Article pubs.acs.org/IC
Multiple Fluorine-Substituted Phosphate Germanium Fluorides and Their Thermal Stabilities Xia Huang,† Biao Liu,† Rong-Chuan Zhuang,‡ Yuanming Pan,§ Jin-Xiao Mi,† and Ya-Xi Huang*,† †
Fujian Provincial Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China ‡ Xiamen Zijin Mining and Metallurgy Technology Co Ltd, Xiamen 361101, China § Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N5E2, Canada S Supporting Information *
ABSTRACT: Anhydrous compounds are crucially important for many technological applications, such as achieving high performance in lithium/sodium cells, but are often challenging to synthesize under hydrothermal conditions. Herein we report that a modified solvo-/ hydro-fluorothermal method with fluoride-rich and water-deficient condition is highly effective for synthesizing anhydrous compounds by the replacement of hydroxyl groups and water molecules with fluorine. Two anhydrous phosphate germanium fluorides, namely, Na3[GeF4(PO4)] and K4[Ge2F9(PO4)], with chainlike structures involving multiple fluorine substitutions, were synthesized using the modified solvo-/hydro-fluorothermal method. The crystal structure of Na3[GeF4(PO4)] is constructed by the common single chains ∞1{[GeF4(PO4)]3−} built from alternating GeO2F4 octahedra and PO4 tetrahedra. For K4[Ge2F9(PO4)], it takes the same single chain in Na3[GeF4(PO4)] as the backbone but has additional flanking GeOF5 octahedra via an O-corner of the PO4 groups, resulting in a dendrite zigzag single chain ∞1{[Ge2F9(PO4)]4−}. The multiple fluorine substitutions in these compounds not only force them to adopt the low-dimensional structures because of the “tailor effect” but also improve their thermal stabilities. The thermal behavior of Na3[GeF4(PO4)] was investigated by an in situ powder X-ray diffraction experiment from room temperature to 700 °C. The modified solvo-/hydro-fluorothermal method is also shown to be effective in producing the most germanium-rich compounds in the germanophosphate system.
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INTRODUCTION Oxyfluoride inorganic compounds are important materials for a wide range of industrial and technological applications, that is, Li/Na ion battery materials,1,2 nonlinear optical crystals,3,4 and catalysts.5,6 Recently, significant efforts have been made to search for new cathode materials in the fluorophosphate system, because they exhibit higher cell potentials than oxyphosphates owing to the fact that F− has one less electron than O2−. Therefore, the substitution of F− for O2− to form a fluorophosphate from an oxyphosphate will increase the number of Li+ ions in the structure, leading to enhanced cell potentials. Additionally, the high electronegativity of F− serves to increase the thermal stability of Li2MPO4F compared to LiMPO4.7 Moreover, fluoro-hydrothermal synthesis is an environmentally friendly method to obtain fluorine-substituted compounds.7,8 However, most of the compounds synthesized using the fluoro-hydrothermal method contain water molecules or hydroxyl groups that can hinder their virtual applications. Thus, new strategies must be exploited for synthesizing anhydrous fluorine-substituted phosphates. Additionally, incorporation of fluorine into inorganic framework structures often changes the structural and compositional aspects of the compounds.9,10 Compared with the bridging manner of O atoms, fluorine is often observed at the terminal © XXXX American Chemical Society
site in the structure, thus reducing the structural dimensionality. Moreover, the levels of framework termination depend upon their fluoride contents and show numerous useful characteristics for functionality and applications. Moreover, the electronwithdrawing characteristic of F− ions may also help to stabilize the metals in coordination centers to higher oxidation states.9,11,12 Thus, following our previous investigations on germanophosphates,13−15 we put our efforts on the synthesis of fluorinesubstituted germanophosphates. F− ions are introduced into the germanophosphate system for exploring new compounds with diverse structures. Although many germanophosphates have been reported, including transition-metal germanophosphates,13−20 alkali metal germane-phosphates,21−25 and germanium phosphates,25−29 most compounds of these three categories have three-dimensional framework structures usually built from GeO4 tetrahedra and GeO6 octahedra, sometimes even GeO5 tetragonal pyramids or trigonal bipyramids, together with PO4 tetrahedra. Fluorine-substituted germanophosphates are still quite rare. There are only two isotypic compounds, K4[MIIGe2F2(OH)2(PO4)2-(HPO4)2]·2H2O (M = Fe, Co),15 Received: September 20, 2016
A
DOI: 10.1021/acs.inorgchem.6b02266 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry consisting of complex oxyfluoride anions reported by us recently. A structural alternation from a two-dimensional (2D) layered structure of oxy-germanophosphate to a onedimensional (1D) band structure was observed simply by the addition of a small amount of KF·2H2O to the reaction mixture. The incorporation of one terminal F atom to the coordination sphere of Ge breaks the linkage between the transition metal and germanium octahedra in the layer to form the band structure. Therefore, this is a promising perspective to explore novel low-dimensional structures via the addition of fluoride in the germanophosphate systems. Motivated by these considerations, we applied a modified solvo-/hydro-fluorothermal synthetic method and obtained two anhydrous multiple fluorine-substituted phosphate germanium fluorides Na3[GeF4(PO4)] and K4[Ge2F9(PO4)] with chainlike structures. Herein we will report on their syntheses, crystal structures, and thermal stabilities.
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EXPERIMENTAL SECTION
2.1. Synthesis. 2.1.1. Synthesis of Na3[GeF4(PO4)]. A modified solvo-/hydro-fluorothermal method was applied to synthesize Na3[GeF4(PO4)] by applying triethylamine (TEA) as the solvent. The typical synthetic procedure is presented in Scheme 1a. First, GeO2
Figure 1. Observed and calculated PXRD patterns of Na3[GeF4(PO4)] (top) and K4[Ge2F9(PO4)] (bottom). Red line: observed PXRD patterns, black line: calculated PXRD patterns. (insets) SEM images of representative single crystals. Cu Kα radiation.
Scheme 1. Synthetic Procedures for (a) Na3[GeF4(PO4)] and (b) K4[Ge2F9(PO4)]
demonstrated to be effective in the synthesis of KTP single crystals.30 The typical procedure for the synthesis of K4[Ge2F9(PO4)] is presented in Scheme 1b. First, GeO2 (0.104 g, 1 mmol) powder was added into a 15 mL Teflon inline (15 mL in volume), and then 2 mL of the mineralizer solution was added into the Teflon inline. After that, TEA (3 mL, AR, 21.6 mmol) was added to the above solution. Finally, H3PO4 (85 wt %, 2 mL, 29.2 mmol) was added dropwise. The resulting sandwiched mixture without stirring, with the molar ratio of GeO2/KF·2H2O/H3PO4/TEA = 1:6:29.2:21.6, was sealed into a stainless steel autoclave, which was heated to and kept at 240 °C for 5 d under static conditions. After that the samples were moved out from the oven and cooled in air; the resulting solid product was filtered, washed with deionized water, and dried in air. Colorless crystals of blocky shapes were observed in high yield (∼85%, based on Ge). The PXRD patterns of the crystals agree well with that calculated from single-crystal XRD analysis (see Figure 1b), which confirms the purity of the sample. The pure powder sample confirmed by PXRD analysis was then used for other characterizations. Further explorations on the synthesis of K4[Ge2F9(PO4)] showed that the form of the KF source played a key role on the crystal quality. By keeping other reagents and the reaction conditions, but using different KF sources, the following phenomena were observed: (1) When applying anhydrous KF as the reagent, only unidentified fine powder was obtained as the solid product. (2) When using the hydrate KF·2H2O together with H2O (1−2 mL) as the reagents, a few colorless needlelike crystals of K4[Ge2F9(PO4)] in a powder of an unidentified phase were obtained. (3) When a solution of KF·2H2O with the concentration of 3 mol/ L was added as the reactant, pure fine powder of K4[Ge2F9(PO4)] was obtained. The quality of crystals from such runs was not good enough for single-crystal X-ray structure determination. The above explorations reveal that a heterogeneous condition allows Ge to stabilize at high fluorine coordination (see below) because of the low solubility of GeO2. Also, the fluoride-rich and water-deficient condition is necessary for producing the anhydrous compounds. 2.2. Methods. Powder X-ray Diffraction. The phase purity was checked by means of PXRD. The diffraction patterns were recorded on an X-ray powder diffractometer of Panalytical X’pert PRO (Cu Kα-
(0.104 g, 0.7 mmol) and NaF (0.40 g, 9.5 mmol) were added in the Teflon inline (15 mL in volume). Then TEA (2 mL, 14.9 mmol) was added to cover the above solid powder. Finally, H3PO4 (85 wt %, 1 mL, 7.3 mmol) was added dropwise. The resulting heterogeneous mixture without stirring in the Teflon inline was fixed into a stainless steel autoclave, which was heated to and kept at 240 °C for 5 d under static conditions in an oven. After that the samples were moved out from the oven and cooled in air; the resulting product was filtered, washed with deionized water, and dried in air. Colorless crystals of blocky shapes in high yield (∼90%, based on Ge) were observed as solid products. The experimental powder X-ray diffraction (PXRD) patterns of Na3[GeF4(PO4)] agree well with those calculated from single-crystal XRD analysis (see Figure 1a). Further investigations show that only hydrated compounds were harvested if any free H2O was added into the reactant mixture (see Supporting Information). And the minimum amount of NaF required for the formation of Na3[GeF4(PO4)] was 0.2 g (4.76 mmol). 2.1.2. Synthesis of K4[Ge2F9(PO4)]. The same modified solvo-/ hydro-fluorothermal method was applied to synthesize K4[Ge2F9(PO4)]. One exception (and a key component) is the source of fluoride. A solution of 3 mol·L−1 KF·2H2O + 0.1 mol·L−1 KH2PO4 + 1 wt % H2O2 was added to the reaction mixture as the mineralizer to improve the crystal quality. This mixture had been B
DOI: 10.1021/acs.inorgchem.6b02266 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry radiation, graphite monochromator) and an X-ray powder diffractometer of Bruker Axs Advance 8 (Cu Kα radiation, graphite monochromator). Scanning Electron Microscopy/Energy-Dispersive Analysis by Xray. The sizes and morphologies of the synthesized compounds were observed using a field emission scanning electron microscope (FESEM, LEO-1530 and SU70). Infrared Spectroscopy. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet iS10 Optical FTIR spectrometer equipped with attenuated total reflectance (ATR) containing Diamond detector in the range from 4000 to 500 cm−1. Thermal Analyses. Thermal analyses were made on a NETZSCH TG-209 F1 thermogravimetric/differential thermal analyzer (TGDTA) in the range from room temperature to 800 °C with a heating rate of 10 K·min−1 in a N2 gas flow of 20 mL·min−1. In Situ Powder X-ray Diffraction. Thermodiffraction data were collected on a Panalytical X’pert PRO diffractometer (Cu Kαradiation, graphite monochromator) equipped with an Anton Paar heating stage (HTK1200N) under a flow of air atmosphere. The powder was placed in a corundum sample holder. The step size was 0.0167° in 2θ, and the heating rate was 10 K·min−1 in the range from room temperature to 700 °C. The delay time between reaching the set temperature and measuring the diffraction pattern was 5 min. All powder patterns recorded at different temperatures were collected under the air atmosphere. 2.3. Crystal Structure Determination. Suitable crystals of Na3[GeF4(PO4)] and K4[Ge2F9(PO4)] (Figure 1 insets) for singlecrystal X-ray diffraction measurements were selected and checked under a polarized microscope. The data were collected on a Bruker diffractometer with graphite monochromatic Mo Kα radiation (λ = 0.710 73 Å, 50 kV/40 mA, scan types: ω). The crystal structures were solved by direct methods and refined by full-matrix least-squares methods using the SHELX programs.31 Na3[GeF4(PO4)] crystallizes in space group Pnma (No. 62), and K4[Ge2F9(PO4)] crystallizes in P21/c (No. 14). Ge, P, and their coordination atoms F and O atoms were all determined from direct methods. The remaining F and O atoms as well as the Na/K atom sites were located from the difference Fourier maps. Relevant crystallographic details are given in Table 1;
selected bond lengths and angles as well as bond valence sum results are given in Tables S1−S4. Additional crystallographic information is available in the Supporting Information.
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RESULTS AND DISCUSSION 3.1. Crystal Structure Description. 3.1.1. Structure Description of Na3[GeF4(PO4)]. Na3[GeF4(PO4)] is characterized by a 1D chainlike structure (Figure 2) built from
Figure 2. (a) The 1D wirelike single-chain ∞1{[GeF4(PO4)]3−} built from alternating GeF4O2 octahedra and PO4 tetrahedra. (b, c) The crystal structure of Na3[GeF4(PO4)], viewed along the c-axis and baxis, respectively, shows two different orientations of the single chains correlated to each other via the a-glide plane perpendicular to the caxis. The symmetry correlation symbols marked in blue color. GeF4O2 octahedra: green, PO4 tetrahedra: orange, P atoms: purple spheres, Ge atoms: turquoise spheres, F atoms: green spheres, O atoms: red spheres, Na atoms: light gray spheres.
fluorinated germanophosphate single chains (denoted as GePOF-chain) surrounded by Na+ ions. The infinite wirelike single chain, ∞1{[GeF4(PO4)]3−} (Figure 2a), is built from alternating GeO2F4 octahedra and PO4 tetrahedra via sharing the common O-corners. Ge is six-coordinated to four F atoms in the equatorial plane and two trans-O apexes in the form of a distorted octahedron. The four F atoms are terminals, which have short Ge−F bonds (1.7924(17) and 1.8063(16) Å). The two trans-O atoms acting as bridges to two P atoms have longer Ge−O bonds (1.840(2) Å) than the Ge−F bonds. The much shorter Ge−F bonds than the Ge−O bonds in the title compounds are quite normal and comparable with those reported in the literature.10 The unique P1 atom exhibits a distorted tetrahedral coordination with four O atoms. Two of them are the terminals with short P−O bonds (1.493(3) and 1.495(3) Å), and the other two are connected to Ge atoms with longer P−O bonds (1.5718(19) Å). The distortion of the PO4 tetrahedron is confirmed by IR experiments (see below). The resulting GePOF single chains run parallel to the b-axis. The central axis of each chain lies on the 21 screw axis. There are two identical single chains with different orientations correlated by the a-glide plane perpendicular to the c-axis (Figure 2b,c). These infinite chains are exclusively isolated by Na+ ions that act as both the charge balance agent and a space filler. This type of single chains is very similar to the single chain in the chalcanthite-group minerals [M2+(H2O)4(SO4)](H2O), where M2+ takes the octahedral coordination and S atom in tetrahedral coordination.32 It is interesting to find that the linkage of Na atoms forms a network consisting of three-ring and six-ring (see Figure 3). Each Na1 is 5-connected to three Na1 and two Na2 atoms, while each Na2 is 4-connected to four Na1 atoms, resulting in a
Table 1. Details of the Data Collections and Selected Refinement Results for Na3[GeF4(PO4)] and K4[Ge2F9(PO4)] formula formula weight space group a/Å b/Å c/Å β/deg V/Å3, Z crystal size/mm temperature/K μ/mm−1 ρ/Mg·cm−3 diffractometer radiation reflections (independent, I > 2σ(I)) Rint/Rσ R1 and wR2 (I > 2σ(I)) R1 and wR2 (all) GOF parameter residual electron density max/ min (e Å−3)
Na3[GeF4(PO4)] K4[Ge2F9(PO4)] 312.53 567.55 orthorhombic, Pnma monoclinic, P21/c (No. 62) (No. 14) 10.952(7) 5.8280(12) 11.343(7) 10.644(2) 5.159(3) 19.625(4) 90 107.275(15) 640.9(7), 4 1162.5(4), 4 0.14 × 0.06 × 0.03 0.22 × 0.12 × 0.08 203(2) 203(2) 5.289 6.869 3.239 3.243 Bruker diffraction CCD area detector Mo Kα radiation, λ = 0.710 73 Å 827, 741 2682, 2311 0.0254/0.0255 0.0262, 0.0604 0.0304, 0.0631 1.086 67 0.547/−0.470
0.0545/0.0828 0.0467, 0.1121 0.0536, 0.1155 1.035 185 1.422/−1.206
C
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4) extending along the b-axis. Three crystallographically distinct Ge atoms and one unique P atom are included in the asymmetric unit of K4[Ge2F9(PO4)]. Three distinct Ge atoms (Ge1, Ge2, and Ge3) sit at the positions with point symmetries −1, −1 and 1, respectively. Ge1 and Ge2 have similar coordination by four fluorine and two oxygen atoms, arranged in a slightly distorted octahedral array, where the two oxygen atoms are in a trans configuration linking to PO4 tetrahedra. Ge3 is six-coordinated by five fluorine atoms and one oxygen atom, and shows a slightly greater angular distortion than its counterparts. The P atom takes a distorted tetrahedral coordination with three bridging O-corners and one Oterminal. Each PO4 group is connected to three Ge-octahedra with the free oxygen atoms alternately pointing up and down along the length of the single chain. The dendrite zigzag single chains ∞1{[Ge2F9(PO4)]4−} are interdigitated together to form a pseudosheet parallel to the (100) plane that is cross-linked to adjacent sheets above and below by the 8-, 9-, and 11coordinated K+ ions. It is also important to note that the single chains in K4[Ge2F9(PO4)] have a Ge/P ratio of 2:1, which is the highest ever found in germanophosphates. This proves that the modified solvo-/hydro-fluorothermal method is effective in producing germanium-rich compounds in the germanophosphate system. 3.1.3. Fluoro-Germanophosphate Single Chain. Similar to other fluoride-substituted high-valence metal phosphates,34,35 fluoride prefers to be terminals that act like “scissors”. A similar case is also apparent in the fluorine-substituted borophosphate (enH2)(BPO4F2),36 where two F atoms coordinated to B behave as terminals and two O atoms link to two P atoms to form the pyroxene-like single chain. All F atoms exclusively coordinated to Ge in the title compounds do not have any further connection. Only the O-vertices have further linkages that result in the dendrite fluoro-germanophosphate single chains. Several fluoro-germanophosphates have been synthesized and characterized by our groups recently. It is interesting to find that fluoride substitution can vary from one to five around each Ge atom in these known compounds (see Figure 5). The single F substitution was found in the 1D single band of K4[MIIGe2F2(OH)2(PO4)2(HPO4)2]·2H2O (M = Fe, Co).15 Two F substitutions were found in two types of anionic GePOF building units, namely, 1D single chain and 2D layer, observed in A 2 GeF 2 (HPO 4 ) 2 ·H 2 O (A = Na, K, Rb, NH 4 , Cs; unpublished results). Four F substitutions lie in the two title compounds: 1D wirelike single chain in Na3[GeF4(PO4)] and 1D dendrite zigzag single-chain ∞1{[Ge2F9(PO4)]4−} in K4 [Ge2 F 9(PO 4)]. Additionally, the variety with five F substitutions is also observed in K4[Ge2F9(PO4)] at the flanking octahedra. Moreover, it is interesting to note that the Ge/P ratio increases with the increasing number of F substitutions; that is, Ge/P = 1:2, observed in K4[MIIGe2F2(OH)2(PO4)2-(HPO4)2]·2H2O (M = Fe, Co)10 and A2GeF2(HPO4)2·H2O (A = Na, K, Rb, NH4, Cs), while Ge/P = 1:1 and Ge/P = 2:1 observed in Na3[GeF4(PO4)] and K4[Ge2F9(PO4)], respectively. These germanium-rich germanophosphates may be caused by the bridging manner of the O atoms and the terminal manner of the F atoms. 3.2. Thermal Stabilities and in Situ Powder X-ray Diffraction Results. The TG-DTA curves of Na3[GeF4(PO4)] and K4[Ge2F9(PO4)] are shown in Figure 6. Both compounds have a slow weight loss before ∼600 °C,
Figure 3. (a) The Na topology with the node type of (3262)·(3263). (b) The correlation between GePOF single chains and Na. Note that the solid lines between Na atoms are for clear vision only (not for bonding). Na atoms: medium gray spheres; GeO2F4 octahedra: green, PO4 tetrahedra: orange.
three- and six-ring net with the node symbol of (3262)·(3263). The correlation between the Na-topology and GePOF single chains can be described as the latter delimited by the former. Ge atoms sit at the center of all the six-rings of the Na-network, while P atoms sit at the center of the half of the three-rings when viewed along the c-axis. This reminds us of a host−guest interaction between GePOF single chains and Na networks. However, in contrast to the general situation, here the Na topology acts as a host, and the GePOF single chain as the guest. 3.1.2. Structure Description of K 4 [Ge 2 F 9 (PO 4 )]. K4[Ge2F9(PO4)] is another anhydrous phosphate germanium fluoride with the highest fluorine substitutions known to date. It is isostructural to the phosphate-aluminofluoride mineral Na2Sr2[Al2(PO4)F9] (Bøggildite)33 and titanium phosphate compounds (NH4)xK4−xTi2(PO4)F9 (x = 0.0, 0.70, 1.00, 1.25).34 The crystal structure (Figure 4) consists of a dendrite zigzag single chain that takes the same single chain in Na3[GeF4(PO4)] as the backbone but has additional flanking GeOF5 octahedra. That is, a chain of alternating corner-sharing GeO2F4 octahedra and PO4 tetrahedra, which is decorated by flanking GeOF5 octahedra attached to the PO4 groups, resulting in a dendrite zigzag single chain ∞1{[Ge2F9(PO4)]4−} (Figure
Figure 4. (a) The 1D single chain ∞1{[Ge2F9(PO4)]4−} built from alternating GeF4O2 octahedra and PO4 tetrahedra is decorated by flanking GeOF5 octahedra attached to the PO4 groups, resulting in a dendrite zigzag single chain extending along the b-axis. (b) The crystal structure of K4[Ge2F9(PO4)], viewed along the a-axis, shows two different arrangements of the single chains correlated to each other via the c-glide plane perpendicular to the b-axis. GeF4O2 and GeF5 O octahedra: green, PO4 tetrahedra: orange, Ge atoms: turquoise spheres, K atoms: dark gray spheres, O atoms: red spheres, F atoms: green spheres, P atoms: purple spheres. D
DOI: 10.1021/acs.inorgchem.6b02266 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 7. In situ PXRD patterns of Na3[GeF4(PO4)] during thermal ramping from 25 to 700 °C show its thermal stabilities and decomposition. Cu Kα radiation (λ = 1.541 78 Å). A simulated PXRD pattern (red) of Na3[GeF4(PO4)] is inserted at the bottom for comparison. The purple, orange, and blue patterns mark the observed phase changes during the increment of temperature.
Figure 5. (a) The 1D single band 1 4− ∞ {[FeGe2F2(OH)2(PO4)2(HPO4)2] } consists of one F-substituted GeO4F(OH) octahedron in K4[MIIGe2F2(OH)2(PO4)2(HPO4)2]· 2H2O (M = Fe, Co).10 (b) The 1D single chain and 2D layer consist of two F-substituted GeO4F2 octahedra in A2GeF2(HPO4)2·H2O (A = Na, K, Rb, NH4, Cs; unpublished results). (c) The 1D single chain 1 3− ∞ {[GeF4(PO4)] } consists of four F-substituted GeO2F4 octahedra in Na3[GeF4(PO4)] (this work). (d) The 1D dendrite zigzag single chain ∞1{[Ge2F9(PO4)]4−} contains both four F-substituted GeO2F4 octahedra and five F-substituted GeOF5 octahedra in K4[Ge2F9(PO4)] (this work). GeF4O2 and GeF5O octahedra: green, PO4 tetrahedra: orange, K atoms: dark gray spheres.
between 500 and 700 °C. When temperature reached 620 °C, Na3[GeF4(PO4)] decomposes completely. Meanwhile, two known germanates, Na4Ge9O20 (PDF No.: 80−0703) and Na4GeO4 (PDF No.: 78−0596), start to form during the decomposition of Na3[GeF4(PO4)]. With further increase in temperature, the two germanates become the main phases accompanied by the disappearance of the intermediate phase. Finally only Na4Ge9O20 (PDF No.: 80−0703) and Na4GeO4 (PDF No.: 78−0596) were observed as the crystalline phases when temperature reached or exceeded 700 °C. Although in situ PXRD was not performed for K4[Ge2F9(PO4)], the XRD patterns recorded after heating the samples at different temperatures in air are illustrated in Figure 8. The powder patterns show that the structure of
Figure 6. TG-DTA curves of Na 3 [GeF 4 (PO 4 )] (top) and K4[Ge2F9(PO4)] (bottom).
Figure 8. PXRD patterns of K4[Ge2F9(PO4)] during thermal ramping from 25 to 700 °C show the thermal stabilities and phase decomposition. Cu Kα radiation (λ = 1.541 78 Å).
followed by a fast weight loss. This is particularly evident from the presence of one endothermic peak at 620 or 664 °C in the DTA curves for sodium and potassium compounds, respectively. The endothermic peaks can be attributed to the release of F atoms, similar to that found in (NH4)xK4−xTi2(PO4)F9 (x = 0.0, 0.70, 1.00, 1.25).34 The total weight losses (obsd: 15.0% for Na3[GeF4(PO4)] at 700 °C, 17.2% for K4[Ge2F9(PO4)] at 800 °C) are less than the calculated ones for the release of all F atoms (cald: 24.3% for Na 3 [GeF 4 (PO 4 )], 30.1% for K4[Ge2F9(PO4)]). This discrepancy may be explained by the slow release rate of the F atoms. Caution! Care should be taken, because heating hydrated/wet f luoride phosphates can produce HF (toxic by inhalation, in contact with skin, or if swallowed). Figure 7 shows that Na3[GeF4(PO4)] during the in situ PXRD experiment remains intact before 500 °C. Above 500 °C, small amounts of Na3[GeF4(PO4)] start to decompose to form an unidentified intermediate phase, which is stable only
K4[Ge2F9(PO4)] was kept unchanged up to ∼500 °C, which is similar to Na3[GeF4(PO4)]. However, when temperature (at 680 °C) went above the endothermic temperature at 664 °C (observed in the DTA curve), its structure collapsed. The major phase in the decomposition products is K2Ge4O9 (PDF No.: 80−0703).32 Overall, both the thermal analyses and in situ PXRD results clearly show that the two title compounds are stable up to at least 600 °C, which is significantly higher than their hydrous counterparts. Hydrous germanophosphate compounds containing the OH group or H2O molecules normally start to decompose before 300 °C.13−15,17 3.3. Infrared Spectroscopy. Figure 9 shows that both title compounds possess similar absorption bands, mainly lying in the range between 1172 and 525 cm−1. These bands can be E
DOI: 10.1021/acs.inorgchem.6b02266 Inorg. Chem. XXXX, XXX, XXX−XXX
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fluorine. Moreover, this method produced the most germanium-rich germanophosphates known to date.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02266. Bond-valence-sum calculation results, EDX data, selected bond lengths and angles, X-ray crystallographic information file (CIF) for Na3[GeF4(PO4)], K4[Ge2F9(PO4)], and hydrated Na3[GeF4(PO4)], as well as IR and TG curves of hydrated Na3[GeF4(PO4)] compound (PDF) X-ray crystallographic information (CIF)
Figure 9. FTIR-ATR spectra of Na 3 [GeF 4 (PO 4 )] and K4[Ge2F9(PO4)].
3−
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attributed to the vibration modes of the PO4 unit and the GeO2F4/GeOF5 groups.38,39 The detailed absorption band assignments are discussed in the following. For simplicity, the number 1/number 2 will represent the bands for Na3[GeF4(PO4)] and K4[Ge2F9(PO4)], respectively. Note that the bands between 1200 and 900 cm−1 can unambiguously be assigned to the components of the PO4 stretching ν1 and ν3 vibrations. Thus, the strong and sharp bands at 1172/1187 and 1076/1078 are attributed to the asymmetric stretching mode ν3(P−O), while 1001/1001 and 939/956 cm−1 are assigned to the symmetric stretching mode ν1(P−O) in the PO43− group. The bands at 670/626, 601/598, and 543/570 cm−1 may be assigned to the asymmetric bending mode ν4(P−O) in the phosphate group as well as Ge−O or Ge−F vibration in the germanate groups. More specifically, bands at 670 cm−1 can be assigned to the symmetric stretching vibration of the Ge−O vibration, 601 cm−1 for the deformation vibration of Ge−O/F, and the band at 543 cm−1 (sharp, strong) corresponding to the asymmetric bending mode of ν4(P−O) in the PO43− group or the symmetric stretching mode of Ge−O. Clearly, no characteristic absorption bands for water or hydroxyl groups (e.g., at ∼1600 and 3200−3600 cm−1) are observed in the spectra, confirming that both compounds are anhydrous. Moreover, the IR results further confirm that both compounds consist of PO43−, GeO2F4, and/or GeOF5 groups.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 0086-13950175872. ORCID
Xia Huang: 0000-0003-3266-6651 Ya-Xi Huang: 0000-0002-5934-6079 Notes
The authors declare no competing financial interest. Further crystallographic information (excluding structure factors) can be obtained free of charge from Fachinformationszentrum Karlsruhe, 76344, Eggenstein-Leopold shafen, Germany (e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository numbers CSD-431958 and CSD-431959 for Na3[GeF4(PO4)] and K4[Ge2F9(PO4)], respectively.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21201144 and 21233004), the Fundamental Research Funds for the Central Universities (Grant No. 2013121020), the National Key Research Program of China (Grant No. 2016YFB0901502), and Natural Science and Engineering Research Council of Canada for financial support.
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CONCLUSION Two highly fluorine-substituted anhydrous phosphate germanium fluorides have been successfully synthesized via a modified solvo-/hydro-fluorothermal method. The title compounds are both characterized by 1D chainlike structures built from infinite GePOF single chains surrounding by Na+ or K+ ions. The tailor effect of fluoride is apparent in both compounds and is responsible for the formation of their lowdimensional structures. Thermal analyses together with in situ high-temperature PXRD experiments on Na3[GeF4(PO4)] show that these fluorine-substituted compounds have much higher thermal stabilities than hydrated germanium phosphates. The successful synthesis of these two high-fluorine-substituted phosphate germanium fluorides suggests that the modified solvo-/hydro-fluorothermal method is an efficient route to prepare anhydrous compounds by the replacement of hydroxyl groups and water molecules with fluorine. It also opens further opportunities to synthesize new germanophosphate with lowdimensional structures by incorporating different amounts of
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