Low-temperature route to prepare rare earth fluorides in a molten NH₄NO₃ system: a systematic study on the effects of NaF/Ln ratio and the reaction temperature and time

Abstract

A low-temperature strategy has been applied to synthesize a host of rare earth fluorides including NaLnF₄ and LnF₃ (Ln = rare earth) with NaF and rare earth nitrates as initial raw materials in NH₄NO₃ flux. It is found that the crystal structures and shape evolutions of rare earth fluorides are highly sensitive to the rare earth ions, NaF/Ln molar ratio, reaction temperature and reaction time. Based on their crystallization behavior, the rare earth fluorides can be divided into four groups. Group I (La and Ce)-based LnF₃ exhibits the trigonal tysonite type of structure (P3̄c1). The small particles are generally spherical, whereas the well matured particles have taken on plate-like shape. Group II (Pr and Nd)-based fluorides crystalize in hexagonal LnF₃ with changeable shapes (quasisphere, embryonic rod form, short prism, small rods), and high reaction temperature, long reaction time and large NaF/Ln ratio are favorable for the formation of large hexagonal NaLnF₄ rods. For group III (Sm, Eu, Gd, Tb, Dy and Ho), the transformation from orthorhombic LnF₃ nanopolyhedra to hexagonal LnF₃ nanorods occurs at a low reaction temperature when the NaF/Ln molar ratio increases from three to six, whereas the extension of reaction time and the enhancement of reaction temperature induce the production of hexagonal-phased NaLnF₄ rod-like particles. Group IV (Y, Er, Tm, Yb and Lu)-based rare earth fluorides prefer to form orthorhombic LnF₃ polyhedra at a low NaF/Ln molar ratio of three, whereas increasing the NaF/Ln molar ratio to six produces quasisphere-like cubic NaLnF₄ particles, and the extended reaction time and increase in reaction temperature result in hexagonal NaLnF₄ rods through Ostwald ripening. The general trend of stable phase and shape evolution of rare earth fluorides can be ascribed to the lanthanide contraction effect and the activation energy barrier to shape transformation.

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Introduction

Rare earth fluorides have recently attracted substantial interests due to their conspicuous luminescence, ferromagnetism, insulating/magnetism properties and potential applications in solid state lasers, energy converters and 3D flat panel displays.^1–11 The crystalline structures and chemical and physical properties of rare earth fluorides can normally be determined by the synthetic techniques and experimental variables including the type and the composition of crude materials, the reaction temperature and the reaction time.^5–8 Thus, it is fundamentally necessary and important to attentively choose a suitable synthetic protocol and manipulate the synthetic conditions accordingly. The low-temperature routes to rare earth fluorides mainly depend on the following methods: solvo-/hydrothermal method, thermal decomposition method, coprecipitation method, Pechini-type sol–gel method, microwave-assisted method, and sonochemical method.^{2–8,10–15} Unfortunately, these methods generally suffer from the inherent drawbacks of rigid synthesis conditions, exorbitant crude materials and equipments, and substantial environmental issues, which are still far from the industrial produce. Encouragingly, the molten-salt route has recently been developed and demonstrated as a facile large-scale technique with simplicity, versatility, low cost, and environmental friendliness.^16–21 Moreover, molten NH₄NO₃ has several inherent superiorities such as abundance in nature, low melting point, and easy deliquescence in water/alcohol system to readily isolate the target product.^22–25 Since the reaction temperature required (below 250 °C) in molten NH₄NO₃ is much lower than that in NaF, NaF-KF and NaNO₃ flux,^26–28 molten NH₄NO₃ has been applied to fabricate a host of fluorides such as h-NaYF₄,²² h-NaBiF₄,²³ tetragonal LiYF₄,²⁴ and c-BaGdF₅.²⁵ Their size and crystalline structure can be controlled extensively through stringent control of the experimental conditions such as the amounts of precursor materials, the reaction temperature and the reaction time. However, this is rather time consuming and has low efficiency; also, it is resource intensive and sometimes generates a degree of serendipity. One promising route that simplifies the synthetic procedure is the integration of sodium and fluorine sources into the same precursor NaF, which might provide a much better control of the synthesis of rare earth materials. Furthermore, it is very important to make a thorough inquiry into the crystallization behavior and to underline the phase composition/morphology evolution rule. Herein, we have demonstrated the use of NaF and rare earth nitrates as unprocessed materials to fabricate rare earth fluorides NaLnF₄ and LnF₃ (Ln = rare earth) in molten ammonium nitrate system. It is found that the type of the selected rare earth ion and the synthetic variables such as NaF-to-Ln(NO₃)₃ (referred as NaF/Ln) molar ratio, the reaction temperature and reaction time play a crucial role in the crystal structure, morphology and size of the resultant products. The crystallization characteristics and phase composition/morphology evolution of rare earth fluorides in the molten NH₄NO₃ system originate from the lanthanide contraction and the energy barrier to form rare earth fluorides.

Experimental

Materials

Reagent-grade NaF, NH₄NO₃, Y(NO₃)₃·6H₂O, La(NO₃)₃·6H₂O, Ce(NO₃)₃·6H₂O, Pr(NO₃)₃·6H₂O, Nd(NO₃)₃·6H₂O, Sm(NO₃)₃·6H₂O, Eu(NO₃)₃·6H₂O, Gd(NO₃)₃·6H₂O, Dy(NO₃)₃·6H₂O, Tb(NO₃)₃·6H₂O, Ho(NO₃)₃·5H₂O, Er(NO₃)₃·5H₂O, Tm(NO₃)₃·5H₂O, Yb(NO₃)₃·5H₂O and Lu(NO₃)₃·3H₂O powders were used to synthesize rare earth fluorides.

Synthetic procedures

The starting materials NaF, NH₄NO₃ and Ln(NO₃)₃·xH₂O (Ln= rare earth) were thoroughly mixed. The mixtures were put into 15 ml capacity crucibles. After the lids were tightly closed, the crucibles were placed in an oven. The tank was sealed and treated at designated temperature for designated time in an oven and then cooled to room temperature naturally. The detailed synthetic conditions including NaF/Ln and NH₄NO₃/Ln molar ratio, the reaction temperature and reaction time are summarized in Tables S1–S5, ESI.† After washing several times with deionized water and ethanol, the precipitates were dried at 70 °C for 12 h in vacuum.

Characterization

The phase structures and purities of the rare earth fluorides were examined by X-ray powder diffraction (XRD) characterization on a Bruker D8-Advance diffractometer with graphite-monochromatized CuKα radiation (40 KV/60 mA, λ = 0.1541 nm). The size, morphology and chemical compositions of products were determined by a transmission electron microscope (TEM, JEOL2010) operating at 200 kV and a JSM 6700F scanning electron microscope (SEM) equipped with the energy dispersive X-ray spectrum (EDS). Structural information was obtained by a high-resolution transmission electron microscope (HRTEM).

Results and discussion

As described below, the phase composition and the morphology evolution of the resultant rare earth fluorides are highly dependent upon the type of chosen rare earth ion and the experiment parameters including the NaF/Ln molar ratio, reaction temperature and time. Accordingly, the as-obtained rare earth fluorides can be divided into four groups, i.e., group I (La and Ce), group II (Pr and Nd), group III (Sm, Eu, Gd, Tb, Dy and Ho) and group IV (Y, Er, Tm, Yb and Lu). The crystallization behaviour and crystal structure/shape evolution rule will be investigated in detail as follows.

For group I, rare earth fluorides tend to crystallize into a trigonal tysonite-type structure and are insensitive to the NaF/Ln molar ratio, reaction temperature and reaction time. In contrast, the morphologies and sizes of the resulting fluorides are highly dependent upon the synthetic conditions (shown in Fig. 1, S1, and Table S1, ESI†). As for the NaF/Ln value of three, the increased reaction temperature and prolonged reaction time drove these quasispherical particles to evolve into large flakes with smooth end plane, followed by increase in size (Table S1, ESI†). In contrast, the NaF/Ln molar ratio of six induces shrinkage in the size (Table S1, ESI†), which may have resulted because excess NaF remarkably increases the crystallization temperature and thus hinders the growth. The high-magnification TEM image of the relatively small particles indicates clear fringes of 0.326 nm (Fig. 1h), corresponding to the distance between the neighbouring (111) planes of h-LaF₃.

Fig. 1 XRD patterns (a) and SEM images of the La-based products obtained under various experiment conditions: (b) 250 °C, 24 h, NaF/NH₄NO₃/La of 3/8/1; (c) 250 °C, 24 h, NaF/NH₄NO₃/La of 6/8/1; (d) 250 °C, 3 h, NaF/NH₄NO₃/La of 3/8/1; (e) 250 °C, 3 h, NaF/NH₄NO₃/La of 6/8/1; (f) 100 °C, 40 min, NaF/NH₄NO₃/La of 3/8/1; (g) 100 °C, 40 min, NaF/NH₄NO₃/La of 6/8/1; (h) HRTEM images of the sample obtained after 40 min at 100 °C for NaF/NH₄NO₃/La of 6/8/1.

Regarding the NaF/Ln molar ratio of three, the resultant group II (Pr and Nd) fluorides exhibited a hexagonal LnF₃ phase (h-LnF₃). A low reaction temperature or short reaction time resulted in some small h-LnF₃ nanospheres, whereas the increased reaction temperature and prolonged reaction time resulted in large h-LnF₃ rods with hexagonal end along with some small nanoparticles (Fig. 2, S2 and Table S1, ESI†). With the NaF/Ln molar ratio of six and gradual increase in the reaction temperature from 100 °C to 200 °C, the product still maintained the nature of the h-LnF₃ phase, whereas the morphologies evolved as follows: quasisphere → embryonic rod form → short prism → small rod → large rod (Fig. 2 and S2, and Table S1, ESI†). Interestingly, the increased reaction temperature substantially improved the size and the ratio of the length to the diameter. Unlike group I, h-NaLnF₄ was detected in severe synthetic conditions (large NaF/Ln molar ratio of six, high temperature and long reaction time (Fig. 2 and S2a, ESI†)). The relevant morphologies are mainly large hexagonal rods with relatively smooth end and side faces as well as small hexagonal nanorods (Fig. 2 and S2, ESI†). Taking Nd for instance to determine the crystal structure of the large and small rods, the fringe distances (d-spacing) of small rods are 0.60 and 0.37 nm in the HRTEM image (Fig. 2j), corresponding to the distance between neighbouring (100) and (002) planes of h-NdF₃ (JCPDS No. 078-1859). The EDS spectra (Fig. 2g) show that they only contain Nd and F, whereas HRTEM images (Fig. 2k) and EDS spectra (Fig. 2l) of the large rods illustrate that the distances between the adjacent lattice fringes of 0.350 nm and 0.295 nm match well with the d₁₀₁ and d₁₀₁ spacings of h-NaNdF₄ (JCPDS card No. 35-1367) and the existence of Na, Nd and F. Thus, it can be concluded that small rod-like particles belong to h-NdF₃, whereas large prisms are ascribed to h-NaNdF₄.

Fig. 2 XRD patterns (a) and SEM images of the Nd-based products obtained under various experiment conditions: (b) 3/1, 250 °C 24 h; (c) 3/1, 140 °C 2 h; (d) 6/1, 100 °C, 24 h; (e) 6/1, 120 °C, 24 h; (f) 6/1, 160 °C, 24 h; (g) 6/1, 200 °C, 24 h; and (h) 6/1, 250 °C 24 h; and HRTEM and EDS spectra of the large rods (i and j) and the small nanorods (k and l) at 6/1, 250 °C, 24 h.

As far as group III and group IV are concerned, the crystallographic structures and the shapes and sizes of the resultant products are closely associated with the NaF/Ln molar ratio, reaction temperature and reaction time (Fig. 3–5, S3–S13, and Tables S2–S5, ESI†). We took Y as an example to study the role of the experimental parameters including the NaF/Ln molar ratio, the reaction temperature and the reaction time for phase structure, morphology and size of the products, which will be discussed in detail as follows.

Fig. 3 XRD patterns (a) and SEM images of the Y-based products obtained under various experiment conditions: (b) 3/1, 120 °C (inset: TEM and HRTEM images); (c) 3/1, 250 °C, 24 h; (d) 4/1, 250 °C, 24 h; (e) 5/1, 250 °C, 24 h; (f) 6/1, 250 °C, 24 h and (g) 8/1, 250 °C, 24 h.

Fig. 4 XRD patterns (a) and SEM images of the Y-based products obtained for (b) 0.5 h; (c) 1 h; (d) 2 h; (e) 6 h; (f) 8 h; (g) 12 h and (g) 18 h at NaF/NH₄NO₃/Y molar ratio of 6/8/1 at 100 °C; HRTEM images of quasispherical particles obtained after 0.5 h at 100 °C (h) HRTEM images of the sample obtained after 40 min at 100 °C for NaF/NH₄NO₃/La of 6/8/1; and the rod-like particles obtained after 18 h at 100 °C (i).

Fig. 5 XRD patterns (a) and SEM images of the Gd-based products obtained under various experiment conditions: (b) 100 °C, 0.5 h; (c) 120 °C, 24 h; (d) 140 °C, 24 h; (e) 160 °C, 24 h; (f) 200 °C, 24 h; (g) at 230 °C, 24 h; and the HRTEM images of the sample obtained after 0.5 h at 100 °C (h) and the sample obtained after 24 h at 230 °C (i).

The as-prepared products with NaF/Y molar ratio of three exhibit the same structure as orthorhombic (o-) YF₃ (JCPDS No. 74-0911) (Fig. 3a). The samples obtained after 2 h at 100 °C exhibit the shape of polyhedra with ca. 120 nm size (Fig. 3b). The HRTEM image indicates that the fringe distances (d-spacing) are 0.311 and 0.372 nm (inset Fig. 3b), corresponding to the distance between neighbouring (111) and (011) planes of o-YF₃ (JCPDS No. 041-0796). Due to the increased temperature of 250 °C, quite a few nanoparticles grew to form large polyhedral particles with ca. 500 nm mean size (Fig. 3c). When the NaF/Y molar ratio was four, the resultant products were a mixture of h-NaYF₄ and o-YF₃ (Fig. 3a, Table S2, ESI†), and h-NaYF₄ exhibited prismatic microrods with diameter of 3.0 μm in length and diameter of ca. 0.8 μm (Fig. 3d). The increased NaF/Y molar ratio of five and above induced the disappearance of the polyhedra o-YF₃ (Fig. 3ef, and Table S2, ESI†). Conversely, upon further increasing NaF/Y molar ratio to eight, the prisms grew in an incomplete manner and their length and diameter decreased to 2.5 μm and 0.6 μm (Fig. 3g), respectively. The variation of the NH₄NO₃/Y molar ratio hardly changed the inherent nature of the hexagonal system, rod-like in shape, and the average diameter of the rod increased, whereas the average length exhibited a significant downward trend when the NH₄NO₃/Ln molar ratio increased from five to eight and ten (Fig. S3 and Table S3, ESI†). Moreover, the resulting products after 20 min at 100 °C exhibited pure c-NaYF₄ (JCPDS No. 28-1192 card) quasisphere-like particles with an average diameter of 120 nm and coarse surfaces (Fig. 4a, b, Table S5, ESI†). The determined interplanar distances of 0.311 nm and 0.280 nm agreed well with (111) and (200) lattice fringes of c-NaYF₄ phase (Fig. 4h). When the time was extended to 1 h, the obtained sample exhibited two distinguished particle shapes, that is, quasisphere-like particles and large rods, which was in good agreement with the presence of two phases observed by X-ray powder diffraction (Fig. 4a). The interplanar spacings of 0.295 nm and 0.350 nm of the rod-like particles were indexed as (101) and (011) lattice fringes of h-NaYF₄ phase (Fig. 4i). This indicates that rod-like particles belong to the h-NaYF₄ phase. A stepwise extension in the reaction time to 24 h and the gradual increase in the reaction temperature from 120 °C through 140 °C to 160 °C resulted in further decrease in the quasisphere-like c-NaYF₄ particles and pronounced growth in the h-NaYF₄ rods (Fig. S4 and Table S4, ESI†). The quasispheric c-NaYF₄ particles were not observed when the reaction temperature was increased to 200 °C (Fig. S5g, ESI†), which was in agreement with the results of the XRD pattern (Fig. 4a). It is suggested that the cubic-to-hexagonal phase conversion is accompanied by dissolution–recrystallization process to minimize the surface energy of the system and Ostwald ripening propels the quasisphere-like particles to convert into rods at the consumption of smaller nanoparticles as well as enhanced sizes.

It should be pointed out that the resultant LnF₃ product of group III and IV with a low NaF/Ln molar ratio of three crystallized in the orthorhombic (o-) system, which is distinct from trigonal LnF₃ of groups I and hexagonal LnF₃ of group II (Fig. S6, S7 and Table S5, ESI†). The low reaction temperature (100 °C) and short reaction time (20 min) produced small nanopolyhedra (Fig. S5, Table S5, ESI†). The increased reaction temperature (250 °C) and extended reaction time (24 h) caused part of the nanopolyhedral particles to transform into large polyhedra (Y, Ho, Er, Tm and Lu), flakes (Sm, Eu, Gd Tb and Yb), or mixed polyhedra and flakes (Dy) (Fig. S6–S8 and Table S5, ESI†). When referring to high NaF/Ln molar ratio of six, all the fluorides of group III and IV with high temperature and long time are present as large rods (Fig. S7 and Table S5, ESI†). Under low reaction temperature (100 °C) and short reaction time (20 min), group III (Sm, Eu, Gd, Tb, Dy and Ho)-based samples were shaped like nanorods (Fig. S9, ESI†). In Eu-based fluorides, the interplanar distance of 0.357 nm observed was consistent with (200) plane of h-EuF₃ (Fig. S9k, ESI†), suggesting that the sample is made up of h-EuF₃, which is in accordance with the XRD results (Fig. S9 and Table S5, ESI†); HRTEM images of Gd-containing samples reveal the presence of two sets of lattice fringes (Fig. 5h). The interplanar distance was determined to be 0.311 nm, which was associated with the (110) plane of h-NaGdF₄, indicating that these rods grow along 001 orientation, whereas the definitive signature of h-GdF₃ was also observed. Thereby, in practice, these rod-like products comprise h-GdF₃ and h-NaGdF₄, which is in agreement with the XRD observations. In contrast, similar to Y-based samples, Er- and Tm-fluorides tended to generate cubic-phased quasispherical c-NaLnF₄ particles, as proved by HRTEM images and XRD (Fig. S8–S10, ESI†).

Notably, the evolution in the crystal phase and shape of NaLnF₄ (such as Er, Tm and Yb) is similar to that of NaYF₄ (Fig. S11–S13 and Table S5, ESI†). The temperature for cubic-to-hexagonal phase complete transition can be dependent upon the chosen rare earth ion and the values are 200 °C, 140 °C and 140 °C after 24 h for Y, Er and Tm, respectively, whereas the temperature and time for all the group III-based h-LnF₃ phases to transform to h-NaLnF₄ are much lower than that for the total phase conversion of group IV-based fluorides. Broadly speaking, the smaller the radii, the higher the temperature for phase transformation. This behavior can be related to a high trend towards electron cloud distortion of the rare earth ions with large ionic radii due to increased dipole polarizability, which is beneficial for hexagonal structures.²⁹

Taken together, the rare earth fluorides LnF₃ and NaLnF₄ were synthesized by a low-temperature route with NaF and rare earth nitrates as precursor materials in a molten ammonium nitrate system (Fig. 6a). Moreover, the stable crystal structure and shape evolution of the resultant products can be adjusted by the choice of rare earth elements and crystal growth conditions (NaF/Ln molar ratio, reaction temperature and reaction time) (Fig. 6b and c). For the Na/Ln molar ratio of three, the lanthanide contraction effect depends upon the stability of the resultant rare earth fluorides.^30–33 Light Ln³⁺ ions (group I and II) with larger ionic radii favour a large ligand number of eleven, as corroborated by the eleven fluoride ions that surround each Ln³⁺ ion in trigonal LaF₃-type and hexagonal structures. In terms of heavy Ln³⁺ ions (from Sm to Tm, Lu), the effects of cation size and polarizability induce the crystal structure to feature lower coordination numbers of nine and the orthorhombic YF₃-type is the preferred structure.²⁹ When referring to NaF/Ln of six, low temperature and short reaction time, lanthanide contraction still plays a predominant role. For group I, II and III, rare earth fluorides adopt the hexagonal LnF₃ structure. Additionally, group IV-based fluorides tend to form cubic NaYF₄ fluorite structures with even low ligand number of eight. Interestingly, increasing the NaF/Ln molar ratio from three to six induces the structure of group III-based LnF₃ to convert from orthorhombic LnF₃ to hexagonal LnF₃. This is related to the amount of fluoride ions in the molten system. The shortage of the fluoride ions (NaF/Ln of three) tends to result in the stable orthorhombic phase with a low ligand number of nine, whereas the hexagonal structure with a large ligand number of eleven is the preferred phase in excess of fluoride ions (large NaF/Ln of six).³⁴ It is well accepted that rare earth fluorides with low temperature and short time are thermodynamically metastable species, whereas high temperature or long time should exhibit kinetic predominance, implying that there is a kinetically prohibitive free energy barrier (i.e., a high activation energy).^19,35 A simplified illustration of the reaction free energy curve is demonstrated in Fig. 6d. Initially, the reaction between NaF and Ln(NO₃)₃ in molten NH₄NO₃ system goes through a low activation energy path and produces h-LnF₃ or c-NaYF₄. Then, the level of difficulty in the second stage highly depends on the kinetic barrier to the formation of h-NaLnF₄. For group I, the kinetic barrier is simply too high to be overcome in the present experimental condition and h-LnF₃ is stable in the present condition. Similarly, the very high activation barrier to generate hexagonal group II-based NaLnF₄ will make this phase conversion kinetically prohibitive under drastic conditions (high reaction temperature and long reaction time). In contrast, group III-based h-LnF₃ compounds go through a relatively low-activation path and readily convert to h-NaLnF₄ rods. It is worthwhile to note that for group II and III, rod-like h-NaLnF₄ species are formed directly from h-LnF₃ because c-NaLnF₄ is not as stable as h-LnF₃. Regarding group IV, quasispherical c-NaLnF₄ particles are formed rather than h-LnF₃ since c-NaLnF₄ is significantly more stable than h-LnF₃. Then, the energy barrier to the formation of h-NaLnF₄ is so high that c-NaLnF₄ is relatively stable. Only under rigorous conditions including high temperature and long time, pure thermodynamically stable hexagonal rods are obtained.

Fig. 6 Synthesis, structural and morphological characterization of rare earth fluoride nanocrystals. (a) Schematic representation of the synthesis method. General trend of stable phases (b) and morphology evolution (c) as a function of the type of rare earth ions, NaF/Ln molar ratio, reaction temperature and reaction time. (d) Schematic illustration of the activation free energy for the controlled synthesis of rare earth fluorides.

Conclusions

Many rare earth fluorides LnF₃ and NaLnF₄ were synthesized through the low-temperature route with NaF and rare earth nitrates as raw materials in molten NH₄NO₃. The crystal structures, morphologies and sizes of the resulting rare earth fluorides are highly sensitive to the species of the rare earth ions and crystal growth conditions including the NaF/Ln molar ratio, the reaction temperature and the reaction time. According to general trend of stable phases and shape evolution of rare earth fluorides, the rare earth elements can be divided into four groups (I: La and Ce; II: Pr and Nd; III: Sm, Eu, Gd, Tb, Dy and Ho, and IV: Y, Er, Tm, Yb and Lu). The resultant group I-based fluorides exhibit the trigonal LnF₃ phase with small quasispherical morphology and large regular platelet particles with smooth end planes under large and small NaF/Ln ratios, respectively. The samples of group II obtained under low temperature comprise quasispherical h-LnF₃ particles, whereas high reaction temperature and long reaction time result in large h-NaLnF₄ hexagonal sheets. Regarding group III and IV, the crystallization behavior is highly sensitive to the synthetic conditions including the NaF/Ln molar ratio, the reaction temperature and the reaction time. The lack of fluoride ions (low NaF/Ln ratio of three) tends to result in o-LnF₃ polyhedra or flakes. Also, h-LnF₃ (Ln = group III) and c-NaLnF₄ (Ln = group IV) exhibit short rod and quasisphere shapes under low reaction temperature (100 °C) and short reaction time (20 min), whereas mixture c- and h-NaLnF₄ (Ln = group III and IV) is shaped like nanorods with sharp ends and quasispherical particles. Regarding the large NaF/Ln molar ratio of six, the extension of reaction time and the increase in the reaction temperature cause h-LnF₃ or c-NaLnF₄ particles to undergo a dissolution–recrystallization process to convert into large h-NaLnF₄ rods. The temperature and time for complete phase conversion required can be decided mainly by the type of rare earth ion. The crystallization nature and shape development is related to the lanthanide contraction as well as the energy barrier to form stable rare earth fluorides. These results substantiate the content of rare earth fluoride material chemistry and conduce to the rules of the growth behaviour.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (61204003), Hunan Provincial Natural Science Foundation of China (Grant No. 2017JJ2326), Train Object Program of Jiangxi Province Young Scientists and Luodi program of Jiangxi Province (No. KJLD13030), respectively.

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns, SEM, TEM, HETEM images of the resulting fluorides under different experimental conditions (NaF/Ln molar ratio, the reaction temperature and the reaction time). See DOI: 10.1039/c8ce01642a

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