Potassium Gadolinium Phosphate with a Tunnel Structure: Synthesis

Samatha Bevara , S. Nagabhusan Achary , Karuna Kara Mishra , T. R. Ravindran , Anil K. Sinha , P. U. Sastry , Avesh Kumar Tyagi. Phys. Chem. Chem. Phy...
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Potassium Gadolinium Phosphate with a Tunnel Structure: Synthesis, Structure, and Optical Properties of K3Gd5(PO4)6 Jing Zhu, Wen-Dan Cheng,* Dong-Sheng Wu, Hao Zhang, Ya-Jing Gong, Hua-Nan Tong, and Dan Zhao

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1649-1652

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences and the Graduate School of the Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ReceiVed March 28, 2006; ReVised Manuscript ReceiVed May 8, 2006

ABSTRACT: A potassium gadolinium phosphate crystal, K3Gd5(PO4)6, has been synthesized by a high-temperature solution reaction and exhibits a new structural family of the alkali-metal-rare-earth phosphate system. Single-crystal X-ray diffraction analysis shows the structure to be monoclinic with space group C2/c and the following cell parameters: a ) 17.470(3) Å, b ) 6.9365(6) Å, c ) 18.096(2) Å, β ) 114.308(4)°, V ) 1998.4(5) Å3, Z ) 4. Its structure features a three-dimensional Gd5(PO4)63- anionic framework with interesting tunnels where the K atoms are located. The framework are constructed from Gd polyhedra and isolated PO4 tetrahedra. We have also investigated the optical properties of K3Gd5(PO4)6 in terms of the measured absorption and emission spectra. Introduction Studies of alkali-metal-rare-earth phosphate materials have attracted continuing attention in recent years because they exhibit diverse structures and interesting physical properties, as well as potential optical applications in both fast scintillators and solid-state laser devices. Several structural families with the general formula MLnP2O7, MLn(PO3)4, M3Ln(PO4)2, etc. (M ) alkali metal, Ln ) rare-earth metal) have been extensively investigated.1-5 These studies showed that these compounds possess various properties which are closely related to their structures. For example, potassium gadolinium polyphosphate, KGd(PO3)4, has a noncentrosymmetrical structure with consequently optical second harmonic generation (SHG).6 It has been reported that the second harmonic efficiency of KGd(PO3)4 is close to that of KDP. In view of the unusual compositional and structural diversity of alkali-metal-rare-earth phosphate compounds, we have continually designed and synthesized new compounds with fascinating properties in the system. K3Gd5(PO4)6 single crystals of good quality were obtained. So far, K3Gd5(PO4)6 has been never reported, and the only reported phosphates containing both potassium and gadolinium are KGd(PO3)47 and K3Gd(PO4)2.8 In this paper, we will describe the synthesis, crystal structure, and optical properties of K3Gd5(PO4)6 by single-crystal X-ray diffraction, elemental analysis, powder X-ray diffraction, and absorption and emission spectra. Experimental Section Synthesis of K3Gd5(PO4)6. Single crystals of K3Gd5(PO4)6 were grown by a high-temperature solution reaction. All reagents were purchased commercially and used without further purification. The analytical reagent starting materials K2CO3, Gd2O3, and NH4H2PO4 were weighed in the molar ratio K/Gd/P ) 22/2/4.5. These starting materials were finely ground in an agate mortar to ensure the best homogeneity and reactivity and then placed in a platinum crucible and heated to 573 K for 4 h in order to decompose K2CO3 and NH4H2PO4. Afterward, the mixture was reground and heated to 1273 K for 24 h. Finally, the temperature was lowered to 1073 K at a rate of 2 K/h and then the mixture was air-quenched to room temperature. A few colorless blockshaped crystals were obtained from the melt of the mixture. * To whom correspondence should be addressed. Fax: +86-591-3714946. E-mail: [email protected].

Table 1. Crystal Data and Structure Refinement Details for K3Gd5(PO4)6 formula formula wt temp (K) wavelength (Å) cryst syst space group unit cell dimens a (Å) b (Å) c (Å) β (deg) V (Å3); Z Dcalcd (g cm-3) µ (mm-1) F(000) cryst size (mm) θ range (deg) limiting indices no. of rflns collected no. of indep rflns refinement method GOF final R indices (I > 2σ(I)) R indices (all data) largest diff. peak and hole (e Å-3)

K3Gd5(PO4)6 1473.37 293(2) 0.71073 monoclinic C2/c 17.470(3) 6.9365(6) 18.096(2) 114.308(4) 1998.4(5); 4 4.897 17.592 2636 0.10 × 0.10 × 0.07 3.20-27.48 -22 e h e 15; -8 e k e 8; -23 e l e 23 7510 2276 (Rint ) 0.0326) full-matrix least-squares on F2 1.006 R1 ) 0.0284, wR2 ) 0.0627 R1 ) 0.0345, wR2 ) 0.0657 1.370 and -1.630

After crystal structure determination, a polycrystalline sample of K3Gd5(PO4)6 was synthesized by solid-state reactions of stoichiometric amounts (K/Gd/P ) 3/5/6) of the analytical reagents K2CO3, Gd2O3, and NH4H2PO4. The pulverized mixture was allowed to react at 883 K for 100 h with several intermediate grindings in an opening Pt crucible. The purity of K3Gd5(PO4)6 was confirmed by powder XRD studies using a Rigaku DMAX2500 diffractometer with Cu KR radiation (step size of 0.05° and range 2θ ) 10-80°). The powder XRD pattern of K3Gd5(PO4)6 is given as Supporting Information. Single-Crystal Structure Determination. A single crystal of K3Gd5(PO4)6 with approximate dimensions of 0.10 × 0.10 × 0.07 mm3 was selected for X-ray diffraction determinations. The diffraction data were collected on a Rigaku Mercury CCD diffractometer with graphitemonochromated Mo KR radiation (λ ) 0.710 73 Å) using the ω scan mode at a temperature of 293 K. The structure of the title compound was solved using direct methods and refined on F2 by full-matrix leastsquares methods with the SHELXL97 program package.9 The position of the gadolinium atom was refined by the application of the direct methods, and the remaining atoms were located in succeeding difference Fourier syntheses. Further details of the X-ray structural analysis are given in Table 1. The atomic coordinates and thermal parameters are

10.1021/cg0601711 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/07/2006

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Figure 1. Projection of the structure of K3Gd5(PO4)6 along the c axis. The K-O and Gd-O bonds are omitted for clarity. Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters for K3Gd5(PO4)6 atom

site

Gd1 Gd2 Gd3 K1 K2 P1 P2 P3 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12

4e 8f 8f 8f 4e 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f

a

x

y

z

Ueqa

0.0000 0.36440(6) 0.2500 0.0060(1) 0.14412(2) -0.41390(4) 0.48099(2) 0.0063 (1) 0.21997(2) 0.12519(4) 0.29232(2) 0.0062(1) 0.0763 (1) 0.9032(2) 0.0745 (1) 0.0186(3) 0.0000 0.8284(3) 0.2500 0.0447(9) 0.1359(1) 0.0846(2) 0.4135(1) 0.0062(3) 0.2082(1) 0.6193(2) 0.34186(9) 0.0057(3) -0.07314(9) 0.4040(2) 0.38281(9) 0.0054(3) 0.1999(3) 0.7861(6) 0.2835(2) 0.0101(9) -0.0999(3) 0.2524(6) 0.3138(2) 0.0094(9) 0.2291(3) 0.0676(6) 0.4261(3) 0.0095(9) 0.1230(3) 0.2515(6) 0.4607(3) 0.013(1) 0.2541(3) 0.6698(6) 0.4313(3) 0.0098(9) -0.1461(3) 0.5391(6) 0.3760(3) 0.0116(9) -0.0047(3) 0.5223(6) 0.3703(3) 0.0094(9) 0.0979(3) -0.1024(6) 0.4270(3) 0.0115(9) -0.0469(3) 0.3055(6) 0.4650(2) 0.0098(9) 0.2551(3) 0.4603(6) 0.3177(3) 0.0104(9) 0.1201(3) 0.5493(6) 0.3299(3) 0.0100(9) 0.0925(3) 0.1277(6) 0.3195(2) 0.0093(9)

Table 3. Selected Bond Distances (Å) and Angles (deg) for K3Gd5(PO4)6a occ 0.5 1 1 1 0.5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

given in Table 2. Selected bond lengths and angles are given in Table 3. To confirm the chemical composition of the title compound, the K3Gd5(PO4)6 single crystal investigated on the diffractometer was analyzed by energy-dispersive X-ray spectrometry (EDX) using a JSM6700F scanning electron microscope. The obtained results are in good agreement with those obtained by the refinement of the crystal structure. No impurity elements have been detected. Further details on crystallographic studies and results of elemental analysis are given as Supporting Information. Optical Measurements. The absorption spectrum was recorded on a Lambda-35 UV/vis spectrophotometer in the wavelength range of 200-1000 nm. The emission spectrum was measured on a FLS920 time-resolved fluorescence spectrometer using an Xe lamp at room temperature.

Results and Discussion Single-Crystal Structure. Crystallographic analysis reveals that the K3Gd5(PO4)6 compound belongs to the monoclinic system with space group C2/c. The structural units of K3Gd5(PO4)6 are Gd polyhedra and isolated PO4 tetrahedra, forming a three-dimensional Gd5(PO4)63- anionic framework containing infinite tunnels along the c axis in which the K atoms are located, as shown in Figure 1. In the tunnel structure, K polyhedra form a wavy chain via sharing O atoms along the c axis (Figure 2). The tunnel structure shows that the K3Gd5(PO4)6

Gd1-O2 Gd1-O2i Gd1-O7 Gd1-O7i Gd1-O11 Gd1-O11i Gd1-O12 Gd1-O12i

2.577(4) 2.576(4) 2.469(4) 2.469(4) 2.376(4) 2.376(4) 2.283(4) 2.283(4)

K1-O9viii K1-O4viii K1-O6i K1-O10ix K1-O9x K1-O8x K1-O2x K1-O8viii K1-O7i

2.673(4) 2.727(5) 2.787(5) 2.812(5) 2.874(4) 3.032(5) 3.073(5) 3.163(5) 3.251(4)

O8-P1-O3 O4-P1-O8 O4-P1-O3 O4-P1-O12 O8-P1-O12 O3-P1-O12

114.3(2) 112.7(3) 111.5(2) 110.3(2) 106.6(2) 100.5(2)

Gd2-O3iv Gd2-O4ii Gd2-O5v Gd2-O5ii Gd2-O6iii Gd2-O7ii Gd2-O8 Gd2-O9iii Gd2-O11ii K2-O12xii K2-O12x K2-O11i K2-O11 K2-O8x K2-O8xii K2-O7i K2-O7 K2-O1 K2-O1i

2.409(4) 2.355(4) 2.495(4) 2.503(4) 2.715(4) 2.585(4) 2.372(4) 2.402(4) 2.602(4) 2.613(4) 2.613(4) 2.783(5) 2.783(5) 2.984(4) 2.984(4) 3.066(4) 3.066(4) 3.302(4) 3.302(4)

O5-P2-O1 O5-P2-O10 O11-P2-O10 O1-P2-O11 O5-P2-O11 O1-P2-O10

114.7(2) 110.9(2) 110.6(2) 109.6(2) 106.3(2) 104.8(2)

Gd3-O6vi Gd3-O2i Gd3-O1ii Gd3-O3 Gd3-O10 Gd3-O12 Gd3-O10vii Gd3-O1vii

2.282(4) 2.355(4) 2.374(4) 2.394(4) 2.400(4) 2.471(4) 2.485(4) 2.583(4)

P1-O4 P1-O8 P1-O3 P1-O12 P2-O5 P2-O1 P2-O11 P2-O10 P3-O9 P3-O7 P3-O6 P3-O2

1.510(4) 1.523(4) 1.553(4) 1.580(4) 1.521(4) 1.532(4) 1.540(5) 1.541(4) 1.525(4) 1.542(4) 1.546(4) 1.551(4)

O9-P3-O7 O6-P3-O2 O9-P3-O2 O7-P3-O6 O9-P3-O6 O7-P3-O2

116.2(2) 112.4(2) 110.6(2) 109.2(2) 104.8(2) 103.7(2)

a Symmetry codes: (i) -x, y, 0.5 - z; (ii) x, -1 + y, z; (iii) -x, -y, 1 - z; (iv) 0.5 - x, -0.5 - y, 1 - z; (v) 0.5 - x, 0.5 - y, 1 - z; (vi) 0.5 + x, -0.5 + y, z; (vii) 0.5 - x, -0.5 + y, 0.5 - z; (viii) x, 1 - y, -0.5 + z; (ix) 0.5 - x, 0.5 + y, 0.5 - z; (x) -x, 1 + y, 0.5 - z; (xi) -x, 2 - y, -z; (xii) x, 1 + y, z; (xiii) x, 1 - y, 0.5 + z; (xiv) -x, -1 + y, 0.5 - z; (xv) -0.5 + x, 0.5 + y, z.

crystal has potential applications in shape-selective catalysis, absorbents, ion-exchange solids, and molecular sieves. A perspective view of the K3Gd5(PO4)6 structure showing the coordinations of the K and Gd atoms is illustrated in Figure 3. Both Gd1 and Gd3 are surrounded by eight O atoms, whereas Gd2 is coordinated by nine O atoms. All Gd-O bond distances are consistent with those reported previously10 and range from 2.282(4) to 2.715(4) Å (Table 3). All Gd polyhedra are greatly distorted. Each Gd1 polyhedron is edge-shared with two Gd2 polyhedra and two Gd3 polyhedrea, and each Gd2 polyhedron is respectively edge-shared with one Gd1, one Gd2, and one Gd3 polyhedron, whereas each Gd3 polyhedron is respectively edge-shared with one Gd1, one Gd2, and two Gd3 polyhedra. The closest Gd-Gd distance is around 4.074 Å. All Gd polyhedra are also linked with isolated PO4 tetrahedra via

Potassium Gadolinium Phosphate

Crystal Growth & Design, Vol. 6, No. 7, 2006 1651

Figure 2. Connection of K polyhedra showing a wavy chain along the c axis.

Figure 4. Absorption spectrum of K3Gd5(PO4)6.

Figure 3. Perspective view of the K3Gd5(PO4)6 structure showing the coordinations of the K and Gd atoms.

sharing O atoms, resulting in a three-dimensional Gd5(PO4)63anionic framework. There are two unique K atom types in the tunnels which the Gd5(PO4)63- anionic framework generates. K1 and K2 are respectively 9- and 10-coordinated with K-O bond distances of 2.613(4)-3.302(4) Å. Each K1 polyhedron is edge-shared with one K2 polyhedron and face-shared with one K1 polyhedron. Each K2 polyhedron is edge-shared with two K1 polyhedra and corner-shared with two other K1 polyhedra. The connection mode among K polyhedra forms a wavy chain along the c axis, as shown in Figure 2. In PO4 tetrahedra, P1 is connected to the O3, O4, O8, and O12 atoms, and P2 is connected to the O1, O5, O10, and O11 atoms, whereas P3 is connected to the O2, O6, O7, and O9 atoms. The P-O bond distances vary from 1.510(4) to 1.580(4) Å, and O-P-O angles range from 100.5(2) to 116.2(2)° (Table 3). This indicates that PO4 tetrahedra are slightly distorted. These PO4 tetrahedra are isolated from each other because they only share O atoms with Gd polyhedra to form a three-dimensional Gd5(PO4)63- anionic framework. Optical Properties. Figure 4 gives the absorption spectrum of K3Gd5(PO4)6 determined by diffuse reflection measurements. F(R) and R are linked by F(R) ) (1 - R)2/2R, where R is the reflectance and F(R) is the Kubelka-Munk remission function. The minima in the second-derivative curves of the KubelkaMunk function are taken as the positions of the absorption bands.11 It is observed that the absorption edge is about 400 nm (3.10 eV), and the strong absorption peak is at about 260 nm (4.78 eV), which is attributed to the transition from 8S7/2 to 6D 3+ ion.12,13 Furthermore, the compound has no 9/2 of the Gd absorption above 400 nm. Figure 5 shows the emission spectrum

Figure 5. Emission spectrum of K3Gd5(PO4)6 (excited at 320 nm).

of K3Gd5(PO4)6 when a ultraviolet light of 320 nm is used to excite it. A broad emission band at around 368 nm (3.38 eV) is observed. The broad band possibly results from 5d-4f electronic transitions and is not related to intraconfigurational 4f-4f transitions of the Gd3+ ion,14 because very strong and sharp optical emissions arising from 4f-4f transitions are not observed in the ultraviolet region. The close distance of Gd-Gd, which has been characterized in the preceding single-crystal structure section, possibly results in a concentration fluorescence arising from f-f electronic transition quenching by the interaction of the Gd3+ ion.15 This indicates that crystal field effects cannot be completely ignored, even for the inner 4f7 orbitals which are shielded by their outer (5s2 and 5p6) filled orbitals. Conclusions In summary, we have synthesized a novel potassium gadolinium phosphate with the chemical formula K3Gd5(PO4)6, which

1652 Crystal Growth & Design, Vol. 6, No. 7, 2006

is a new structural family of the alkali-metal-rare-earth phosphate system. Structural and optical properties have been reported. It is interesting to note that Gd polyhedra and isolated PO4 tetrahedra make up a three-dimensional Gd5(PO4)63anionic framework containing infinite channels along the c axis in which the K atoms are located. The spectral measurements and analyses indicate that the absorption edge is about 400 nm (3.10 eV), and the emission peak localized at 368 nm arises from 5d-4f electronic transitions of the Gd3+ ion. The investigations on the synthesis, structure, and optical properties of this novel compound will promote further development of M3Ln5(PO4)6 (M ) alkali metal, Ln ) rare-earth metal) phosphate materials. Acknowledgment. This investigation was based on work supported by the National Basic Research Program of China (No. 2004CB720605), the National Natural Science Foundation of China under projects 20373073 and 90201015, the Science Foundation of the Fujian Province (No. E0210028), and the Foundation of State Key Laboratory of Structural Chemistry (No. 060007). Supporting Information Available: A CIF file giving X-ray crystallographic data and figures giving full results of elemental analysis and the experimental and simulated powder XRD patterns for K3Gd5(PO4)6. This material is available free of charge via the Internet at http:// pubs.acs.org.

Zhu et al.

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