New synthetic [LREE (LREE = La, Ce, Pr, Sm), Pb]-phosphate phases

Comparison of the patterns and identification of synthesis products based on PXRD is shown in Figs. 1 and 2. Control synthesis from solutions containing only Pb and PO₄ ions resulted in precipitation of PbHPO₄. This is a synthetic analogue of ‘phosphoschultenite’ (ICDD card No 06-0274). The name of the phase, proposed by Młynarska et al. (2014) and Wudarska et al. (2022) is not yet approved by IMA. The other control synthesis from solutions containing only LREE and PO₄ ions resulted in precipitation of hydrated phosphates with a monoclinic, rhabdophane structure: LaPO₄·0.66H₂O, CePO₄·0.66H₂O, PrPO₄·0.66H₂O and SmPO₄·0.66H₂O (based on earlier work by Mesbah et al. (2014) with which our results showed excellent agreement). The PXRD patterns were identical with a slight shift towards high angles resulting from decreasing ionic radius of lanthanides from La to Sm (Fig. 2).

Figure 1.

Figure 2.

In contrast, phosphate syntheses using solutions containing both Pb and LREE ions yielded crystalline precipitates with PXRD patterns very similar to each other (Fig. 3) but distinct from both PbHPO₄ and rhabdophanes. A slight shift in peak positions towards high angles resulting from different unit cell parameters of structures accommodating LREE with decreasing ionic radius was observed. This was identical to the shift observed for control rhabdophanes, indicating that La, Ce, Pr and Sm coprecipitated with Pb phosphates as analogous crystalline phases. The similarities and differences between the patterns are illustrated in Fig. 4 using La and Sm phosphates. The patterns of (LREE,Pb)-phosphates were very different from the patterns of control ‘phosphoschultenite’ indicating that this was not a REE-substituted PbHPO₄. They were also significantly different from other phases such as xenotime (HREEPO₄, tetragonal), monazite (LREEPO₄, monoclinic), or mixite group minerals (e.g., petersite Cu₆Ce(PO₄)₃(OH)₆·3H₂O).

Figure 3.

Figure 4.

Despite the differences in the 10 – 30° 2Θ range, the diffraction patterns of (LREE,Pb)-phosphates show some resemblance to control rhabdophanes. Pb-containing rhabdophanes have been previously described by Walenta (2003). To explain the differences in PXRD patterns, Rietveld modelling of rhabdophanes with LREE partially substituted by Pb was attempted but did not give successful convergence (Fig. S1). This could be partly due to the poor crystallinity of the phases and the low quality of the PXRD data. The similarity of patterns between all four (LREE,Pb)-phosphates, however, indicates that the precipitate probably consists of a single phase, rather than a mixture of two phosphates. Indexing attempts by GSAS-II (Fig. S2) resulted in all cases in poor convergence at orthorhombic system, Cmmm space group (Table 2). Unit cell parameters vary linearly with the ionic radius of LREE (Fig. S3) This needs to be confirmed in future by optimizing the synthesis procedure to produce larger grains for single-crystal X-ray diffraction analysis.

To observe the possible effect of pH, the synthesis of La and Sm phosphates was repeated identically at pH 2, 3 and 4. No effect of pH was observed on the composition or structure of phosphates synthesized in the absence and presence of Pb at different pH. Comparison of diffraction patterns showed peak broadening with increasing pH which was particularly apparent at pH = 4 (Fig. 5). The feature is the most pronounced for rhabdophanes. This was partly due to the decreasing size of the crystallites with increased pH (consistent with the results of SEM imaging presented below), and partly due to the poor crystallinity associated with the higher degree of hydration (in accordance with the results of DTA/TG).

Figure 5.

SEM imaging confirmed the interpretation of the diffraction curves. All rhabdophanes crystallized in the similar form of 1-2 μm long circular needles (Fig. 6) similar to rhabdophanes reported in the literature (Gausse et al., 2016; Patra et al., 2005; Terra et al., 2003). Increasing pH resulted in the formation of smaller and more aggregated fibrous forms (Fig. S4) as previously reported by Sordyl et al. (2022) and in accordance with the broadened peaks on PXRD patterns. In contrast, phosphates containing LREE and Pb crystallized as needles in spherical aggregates (Fig. 7). The diameter of the aggregates was ~0.5 μm and decreased with increasing pH of the synthesis (Fig. S5).

Figure 6.

Figure 7.

Elemental microanalysis of the La-P, Ce-P, Pr-P and Sm-P using SEM/EDS (Fig. S6, Table S1) revealed that the molar ratio of LREE:P was close to 1. This indicated the chemical formula: (LREE)PO₄·nH₂O consistent with rhabdophanes. The molar ratio of O:P increased at higher pH (for La and Sm phases), which may indicate the increasing hydration of the samples.

Wet chemical analysis of LaPb-P, CePb-P, PrPb-P and SmPb-P indicated that molar ratio of (LREE+Pb):P was between 1.27 and 1.38. The content of LREE with respect to Pb was somewhat higher for SmPb-P and PrPb-P (0.73 and 0.77, respectively) than for LaPb-P and CePb-P (0.68 and 0.65, respectively). Based on these results, calculated chemical formulas of newly discovered phases are: La₂Pb₃(PO₄)₄·nH₂O, Ce₂Pb₃(PO₄)₄·nH₂O, Pr₂Pb₃(PO₄)₄·nH₂O and Sm₂Pb₃(PO₄)₄·nH₂O. This way, the charges are balanced and molar ratios assumed in the chemical formula are close to these resulting from chemical analysis: (LREE+Pb):P equal to 1.25 and LREE:Pb equal to 0.67. This formula is different from natural phase Pb(Ce,REE)₃(PO₄)₃(OH)₂·nH₂O, known as UM2006-35-PO:HPbREEY, described by Plasil et al. (2009).

The IR spectrum of each phase is dominated by strong phosphate bands (broad, not fully resolved P-O ν₁ symmetric and ν₃ asymmetric stretching modes between 920 and 1070 cm^–1) and a broad band around 3455 cm^-1 which was assigned to O-H stretching (ν₁/ν₃) modes in water molecules indicating that analyzed phases are hydrated (Fig. 8). The relatively small band apparent at around 1630 cm^-1 was attributed to H-O-H bending (ν₂) mode. All these phases are hydrated (Clavier et al., 2018). The relative intensity of these bands increased with the increase of pH indicating that phases precipitated at higher pH are more hydrated. However, no shift in position of bands was observed as a function of pH. Two or three bands, depending on the sample, around 540, 570 and 610 cm^–1 were assigned to O-P-O ν₄ asymmetric bending mode. Spectra of phases lacking Pb contained broad, low intensity band with maximum at around 2000 cm^–1 that was attributed to series of overtones of phosphate group (Manecki et al., 2000). Those overtone bands were not present in the spectra of phosphates containing Pb. This difference can be associated with high stability of Pb(II) – PO₄ bonds due to the presence of lone pair electrons (LEP) in coordination sphere of the Pb(II) by phosphate groups (Krivovichev, 2003). Also, splitting of two very intense bands (around 615 and 540 cm^–1 for Pb-free phases) into three of lower intensity (around 600, 570 and 530 cm^–1 for phosphates containing Pb) may suggest deformation of inner structure resulting from the presence of Pb ions. Small shift of the position of strong phosphate modes between 920 and 1070 in the phosphates containig Pb indicated partial substitution of Pb for LREE in the structure of these hydrated phosphates. The band at 1385 cm^-1 was attributed to the NO₃^- group. Most likely it was the artifact of residual NaNO₃ that was not washed out completely after the synthesis (Diaz-Guillén et al., 2007). The spectra of control samples confirmed the formation of rhabdophanes (Diaz-Guillén et al., 2007; Heuser et al., 2014). The spectra of (LREE,Pb)-phosphates could not be assigned to any known mineral or synthetic phase.

Figure 8.

The thermogram plots of (LREE,Pb)-phosphates and Pb-free controls are shown in Fig. 9 (TG, DTA and QMS, m/z = 18 – assigned as water vapor). The weight loss in two or three steps was observed similarly to the previously reported findings (Lucas et al., 2004). Temperature ranges of those steps were estimated based on DTA and QMS plots. For Pb-containing phases, the first two steps could not be precisely distinguished and were merged in one. The first step occurred below 200°C with an increase of intensity of the QMS plot and was associated with release of residual moisture adsorbed at the surface of the powders. The amount of adsorbed water varied between 3 and 10 wt% depending on crystal size affected by pH of the synthesis: the higher pH the smaller crystallites and the higher content of residual water. This varied mostly for control LREE rhabdophanes, while for (LREE,Pb)-phosphates variation of moisture content was minimal.

Figure 9.

The second step occurred in the 170-300°C range and was most probably associated with the dehydration. The weight loss of 3-4 wt% was determined for control Pb-free phases. The lowest weight loss was observed for Ce-P at around 3.1 wt%. For phases containing Pb, cumulative weight loss during those two steps was on average between 4-5 wt%. Only SmPb-P synthesized at pH=4 showed weight loss around 8 wt% with a significant share below 200°C (possibly residual moisture). The lowest weight loss was observed for PrPb-P at around 4.1 wt%.

The third step occurred between 450 and 650°C and was accompanied by a water QMS signal with a maximum around 550-580°C. Based on the presence of O-H stretching band around 3455 cm^–1 in FTIR spectra (Fig. 8), this increased QMS signal could be associated with dehydroxylation. In majority of analysed phases, no mass changes could be associated with this effect. Possible presence of hydroxyl groups is not certain but is negligible – the QMS signal and weight loss in this range were not as significant as in the first and second steps. Phases containing Ce and Pr did not exhibit increase in water QMS signal in this temperature range.

Endothermic effects present on DTA plots of phases containing Pb appeared above 850°C with varying temperatures of maximal intensities for each effect for different pH conditions during synthesis. For LaPb-P, the temperature of those effects increased with pH, while for SmPb-P temperature dropped. For CePb-P and PrPb-P those effects appeared near 1000°C. These effects appeared probably during phase transformations from the obtained (LREE)Pb-P phases to LREE oxides and anhydrous phosphates.

A total weight loss measured during the second step allows to determine the hydratation ratio n of analysed Pb-free control phosphates. The average n ratio is equal to 0.5 mole, resulting in the composition (LREE) PO₄·0.5H₂O. Total cumulative weight loss in the first and second steps correspond to the n ratio of Pb-containing phases. The average n ratio is equal to 3.5 mole for LaPb-P, 3.3 mole for CePb-P, 3.1 mole for PrPb-P, and 3.3 mole for SmPb-P. Therefore, the calculated chemical formulas of newly discovered phases are La₂Pb₃(PO₄)₄·3.5H₂O, Ce₂Pb₃(PO₄)₄·3.3H₂O, Pr₂Pb₃(PO₄)₄·3.1H₂O and Sm₂Pb₃ (PO₄)₄·3.3H₂O. A trend can be seen in the general decreasing hydration ratio of REE-phosphates phases in the order from lighter to heavier REE. This is similar to previously reported Pb-free hydrous REE-phosphate phases (Shelyug et al., 2018).