SUN Tong-Qing WANG Qian-Qian KONG Yong-Fa XU Jing-Jun

(School of Physics and the MOE Key Laboratory of Weak-light Nonlinear Photonics, Nankai University, Tianjin 300071, China)

ABSTRACT A potassium silicophosphate, K2SiP4O13, has been synthesized in molten polyphosphoric acid. It crystalizes in the triclinic space group (No. 2) with a = 4.8327(10), b = 7.7403(15), c = 14.485(3) Å, α =82.29(3)°, β = 83.31(3)°, γ = 81.95°, V = 529.02(19) Å3, Z = 2. The crystallographic structure features 2D layers of[SiP4O13]∞ in the ab plane with counter cations K+ residing among the layers, and the anionic framework of[SiP4O13]∞ is composed of six-fold coordinated Si atoms and tetraphosphate anions by sharing vertex O atoms. The title compound was characterized by powder X-ray diffraction, IR and Raman spectroscopies, UV-vis diffuse reflectance spectroscopy, thermogravimetry and differential scanning calorimetry.

Keywords: synthesis, silicophosphate, SiO6 octahedral coordination, crystal structure;

1 INTRODUCTION

Silicophosphates, also named as phosphosilicates, specially refer to the compounds with bridging P−O−Si bonds between the moieties of phosphate and silicate[1]. Non-crystalline silicophosphates have gained great industrial and academic interest for excellent chemical stability, mechanical property and photosensitivity, and can be used as optical fibers,biomaterials, luminescent materials and so on[2-4]. However,crystalline silicophosphate compounds have been developed backwardly. Though there are a large number of synthetic compounds and natural minerals in the system containing P,Si, and O elements, most of them don’t belong to silicophosphates for being without P-O-Si bonds. One case is that the anionic groups of phosphorous and silicon are isolated by metal cations, and they don’t share any vertex oxygen atoms to form P-O-Si bonds[5]. Another case is that phosphorous and silicon atoms disorderedly occupy the same lattice positions of crystal structure due to their close ionic radii[6,7].By contrast, the proportion of silicophosphates, with bridging P-O-Si bonds, are not so much in the inorganic crystalline compounds containing P, Si and O.

As we all know, the bonding between Si and O atoms are typically four-fold coordinated. The six-fold coordination of silicon usually exists in several high pressure materials, for example, MgSiO3ilmenite and SiO2stishovite[8,9]. The few oxides with SiO6that can stably exist at ambient pressure almost have P-O-Si bonds, for instance, SiP2O7[10],Si5O(PO4)6[11], (Re2O5)Si2[Si2O(PO4)6][12], Rb2SiP4O13and BaH2Si(P2O7)2[13]. So, the field of silicophosphate is distinctive from the view point of structural chemistry.

The varieties of connection mode among PO4, SiO4and SiO6by sharing vertex oxygen atoms would give rise for structure and property variations, thus more versatile applications could be foreseeing. For example, similar to aluminosilicates, silicophosphates with different frameworks and topologies may have an important place in various scientific applications, such as molecular sieves, absorbents, and ionic conductors[1,14]. Therefore, it is necessary to explore new silicophosphate compounds. Recently, the discoveries of Na4Si2PO4F9[15], K4Si3P2O7F12and BaSiP2O8[16]intensively catch the eyes of researchers for the six-fold coordinated Si and P-O-Si bonds. In this paper, we reported the synthesis and structure of a new silicophosphate K2SiP4O13, and analyzed its structural features, and characterized it by IR and Raman spectroscopies, diffuse reflectance spectroscopy and thermal analysis.

2 EXPERIMENTAL

2. 1 Synthesis

Single crystals of K2SiP4O13were synthesized by the flux method in molten polyphosphoric acid. All the reagents used in synthesis were purchased from commercial sources without further treatment. K2CO3(4.561 g, 33 mmol), SiO2(0.601 g,10 mmol) and phosphoric acid (85 wt%, 8.6 mL, 126 mmol)were mixed by stirring in a Pt crucible at room temperature.The crucible was placed in a muffle furnace oven and then heated to 400 °C in 6 h. After having been kept at 400 °C for 2 days, the temperature of the furnace was gradually decreased to 30 °C in about 10 h. The resulting products in the crucible were washed by using hot water for eliminating polyphosphoric acid and its potassium salts, and the single crystals were separated and dried in air. Eventually, colorless and narrow crystal slices of K2SiP4O13were obtained with a yield of about 90% based on SiO2.

2. 2 Crystal structure determination

A single crystal of K2SiP4O13with the size of 0.30mm ×0.24mm × 0.05mm was selected for crystal structure determination. The single-crystal X-ray diffraction (XRD)data were collected on an Agilent Gemini E diffractometer with graphite-monochromated MoKα radiation (λ = 0.71073 Å) using the ω-scan technique. The crystal was kept around 106 K during data collection. In the range of 6.36≤2θ≤57.96°, a total of 3613 reflections were collected and 2415 were independent with Rint= 0.0265, of which 2095 were observed with I ＞ 2σ(I). The absorption correction of multi-scan was performed. Using Olex2[17], the structure was solved with the SHELXT[18]structure solution program using intrinsic phasing, and was refined with the SHELXL[19]refinement package using full-matrix least squares minimization on F2. All atoms were refined with anisotropic thermal parameters. The final full-matrix least-squares refinement converged to R = 0.0352, wR = 0.0814 for the observed reflections with I ＞ 2σ(I), and R = 0.0419, wR =0.0863 for all data. The largest diffraction peak and hole are 0.52 and −0.56 e·Å−3, respectively. The structural data were checked by the PLATON program[20], and no higher symmetries were found. Selected bond lengths and bond angles of K2SiP4O13are given in Table 1.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

2. 3 Characterization of K2SiP4O13

The powder XRD patterns were collected on a Bruker D8 Advance diffractometer with CuKα radiation (λ = 1.5418 Å)at room temperature in the 2θ range of 5～70° with a step size of 0.02°. The IR spectrum was recorded on a Bruker TENSOR II FTIR spectrophotometer in the range of 400～4000 cm−1with KBr pressed pellet. The Raman spectrum was measured at room temperature in the range from 100 to 3000 cm−1, using a Renishaw in via spectrometer under the excitation of an argon ion laser with the wavelength of 785 nm. The UV-vis diffuse reflectance spectrum was scanned in the range of 200～800 nm at room temperature on a Hitachi U-4100 spectrophotometer, with a powder pellet of BaSO4as a standard (100% reflectance). The thermogravimetric (TG)and differential scanning (DSC) calorimetric analyses were carried out on a Setaram Setsys Evolution thermal analyzer from METTLER TOLEDO, and the samples were placed in alumina crucibles and heated at a rate of 10 °C/min from room temperature to 1000 °C under an atmosphere of Ar.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

K2SiP4O13crystallizes in the triclinic system,(No. 2)space group with a = 4.8327(10), b = 7.7403(15), c =14.485(3) Å, α = 82.29(3)°, β = 83.31(3)°, γ = 81.95°, V =529.02(19) Å3and Z = 2. Its asymmetric unit comprises of two K, one Si, four P, and thirteen O atoms, and all of them locate on general lattice positions. K2SiP4O13crystal features 2D layers of [SiP4O13]∞along the c axis, and counter cations K+reside among the layers.

Each P atom in K2SiP4O13is covalently bonded with four O atoms to form basic PO4tetrahedron, and the four unique tetrahedra are linked by sharing three vertex O atoms to build a short zigzag chain, i.e. tetraphosphate anion group [P4O13]6−,which is quite comparable to a fragment of a long-chain polyphosphate[21,22]. The [P4O13]6−groups are isolated from each other, and are linked by Si atoms. Each [P4O13]6−interconnects with four Si atoms, and vice versa, which leads to the formation of a 2D layer of [SiP4O13]∞polyanion in the ab plane, as shown in Fig. 1(a). The Si atom in K2SiP4O13is in an octahedral coordination with the O atoms from six adjacent PO4. The six O atoms almost uniformly surround around Si with nearly identical distances. The distortion index of SiO6was calculated based on the methodology proposed by Halasyamani[23], and its value was 0.04, which means that the SiO6octahedron is basically undistorted. The layers of[SiP4O13]∞stack along the c axis, and K(1) and K(2) atoms occupy the void among the layers. Not only do they balance the electronic charges, but also join the layers by the electrostatic force between themselves and O2−to build the 3D framework of K2SiP4O13, as shown in Fig. 1(b). The coordination environments around the K atoms are rather irregular, and the K-O distances vary in a rather wide range of 2.676(2)～3.478(2) Å.

Fig. 1. (a) View of the [SiP4O13]∞ layer in the ab plane and the K+ ions in the cavities,(b) Projection of the K2SiP4O13 crystallographic structure along the a axis

The bond valences (BV)[24]and bond valence sums (BVS)of all atoms in K2SiP4O13were calculated, with the results listed in Table 2. The calculated BVS value of each atom is in good agreement with its corresponding oxidation number. It should be noted that thirteen independent O atoms can be divided into three groups according to their bonding characters. The first group consists of the bridge oxygen atoms of P-O-P, i.e., O(4), O(7) and O(10), which have the longest bond lengths of P-O in each PO4tetrahedron. The second one includes the bridging O atoms bonding with P and Si: O(1), O(2), O(5), O(9), O(11), and O(12). Finally, the terminal oxygen atoms of PO4tetrahedra, i.e. O(3), O(6), O(8)and O(13), belong to the third group, which has the shortest distances of P-O and correspondingly, the highest BV values in each tetrahedron. The average values of BVS for these three groups are 2.13, 2.03, and 1.88, respectively, which reflects well the differences of O atoms on the bonding environments in K2SiP4O13that brings rich stretching vibrations in the IR and Raman spectra.

Table 2. Bond Valences (BV) and Bond Valence Sums (BVS) in K2SiP4O13

K2SiP4O13crystal is isostructural with its homologue,Rb2SiP4O13[13], of which cell parameters are a = 4.8327(10), b= 7.7403(15) and c = 14.485(3) Å. In comparison to those of Rb2SiP4O13, the dimensions of a and b axes of K2SiP4O13don’t generate obvious changes (less than 0.5%) though the ion radius of K+(1.51 Å) is obviously smaller than that of Rb+(1.61 Å). Meanwhile, the dimension of the c axis decreases 4%, which implies that the 2D network of [SiP4O13]∞is rigid parallel to the ab plane and the size of the cavities along the c axis occupied by the alkali metal cations is delimited by it.When Na+with smaller size (1.18 Å) residing the cavities, the long distances of Na-O would make the structure instable. On the contrary, the cavities could not accommodate bigger cation, such as Cs+(1.74 Å). It may be the reason that the isostructural silicophosphates of Na+and Cs+have not been successfully synthesized. With the substitution of Si4+(0.40 Å)by Ge4+(0.53 Å) or Ti4+(0.61 Å), the sizes of the cavities would increase and could hold bigger alkali metal cation.Therefore, Cs2GeP4O13[25]and Cs2TiP4O13[26]that are isostructural with K2SiP4O13and Rb2SiP4O13can stably exist.

The sample for powder XRD was prepared by pulverizing and grinding the synthesized crystals, and Fig. 2 presents the experimental XRD pattern accompanied with the simulated one by Powder Cell[27]based on the single-crystal structure.The positions of diffraction peaks between them are completely identical with each other, and so do the intensities basically except for the multiple diffraction of (001), whose experimental peaks are greatly stronger than the simulated ones. This shows a strong preferred orientation in the tested powder XRD sample. The [SiP4O13]∞layer is fully composed of strong covalent interactions of Si-O and P-O in the ab plane. By contrast, the layers are interconnected along the c axis only by weak electrostatic forces between K+and O2−. So,K2SiP4O13is very apt to cleavage parallel to the (001) crystal face. The slice-like crystallization habit of K2SiP4O13is also related to its special layered structure.

Fig. 2. Experimental XRD pattern of the pulverized K2SiP4O13 crystals

3. 2 IR and Raman spectra

IR and Raman spectra of K2SiP4O13are presented in Fig. 3.On the basis of literature reports[3,4,7,28-32], the stretching vibrations in the IR spectrum could be divided mainly to four regions. The broad band in region I is attributed to the O-P-O stretching vibrations of the terminal oxygen atoms, which are sometimes called as doubly bonded oxygen vibrations. The intensive band around 1060 cm−1in region II is assigned to the O-P-O stretching vibrations of the bridging oxygen atoms in PO4tetrahedra while the band around 940 cm−1(region III)to the O-Si-O stretching vibrations in the SiO6octahedron.The bands in region IV are attributed to the stretching vibrations of P-O-P and P-O-Si. The assignments of the stretching vibrations in the Raman spectrum are principally identical with those in the IR spectrum. The bands below 600 cm−1are attributed to the bending vibrations of PO4and SiO6structural units and the harmonics of stretching vibrations.

Fig. 3. IR and Raman spectra of K2SiP4O13

3. 3 UV-vis diffuse reflectance spectrum

Fig. 4 shows the UV-vis diffuse reflectance spectrum of K2SiP4O13. The absorption spectrum was calculated from the diffuse reflectance spectrum using the Kubelka-Munk function: F(R) = (1 − R)2/2R = K/S, where R represents the reflectance, K the absorption, and S the scattering. In a F(R)versus E(eV) plot, extrapolating the linear part of the rising curve to zero provides the onset of absorption. Based on the inset of Fig. 4, the optical band gap of K2SiP4O13is estimated to be 4.85 eV, which is much wider than that of Cs2GeP4O13(4.07 eV)[25]. However, the band gap of K2SiP4O13is narrow by contrast with the phosphates of alkali metals or alkali earth metals[33,34], which should be related to the electronic transition of the Si-O bonds[35].

Fig. 4. UV-vis diffuse reflectance spectrum of K2SiP4O13

3. 4 Thermal properties

Fig. 5 presents the TG-DSC curves of K2SiP4O13. A sharp endothermic peak was observed at 730 °C while the sample weight didn’t show a representative change in the whole temperature range. The isostructural compound Rb2SiP4O13was reported to decompose at 723 °C , however, no more information was mentioned[13]. In order to clarify whether the title compound melts or decomposes at high temperature, a sample of pulverized K2SiP4O13crystal was heated at 800 °C for two hours, and then the sample was very quickly decreased to room temperature. White solidified polycrystalline substances mixed glassy things were observed. The sample after heat-treating was analyzed by powder XRD. As shown in the inset of Fig. 5, K2SiP4O13should decompose in accordance with the reaction: K2SiP4O13→ Si5O(PO4)6+amorphous phase. The amorphous phase probably contained phosphorus and potassium oxides since the sample weight remained roughly constant. The approximate decomposition temperature of K2SiP4O13and Rb2SiP4O13indicates that the rigid 2D network of [SiP4O13]∞could play a crucial role in their thermal stability. After comparing with the structure of Si5O(PO4)6[11], it should be mentioned that the majority of Si atoms originally in K2SiP4O13still kept an octahedron coordination after heat decomposition while the short zigzag chain of [P4O13]6−was broken.

Fig. 5. TG and DSC curves of the K2SiP4O13 pulverized crystals and the powder XRD pattern of their thermal decomposition products (inset)

4 CONCLUSION

In summary, we have synthesized a new silicophosphate compound, K2SiP4O13, by the flux method. The crystallographic structure features a 2D network of [SiP4O13]∞stacking along the c axis, which is composed of tetraphosphate anions interconnected by Si atoms in a hardly distorted octahedral coordination. The IR and Raman vibration spectra reflect the structural characteristics of K2SiP4O13. The compound is thermally stable up to 730 °C , and then decomposes to Si5O(PO4)6. Its optical band gap was estimated to be about 4.85 eV by diffuse reflectance spectroscopy.