which is due to the reduction of Sm³⁺ ions in the melt [38,39]. In the same figure the representative SWV shown for equilibrated LiCl-KCl-SmCl₃ melt (~3.08 × 10⁻⁴ mol cm⁻³, marked b) exhibits a cathodic peak at −0.67 V, which confirms the formation of SmCl₃ from Sm₂O₃.

Osteryoung and Ramaley et al [41,42] showed that for the ideal cases involving reversible system, the differential peak current δi_p is directly proportional to the concentration of the electroactive species and square root of pulse frequency ( f) following the Eq. (4) [43]

where, Γ=exp(nFΔE2RT)

wWhere ΔE is the amplitude of square wave potential, A is the electrode area (cm²), C₀ is the solute concentration (mol cm⁻³), D is the diffusion coefficient (cm² s⁻¹).

Fig. 7 shows the plot of δi_p vs f for LiCl-KCl-SmCl₃ (commercial, 6.5 × 10⁻⁵ mol cm⁻³) and equilibrated LiCl-KCl-SmCl₃ (~3.08 × 10⁻⁴ mol cm⁻³), respectively. Linear dependency of δi_p vs f is observed in both the cases, implying the reversibility of the process. The number of electrons involved in the electrode reaction is determined by measuring the half-peak width W_1/2 (V) in the linear region of the above plot from the Eq. (5)

For a SWV in LiCl-KCl-SmCl₃ (commercial, 6.5 × 10⁻⁵ mol cm⁻³) at frequency of 12 Hz and amplitude of 30 mV, the value of n is estimated as 0.98 ± 0.03, which confirms the fact that Sm³⁺ is reduced to Sm²⁺. The number of electron transfer for equilibrated LiCl-KCl-SmCl₃ (~3.08 × 10⁻⁴ mol cm⁻³) is determined as 0.97 ± 0.02 for a SWV at frequency of 12 Hz and amplitude of 40 mV, which is similar to commercial SmCl₃ reduction and reflects the reduction of Sm³⁺ to Sm²⁺. The values of n determined with Eq. (2) and (5) are very close (see Table 4). In the process, CV and SWV studies confirm the presence of Sm³⁺/Sm²⁺ redox couple in LiCl-KCl validating the conversion of Sm₂O₃ to SmCl₃.

Powder XRD analyses are performed with the products obtained after conversion of Sm₂O₃ to SmCl₃ to ascertain the conversion reaction. XRD of LiCl-KCl salt (Table 3, E-2-10) after 6 h of equilibration of Sm₂O₃ and AlCl₃ (Fig. 8a) shows the diffraction peaks for only LiCl and KCl. Hence, another experiment is carried out in absence of LiCl-KCl (Table 3, E-2-S) for eliminating the salt matrix effect in XRD pattern. The reaction is carried out by placing homogenously mixed Sm₂O₃ and AlCl₃ (1: 1.5 wt. ratio) for 2 h at 773 K followed by cooling and grinding. The procedure is repeated every 2 h and total 12 h of heating is performed. The XRD pattern of the product is shown in Fig. 8b where presence of SmCl₃ is observed. Powder XRD analysis of the product also proves the conversion of Sm₂O₃ to SmCl₃ with the aid of AlCl₃.

Conversion reactions of Sm₂O₃ to SmCl₃ are carried out in LiCl-KCl media using different concentrations of AlCl₃ at 773 K (see Table 1), duration of the reaction (see Table 2) and the amount of LiCl-KCl (see Table 3).

In the present study, sample F-1 (water dissolved part) is analysed by fluorescence spectroscopic method to ascertain the conversion of Sm₂O₃ to SmCl₃. Fig 9 shows the emission spectra recorded with the F-1 (marked a, Table 2, E-2-6) in the wavelength range 530–675 nm with an excitation wavelength (λ_ex) of 402 nm and compared with the standard SmCl₃ (commercial) solution in the same wavelength range (plot, marked b). There are three emission peaks (in both the cases) located at 559 nm, 594 nm and 643 nm correspond to the Sm³⁺ intra-4f transition from the excited levels to lower levels, the ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 transitions, respectively. It is seen from the figure that the characteristic emission peaks of Sm in the filtrate are well matched with that of the standard and it is in line with the earlier studies on fluorescence spectra of Sm³⁺ ions in molten LiCl-KCl as well as in different phosphors [44,45]. This clearly indicates to the presence of SmCl₃ in the F-1 and confirms the conversion of Sm₂O₃ to SmCl₃.

Fig. 10 shows absorption spectrum recorded for F-1 (marked a in the plot, Table 2, E-2-6) over 330–450 nm. For comparison, spectrum is recorded for standard SmCl₃ (commercial) solutions and is shown in Fig. 10 (marked b). Several absorption peaks are observed between 330–450 nm correspond to the typical f-f transition of Sm³⁺ ions. The peaks centered at 345, 363, 375, 391, 402, 417 and 441 nm are attributed to ⁶H_5/2→ ³H_7/2, ⁶H_5/2→ ⁴F_9/2, ⁶H_5/2→ ⁴D_5/2, ⁶H_5/2→ ⁶P_7/2, ⁶H_5/2→ ⁴K_11/2, ⁶H_5/2→ ⁶P_5/2 + ⁴M_19/2 and ⁶H_5/2→ ⁴G_9/2 + ⁴I_15/2 transitions [44]. It is seen from the figure that the characteristic absorption peaks of Sm in the filtrate are well matched with that of the standard. This clearly indicates the presence of SmCl₃ in the filtrate. The peak absorbance of the strongest absorption peak at 402 nm, which corresponds to ⁶H_5/2→ ⁴K_11/2 transition is used here for quantification of SmCl₃. Absorbance spectra are measured by varying concentrations (0.250–10 mg mL⁻¹ ) of standard SmCl₃ (commercial) solutions.

Fig. 11 shows a linear calibration plot for standard SmCl₃ (commercial) solutions in the range 0.250–10 mg mL⁻¹ used for quantitative analysis (marked a). As the filtrate always contains LiCl-KCl salt as matrix and some amount of unreacted AlCl₃, the effect of them is examined by plotting calibration plots in the presence of salt and AlCl₃. Fig. 11 (marked b) shows the calibration plot of SmCl₃ in presence of 150 mg mL⁻¹ salt and 9 mg mL⁻¹ of AlCl₃. These concentrations of salt and AlCl₃ are chosen as the concentration of salt and AlCl₃ in the filtrate is in the range 120–150 mg mL⁻¹ and 4–9 mg mL⁻¹ (assuming 50% consumption of AlCl₃), respectively. The uncertainty in each data point is ≤ 2%. It is seen that the slope of both the plots remains almost same (within ~ 0.3%). This clearly indicates that the absorbance values do not vary for salt up to 150 mg mL⁻¹ and AlCl₃ up to 9 mg mL⁻¹. Absorbance values are recorded in the F-1 (Table 1: sample E-1 to E-3, Table 2: sample E-2-1 to E-2-6, Table 3: sample E-2-7 to E-2-S) as well as in the F-2 and from the calibration plot (see Fig. 11, marked b), the concentration of Sm is determined in both the filtrates. The analysis of F-1 shows the amount of SmCl₃ which is converted from Sm₂O₃ after chlorination.

Using the absorbance values in F-1, amount of SmCl₃ produced after equilibration in LiCl-KCl is calculated from the calibration plot (Fig 11, marked b). The conversion efficiency is calculated using the equation below

When stoichiometric amount of AlCl₃ are added in the reaction (Table 1, E-1), the conversion efficiency of Sm₂O₃ is found to be 67.4 (± 0.9) wt.%. Some amount of AlCl₃ loss may have occurred due to the sublimation of AlCl₃ as white vapours are observed when it is added into the melt at 773 K [46]. To improve the efficiency of the conversion process, the amount of AlCl₃ is increased from the stoichiometric amount in the conversion reactions. 85.3 (± 1.5) and 83.6 (± 1.9) wt.% conversion is found when of AlCl₃ is double and triple of the stoichiometric amount, respectively. The conversion efficiencies obtained by UV-Vis spectroscopic analyses are compared with ICP-AES analysis (Table 1) and these are found to be in good agreement.

The amount of AlCl₃ present in the melt after conversion experiments are analysed by ICP-AES. The analysis shows absence of Al in sample F-1 when stoichiometric amount of AlCl₃ is used. This reflects that the stoichiometric amount has been utilized fully and may not be sufficient for full conversion of Sm₂O₃. When amount of AlCl₃ is increased, the analysis shows the presence of ~ 6 wt.% and ~ 20 wt.% of initial AlCl₃ in the F-1 for reactions with double and triple of the stoichiometric of AlCl₃, respectively. This reflects that in experiment with triple of stoichiometric of AlCl₃, the unreacted amount of AlCl₃ in the melt only increases, but fails to increase the conversion efficiency. Hence, double the stoichiometric amount of AlCl₃ is sufficient for efficient conversion reaction.

To improve the conversion efficiency further, experiments are performed with 50 g of LiCl-KCl with double the stoichiometric amount of AlCl₃ and duration of reaction is varied from 0.25 h to 12 h (Table 2, sample E-2-1 to E-2-6). Fig. 12 shows the plot of conversion ratio of Sm₂O₃ in salt phase (UV-Vis analysis) vs. duration of reaction. It shows that the reaction reaches to 94.8 (± 0.9) wt.% of conversion of Sm₂O₃ within 15 min (Table 2, E-2-1). Almost same conversion efficiency is observed even after increasing the duration of reaction up to 12 h. It is noted that the values of the conversion efficiencies are close to the values obtained from ICP-AES analysis (Table 2). As the conversion reaction reaches ~ 95 wt.% efficiency within 15 min, it may affect the conversion process of other lanthanides and actinides when Sm₂O₃ is present in mixture of oxides. Analysis of Sm in sample F-2 shows 3.3 – 5.0 wt.% Sm₂O₃ present as unreacted in the melt after conversion reactions, which corresponds to 98–99 wt.% of total Sm₂O₃ used for experiments.

It is observed that the amount of LiCl-KCl salt also has impact on the conversion process. When 30 g of LiCl-KCl salt is used for chlorination reaction (Table 3, sample E-2-7), the oxide particles are found to be floating on the top surface of the molten salt and stirring is difficult in the colloidal mixture. As the reactants do not mix properly in less amount of molten salt, adherence of the reactants has been observed on the side of the crucible leading to decrease in process yield and therefore, maximum ~ 83–85 wt.% conversion of oxide is achieved. To get further improvement in the conversion, the quantity of solvent (salt) is increased and equilibration reactions are conducted using double the stoichiometric amount of AlCl₃. Fig. 13 shows the plot of conversion ratio of Sm₂O₃ in salt phase (UV-Vis analysis) as a function of mass of LiCl-KCl salt. It can be seen that about 95 wt.% conversion is achieved when amount of salt is raised to 50 g. No further improvement is recorded even when salt amount is raised to 100 g. The ICP-AES analysis also reflects the same and the values obtained are provided in the Table 3.

This indicates that 30 g of LiCl-KCl salt is insufficient for proper mixing of the reactants and hence the reaction cannot attain maximum efficiency. Conversion reaction is carried out in absence of LiCl-KCl (Table 3, sample E-2-S). Analysis of the product shows around 64.5 wt.% of samarium oxides has been converted to chlorides. In absence of any medium, AlCl₃ vaporised at much higher rate than in dissolved condition in molten LiCl-KCl. As solvent increases the chance of collisions of reactants, conversion efficiency of oxides in LiCl-KCl is higher than the reaction in absence of LiCl-KCl.