Phase Diagram of the System LaCl₃-CaCl₂-NaCl

Abstract

The phase diagram of ternary system LaCl3-CaCl2-NaCl has been constructed from the phase diagrams of the three binary systems and of thirteen quasi-binary systems determined by DTA. For the binaries LaCl3-CaCl2 and CaCl2-NaCl eutectic points were observed at 651 °C , 35.1 mol% LaCl3 and at 508 °C , 49.9 mol% NaCl, respectively. For LaCl3-NaCl, a peritectic point besides the eutectic point at 545 °C , 36.1 mol% LaCl3 was found at 690 °C , 57.5 mol%, which is attributable to the formation of the peritectic compound 3 LaCl3 · NaCl. The phase diagram of the ternary system has a ternary eutetic point and a ternary peritectic point due to 3 LaCl3-NaCl, the form er at 462 °C and 12.1 - 3 9 .7 - 4 8 .2 mol% (LaCl3-CaCl2-NaCl) and the latter at 612 °C and 26.9 - 55.1 - 18.0 mol%.

Local structure of molten LaCl3 analyzed by X-ray diffraction and La–LIII absorption-edge XAFS technique

“…In contrast, the corresponding CN in solid LaCl 3 was 9, as described in [26]. This small difference in CN from 9 (solid) to 8.2 (melt) would be rather difficult to be explained from the large expansion in molar volume on melting, 19.1% [29] since the change of CN from 6 (solid) to 4.0-4.5 (melt) on melting of several alkali halides corresponded to the volume change of about 20%. There was also found a discrepancy between the results of XRD and XAFS concerning the coordination number of La-Cl pair.…”

“…From detailed investigation of the peak heights and half widths in Fig. 5, the local structure of solid LaCl 3 is found to be different from that at RT, giving decreases of coordination number (6.00 + 3.21 → 8.03) and interatomic distance (0.278 nm, 0.305 nm → 0.279 nm), implying the existence of another stable state in solid LaCl 3 before melting at 1150 K [28]. The decrease in coordination number for La-Cl pair from RT to 1173 K is thought to be due to the increase in molar volume and to be well interpreted by that on melting, for instance, 19.1% [29].…”

Thermodynamic assessment of LiCl–NdCl3 and LiCl–PrCl3 quasi-binary systems

“…According to the unpublished work of Sun and Qiao [31] on molten salt phase diagram evaluation by pattern recognition, the existence of the compound LiNd 2 Cl 7 in the LiCl-NdCl 3 system was strongly supported. It is intriguing that the compound LiNd 2 Cl 7 reported by Kim and Okamoto [19] and Sun and Qiao [31] was not confirmed by XRD or even not detected by other experimental researchers [17,18,[20][21][22][23][24][25][26][27][28][29][30]. Two probable reasons may be provided: one is that the compound only exists in a very limited temperature range, the phase transition should have occurred during the quenching of the XRD sample.…”

“…The other one is that the amount and purity of melts (NdCl 3 , LiCl) in the sample affect the DTA and XRD measurement seriously. As a matter of fact, the measured liquidus line in the NdCl 3 -rich side as well as the reported melting temperature of pure NdCl 3 differed greatly between the different investigations [17][18][19][32][33][34][35][36][37]. The abnormal angle in the measured NdCl 3 -rich side liquidus line from Zhang et al [17] or the systematically lower temperatures reported by Igarashi and Mochinaga [18], respectively, should be related to the poor purity of samples, especially NdCl 3 .…”

“…As a matter of fact, the measured liquidus line in the NdCl 3 -rich side as well as the reported melting temperature of pure NdCl 3 differed greatly between the different investigations [17][18][19][32][33][34][35][36][37]. The abnormal angle in the measured NdCl 3 -rich side liquidus line from Zhang et al [17] or the systematically lower temperatures reported by Igarashi and Mochinaga [18], respectively, should be related to the poor purity of samples, especially NdCl 3 . The NdCl 3 as well as PrCl 3 samples used by Kim and Okamoto [19] were purified by sublimation in vacuum 10 −3 Pa at 1273 K, and LiCl was dried at 573 K in vacuum better than 10 −1 Pa for two days.…”