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Fan Li, Zhiyuan Pan, Sihan Feng, Qi Zhu, Xudong Sun, [李 継光](https://orcid.org/0000-0002-5625-7361)

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[Mild-hydrothermal synthesis of RE(OH)SO4 layered compounds (RE = La-Tb), crystal structure, thermolysis, and photoluminescence](https://mdr.nims.go.jp/datasets/98eb00b4-6840-40e1-a3c3-a483c8559d6e)

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1  Mild-hydrothermal synthesis of RE(OH)SO4 layered compounds (RE = La-Tb), crystal structure, thermolysis, and photoluminescence  Fan Li,a Zhiyuan Pan,a Sihan Feng,a Qi Zhu,a Xudong Sun,a Ji-Guang Lib,∗ a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China b Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan          *Corresponding author Dr. Ji-Guang Li National Institute for Materials Science Tel: +81-29-860-4394 E-mail: li.jiguang@nims.go.jp mailto:li.jiguang@nims.go.jp2  Abstract A series of RE(OH)SO4 layered compounds (RE = La-Tb lanthanides) were successfully synthesized via hydrothermal reaction under near-neutral solution pH and the mild temperature of 180 °C for 24 h, and the temperature/pH/time courses of phase/morphology evolution were clarified with RE = Tb for example. The compounds were characterized in detail for crystallite morphology, crystal structure, thermal behavior and vibrational property, and the intrinsic influence of lanthanide contraction was unveiled. Tb(OH)SO4 was analyzed to be isostructural with its La-Gd analogues (CN = 9 for Tb), instead of the reportedly Dy-Yb ones (CN = 8), and have parameters of a = 4.3806 Å, b = 12.1729 Å, c = 6.7674 Å and  = 106.3967° for its monoclinic unit cell (P21/n space group). The Eu and Tb compounds exhibited red (λex = 395 nm, λem = 616 nm) and green (λex = 369 nm, λem = 545 nm) emissions, and have fluorescence lifetimes of ~0.84 and 1.07 ms and chromaticity coordinates of around (0.632, 0.365) and (0.295, 0.481), respectively.      Keywords: RE(OH)SO4; Crystal structure; Hydrothermal crystallization; Phase/morphology evolution   3  1. Introduction Layered rare earth hydroxides (LREH) received broad research interest because of their peculiar layered structure, rich interlayer chemistry and abundant functionalities of the RE elements. Typical examples of this category of compounds may include RE(OH)2A, where A is NO3− or a halogen anion [1-3], RE2(OH)5A∙nH2O (A−-LREH, n ~ 1.5) [4-6], RE2(OH)4SO4∙nH2O (SO42−-LREH, n = 0 or 2) [7-9] and RE(OH)SO4 [10]. RE(OH)SO4 received attention due to its potential applications in catalysis and electrical/optical devices [10, 11]. For example, Ce doped La(OH)SO4 was suggested to be a promising black light radiation material for insect killing [12, 13]. Meanwhile, with rising importance of the sacrificial template method for materials synthesis, RE(OH)SO4 can be a precursor better than the aforesaid SO42−-LREH and A−-LREH for RE fluorides, phosphates, vanadates, molybdates, tungstates and so on, due to its lower OH−/RE3+ molar ratio [14-16]. This is because the released excessive OH− would compete with the anions in the targeted compound for RE3+ coordination, which reduces the kinetics of phase conversion and even causes an incomplete reaction [16, 17]. The crystal structure RE(OH)SO4 has been studied by several individual research groups. For example, Haschke et al. [18] solved the structures of La(OH)SO4, Pr(OH)SO4 and Nd(OH)SO4, Jacobson et al. [19] and Feng et al. [20] respectively reported the structures of Y(OH)SO4 and Ce(OH)SO4, Xu et al. detailed the structures of Eu(OH)SO4 and Dy(OH)SO4 [11, 21], and Zehnder et al. investigated the effect of lanthanide contraction on crystal structure of this series of compounds for RE = Pr-Yb lanthanides [22]. It was suggested by 4  these earlier studies that, although they are all monoclinic (P21/n space group), the larger RE3+ (RE = La-Gd) and smaller RE3+ (RE = Tb-Yb and Y) form two separate types of crystal structures, where the RE3+ is 9-fold and 8-fold coordinated by oxygen atoms, respectively. Another difference is that the RE(OH)SO4 of larger RE3+ possesses a unit cell accommodating four complete formulas [18, 20, 21, 23] while that of the smaller RE3+ has a unit cell containing eight formulas [11, 19]. Zehnder et al. [22] speculated that such a "break" between Gd and Tb is associated with lattice energy and lattice strain. The family of RE(OH)SO4 compounds were reported to form in the oxide-hydroxide-sulfate reaction system during hydrothermal equilibria study, which employed harsh reaction conditions such as high temperature ( 450 °C), high pressure ( 120 MPa) and long reaction duration (longer than 6 days) [18, 21, 22]. Our systematic study on the synthesis of RE2(OH)5A∙nH2O (n ~ 1.5) and RE2(OH)4SO4∙nH2O (n = 0 or 2) LREHs clearly manifested the influence of lanthanide contraction and identified that solution pH, aside from temperature, is a decisive factor for the intended compound to be formed, and the results were rationalized by considering cation hydrolysis and coordination competition [7, 8, 24-26]. In view of the previous results, we performed hydrothermal reaction of the RE(NO3)3-(NH4)2SO4-NH4OH system for the full series of lanthanides (excluding radioactive Pm, including Y) under near-neutral conditions, and phase-pure RE(OH)SO4 was successfully obtained for RE = La-Tb via reaction at the mild temperature of 180 °C for 24 h. The products were characterized in detail to manifest the intrinsic influence of lanthanide contraction on crystal structure, thermal behavior, vibrational property and crystallite morphology. It was clearly shown that 5  Tb(OH)SO4 is isostructural with its La-Gd analogues instead of the Dy-Yb ones, implying that two types of crystal structures may exist for Tb(OH)SO4 and even Dy(OH)SO4. Furthermore, the pH-, time- and temperature-course of phase and morphology evolution was clarified with RE = Tb for example, and the RE = Eu and Tb compounds were also investigated for their photoluminescence properties.  2. Experimental Section 2.1. Hydrothermal synthesis Analytical grade (NH4)2SO4 and NH4OH were purchased from Sinopharm Co., Ltd (Shanghai, China), and 99.99% pure Ce(NO3)36H2O, Pr6O11, Tb4O7 and RE2O3 were purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China). The rare earth oxides listed above were separately dissolved in a proper amount of HNO3 to form nitrate solution. Ultra-pure water (resistivity  18 Mcm) was used throughout the experiments. In a typical synthesis of RE(OH)SO4, 3.1 mmol of (NH4)2SO4 was dissolved in 60 mL aqueous solution containing 3 mmol of RE3+ under magnetic stirring, followed by pH adjustment with NH4OH until the solution changed from clear to cloudy. The resultant mixture was homogenized via constant magnetic stirring for 30 min before being transferred into a Teflon lined stainless steel autoclave (100 mL capacity) for 24 h of reaction in an electric oven preheated at 180 °C. After the reaction, the precipitate was collected via centrifugation, washed with water and ethanol successively, and then dried at 60 °C for 12 h to yield powder products. With RE = Tb for example, a series of experiments were carried out 6  to investigate the influence of solution pH (6.5-10.0), reaction time, and reaction temperature (70-180 °C).  2.2. Characterization Phase identification was performed via X-ray diffractometry (XRD, SmartLab, Rigaku, Tokyo, Japan) under 40 kV/200 mA, using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm) and a scanning speed of 6° 2 /min. The XRD data for structure analysis were collected via step-scan over the 2θ range of 10-100°, using a step width of 0.02° and a counting time of 10 s per step. Crystal structure refinement was performed by the Rietveld technique as implemented in the TOPAS v4.2 software suite (Bruker, Germany). Thermogravimetry/differential thermal analysis (TG/DTA, Model SETSYS Evolution-16, Setaram, France) was performed at a constant heating rate of 10 °C/min in flowing simulated air (50 mL/min). Fourier transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Fisher Scientific, Waltham, USA) was conducted using the standard KBr pellet method. Product morphology was analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo) under an acceleration voltage of 15 kV. Photoluminescence and fluorescence decay were analyzed with a Model FP-8600 fluorospectrophotometer (JASCO, Tokyo) equipped with a 60 mm-diameter integrating sphere, using a 150 W xenon lamp for excitation, a slit width of 5 nm, and a scan speed of 500 nm/min.  Results and discussion 3.1 Characterization of the hydrothermal products Hydrothermal reaction was carried out for the series of lanthanides (excluding radioactive Pm) and Y under 180 °C for 24 h in an attempt to obtain RE(OH)SO4. It was found that, for 7  different type of RE, the pH value for the transparent RE3+ solution to turn turbid (turbidity pH) is different, and the pH gradually decreased from 7.7 to 6.1 as the size of RE3+ decreased from La3+ to Lu3+, noticing that Y3+ and Ho3+ are very similar in ionic radius. This is consistent with the fact that smaller RE3+ has a stronger hydrolysis capacity due to lanthanide contraction [27]. Fig. 1 shows the XRD patterns of the series of hydrothermal products, togther with the turbidity pH. It can be seen that the La-Tb products (Fig. 1a) conform to the monoclinic La(OH)SO4 standard (PDF No. 04-015-7588), and the diffraction peaks tend to move to larger diffraction angles with decreasing RE3+ size. The diffraction pattern of the Dy product basically matches with the Eu(OH)SO4 standard (PDF No. 04-014-5923), except for a diffraction shift towards larger angles and the existence of additional weak peaks (marked with “”). This indicates that the Dy product might be a mixture of Dy(OH)SO4 and unknown impurity phase. The diffraction patterns of the RE = Ho-Lu and Y products (Fig. 1b) can not be indexed with any of the RE-containing compounds in the ICDD database or literature, but the coexistence of widened and sharp diffraction peaks indicates that they might be phase mixtures or a single phase of significantly anisotropic crystallite morphology (such as two-dimensional) in each case. As said earlier, the RE(OH)SO4 compounds of lighter (RE = La-Gd) and heavier (RE = Tb-Yb, including Y) lanthanides are separately isostructured and have REO9 and REO8 polyhedrons, respectively, although they are all monoclinic and belong to P21/n space group [13, 18-20, 22]. Zehnder et al. [22] suggested that the “break” between Gd and Tb is owing to the excessive strain and crystal energy of the REO9 polyhedron caused by the reduction of RE3+ radius. We carefully compared the XRD pattern of the Tb(OH)SO4 prepared in this work with that of the Tb(OH)SO4 reported by Zehnder et al. (PDF No. 04-8  018-2414; Fig. S1). The significant differences between the two patterns indicated that the Tb(OH)SO4 of this work belongs to the La-Gd series rather than the Tb-Yb ones and there are two crystal forms for Tb(OH)SO4 and even Dy(OH)SO4 (Fig. 1b).    Fig. 1. XRD patterns of the hydrothermal products of RE = La-Tb (a) and RE = Dy-Lu and Y (b), with those of the monoclinic structured La(OH)SO4 (PDF No. 04-015-7588) and Eu(OH)SO4 (PDF No. 04-014-5923) standards included for comparison.  Fig. 2. FTIR spectra of the hydrothermal products for RE = La-Dy. The stretching vibration of OH− (ν1) can also be used to distinguish the two crystal forms, 5 10 15 20 25 30 35 40 45 50 55 60 2 /(°)(020)(011)(021)(-1-11)(120)(002)(040)(-112)(041)PDF (La(OH)SO4): No. 04-015-7588(101) La, pH = 7.7 Ce, pH = 7.6 Pr, pH = 7.3 Intensity/a. u.Nd, pH = 7.2(a) Sm, pH = 7.0 Eu, pH = 6.9 Gd, pH = 6.7  Tb, pH = 6.55 10 15 20 25 30 35 40 45 50 55 602 /(°)(020)(011)(021)(-1-11)(120)(002)(040)(-112)(041)(101)PDF (Eu(OH)SO4): No. 04-014-5923Dy, pH = 6.5Y, pH = 6.7Intensity/a. u.(b)Ho, pH = 6.4Er, pH = 6.3Tm, pH = 6.2 Yb, pH = 6.2Lu, pH = 6.14000 3800 3600 3400 1600 1400 1200 1000 800 600Transmittance/a. u.Wavenumber/cm-1 Tb Gd Eu Sm Nd Pr Ce LaOHSO4 DyLaCePrNdSmEuGdTbDy3488 1057 596 718 820 1600-1370 9  as it was reported that the ν1 would be a single state (unimodal, in the regions of ~3455-3505 cm−1) for the REO9 type and would be a dual state (bimodal, in the regions of ~3455-3505 cm−1 and 3540-3570 cm−1) for the REO8 type [22]. FTIR analysis was thus conducted for the La-Dy products and the resulting spectra are shown in Fig. 2. It is seen that the ν1 vibration of OH− in each case is unimodal (~3488 cm−1), which obviously confirms that the as-prepared RE(OH)SO4 (La-Dy) compounds all belong to the REO9 crystal form. In addition to ν1, the bending vibration (ν2) of OH− was also observed in the region of ~663-906 cm−1. Additionaly, the characteristic vibrations of SO42− are well identifiable at ~1001 cm−1 (ν1, medium strong) and in the regions of ~1019-1270 cm−1 (ν3 strong) and 565-653 cm−1 (ν4, strong) [28]. The Dy product differs from the La-Tb ones by showing additional absorptions in the range of ~1370-1600 cm−1. These vibrations, attributable to NO3− (from RE nitrate) and CO32− (from atmospheric CO2) [28, 29], may partially reflect the chemical composition of the impurity phase. Shifting of the bending mode (ν2) of OH− to a larger wavenumber with decreasing RE3+ size is much faster than that of the SO42− vibrations, which is owing to higher structure rigidity of the [SO4] tetrahedron. In order to clarify structure details, Rietveld refinement of the XRD pattern was conducted for Tb(OH)SO4, using Eu(OH)SO4 (COD-2211386) as initial structure model. The results (Table 1 and Fig. S2) indicated that the compound is single phasic and its diffractions can be readily indexed in the monoclinic unit cell (P21/n space group). The results of peak indexation in the range of 2θ = 5-45° are shown in Fig. S3, and Table S1 and Table S2 summarize the derived structure details, including atomic coordinates, d-spacing, and Miller index. 10  Table 1. The results of Rietveld structure refinement for the Tb(OH)SO4 compound. symmetry monoclinic space group P21/n a (Å) 4.3806(1) b (Å) 12.1729(1) c (Å) 6.7674(2)  (°) 106.3967(13) V (Å3) 346.19(1) Z 4 2θ-range (°) 10-100 Rwp (%) 9.91 Rexp (%) 5.03 Rp (%) 6.82 χ2 1.97 RB (%) 1.17   Fig. 3. Schematic illustration of the coordination environment of Tb3+ (a), the coordination mode of sulfate group (b), and the crystal structure viewed along axis a (c) and axis b (d) for the Tb(OH)SO4 compound. Fig. 3 shows the crystal structure and polyhedron geometries of Tb(OH)SO4, which were 11  visualized by the Diamond software. Similar to La(OH)SO4 [23] and Eu(OH)SO4 [21], the framework of the Tb(OH)SO4 compound features [TbO9] polyhedron and [SO4] tetrahedron. Each [TbO9] is connected to six [SO4] by sharing vertices, among which five are from the same layer and one from the adjacent layer (Fig. 3a,c). Meanwhile, all the O atoms of the sulfate group take part in Tb coordination, three of which participate in the construction of the same layer by connecting five Tb3+ ions and the other one bridges one Tb3+ in the adjacent layer along the b axis (Fig. 3b,c). The coordination of Tb3+ is finally completed by the O atoms from three hydroxide ions, which act as bridging ligands between three Tb3+ ions (Fig. 3a,d). The [TbO9] polyhedrons are connected via alternative edge- and face-sharing to form infinite chains along the c axis (Fig. 3c,d), and the chains then form ac layers by vertex sharing along the a axis (Fig. 3d). The layers confined to ac planes are finally tied together by the bridging [SO4] tetrahedrons along the b axis to form a layered crystal structure.   Fig. 4. Correlation of lattice constants (a, b and c), axis angle () and cell volume (V) with the ionic radius of RE3+ (CN = 9). Fig. 4 shows the lattice parameters derived with Jade 6 software for the series of 1.10 1.12 1.14 1.16 1.18 1.20 1.224.354.404.454.504.5512.212.312.412.512.612.76.786.846.906.967.02106.08106.21106.34106.47352363374385 a /ÅIonic size/ÅGdTbEu SmLaCePrNd b /ÅLaCePrNdSmEuGdTbc /ÅLaCePrNdSmEuGdTb  /(°)LaCePrNdSmEuGdTb  V/Å3 LaCePrNdSmEuGdTb12  RE(OH)SO4, where it is seen that lattice constants (a, b and c) and cell volume (V) tend to linearly decrease as the radius of RE3+ decreases while axis angle β showed an opposite tendency. The contraction of unit cell explains the diffraction shift of RE(OH)SO4 to higher angles as RE3+ size decreases (Fig. 1), which is consistent with lanthanide contraction. FE-SEM analysis (Fig. 5) demonstrated the La-Tb products are mostly composed of giant laths with a side length up to ~50 μm in each case, which is the typical morphology of RE(OH)SO4 and is consistent with previous reports [13, 30]. Two types of distinctly different particles/crystallites were observed in the Dy product, with the giant laths and flower-like clusters (~2 μm in size) presumbly belonge to Dy(OH)SO4 and the impurity phase, respectively, in accordance with the results of XRD and FTIR analyses (Fig. 1b and Fig. 2). The two-dimensional (2D) growth behavior of the RE(OH)SO4 crystals is an externalization of ther layered crystal structure, which is similar to the situations of SO42−-LREH and A−-LREH layered compounds [31, 32].    13       Fig. 5. FE-SEM morphologies of the hydrothermal products for RE = La-Dy.    Fig. 6. TG/DTA curves for Tb(OH)SO4 (a) and XRD patterns of the products obtained by calcining Tb(OH)SO4 at different temperatures for 1 h (b). The thermal behaviors of the series of RE(OH)SO4 were comparatively investigated by TG/DTA in simulated air, and the results are shown in Fig. 6a for the Tb(OH)SO4 representative and in Fig. S4 for the rest. It is seen that thermal decomposition similarly proceeds via three distinct stages (Fig. 6a), and each stage is accompanied by an endothermic peak. The three successive stages correspond to dehydroxylation (removal of OH−) to form a 150 300 450 600 750 900 1050 1200 1350707580859095100Weight/%Temperature/°C GStage IStage IIStage IIIPeak I(a)-10-8-6-4-2024DTA/(mV/mg)6319771230Tb Peak II Peak III5 10 15 20 25 30 35 40 45 50 55 60 2 /(°)PDF (Tb2(SO4)3): No. 39-0308(b) 700 °C Intensity/a. u.PDF (Tb2O2SO4): No. 23-0506 1020 °C  PDF (Tb4O7): No. 32-128614  nominal composition of RE2O(SO4)2, desulfurization of RE2O(SO4)2 to yield RE2O2SO4, and desulfurization of RE2O2SO4 to produce RE oxide, respectively. Noteworthy is that RE2O(SO4)2 could be a compound or a mixture of equi-molar RE2O2SO4 and RE2(SO4)3. For a better understanding, the Tb(OH)SO4 representative was calcined at a temperature (700 °C) covered in stage I for 1 h. XRD analysis (Fig. 6b) found that the product is actually a mixture of RE2O2SO4 and RE2(SO4)3, indicating that the latter is the actual case. In addition, the product calcined at 1020 °C is mainly composed of Tb2O2SO4 (Fig. 6b), indicating that RE(OH)SO4 was indeed converted to RE2O2SO4 after stage II. In addition, the weak diffractions characteristic of Tb4O7 indicates the commencement of stage III (Fig. 6b), which verifies that stage III indeed corresponds to the conversion of RE2O2SO4 to oxide. From the above discussion and the weight loss observed for each stage, the procedure of RE(OH)SO4 decomposition can be detailed as follows: (1) RE(OH)SO4 → 0.25RE2O2SO4 + 0.25RE2(SO4)3 + 0.5H2O (Stage I); (2) RE2(SO4)3 → RE2O2SO4 + 2SO3 (Stage II); (3) RE2O2SO4 → RE oxide + SO3 (Stage III). The decomposition data are tabulated in Table 2 for the entire series of RE(OH)SO4 except for RE = Ce. The Ce sample was not discussed here because it is easily oxidized to CeO2 in air. Since the thermal stability of RE2O2SO4 increases with increasing RE3+ radius [7, 33], the La-Nd samples showed the first two stages of decomposition in the measured temperature range of 30-1380 °C while a complete three-stage decomposition was observed for the Sm-Tb samples. In the latter case, the shifting towards a higher temperature of peak III with increasing RE3+ size well corresponds to previous reports [7, 33]. Besides, the tending to be higher temperature of dehydroxylation with decreasing 15  RE3+ radius (peak I, Table 2), as observed for SO42−-LREH and RE2(OH)2CO3SO4·nH2O (REOCSH, RE = Gd-Lu) [7, 8, 34], is in line with the Pearson hard-soft-acid-base (HSAB) theory by considering lanthanide contraction, though the occurrence temperature of peak II did not show a clear trend (Table 2). The good correspondence of the observed (Table 2) and theoretical (Table S3) weight losses further confirmed the proposed decomposition procedures.  Table 2 A summary of thermal decomposition data for the RE(OH)SO4 compounds (RE = La-Tb, excluding Ce) up to 1380 °C RE Peak Ⅰ (°C) Peak Ⅱ (°C) Peak III (°C) Weight loss Ⅰ (%) Weight loss Ⅱ (%) Weight loss III (%) Sum of weight loss (%) La 607 1061 --- 3.49 15.10 Incomplete 18.59 Pr 598 993 --- 3.43 15.00 Incomplete 18.43 Nd 606 1004 --- 3.40 14.81 Incomplete 18.21 Sm 606 996 1316 3.39 14.71 14.58 32.68 Eu 608 948 1295 3.34 14.38 14.41 32.13 Gd 621 1019 1290 3.23 14.26 14.29 31.78 Tb 631 977 1230 3.29 14.57 13.13 30.99 3.2 The effect of solution pH and reaction temperature on the phase and morphology evolution of RE(OH)SO4 Up to now, a systematic study on the effects of hydrothermal parameters such as soution pH and temperature on the synthesis of RE(OH)SO4 is still lacking. Therefore, we conducted a detailed investigation with RE = Tb as a representative. Fig. 7 shows the XRD patterns of the products obtained via reaction at 180 °C under different pH values. It can be seen that in the wide pH range of 6.5-10, Tb(OH)SO4 is only obtainable at pH = 6.5 and no precipitate was obtained at pH ≤ 6. The pH = 7 product is an unknown phase, and FTIR analysis (Fig. S5) showed that it contains SO42−, OH−, NH4+ and H2O. The pH = 8 and 9 products are SO42−-LTbH (Tb2(OH)4SO4), and their FTIR spectra (Fig. S5) indeed showed the typical vibrations of SO42−-LTbH [8]. The pH = 10 product is also an unknown phase, and FTIR analysis identified the exixtence of SO42−, OH−, NH4+ and H2O, but the vibrations of OH− is more 16  abundant. Although the RE(NO3)3-(NH4)2SO4-NH4OH reaction system is simple, the above results indicated that the phase evolution of hydrothermal product is very rich and complicated. A weakly acidic environment is conducive to Tb(OH)SO4 formation, which is similar to the cases of REOCSH (RE2(OH)2CO3SO4·nH2O; pH = 6) [34, 35]. Since the OH−/RE3+ molar ratio (1:1) of  Tb(OH)SO4 is the same as that of REOCSH but is lower than that (2:1) of SO42−-LREH, it is thus understandable in view of coordination competition that a higher solution pH is required for SO42−-LTbH to form. FE-SEM analysis found that the pH = 7 product is composed of sheets with a length of ~1-3 μm and a width of ~100-500 nm, the pH = 8 and 9 products (SO42−-LTbH) contian blocks of ~10-30 μm in length and ~5-10 μm in width, and the pH = 10 product consists of needle-like objects of bout ~5-20 μm long. The morphology of Tb(OH)SO4 (pH = 6.5) is the same as that in Fig. 5 and is not detailed here. The pH = 7 and 10 products could both be single phasic, as inferred from their uniform morphologies, though more studies are needed for clarification.   Fig. 7. XRD patterns of the products obtained under 180 °C and different solution pH. 5 10 15 20 25 30 35 40 45 50 55 60 2 /(°)PDF (Eu(OH)SO4): No. 04-014-5923 pH = 6.5 pH = 7 Intensity/a. u.pH = 8 pH = 9  pH = 1017   Fig. 8. FE-SEM morphologies of the products obtained via reaction at 180 °C and the different solution pH values of 6 (a), 7 (b), 8 (c) and 10 (d).  To better understand Tb(OH)SO4 formation, we tracked the time-course phase and morphology evolution under the fixed reaction conditions of 180 °C and pH = 6.5. As shown in Fig. 9, the 35 min product (Fig. 9a) is a fibrous aggregate, which turned into flower-like clusters with a diameter of ~5 μm when the reaction time reached 1 h (Fig. 9b). XRD analysis (Fig. S6) showed the 35 min product is amorphous while the 1 h one is very similar to that of the aforesaid pH = 7 product (Fig. 7). The primary architecture units of the flower-like clusters (the inset in Fig. 9b) are also consistent with the lamellar morphology of the pH = 7 product, indicating that they might be the same phase. After a duration of 2 h, the flower-like clusters were observed to further nucleate and grow, with the individual particles reached up to ~10 μm in diameter (Fig. 9c). After 4 h of reaction, the flower-like clusters were found to coexist with irregular lath-like particles, where the size of the former hardly changed but the length of the latter reached ~10 μm (Fig. 9d). As the lath-like morphology is typical of RE(OH)SO4 in this work (Fig. 5), the appearance of mixed morphologies thus indicates that Tb(OH)SO4 was crystallized through a typical dissolution-reprecipitation (DR) process. The already quite big Tb(OH)SO4 particles/crystallites in the 4 h product may imply a fast growth following nucleation. The XRD pattern (Fig. S6) of the 2 h product is similar to that of the 4 h product, where the weak peak at 2 = 14.52° corresponds to the (020) plane of Tb(OH)SO4. 18  The content of Tb(OH)SO4 in the 6 h product increased significantly, and the observed breakage of flower-like clusters was resulted from dissolution (Fig. 9e). This is because the nucleation and growth of Tb(OH)SO4 during the DR process continuously consumes solutes, which drives the earlier precipitate (clusters) to dissolve. After 10 h of reaction, the Tb(OH)SO4 microcrystals grew further, and giant laths of ~60 μm were formed (Fig. 9f). The relative content changes of the flower-like clusters and Tb(OH)SO4 during the whole DR process are consistent with the results of XRD analysis (Fig. S6).       Fig. 9. FE-SEM morphologies of the products obtained by 180 °C hydrothermal reaction under pH = 6.5 for 35 min (a), 1 h (b), 2 h (c), 4 h (d), 6 h (e) and 10 h (f). The insets in (b), (c) and (d) are closer views.  As aforesaid, Tb(OH)SO4 was crystallized through a DR process, which requires the energy provided by the thermal field to cross a certain barrier. Therefore, hydrothermal temperature would have an important influence on Tb(OH)SO4 formation. We thus tracked the temperature-course of phase and morphology evolution. As shown in Fig. 10, the 70 and 19  100 °C products are amorphous, and FE-SEM analysis showed that they are irregular aggregates (Fig. S7a, b). The diffraction pattern of the 120 °C product (Fig. 10) is similar to those of the aforesaid pH = 7 (Fig. 7) and 1 h (Fig. S6) products, except that the diffraction peaks are sharper. Different from the 1 h product (flower-like clusters, Fig. 9b), the 120 °C product is composed of spheroidal particles of ~50 μm and also a small amount of aggregates (Fig. S7c). The primary architectural unit of the spheroids is significantly larger and wider than that of the 1 h product, which is consistent with the sharper diffraction of the 120 °C product. The 150 °C product only showed weak (020) diffraction of Tb(OH)SO4 (Fig. 10), and largely contains flower-like clusters (~10 μm), together with some Tb(OH)SO4 laths (Fig. S7d), indicating that the DR process only partially occurred. Therefore, it can be concluded that at least ~180 °C is needed for the DR process to complete.   Fig. 10. The XRD patterns of the products obtained at different hydrothermal temperatures.  5 10 15 20 25 30 35 40 45 50 55 60 2 /(°)70 °C Intensity/a. u.100 °C120 °C  (020)150 °C20  3.3 Photoluminescence properties of Eu(OH)SO4 and Tb(OH)SO4        Fig. 11. PLE and PL spectra of the Eu(OH)SO4 (a) and Tb(OH)SO4 (b) compounds, together with the CIE chromaticity diagram (c) and fluorescence decay behaviors (d) of luminescence.  Fig. 11 shows the photoluminescence excitation (PLE) and emission (PL) spectra of the Eu(OH)SO4 and Tb(OH)SO4 compounds. Noteworthy is that the other RE(OH)SO4 products are not luminescent even though their RE3+ ions are optically active (RE = Ce, Pr, Sm, Gd, Dy). For Eu(OH)SO4 (Fig. 11a), the PLE spectrum obtained by monitoring the 616 nm red emission (5D0 → 7F2 transition) displayed a series of sharp lines in the ~270-500 nm region for the intra-4f6 transitions of Eu3+ [36-38], with the 7F0 → 5L6 excitation at ~395 nm being 250 300 350 400 450 500 550 600 650 700 750 800Intensity/a. u.Wavelength/nmlem = 616 nm lex = 395 nm395 nm 7F0→5L6 616 nm 5D0→7F2 5D0→7F1 5D0→7F4PLE PL 7F0→5D2 7F0→5D37F0→5L77F0→5D47F0→5H6 5D0→7F3 5D0→7F07F0→5F2(a)200 250 300 350 400 450 500 550 600 650 700Intensity/a. u.Wavelength/nmlem = 545 nm lex = 369 nmPLE PL369 nm 7F6→5L10545 nm 5D4→7F55D4→7F45D4→7F35D4→7F6 7F6→5D3 7F6→5GJ 7F6→5D1, 0 7F6→5HJ 7F6→5I J(b) Low-spin High-spin 7F6→5D215 20 25 30 35 40 EuOHSO4 Fitting data  Intensity/a. u.Decay time/ms模型 ExpDec1方程 y = A1*exp(-x/t1) + y0绘图 Euy0 -0.26599 ± 0.13817A1 8.44307E9 ± 5.03934E8t1 0.84315 ± 0.00296Reduced Chi-Sqr 7.72279R平方(COD) 0.99856调整后R平方 0.99856(d)t = 0.84(0.03) ms; c2 = 0.999 TbOHSO4 Fitting data   模型 ExpDec1方程 y = A1*exp(-x/t1) + y0绘图 Tby0 -0.08264 ± 0.21623A1 4.00216E8 ± 1.46741E7t1 1.07172 ± 0.00294Reduced Chi-Sqr 18.15218R平方(COD) 0.99912调整后R平方 0.99912t = 1.07(0.01) ms; c2 = 0.99921  the strongest. The PL spectrum obtained under UV excitation at 395 nm exhibited the typical 5D0 → 7FJ (J = 0-4) emissions of Eu3+ at 579, 595, 616, 651 and 697 nm, as labeled in the figure, with the 5D0 → 7F2 red emission (616 nm) being the most prominent. Since Eu3+ would occupy low-symmetric C1 sites in Eu(OH)SO4, the parity forbidden 5D0 → 7F2 electric dipole transition is stronger than the parity allowed 5D0 → 7F1 magnetic dipole transition [39, 40]. For Tb(OH)SO4 (Fig. 11b), the PLE spectrum recorded by monitoring the 545 nm green emission (5D4 → 7F5 transition) of Tb3+ contains two broad bands centered at ~214 and 252 nm for the spin-allowed (low-spin) and spin-forbidden (high-spin) 4f8 → 4f75d1 intra-configurational transitions of Tb3+ [41], respectively, and also a series of sharp peaks in the longer wavelength region for the intra-4f8 transitions of Tb3+, as assigned in the figure, with the 7F6 → 5L10 transition at ~369 nm being the most prominent [42, 43]. Under 369 nm excitation, Tb(OH)SO4 showed luminescence via transition from the 5D4 excited state to 7FJ (J = 3-6) ground multiples of Tb3+, as indicated in the figure, with the 5D4 → 7F5 green emission at ~545 nm being the strongest. The Eu and Tb compounds were analyzed from their PL spectra to have Commission Internationale de L'Eclairage (CIE) chromaticity coordinates (Fig. 11c) of around (0.632, 0.365) and (0.295, 0.481), typical of orange red and green colors, respectively. Fluorescence decay analysis found that the main luminescence of Eu3+ (λem = 616 nm, λex = 395 nm) and Tb3+ (λem = 545 nm, λex = 369 nm) both decreased in a single exponential manner (Fig. 11d), and have lifetime values of 0.84±0.03 and 1.07±0.01 ms, respectively.  4. Conclusion 22  A family of RE(OH)SO4 layered compounds (RE = La-Tb lanthanides) were successfully obtained via mild hydrothermal reaction at 180 °C for 24 h. The solution pH for RE(OH)SO4 formation was found to decreass with decreasing ionic radiu of RE3+. The Tb compound was analyzed to be isostructural with its La-Gd analogues rather than the reportedly Dy-Yb ones, and it has a monoclinic unit cell (P21/n space group) of a = 4.3806 Å, b = 12.1729 Å, c = 6.7674 Å and  = 106.3967°, where Tb3+ is 9-fold coordinated. An increasing temperature of dehydroxylation and a decreasing temperature of complete desulfurization were observed at a smaller RE3+. Investigation of the time-course and temperature-course of phase/morphology evolution showed that RE(OH)SO4 was formed via a typical dissolution-reprecipitation process. The Eu and Tb compounds showed red (616 nm) and green (545 nm) emissions upon UV exciation, with fluorescence lifetimes of ~0.84 and 1.07 ms and chromaticity coordinates of around (0.632, 0.365) and (0.295, 0.481), respectively. 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