Monoclinic Form of the Rhabdophane Compounds: REEPO4Â·0.667

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The monoclinic form of the rhabdophane compounds: REEPO4 . 0.667 H2O Adel Mesbah, Nicolas Clavier, Erik Elkaim, Gausse Clemence, Ilyes Benkacem, Stéphanie Szenknect, and Nicolas J Dacheux Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg500707b • Publication Date (Web): 02 Sep 2014 Downloaded from http://pubs.acs.org on September 4, 2014

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Crystal Growth & Design

The monoclinic form of the rhabdophane compounds: REEPO4 . 0.667 H2O Adel Mesbah†*, Nicolas Clavier†, Erik Elkaim‡, Clemence Gausse†, Ilyes Ben Kacem†, Stephanie Szenknect†, and Nicolas Dacheux†. †

ICSM, UMR 5257 CNRS / CEA / UM2 / ENSCM, Site de Marcoule - Bat 426, Bp 17171, 30207 Bagnols/Ceze, France ‡

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France Abstract: Hydrated rhabdophane with a general formula REEPO4 . nH2O (REE: La→Dy) has been always

considered to crystallize in the hexagonal system. A recent re-examination of this system by the use of synchrotron powder data of the SmPO4 . 0.667 H2O compound led to a structure crystallizing in the monoclinic C2 space group with a = 28.0903(1) Å, b = 6.9466(1) Å, c= 12.0304(1) Å, β = 115.23(1)° and V = 2123.4 (1) Å3 with 24 formula units per unit cell. The structure consists of infinite channels oriented along the [101] direction and formed by the connection of Sm-polyhedra and P-tetrahedra by sharing O-edges. The water molecules filling the space have been localized for the first time. The monoclinic form of the hydrated rhabdophane was confirmed by studying the series with REE: La→Dy. Moreover, the dehydration of SmPO4 . 0.667H2O led to the stabilization of an anhydrous form SmPO4 in the C2 space group with a = 12.14426 (1) Å, b = 7.01776(1) Å, c= 6.34755(1) Å, β = 90.02 (1)°, V = 540.97(1) Å3 and Z =6.

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1. Introduction The chemistry of Rare-Earth Elements phosphate compounds (REEPO4 . nH2O) has been widely studied in the past decades, among them: monazite, xenotime and rhabodophane phases.1-6 This interest stems not only from their easy way of synthesis leading to homogenous compounds but also from their low solubility constants

7-9

and their various luminescent properties.

10-15

In

consequence, they allow a wide range of possible applications such as cell biology, photon up conversion, catalysts, laser materials, proton conduction.15-19 From a geochemical point of view, monazite (LnPO4) represents one of the most abundant sources of lanthanide observed in nature. This mineral also appears to be a major source of thorium on earth.6, 20 Moreover, considering its high chemical durability and structural flexibility, monazite-based ceramics are also considered to be used as specific matrices for the conditioning of high level radioactive wastes.21, 22 Hydrated compounds with general formula of LnPO4 . nH2O (Ln = La - Gd), namely rhabdophane-type, are usually considered as lowtemperature precursors for monazites. The advantage of their use arises from their simple synthesis through wet chemistry routes and their easy conversion into monazite through hightemperature heat treatment. Moreover, they could play an important role during the alteration of monazites as neoformed phases, and are thus controlling the solubility of actinides during their release.20,

21, 23

Indeed, owing to their very low solubility constant, the formation of hydrated

lanthanide phosphates at the surface of the leached pellets usually leads to the formation of protective layers, which can delay significantly or even stop the release of the leached radioactive elements. 23-25 The nature of the neoformed phase LnPO4 . n H2O depends strongly on the ionic radii and also on the temperature during the leaching phenomena.23 For example, monazite is precipitated at 393K for the lighter lanthanide elements (La, Ce) while rhabdophane compounds are formed from neodymium to dysprosium and the xenotime phases for heavy lanthanides (Ho-Lu). For higher temperatures, the monazite-type materials were found to be the stable phase instead of the rhabdophane.23 However, when studying the solubility of some hydrated lanthanide phosphates with implications for diagenesis and sea water concentrations, it was suggested that the rhabdophane/monazite relationship was still unclear; rhabdophane being often considered as a “meta-stable” phase compared to monazite in geochemical environment.7, 8, 23, 26


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Regarding to their structural properties, the LnPO4 . n H2O compounds display a rich variety of crystallographic structures depending on the nature of the RE element. The structure of hydrated REEPO4 . nH2O (REE = La → Dy) was reported to be hexagonal (P6222 or P3121) by Mooney3, 4, whereas the anhydrous form (monazite) crystallizes in the monoclinic P21/c structure. For the heavy 4f elements with REE = Tb → Lu and Sc, Y, the REEPO4 compounds (xenotime) crystallize in the tetragonal (I41/amd) zircon structure type1,

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. Moreover,

GdPO4 . 2H2O28 (weinshenkite) was reported in a monoclinic system and an orthorhombic system was suggested for DyPO4 . 1.5 H2O but the crystallographic structure needs to be confirmed.28, 29 The crystallographic structure of the rhabdophane was reported by Mooney in the hexagonal system from powder X-ray data for the compounds LnPO4 . 0.5 H2O (Ln = La, Ce and Nd). Therefore two close models have been proposed in the P31213 and P62224 space groups, but without any accurate location of the water molecules despite the confirmation of their presence in the zeolitic channels according to Mooney4. Almost all of the studies on the rhabdophane structure type based materials have been dedicated to the impact of the crystal shape, composition on their luminescence properties and have taken the hexagonal model as structure, despite that the crystallographic positions of the water molecules remain unknown. Recently, our attempts to perform a Rietveld refinement in the hexagonal system of the PXRD data collected on well crystallized powder led to unsatisfactory results in particular on data recorded using synchrotron radiation. Consequently, a particular attention was paid to the rhabdophane structure in this paper. The structure of the SmPO4 . 0.667 H2O compound was solved from powder data recorded by the use of synchrotron radiation (Soleil, France). It led to a monoclinic C2 structure, with ordered water molecules. This C2 monoclinic structure was also verified for the lanthanides series with Ln = La → Dy. Then a thermal study combining PXRD, TGA and dilatometry was performed, showing a complex dehydration-hydration process depending on the number of water molecules in the channels. Moreover, from PXRD data collected at 300 °C, a new structure crystallizing in the monoclinic C2 space group is proposed after the complete elimination of the water molecules. 3 ACS Paragon Plus Environment


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Finally, the irreversible transition between the anhydrous rhabdophane to monazite was reached as was showed previously by Jonasson and Vance.30 2. Experimental section 2.1. Synthesis The following reactants were used as starting materials: LaCl3 . 7H2O (99.9%), CeCl3 . 7H2O (99.9%), PrCl3 . xH2O (99.9%), NdCl3 . 6H2O (99.9%), SmCl3 . 6H2O (99.9%), EuCl3 . 6H2O (99.9%), GdCl3 . 6H2O (99%), TbCl3 . 6H2O (99.9%), DyCl3 . 6H2O (99.99%) all supplied by Sigma Aldrich. Acid solutions were prepared from H3PO4 (85% Normapur) and HCl (37% Carbo Erba). Because of the hygroscopic character of the lanthanide powders, all the lanthanides salts were dissolved in 1M HCl. The final concentration comprised between 0.5 a 1 M for each resulting solution was further determined by ICP-AES. Each LnPO4 . nH2O compound (Ln = La to Dy) was synthesized by pouring a solution of LnCl3 (4 mmol) into 5M H3PO4 acid considering a 3% molar excess. The mixture was stirred for 15 min at 60 °C, transferred into a Teflon container and placed in oven for 15 days at 90 °C. Then, the resulting powders were washed by centrifugation (4500 rpm, 5 min), twice with deionized water and a final cycle with the use of ethanol. The powders were finally dried in air at 90 °C in oven overnight. 2.2. X-ray diffraction study. Synchrotron powder diffraction Synchrotron powder diffraction pattern of the compound SmPO4 . 0.667 H2O was collected on beamline "Cristal" at synchrotron Soleil in high angular resolution mode using the 2 circles diffractometer equipped with its 21 Si(111) perfect crystals rear analyzer. Data were recorded at room temperature with the powder inside a glass capillary (diameter of 0.3mm) mounted on a spinner to improve particle statistics. A wavelength of 0.6967 Å was selected and calibrated using LaB6 standard NIST powder. With this setup a diagram was collected up to 2θ = 60° in about two hours.


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Laboratory powder diffraction Powder X-Ray Diffraction (PXRD) patterns of all the synthesized LnPO4 . nH2O compounds were collected by the use of the Panalytical X'pert pro diffractometer, equipped with Cu Kα radiation (λ = 1.54184 Å) and the X'Celerator solid detector. All the data were recorded in the Bragg–Brentano geometry at room temperature with 2θ ranging from 5 to 120° by steps of ∆θ = 0.017° and a global counting time of 3 hours per sample. Structure solution: Regarding the unsuccessful attempts to refine the Synchrotron powder pattern of SmPO4 . 0.667 H2O in the hexagonal structure proposed previously by Mooney

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, it appeared

necessary to solve the structure ab initio. Therefore, our synchrotron powder pattern of the SmPO4 . 0.667 H2O compound was indexed in the C2 space group. The suggested unit cell parameters were refined successfully by the Le Bail method with the use of the fullprof_suite program32 and were found to be a = 28.0903(1) Å, b = 6.9466(1) Å, c = 12.0304(1) Å, β = 115.23(1)°. The overall structure was solved by direct methods using the program EXPO 2004.33 The observed, calculated patterns and the difference is viewed in Figure 1 and zooms on selected angular regions are showed in Figures 2 and 3, confirming the superstructure in the C2 space group rather than the hexagonal system. The final refinement was performed using the Rietveld method by using the fullprof_suite program.32 Atomic positions were standardized by the use of structure Tiddy program function in Platon

35

34

in Platon35. The structure was finally checked by the AddSym

and confirmed the C2 space group. The crystal data and the refinement

parameters are reported in the Table 1 and in the CIF files joined in the supplementary materials. 2.3. TGA analyses Thermogravimetric analyses were undertaken thanks to a Setaram Setsys Evolution equipped with a type-S thermocouple (Pt/Pt-10%Rh). After recording of the baseline using an empty alumina crucible (100 µL), weight loss was measured during a heat cycle under moist air. Sample was first heated up to 300°C to reach the complete dehydration (2°C.min-1), then cooled down at room temperature with the same rate to study the rhabdophane rehydration. A second heating step up to 300°C was further performed in the same conditions to check the reversibility of the hydration/dehydration process. 5 ACS Paragon Plus Environment


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3. Results and discussion 3.1 TGA. Thermogravimetric measurements were performed on five hydrated rhabdophane compounds LnPO4 . nH2O (Ln = La, Ce, Nd, Sm and Eu). The experimental amount of water molecules in each compound is summarized in Table 2 and the TGA curve corresponding to the thermal behavior of the SmPO4 . 0.667 H2O compound is viewed in Figure 4. According to our results, all the rhabdophane powders (LnPO4 . nH2O, with Ln = La, Ce, Nd, Sm and Eu) were found to be hydrated with n value ranging from 0.6 to 1. Moreover, the water molecules were eliminated in two steps. Firstly, 0.1 to 0.3 H2O molecule was removed between RT and 150°C, leaving in the channels ~ 0.5 H2O. Then, the remaining water was eliminated between 150 and 240 °C. This two steps dehydration process was already observed in the literature for the hydrated rhabdophane phases.30, 36-38 Nevertheless, the first weight loss of the water molecules was mainly assigned to the water adsorbed onto the powders’ surface due the storage conditions38. However, given the consistency of the dehydration process in all the rhabdophane series, the existence of a subhydrated structure could not be discarded. Thus, the two-step dehydration of the powders could be assigned to the successive transformations of a subhydrate structure to an hemihydrate form then finally to the anhydrous compound. 3.2 Structure The structure of the SmPO4 . 0.667 H2O compound crystallizes in the monoclinic system in the C2 space group, with 24 formula units in the unit cell with a = 28.0903(1) Å, b = 6.9466(1) Å, c = 12.0304(1) Å, β = 115.23(1)° and V = 2123.4 (1) Å3. The asymmetric unit contains six Sm atoms, five phosphate groups (PO4)3- and four water molecules (H2O). A general view of the structure along the [101] direction is presented in Figure 5 and metrical data are given in Table 3. The atoms Sm1, Sm4, Sm5 and Sm6 are coordinated to nine oxygen atoms, eight being provided by the phosphate groups and one coming from water molecule to form a polyhedra close to be monocapped square antiprism with an angular and planar side. On the other hand, the atoms Sm2, Sm3 atoms are surrounded by eight oxygen atoms provided only by the phosphate groups leading to the formation of a polyhdra with a geometry similar to a square antiprism. The local coordination of each Sm atoms (Sm1 to Sm6) atoms is viewed in Figure 6. 6 ACS Paragon Plus Environment

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The Sm − O inter atomic distances range between: 2.406(18) − 2.66(2) Å (Sm1), 2.279(18) − 2.603(20) Å (Sm2), 2.38(2) − 2.739(20) Å (Sm3), 2.32(3) − 2.61(2) Å (Sm4), 2.261(14) − 2.749(18) Å (Sm5), 2.35(3) and 2.71(2) Å for Sm6. These values are typical for SmIII− O distances and could be compared to those reported in the literature for Sm2O3 (2.277−2.622 Å) and the SmPO4 in its monazite structure (2.397−2.774 Å).1 The overall structure consists of infinite channels oriented along the [101] direction formed by the connection of six infinite chains. These chains are composed of Sm polyhedra and PO4 tetrahedra through the share of O edges along the [101] direction. Crystallographically, the structure contains two different chains. The first one, namely Ch1, contains water molecules linked

to

the

Sm

atoms

with

the

sequence:

Sm1O8Ow−P1O4−Sm4O8Ow−P3O4−Sm6O8Ow−P4O4−Sm5O8Ow−P5O4−Sm1O8Ow−P1O4. Conversely, in the second chain (Ch2), the Sm polyhedra are free from water and connected as follows:

Sm2O8−P2O4−Sm3O8−P6O4−Sm3O8−P2O4−Sm2O8−P7O4−Sm2O8−P2O4.

Thereafter, a complex connection between both chains leads to the formation of channels. In fact, the chains Ch1 are connected to each other along the b axis by sharing oxygen atoms via corners. The orientation of the water molecules is switched from left to right as showed in Figure 5. The chains Ch1 and Ch2 are also linked via O corners following the [011] direction and then Ch2 chains bridge the parallel Ch1 chains to form infinite channels oriented along the [101] direction. Consequently, each channel is composed by six chains, 4 Ch1 and 2 Ch2. Inside each channel, the water molecules form a kind of infinite wires, with an alternation of short and long distances as showed in Figure 7. These Ow-Ow distances are typical of the presence of H bonds network with three short distances of Ow1−Ow1: 2.733(12) Å, Ow2−Ow2: 2.754(10) Å and Ow3−Ow4: 2.726(12) Å and these pairs are separated by longer distances of 4.478 Å (Ow4−Ow2) and 3.734 Å (Ow1−Ow3).

3.2 Configuration of the water molecules Regarding to the structural properties of SmPO4 . 0.667 H2O, the stabilization of the monoclinic form rather than the hexagonal structure is certainly caused by the arrangement of the water molecules inside the channels which led to super structure of the hexagonal compound reported initially by Mooney.3, 4 In the structure published by Mooney, the water molecules were observed experimentally, but have not been localized in the channels. 7 ACS Paragon Plus Environment


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The difficulties related to the determination of the true space group, and consequently the accurate crystal structure have been already reported in the literature. We can cite, as example, the case of the CaSO4 . n H2O compounds for which controversial discussions occurred about the true crystal structure ( hexagonal or not) and the role of the water amount inside the channels. It was finally shown that CaSO4 . 0.5 H2O 39-41 crystallizes in a C2 space group, while the CaSO4 . 0.625 H2O39, 41 crystallizes in a P3221 space group by doubling the original hexagonal unit cell parameters. However, both structures derivate from the original hexagonal model,3, 4 but differ markedly in terms of the water molecules arrangement. For comparison purposes, the distribution of the water molecules within the channels in the SmPO4 . 0.667 H2O, CaSO4 . 0.625H2O and CaSO4 . 0.5H2O structures is viewed in Figure 8, respectively. In the case of CaSO4 . 0.625 H2O, there are two different channels. Each of them is composed by the connection of six infinite chains. In the first channel, three chains contain water molecules linked to the half of the calcium atoms. While the second channel is similar to that found in the hydrated rhabdophane SmPO4 . 0.667 H2O, the structure of CaSO4 . 0.5 H2O exhibits only one kind of infinite chains. In each chain, the water molecules are connected to half of the calcium atoms as viewed in the Figure 7. Thus in our case, the structure of the hydrated rhabdophane shows similarities with that of the sulfate calcium subhydrate with the existence of two different chains forming the channels. The comparison between these three structures shows the different possibilities of the water arrangement within the structure of these materials. Consequently, it complicates the interpretation of the PXRD patterns. Moreover, as it has been reported in the case of CaSO4 . n H2O, the variation of the water content in the structure could easily lead to a change of the crystallographic structure. Thus, in our case the loss of 0.1 water molecules observed by TGA at low temperature could probably be accompanied by the modification of the structure.

3.2 The monoclinic form of LnPO4 . n H2O (Ln = La → Dy) Based on the structure of SmPO4 . 0.667 H2O, there was an obvious need to check if the LnPO4 . n H2O series with Ln = La → Dy follow the same trend from the crystallographic point of view. Therefore, PXRD patterns were collected on all the synthesized rhabdophane phases 8 ACS Paragon Plus Environment

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and the data were refined by the Rietveld method with the use of the model obtained in C2 space group including the water molecules in the channels. The volume variation of LnPO4 . n H2O (La → Dy) depending on the ionic radii is viewed in Figure 9. A thorough analysis of the PXRD data shows the presence of peaks overlapping instead the presence of single reflections. In addition, the Rietveld refinement confirms that all the compounds of the rhabdophane series are certainly crystallizing in a monoclinic form rather than in the hexagonal system. As expected, the unit cell parameters vary with the nature of the lanthanide element in the structure (Table 4). Although the variation looks linear, the meaning of the obtained values should be examined cautiously. In fact the exact number of water molecules was not refined with high accuracy. Therefore, the values of the volume could deviate slightly for each compound. 3.3 Thermal behavior Since the stabilization of the monoclinic structure was assigned to the presence of water molecules in the channels, it appeared necessary to identify the different steps leading to the monazite structure type. Therefore in situ PXRD experiments assisted by TGA and dilatometric measurements were carried out. The collected PXRD data vs temperature for SmPO4 . nH2O are gathered in Figure 10. Heating the powder up to 300 °C yields to a first structural transformation around 160 °C corresponding to the elimination of water molecules and to the stabilization of the anhydrous SmPO4. Moreover, the reversibility of the dehydration−hydration process was observed during the cooling step around 120 °C. Given the results obtained from TGA experiments, in air, the rehydration of the Sm-rhabdophane reached a rate of 1.5 10-3 mole.min-1 . However, the quantity of re-absorbed water did not reach the initial amount of 0.667 H2O per formula unit and was still lower than 0.5 water molecule. On this basis, and considering the quality of the PXRD data and the behavior of the CaSO4 . n H2O compounds, we can not exclude that the rehydration led to the stabilization of a hemihydrated rhabdophane different from that of SmPO4 . 0.667 H2O. Although the rehydration step showed the compound did not reach the initial amount of water inside the channels. Unfortunately, the data collected by using a laboratory diffractometer (Bruker D8 advance) did not allow us to evidence the crystallographic structure of the potential SmPO4 . 0.5H2O. However, the use of synchrotron data may allow the observation and the characterization of such phase. 9 ACS Paragon Plus Environment


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Finally, as expected, heating the sample up to 1000 °C led to the irreversible transformation of the rhabdophane to the monazite structure (P21/c). Then, in order to characterize the structure of the anhydrous rhabdophane SmPO4, a PXRD pattern was collected at 300 °C to ensure the complete removal of water molecules. Attempts to refine the powder patterns in the monoclinic superstructure by removing the water molecules did not lead to a satisfactory refinement, neither the model reported previously by Mooney in the hexagonal system. However, the indexation of the powder pattern suggested another set of unit cell parameters in the monoclinic C2 space group with a = 12.14426 (1) Å, b = 7.01776(1) Å, c= 6.34755(1) Å, β = 90.02 (1)° and V = 540.97(1) Å3. An initial model of SmPO4 was proposed by EXPO 2004 (direct methods) and the final one was then refined successfully by Fullprof_suite program with Z = 6. The structure data and refinement parameters are gathered in Table 5 and in the joined CIF file (supplementary materials). However, although the consistency of the refinement of the anhydrous form in a monoclinic system, due to the quality of the data collected at 300 °C in an Anton Parr chamber, we should keep in mind the possibility of another superstructure. The use of synchrotron data will thus be very helpful. The crystallographic structure of the anhydrous SmPO4 compounds contains two samarium, two phosphorus and four oxygen atoms. Each Sm atom is coordinated to eight oxygen atoms provided by the phosphate groups by the share of edges down the c axis leading to two independent and infinite chains. The overall structure consists of channels formed by the connection of those chains by the share of oxygen corners. These channels were found to be totally free from the water molecules in agreement with the TGA results which showed the full elimination of water molecules above 160 °C. A general projection of the anhydrous rhabdophane along the c axis is viewed in figure 11. This set of unit cell parameters found in the anhydrous rhabdophane was already observed in the case of the anhydrous CaSO4 compound with a = 12.0777(7) Å, b = 6.9723(4) Å, c = 6.3040(2) Å from neutron powder diffraction data.42 In fact Bezou et al. showed that this latter compound crystallizes in the orthorhombic C222 space group with two independent chains forming infinite channels. Moreover, the variation of the unit cell parameters versus the temperature was obtained from the refinement of the collected PXRD (figure 12). For clarity the unit cell volumes of the hydrated rhabdophane and the monazite have been normalized to six formula units. The structural 10 ACS Paragon Plus Environment

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transition due to the dehydration step is characterized by an increase of the unit cell of about 1%. This is also confirmed by dilatometric measurement carried out on a SmPO4 pellet (figure 13), which shows a swelling of about 1% between 200 and 400°C.21 However, regarding these data, the transition from the subhydrated to the hemihydrate could not be detected. As a summary, the transformation of the rhabdophane phase to the monazite compound over the thermal treatment follows different steps including the transition from SmPO4 . 0.667 H2O to SmPO4. 0.5 H2O then to the anhydrous SmPO4 before the final irreversible conversion to the monazite structure. The rhabdophane phases were mainly synthesized in aqueous solution which explains the presence of water molecules inside the channels. However, from the synthesis point of a view, the control of the amount of the water molecules inside the channels seems to be difficult so far. However, it could be possible during the de dehydration and/or the rehydration steps or also by varying the relative humidity as showed by Schmidt et al in the case of the CaSO4 . 0.5H2O. 39, 41 The stabilization of the rabdophane compounds by precipitation in aqueous solution represents a reliable procedure leading to pure phases with variety of textural and microstructural properties. Moreover, it is in agreement with the different observations found in nature. 7, 8, 23, 26

4. Conclusion An examination of the hydrated rhabdophane structure (SmPO4 . 0.667 H2O) was carried out by the synchrotron powder diffraction data collected at Soleil, France. The study showed that this compound crystallizes in a monoclinic system (C2 space group) instead of the hexagonal structure proposed previously by Mooney.

3, 4

Moreover, for the first time, the water molecules

inside the channels were localized accurately. The organization of water molecules in the channels shows similarities with that found in the CaSO4 . 0.625 H2O compound. Moreover, the combined in situ PXRD, TGA and dilatometry analyses showed a reversible hydrationdehydration process which stabilizes the anhydrous form of the rhabdophane compound with also a swelling of the volume of about 1% owing to the departure of the water molecules. Based on the data collected at 300°C, the structure of the anhydrous compound appears also to be more likely monoclinic. Regarding to these results, it is always very useful to collect good powder data



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in order to avoid any traps during the unit cell indexation. Finally, like the CaSO4 . n H2O case, a new challenge is given in order to identify and solve the possible hemihydrate rhabdophane. Supporting Information. Crystallographic file in CIF format for SmPO4 . 0.667 H2O and SmPO4. This material is available free of charge via the internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge B. Corso (ICSM) for his valuable contribution during the in situ PXRD data collection.


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1. Ni, Y. X.; Hughes, J. M.; Mariano, A. N., American Mineralogist 1995, 80, 21-26. 2. Veilly, E.; du Fou de Kerdaniel, E.; Roques, J.; Dacheux, N.; Clavier, N., Inorganic Chemistry 2008, 47, 10971-10979. 3. Mooney, R. C. L., Journal of Chemical Physics 1948, 16, 1003-1003. 4. Mooney, R. C. L., acta Crystallographica 1950, 3, 337-340. 5. Clavier, N.; Podor, R.; Dacheux, N., Journal of the European Ceramic Society 2011, 31, 941-976. 6. Boatner, L. A., Synthesis, structure, and properties of monazite, pretulite, and xenotime. In Phosphates: Geochemical, Geobiological, and Materials Importance, Kohn, M. J.; Rakovan, J.; Hughes, J. M., Eds. Mineralogical Soc America: Washington, 2002; Vol. 48, pp 87-121. 7. Jonasson, R. G.; Bancroft, G. M.; Nesbitt, H. W., Geochimica et Cosmochimica Acta 1985, 49, 2133-2139. 8. Byrne, R. H.; Kim, K.-H., Geochimica et Cosmochimica Acta 1993, 57, 519-526. 9. Roncal-Herrero, T.; Rodríguez-Blanco, J.; Oelkers, E.; Benning, L., Journal of Nanoparticle Research 2011, 13, 4049-4062. 10. Zhang, L.; Yin, M.; You, H.; Yang, M.; Song, Y.; Huang, Y., Inorganic Chemistry 2011, 50, 10608-10613. 11. Zollfrank, C.; Scheel, H.; Brungs, S.; Greil, P., Crystal Growth & Design 2008, 8, 766770. 12. Luwang, M. N.; Ningthoujam, R. S.; Jagannath; Srivastava, S. K.; Vatsa, R. K., Journal of the American Chemical Society 2010, 132, 2759-2768. 13. Luwang, M. N.; Ningthoujam, R. S.; Srivastava, S. K.; Vatsa, R. K., Journal of the American Chemical Society 2011, 133, 2998-3004. 14. Fang, Y. P.; Xu, A. W.; Song, R. Q.; Zhang, H. X.; You, L. P.; Yu, J. C.; Liu, H. Q., Journal of the American Chemical Society 2003, 125, 16025-16034. 15. Maas, H.; Currao, A.; Calzaferri, G., Angewandte Chemie-International Edition 2002, 41, 2495-+. 16. Patra, C. R.; Bhattacharya, R.; Patra, S.; Basu, S.; Mukherjee, P.; Mukhopadhyay, D., Clinical Chemistry 2007, 53, 2029-2031. 17. Heer, S.; Lehmann, O.; Haase, M.; Gudel, H. U., Angewandte Chemie-International Edition 2003, 42, 3179-3182. 18. Takita, Y.; Sano, K.-i.; Muraya, T.; Nishiguchi, H.; Kawata, N.; Ito, M.; Akbay, T.; Ishihara, T., Applied Catalysis A: General 1998, 170, 23-31. 19. Norby, T.; Christiansen, N., Solid State Ionics 1995, 77, 240-243. 20. Bregiroux, D.; Terra, O.; Audubert, F.; Dacheux, N.; Serin, V.; Podor, R.; BernacheAssollant, D., Inorganic Chemistry 2007, 46, 10372-10382. 21. Terra, O.; Clavier, N.; Dacheux, N.; Podor, R., New Journal of Chemistry 2003, 27, 957967. 22. Dacheux, N.; Clavier, N.; Podor, R., American Mineralogist 2013, 98, 833-847. 23. Du Fou de Kerdaniel, E.; Clavier, N.; Dacheux, N.; Terra, O.; Podor, R., Journal of Nuclear Materials 2007, 362, 451-458. 24. Poitrasson, F.; Oelkers, E.; Schott, J.; Montel, J. M., Geochimica et Cosmochimica Acta 2004, 68, 2207-2221. 25. Oelkers, E. H.; Poitrasson, F., Chemical Geology 2002, 191, 73-87.



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26. Braun, J.-J.; Viers, J.; Dupré, B.; Polve, M.; Ndam, J.; Muller, J.-P., Geochimica et Cosmochimica Acta 1998, 62, 273-299. 27. Hezel, A.; Ross, S. D., Journal of Inorganic & Nuclear Chemistry 1967, 29, 2085-&. 28. Assaaoudi, H.; Ennaciri, A.; Rulmont, A.; Harcharras, M., Phase Transitions 2000, 72, 113. 29. Donaldson.J.D; Hezel, A.; Ross, S. D., Journal of Inorganic & Nuclear Chemistry 1967, 29, 1239-1242. 30. Jonasson, R. G.; Vance, E. R., Thermochimica Acta 1986, 108, 65-72. 31. Mooney, R., acta Crystallographica 1950, 3, 337-340. 32. Frontera, C.; Rodriguez-Carvajal, J., Physica B: Condensed Matter 2003, 335, 219-222. 33. Altomare, A.; Caliandro, R.; Camalli, M.; Cuocci, C.; Giacovazzo, C.; Moliterni, A. G. G.; Rizzi, R., Journal of Applied Crystallography 2004, 37, 1025-1028. 34. Gelato, L. M.; Parthé, E., J. Appl. Crystallogr. 1987, 20, 139. 35. Spek, A. L., PLATON, A Multipurpose Crystallographic Tool. 2008. 36. Kijkowska, R., Thermochimica Acta 2003, 404, 81-88. 37. Glorieux, B.; Matecki, M.; Fayon, F.; Coutures, J. P.; Palau, S.; Douy, A.; Peraudeau, G., Journal of Nuclear Materials 2004, 326, 156-162. 38. Lucas, S.; Champion, E.; Bernache-Assollant, D.; Leroy, G., Journal of Solid State Chemistry 2004, 177, 1312-1320. 39. Schmidt, H.; Paschke, I.; Freyer, D.; Voigt, W., Acta Crystallographica Section BStructural Science 2012, 68, 92-92. 40. Weiss, H.; Brau, M. F., Angewandte Chemie-International Edition 2009, 48, 3520-3524. 41. Schmidt, H.; Paschke, I.; Freyer, D.; Voigt, W., Acta Crystallographica Section B 2011, 67, 467-475. 42. Bezou, C.; Nonat, A.; Mutin, J. C.; Christensen, A. N.; Lehmann, M. S., Journal of Solid State Chemistry 1995, 117, 165-176.


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Figure captions: Figure 1: Rietveld plot showing the observed, calculated and the difference for the SmPO4 . 0.667 H2O. Figure 2: Observed and calculated patterns for SmPO4 . 0.667 H2O after Rietveld refinement considering hexagonal (top) and monoclinic (bottom) structures. Figure 3: Observed and calculated pattern of SmPO4 . 0.667 H2O after Rietveld refinement considering hexagonal (top) and monoclinic (bottom) solutions showing the indexation of selected very weak peaks only in the monoclinic model. Figure 4: TGA weight loss curve showing the hydration reversibility in the SmPO4.nH2O compound. Figure 5: General view of the structure of SmPO4 . 0.667 H2O along the [101] direction. Figure 6: Local coordination of Sm atoms in the SmPO4 . 0.667 H2O structure. Figure 7: Water molecules network inside the channels in the SmPO4 . 0.667 H2O structure. Figure 8: Distribution of water molecules within the channels in the SmPO4 . 0.667 H2O, CaSO4 . 0.625 H2O, CaSO4 . 0.5 H2O structures. Figure 9: Refined unit cell volume vs the average lanthanide radius (1/3 rLnCN8 + 2/3 rLnCN9). Figure 10: In situ PXRD patterns showing the different structural transitions from hydrated rhabdophane SmPO4 . nH2O to monazite. Figure 11: Representation of the structure of anhydrous rhabdophane SmPO4 along the c axis. Figure 12: Variation of the unit cell volume versus heating temperature showing the transformation of SmPO4 . 0.667 H2O rhabdophane to SmPO4 monazite (left), and a zoom at low temperature showing the reversible swelling of the unit cell corresponding to the transformation between hydrated and anhydrous rhabdophane. Figure 13: Dilatometric experiment obtained on the compound SmPO4 . 0.667 H2O showing a swelling of 1% corresponding to the dehydration and the structure transition into anhydrous rhabdophane (SmPO4).



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Table 1: Crystal data and structure refinement of SmPO4 . 0.667 H2O Compound

Sm-rhabdophane

Formula

SmPO4 . 0.667 H2O

Formula weight (g.mol-1)

532.05

T(K)

293 K

System

Monoclinic

Space group

C2

a (Å)

28.0904(1)

b (Å)

6.9466(1)

c (Å)

12.0304(1)

β (°)

115.23(1)

V (Å3)

2123.4(1)

Z / Dx (g cm-3)

24 / 4.8

Wavelength (Å)

0.696700

Rp

0.068

Rwp

0.091

RBragg

0.041

RF

0.059


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Table2: Characteristics of the dehydration steps of LnPO4 . n H2O, obtained from TGA experiments

La Ce Nd Sm Eu

Water loss 1 (mol H2O) 0.3 0.2 0.3 0.1 0.3

Temperature range (°C)

RT – 150 RT – 150 RT – 140 RT – 120 RT - 110

Water loss 2 (mol H2O) 0.5 0.5 0.5 0.5 0.7

Temperature range (°C)

150 – 240 150 – 230 140 – 240 140 – 220 110 - 210

Rehydration onset temperature (°C) 140 130 140 120 115

Rehydration rate at RT (mol.min-1) 2.3 10-3 1.5 10-3 1.8 10-3 1.5 10-3 1.5 10-3



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Table 3: Selected interatomic distances (Å) in SmPO4 . 0.667 H2O Atom−Atom Sm1 − O12 Sm1 − O13 Sm1 − O41 Sm1 − O43 Sm1 − O51 Sm1 − O54 Sm1 − O61 Sm1 − O71 Sm1 − Ow1

Distance 2.339(16) 2.595(17) 2.453(16) 2.55(2) 2.409(17) 2.47(2) 2.42(2) 2.44(3) 2.455(16)

Atom−Atom Sm4 − O11 Sm4 − O14 Sm4 − O21 Sm4 − O23 Sm4 − O32 Sm4 − O32 Sm4 − O33 Sm4 − O33 Sm4 − Ow3

Distance 2.580(15) 2.54(2) 2.407(18) 2.29(3) 2.481(16) 2.539(17) 2.557(19) 2.410(15) 2.502(15)

Sm2 − O11 Sm2 − O23 Sm2 − O24 Sm2 − O31 Sm2 − O41 Sm2 − O53 Sm2 − O71 Sm2 − O72

2.28(2) 2.650(18) 2.63(2) 2.31(2) 2.44(3) 2.36(2) 2.582(17) 2.44(2)

Sm5 − O22 Sm5 − O24 Sm5 − O42 Sm5 − O42 Sm5 − O44 Sm5 − O44 Sm5 − O52 Sm5 − O53 Sm5 − Ow4

2.26(2) 2.371(18) 2.607(18) 2.259(15) 2.533(16) 2.774(16) 2.393(16) 2.69(3) 2.471(14)

Sm3 − O14 Sm3 − O21 Sm3 − O22 Sm3 − O34 Sm3 − O43 Sm3 − O52 Sm3 − O61 Sm3 − O62

2.497(19) 2.59(2) 2.668(18) 2.47(2) 2.510(19) 2.43(3) 2.39(2) 2.692(18)

Sm6 − O12 Sm6 − O13 Sm6 − O31 Sm6 − O34 Sm6 − O51 Sm6 − O54 Sm6 − O62 Sm6 − O72 Sm6 − Ow2

2.479(15) 2.559(17) 2.67(2) 2.363(15) 2.596(17) 2.53(2) 2.37(3) 2.48(2) 2.502(15)


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Table 4: Unit cell parameters of the LnPO4 . 0.667 H2O (Ln = La-Dy) rhabdophane-type compounds collected at room temperature and refined on the basis of the C2 space group. Element Ln

a (Å)

b (Å)

c (Å)

β (°)

Volume (Å3)

La

28.7317(1)

7.1060(3)

12.3121(6)

115.26(1)

2273.2(2)

Ce

28.5690(16)

7.0729(3)

12.2109(5)

115.14(1)

2233.5(2)

Pr

28.4289(10)

7.0321(4)

12.1830(7)

115.24(1)

2203.0(2)

Nd

28.2769(6)

7.0033(3)

12.1327(4)

115.28(1)

2172.5(1)

Sm

28.0844(4)

6.9454(1)

12.0287(1)

115.23(1)

2122.3(1)

Eu

28.0245(4)

6.9212(1)

11.9853(2)

115.22(1)

2103.1(1)

Gd

27.9470(4)

6.9025(1)

11.9528(2)

115.22(1)

2085.8(1)

Tb

27.8796(7)

6.8730(2)

11.8787(3)

115.11(1)

2060.9(1)

Dy

27.8226(5)

6.8526(1)

11.8198(1)

115.09(1)

2040.8(1)



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Table 5: Crystal data and structure refinement of the anhydrous rhabdophane SmPO4. Compound

Sm-rhabdophane

Formula

SmPO4

Formula weight (g.mol-1)

245.33

T(K)

573 K

System

Monoclinic

Space group

C2

a (Å)

12.1443(1)

b (Å)

7.0177(1)

c (Å)

6.3475(1)

β (°)

90.02(1)

V (Å3)

540.97(1)

Z / Dx (g cm-3)

4.519

Wavelength (Å)

CuKα1,2

Rp

0.049

Rwp

0.066

RBragg

0.116

RF

0.177


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Synopsis: The crystallographic structure of Sm-rhabdophane, namely SmPO4 . 0.667 H2O, was solved from synchrotron powder data in a monoclinic C2 space group with a = 28.0903(1) Å, b = 6.9466(1) Å, c= 12.0304(1) Å, β = 115.23(1)° and V = 2123.4 (1) Å3 with Z = 24. Heating the rhabdophane powder led to the identification of an anhydrous form above 160°C and a new structure crystallizing in C2 space group, with six formula units in a = 12.14426 (1) Å, b = 7.01776(1) Å, c= 6.34755(1) Å, β = 90.02 (1)°, V = 540.97(1) Å3.



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Figure 1: Rietveld plot showing the observed, calculated and the difference for the SmPO4 . 0.667 H2O. 269x207mm (150 x 150 DPI)

ACS Paragon Plus Environment

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Figure 2: Observed and calculated patterns for SmPO4 . 0.667 H2O after Rietveld refinement considering hexagonal (top) and monoclinic (bottom) structures. 243x227mm (72 x 72 DPI)



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Figure 3: Observed and calculated pattern of SmPO4 . 0.667 H2O after Rietveld refinement considering hexagonal (top) and monoclinic (bottom) solutions showing the indexation of selected very weak peaks only in the monoclinic model. 122x209mm (72 x 72 DPI)


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Figure 4: TGA weight loss curve showing the hydration reversibility in the SmPO4.nH2O compound. 269x187mm (150 x 150 DPI)



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Figure 5: General view of the structure of SmPO4 . 0.667 H2O along the [101] direction. 281x186mm (72 x 72 DPI)


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Figure 6: Local coordination of Sm atoms in the SmPO4 . 0.667 H2O structure. 186x195mm (72 x 72 DPI)



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Figure 7: Water molecules network inside the channels in the SmPO4 . 0.667 H2O structure. 362x119mm (72 x 72 DPI)


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Figure 8: Distribution of water molecules within the channels in the SmPO4 . 0.667 H2O, CaSO4 . 0.625 H2O, CaSO4 . 0.5 H2O structures. 172x384mm (72 x 72 DPI)



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Figure 9: Refined unit cell volume vs the average lanthanide radius (1/3 rLnCN8 + 2/3 rLnCN9). 266x220mm (72 x 72 DPI)


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Figure 10: In situ PXRD patterns showing the different structural transitions from hydrated rhabdophane SmPO4 . nH2O to monazite. 244x195mm (72 x 72 DPI)



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Figure 11: Representation of the structure of anhydrous rhabdophane SmPO4 along the c axis. 270x174mm (72 x 72 DPI)


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Figure 12: Variation of the unit cell volume versus heating temperature showing the transformation of SmPO4 . 0.667 H2O rhabdophane to SmPO4 monazite (left), and a zoom at low temperature showing the reversible swelling of the unit cell corresponding to the transformation between hydrated and anhydrous rhabdophane. 357x135mm (72 x 72 DPI)



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Figure 13: Dilatometric experiment obtained on the compound SmPO4 . 0.667 H2O showing a swelling of 1% corresponding to the dehydration and the structure transition into anhydrous rhabdophane (SmPO4). 290x220mm (72 x 72 DPI)


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Graphical abstract 321x233mm (72 x 72 DPI)


Monoclinic Form of the Rhabdophane Compounds: REEPO4Â·0.667

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