YANG Dong ZHUANG Rong-Chun MI Jin-Xio HUANG Y-Xi②

a (Fujian Key Laboratory of Advanced Materials (Xiamen University), Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China) b (State Key Laboratory of Comprehensive Utilization of Low-grade Refractory Gold Ores, Shanghang 364200, China)

ABSTRACT Two new alkali metal germanophosphates, namely, Na3[Ge(OH)(PO4)2]·2H2O and Li2Na[GeO(HPO4)(PO4)], have been prepared by solvothermal method, and their crystal structures were determined by single-crystal X-ray diffraction. The title two compounds crystalize in the same orthorhombic space group Pbcm (No. 57) and feature similar chain-like structure which is built from zig-zag GeO6 octahedral thread loop branched by PO4 tetrahedra. For Na3[Ge(OH)(PO4)2]·2H2O, a = 10.1650(9), b = 13.1975(12), c = 6.9751(7) ?, V = 935.73(15) ?3, Z = 4, R = 0.0356 and wR = 0.1109; and for Li2Na[GeO(HPO4)(PO4)], a = 6.9855(5), b = 14.5809(18), c = 6.6620(5) ?, V = 678.56(11) ?3, Z = 4, R = 0.0286, and wR = 0.0762. The partial substitution of Na ions by Li ions not only significantly influences the total structural features and the water molecule contents, but also impacts on their thermal stabilities. Li2Na[GeO(HPO4)(PO4)] is thermally stable up to 400 ℃, whereas only 150 ℃ for Na3[Ge(OH)(PO4)2]·2H2O.

Keywords: alkali metal, chain-like structure, germanophosphates, thermal stability;

1 INTRODUCTION

The development of inorganic metal phosphates plays an important role in improving our understanding of nonlinear optics, catalysts, Li/Na ion batteries and luminescence[1-8]. Recently, as a branch of metal phosphates, germano- phosphates (GePOs) have attracted increasing attention due to its structural diversity. For instance, the first zinco- germanophosphate (DABCO)·ZnGe(HPO4)3features a zeolite molecular sieve structure[9]; dimensional reduction from 2D layer to 1D band for germanophosphates induced by the “tailor effect” of F-ions has been found in K[MIIGe(OH)2(H0.5PO4)2] (M = Fe, Co) and K4[MIIGe2F2(OH)2(PO4)2(HPO4)2]·2H2O (M = Fe, Co)[10]; structural assembly from phosphate to germanophosphate by applying GeO6octahedra as a binder has been found in K3[MII4(HPO4)2][Ge2O(OH)(PO4)4]·xH2O (MII= Fe, Cd; x = 2 for Fe and 3 for Cd)[11]. Moreover, the structural diversity of GePOs leads to various potential applications. More and more GePOs are capable of being applied in the field of lithium ion batteries, nonlinear optics and luminescence, i.e. Li1+xAlxGeyTi2-x-y(PO4)3with high lithium ionic conductivity, Ba-substituted Li1+xAlxGe2-x(PO4)3(x = 0.5) solid electrolytes and NASICON-structured LiGe2(PO4)3lithium ion batteries[12-14]; K(Ga0.5Ge0.5)(F0.5O0.5)(PO4) compound with KTiOPO4structure can be applied to nonlinear optics; Sn2Ge(PO4)2(OH)2might be pure inorganics for potential intrinsic green light-phosphors[6,15]. Obviously, most of the GePOs with potential applications lie in the compounds without containing water molecules or hydroxyl group. Therefore, how to design water-free GePOs becomes to be a research interest.

Inspired by the applications of GePOs and the interest to design water-free GePOs, we have developed a modified solvo-/hydro-fluorothermal method under fluoride-rich and water-deficient condition, which can be highly effective for synthesizing anhydrous compounds by the replacement of hydroxyl groups and water molecules with fluorine. Our group had previously reported two anhydrous multiple fluorine-substituted phosphate germanium fluorides Na3[GeF4(PO4)] and K4[Ge2F9(PO4)] with higher thermal stability than their hydrated germanophosphates[16]. And we also had found out that the synergetic effects of fluoride and alkali cations led to structural changes from chain-like to layered structures in a series of five novel fluorogerma- nophosphates: A2[GeF2(HPO4)2] (A = Na, K, Rb, NH4, and Cs)[17].

As fluorine is highly toxic, it is more desirable to obtain fluorine-free compounds with high thermal stability. Based on the previous investigations, in order to enrich the structure chemistry of GePOs, we attempt to investigate the possibility of alkali metal ion substitution to synthesize less-water compounds considering the regulatory effect of alkali metal ions on structure. As a result, two novel alkali metal germanophosphates, Na3[Ge(OH)(PO4)2]·2H2O (denoted as 1) and Li2Na[GeO(HPO4)(PO4)] (denoted as 2), have been synthesized by solvothermal method. In this paper, the synthesis, crystal structure, and thermal stability of the two title compounds have been discussed. It is worthy to note that Li2Na[GeO(HPO4)(PO4)] is the first reported germano- phosphate containing mixed alkali metals.

2 EXPERIMENTAL

2. 1 Synthesis

Single crystals of compounds 1 and 2 were synthesized via solvothermal method. The synthetic procedures of these two compounds are presented in Scheme 1.

Scheme 1. Synthetic procedures of (a) Na3[Ge(OH)(PO4)2]·2H2O and (b) Li2Na[GeO(HPO4)(PO4)]

For Na3[Ge(OH)(PO4)2]·2H2O: Firstly, GeO2(0.104 g, 1.0 mmol) was added to the Teflon liner (10 mL in volume) with pre-configured 0.5 mL NaOH (4 mol·L-1, aq) solution by stirring to form a slurry. Then, solvents triethylamine (TEA, 1 mL, 7.4 mmol) and H3PO4(85wt%, 0.5 mL, 3.7 mmol) were dropped in above the slurry separately without any stirring, resulting in the mixture with a molar ratio of GeO2/NaOH/TEA/H3PO4= 1:2:7.2:3.7. The sealed Teflon-lined autoclaves were placed in an oven and heated at 230 ℃ for 7 days. After that, it was cooled down to room temperature in air. Colorless and transparent block-like crystals (Fig. 1a) were obtained after filtration, washed with deionized water and dried in air. The purity of the obtained sample has been checked by powder X-ray diffraction (PXRD) measurement. Fig. 2 shows that the observed pattern of compound 1 fits quite well with the simulated one based on the single-crystal diffraction below.

Fig. 1. SEM images of (a) Na3[Ge(OH)(PO4)2]·2H2O and (b) Li2Na[GeO(HPO4)(PO4)]

Fig. 2. Experimental (black dot) and calculated (red line) X-ray diffraction patterns of (a) Na3[Ge(OH)(PO4)2]·2H2O and (b) Li2Na[GeO(HPO4)(PO4)]. The difference profile (blue) and background (black) from Rietveld refinement (Rp = 6.057, Rwp = 5.704, χ2 = 2.152 for Na3[Ge(OH)(PO4)2]·2H2O, and Rp = 7.996, Rwp = 10.982, χ2 = 1.629 for Li2Na[GeO(HPO4)(PO4)]) are given at the bottom. The Bragg positions are indicated by the vertical bars below the patterns

Single crystals of Li2Na[GeO(HPO4)(PO4)] were obtained under similar preparation procedure as Na3[Ge(OH)- (PO4)2]·2H2O. However, here the combination of LiOH·H2O (0.200 g, 4.8 mmol) and NaOH (0.096 g, 2.4 mmol) took the place of NaOH (4 mol·L-1, aq) solution. In addition, the amount of H3PO4used is twice of that for Na3[Ge(OH)- (PO4)2]·2H2O. The optimized reaction conditions are as follows: a mixture of GeO2(0.104 g, 1.0 mmol), LiOH·H2O (0.200 g, 4.8 mmol), NaOH (0.096 g, 2.4 mmol), triethyl- amine (TEA, 1 mL, 7.4 mmol) and H3PO4(85wt%, 1 mL, 7.3 mmol) was kept at 230 ℃ for 7 days. Colorless and trans- parent prismatic crystals (Fig. 1b) were gotten after reactions. The experimental powder X-ray diffraction pattern showed in Fig. 2 is in good agreement with the simulated one based on the single crystal crystallographic data which further confirm its purity.

To understand the effect of pH on the formation of two compounds, we performed a series of experiments by changing the amount of H3PO4(0.25～1.5 mL). Because the reaction is heterogeneous, TEA and H3PO4were dropped without stirring and GeO2has low solubility, the accurate pH value cannot be measured before the reaction. Investigations indicate that the most suitable synthetic conditions are the above synthetic conditions, the pH values of the solutions after reaction were in the range of 5.5～6.5 which is at the weakly acidic and nearly neutral condition. Strong acidic and basic conditions were not able to result in the title two compounds.

2. 2 Characterization methods

2. 2. 1 Powder X-ray diffraction (PXRD)

PXRD measurements were performed on a Bruker D8 Advance powder X-ray diffractometer with CuKα radiation (λ = 1.5418 ?, Ni filter, 40 kV/40 mA). The relative parameters for collecting the patterns were below: the scan step width was 0.01° and the fixed counting time was 0.3 s per step in the 2θ range from 5° to 60°.

2. 2. 2 Scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDX)

A field emission scanning electron microscope (FE-SEM, SU70) was utilized to characterize the crystalline morpholo- gies and elemental compositions.

2. 2. 3 Fourier transform infra-red (FTIR) spectroscopy

The FTIR spectra were carried on a Nicolet iS10 FT-IR spectrometer in the range from 4000 to 400 cm-1. The title compounds were well-ground and mixed homogeneously with KBr firstly and further pressed into pellets for the measurement.

2. 2. 4 Thermal analysis

A NETZSCH TG-209 F1 thermogravimetric/differential thermal analyzer (TG/DTA) was used to analyze the thermal behavior of the title compounds at a heating rate of 10 ℃·min-1in a nitrogen atmosphere (50 mL·min-1) from 35 to 930 ℃.

2. 3 Crystal structure determination

Colorless, transparent and faultless crystals (0.15 × 0.14 × 0.03 mm3and 0.22 × 0.09 × 0.08 mm3for compounds 1 and 2, respectively) have been picked up for data collection on an Oxford Gemini S Ultra single-crystal X-ray diffractometer equipped with graphite-monochromatic MoKα radiation (λ = 0.71073 ?, 50 kV/40 mA) at 193(2) K.

For compound 1, in the range of 3.09≤θ≤29.04o, a total of 2312 reflections were collected and 1156 were independent with Rint= 0.0322, of which 1015 were observed with I > 2σ(I). Multi-scan absorption correction has been performed. The crystal structure was solved by direct methods and refined by full-matrix least-squares technique applying the SHELX programs included in the WinGX package, and further checked for missing symmetry elements by using PLATON[18,19]. The structure has been solved in orthorhombic space group Pbcm (No. 57) with a = 10.1650(9), b = 13.1975(12), c = 6.9751(7) ?, V = 935.73(15) ?3and Z = 4. Na, Ge, P and some coordinated O atoms were determined by direct methods. The rest O atom sites were located from difference Fourier maps. H atoms coordinated to O(7) as hydroxyl group and O(8) and O(9) as water molecule were able to locate from difference Fourier maps and their O-H distances and displacement parameters were con- strained to be 0.82(2) ? and 0.05 e·?-3. However, H(9B) atom coordinated to O(9) could not find hydrogen bond acceptor due to the nearest atom of Na(2) with the distance of 2.433(3) ?. The final full-matrix least-squares refinement converged to R = 0.0356, wR = 0.1057 for 1015 observed reflections with I > 2σ(I), and R = 0.0410, wR = 0.1109 for all data (2312), S = 1.069, Dc= 2.730 g·cm-3, μ(MoKα) = 3.81 mm-1and F(000) = 752. The largest diffraction peak and hole are 0.99 and -0.77 e·?-3, respectively.

For compound 2, a total of 1811 reflections were collected in the range of 2.79≤θ≤29.36o, of which 898 were inde- pendent with Rint= 0.0220 and 782 were observed with I > 2σ(I). The structure has been solved in orthorhombic space group Pbcm (No. 57), with a = 6.9855(5), b = 14.5809(18), c = 6.6620(5) ?, V = 678.56(11) ?3and Z = 4. Na, Ge, P and some coordinated O atoms were determined from direct methods. The rest O atom and Li atoms were located from difference Fourier maps. H(3) atoms coordinated to the O(3) as hydroxyl group were able to locate from difference Fourier maps and their O-H distances and displacement parameters were constrained to be 0.82(2) ? and 0.05 e·?-3. The final full-matrix least-squares refinement converged to R = 0.0286, wR = 0.0739 for 1015 observed reflections with I > 2σ(I), and R = 0.0340, wR = 0.0762 for all data (1811), S = 1.106, Dc= 3.097 g·cm-3, μ(MoKα) = 5.08 mm-1and F(000) = 608. The largest diffraction peak and hole are 0.67 and -0.68 e·?-3, respectively.

Selected bond distances and bond angles of compounds 1 and 2 are given in Tables 1 and 4, respectively. The hydrogen bonds of 1 and 2 are listed in Tables 2 and 5, respectively.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) of Na3[Ge(OH)(PO4)2]·2H2O

Symmetry codes: (i) -x+1, -y, -z+1; (ii) x, y, -z+3/2; (iii) x, y, z+1; (iv) x, y, -z+1/2; (v) -x+1, -y, z+1/2; (vi) x, -y+1/2, -z+1; (vii) x, -y+1/2, z+1/2; (viii) -x+1, -y, z-1/2; (ix) x-1, y, z; (x) x-1, -y+1/2, -z; (xi) -x+1, y-1/2, -z+1/2; (xii) -x+1, y+1/2, -z+1/2; (xiii) x, y, z-1; (xiv) x, -y+1/2, z-1/2; (xv) x, y, -z-1/2; (xvi) x+1, y, z; (xvii) -x+1, y-1/2, z-1; (xviii) x, -y+1/2, -z

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°) of Na3[Ge(OH)(PO4)2]·2H2O

Table 3. Bond Valence Sum (BVS) of Na3[Ge(OH)(PO4)2]·2H2O

Table 4. Selected Bond Lengths (?) and Bond Angles (°) of Li2Na[GeO(HPO4)(PO4)]

To be continued

Table 5. Hydrogen Bond Lengths (?) and Bond Angles (°) of Li2Na[GeO(HPO4)(PO4)]

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

3. 1. 1 Crystal structure of Na3[Ge(OH)(PO4)2]·2H2O (1)

The bond lengths of Ge-O and P-O are in the ranges of 1.859(2) ～1.8963(12) and 1.516(3) ～ 1.567(2) ?, respectively. Meanwhile, the O-Ge-O bond angles range from 88.45(12) to 180.0° and the O-P-O bond angles change from 105.85(11) to 113.98(18)o. Both the bond lengths and bond angles are in good agreement with those previously reported germanophosphates[8,10,20]. And bond valence calculation[21,22]results of Na3[Ge(OH)(PO4)2]·2H2O (given in Table 3) reveal that Na, Ge, P and O are in their expected values (+1, +4, +5, -2, respectively)[17,20,23].

3. 1. 2 Crystal structure of Li2Na[GeO(HPO4)(PO4)] (2)

The bond lengths of Ge-O and P-O are in the ranges of 1.8149(12)～1.917(2) and 1.511(3)～1.562(2) ?, respec- tively. Meanwhile, the O-Ge-O bond angles range from 85.23(12) to 180.0° and the O-P-O bond angles fall in the 105.18(12)～114.1(2)o range. Both bond lengths and bond angles are in good agreement with those previously reported germanophosphates[8,10,20]. And bond valence sum (BVS) calculation[21,22]results of Li2Na[GeO(HPO4)(PO4)] are given in Table 6. The BVS values are 0.94～1.11 for Li+cations, 1.22 for Na+cations, 4.24 for Ge4+ions and 4.80～4.86 for P5+cations, which are in good agreement with the expected values[7,17,23,24].

Table 6. Bond Valence Sum (BVS) of Li2Na[GeO(HPO4)(PO4)]

3. 1. 3 Crystal structure comparison

Both of the title compounds crystallize in the same space group Pbcm and consist of pretty similar infinite zig-zag germanophosphate single chains with a molar ratio of Ge/P = 1:2 as the structure building units. However, adjusting the synthetic raw materials with the addition of LiOH·H2O as reactant leads to a new compound Li2Na[GeO(HPO4)(PO4)] (2) with different cell parameters and water contents compared to Na3[Ge(OH)(PO4)2]·2H2O (1). The comparison of their crystal structures indicates that Li(1) and Li(2) atoms in compound 2 take the place of Na(2) positions in compound 1 (Fig. 3). Moreover, Li+and Na+ions have dramtic different coordination environments. Na(2) atoms are in six-coor- dination. It not only coordinates to the O atoms from germanophosphates (GePO) single chains, but also coor- dinates to two H2O molecules which separate the chains into pseudo-layers perpendicular to the a-axis. In contrast, because of the smaller ionic radius of Li+ions, both Li(1) and Li(2) are only in five coordination. Thus, Li atoms only 5-coordinate to the O atoms from GePO single chains which do not need any more water molecules to stabilize the crystal structure. The GePO single chains in compound 1 are interlinked by the hydrogen bonds originated by OH groups into pseudo-layers parallel to the bc plane due to the separation of Na+ions and H2O molecules, whereas in compound 2, the GePO single chains link the neighboring single chains via hydrogen bonds to result in a pseudo- network structure. Consequently, the a-axis of compound 1 (10.1650(9) ?) is much larger than that of 2 (6.9855(5) ?), and compound 1 (3.81 Mg·m-3) has much smaller density than compound 2 (5.08 Mg·m-3). Therefore, the substitution of Na ions by Li ions results in the structure evolution from pseudo-layered structure for compound 1 (Fig. 3c) to the pseudo-network structure for 2 (Fig. 3f). It is worthy to note that water molecules in compound 1 could not be removed without destroying the structure. The details will be discussed in the section of 3.2.

3. 2 Thermal stabilities

The TG curve of Na3[Ge(OH)(PO4)2]·2H2O (1) presented in Fig. 4a contains four steps of weight loss. In the first step the weight loss is about 2.04% in the range of 105～135 ℃ attributed to the evaporation of 0.5×H2O molecule (calcd.: 2.34%). The second step occurs around 6.61% from 135 to 360 ℃, in good agreement with the calculated weight loss of 1.5×H2O molecules (calcd.: 7.02%). The third step has slow weight loss of approximately 3.02% which could be the condensation of one hydroxyl group (calcd.: 2.34%). The percentage of the above total three steps (obsd.: 11.67%) is close to the calculated one (calcd.: 11.70%). The uncommon water release, first 0.5×H2O and then 1.5×H2O, could be explained as follows. There are two kinds of positions for water molecules in the crystal structure. One has a H atom without forming hydrogen bond because of no acceptor, so 0.5H2O water molecule is lost at the first step, and the rest water molecules (1.5H2O) forming hydrogen bonds are released subsequently in the second step.

In order to understand the thermal behavior of compound 1, PXRD measurements of the samples after heating to different temperature (150, 250 and 550 ℃ according to TG curves) have been conducted. All samples were heated up from 30 ℃ at a heating rate of 5 ℃·min-1, and then kept at different temperature for 1 hour. The PXRD results (Fig. 4b) show that the crystal structure of compound 1 has been changed at 150 ℃, since a new peak emerged (marked with asterisk(*)) due to the removal of crystal water, which is also consistent with the first step of weight loss in the temperature range of 100～150 ℃. When heated to 250 ℃, the crystal structure nearly decomposes to unidentified phase. And after heating to 550 ℃, the structure collapses completely, with the residue identified to be GeO2(JCPDS No: 36-1463)[25].

Compared with compound 1, the TG curve of Li2Na[GeO(HPO4)(PO4)] (2) showed in Fig. 4c only includes one step of weight loss (2.96%) in the 420～530 ℃ region, which is consistent with the calculated value of 2.84%. As for the second step, the slow and lasting weight loss is verified by the PXRD patterns which display that the structure of compound 2 starts to decompose, and the residue at 550 ℃ is a mixture of compound 2, GeO2[25]and some unidentified phases. It needs to note that the weight loss (ca. 1.0%) below 400 ℃ is mainly due to the instrumental error which can be confirmed by the PXRD results that the sample after heating to 400 ℃ for 1 h still keeps the structure of Li2Na[GeO(HPO4)(PO4)] (Fig. 4d).

Fig. 4. TG analysis curves and PXRD patterns of Na3[Ge(OH)(PO4)2]·2H2O (a, b) and Li2Na[GeO(HPO4)(PO4)] (c, d). (a) TG analysis curves of compound 1; (b) PXRD patterns of compound 1 after heating to different temperature; (c) TG analysis curves of compound 2; (d) PXRD patterns of compound 2 after heating to different temperature. The asterisk (*) indicates a new peak appearing for compound 1; Rhombus symbol (?) indicates the reflection of compound 2; The triangle symbol (▼) indicates the reflection of GeO2

Obviously, th e thermal stability of compound 2 (up to 400 ℃) is much higher than that of compound 1 (up to 150 ℃). The PXRD results do agree well with the thermal behaviors, which could be explained in both synthesis methods and structural features. Firstly, there is more solution in the synthesis environment of compound 1, and sodium hydroxide solution is the sodium source, while compound 2 can be formed without adding extra free water. It is well known that the thermal stability of non-aqueous structures is remarkably better. Secondly, the water molecules are connected to Na+ions by sharing O atoms, and the crystal structure of compound 1 would be collapsed once losing water molecules during the heating process. In addition, the crystallographic position of Na(2) atom in compound 1 replaced by Li atoms with smaller radius results in a more tightly connected crystal structure of compound 2. Therefore, different crystal structures arise from the synthetic conditions, and further influence the thermal stability.

3. 3 Fourier transform infra-red (FTIR) spectroscopy

The FTIR spectra of the two title compounds are illustrated in Fig. 5. They show dramatic different absorption bands in the high wavenumber region. Compound 1 has additional strong and sharp absorption bands at 3561, 3527, and 1636 cm-1, which are attributable to the stretching vibrations and bending modes of the free water molecule of compound 1[26]. Moreover, two separated bands at 3561 and 3527 cm-1having different absorption intensity confirm two types of H2O molecules in compound 1. On the other hand, both compounds have broad and weak absorption bands at 3417/3421 cm-1for compounds 1 and 2, respectively, which can be assigned to the hydrogen bonds arising from OH groups. Besides, the absorption bands appearing in low- frequency regions can mainly attribute to the vibrations of PO4and GeO4(OH)2groups for compound 1, and PO4, HPO4and GeO6groups for 2. In particular, the sharp and strong bands at 767 and 811 cm-1for 1 and 2 respectively are the characteristic vibration peaks of Ge-O[27,28]. Thus, IR results confirm that compound 1 contains crystal water molecules and hydroxyl groups whereas compound 2 only has hydroxyl groups. Both compounds compose characteristic absorption bands for Ge-O and P-O bonds.

Fig. 5. FTIR spectra of Na3[Ge(OH)(PO4)2]·2H2O and Li2Na[GeO(HPO4)(PO4)]

4 CONCLUSION

In summary, two novel alkali metal germanophosphates, Na3[Ge(OH)(PO4)2]·2H2O and Li2Na[GeO(HPO4)(PO4)], have been synthesized by solvothermal method. Both of them crystallize in the same orthorhombic space group Pbcm with similar infinite zig-zag germanophosphate single chains. The study of crystal structures and thermal behaviors reveals that the substitution of Li+ions for Na+ions does significantly influence the total structural features and water molecule contents. Moreover, it also impacts on their thermal stabilities. The successful syntheses of the two title compounds with similar structural features but different water contents and alkali metal types suggest that the replacement of alkali metal through this modified solvothermal method is an efficient strategy to prepare new metal phosphates with higher thermal stability. Moreover, to the best of our knowledge, Li2Na[GeO(HPO4)(PO4)] is the first mixed alkali metal germanophosphate up to now.

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