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Pb10V6O25: A new lead vanadate with apatite structure
Journal of Molecular Structure 1151 (2018) 223e229
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Pb10V6O25: A new lead vanadate with apatite structure Zhizhong Zhang a, Xiaoyu Dong b, Zhaohui Chen a, *, Yunjing Shi b, Qun Jing a, ** a b
College of Chemistry and Chemical Engineering, School of Physics and Technology, Xinjiang University, 666 Shengli Road, Urumqi 830046, China Engineering Department of Chemistry and Environment, Xinjiang Institute of Engineering, 236 Nanchang Road, Urumqi 830091, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2017 Received in revised form 12 September 2017 Accepted 13 September 2017 Available online 18 September 2017
The apatite-type crystal Pb10V6O25 has been grown from high temperature solution by spontaneous crystallization. It crystallizes in the hexagonal space group P63/m with lattice parameters a ¼ 10.1045(6) Å, c ¼ 7.3503(6) Å and Z ¼ 1. The structure of Pb10V6O25 is composed of Pb(1)O9 polyhedra and VO4 tetrahedra, which form infinite Pb(1)V6O24 three-dimension (3D) framework with hexagonal star tunnels along the c axis, where Pb(2)3O(4) groups are filled. The band gap for Pb10V6O25 is calculated to be 2.86 eV, from the UVeViseNIR diffuse reflectance spectrum. First-principles calculations are performed to elucidate the electronic structure and optical properties. Thermal behavior and vibration spectroscopy of Pb10V6O25 are also reported in this work. © 2017 Elsevier B.V. All rights reserved.
Keywords: Pb10V6O25 Crystal structure Optical properties Theoretical calculation
1. Introduction Vanadium-containing compounds have been investigated for a variety of applications, including as magnetic materials [1e4], catalysts [5e8], cathode materials [9e12], ion-exchange materials [13,14] birefringent materials [15,16], and nonlinear-optical materials [17e19]etc. The diverse applications attribute to the origination of various coordination environments and oxidation states found in vanadates. In general, vanadium may exist as V0 (metallic), V2þ, V3þ, V4þ and V5þ depending on the synthesis procedure and chemical environment [20]. Therefore, it can form tetrahedron, pentahedron, triangular pyramids, and octahedron, and has many interesting properties [21]. In recent years, researchers have synthesized a series of vanadium oxide-based compounds with apatite structures, such as A5(VO4)3F (A ¼ Ca, Sr, Ba) [22e24], A5(VO4)3Cl (A ¼ Ca, Sr, Ba) [25,26], Pb5(VO4)3Cl [27]and Ca5(VO4)3OH [28]. Apatites are important materials with applications in medical prostheses, environmental remediation and catalysis [29e34]. In the aspect of optical applications, their robust physical properties coupled with various coordination environments have made them suitable vanadate hosts for optically characteristics [35,36].
In order to further investigate the crystal structures with physical properties, we have extended the study on the synthesis of vanadium-containing compounds with apatite structure. Single crystal of the title compound is obtained firstly during our attempt to explore new compounds in the PbeVeO system. In this paper, the synthesis, crystal structure, vibrational spectroscopy, UVeViseNIR diffuse reflectance spectrum and theoretical calculations of the title compound are reported. 2. Experimental section 2.1. Synthesis Polycrystalline samples of Pb10V6O25 were synthesized by solidstate reaction method. A stoichiometric mixture of PbO (99.5%, Shanghai Shanpu Chemical Co., Ltd), and V2O5 (99.5%, Shanghai Shanpu Chemical Co., Ltd) was thoroughly ground and put into a corundum crucible. The sample was first preheated at 300 C for 10 h and then heated at 500 C for 24 h. Afterwards it was sintered at 600 C and kept at this temperature for 72 h with several intermediate mixings and grindings, the pure Pb10V6O25 were obtained. 2.2. Powder X-ray diffraction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Chen), [email protected] (Q. Jing). http://dx.doi.org/10.1016/j.molstruc.2017.09.036 0022-2860/© 2017 Elsevier B.V. All rights reserved.
The powder X-ray diffraction (XRD) data were collected at room temperature in the angular range of 2q ¼ 10e70 with a step width of 0.02 and a fixed counting time of 1 s/step using an automated Bruker D2 X-ray diffractometer with Cu Ka radiation (l ¼ 1.5418 Å).
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Fig. 1 shows the observed powder XRD patterns of Pb10V6O25 together with those calculated from the single crystal data for comparison. It is clear that the observed XRD patterns are in good agreement with the theoretical ones, confirming the pure powder of this compound. 2.3. Single crystal growth Single crystals of Pb10V6O25 were grown by the spontaneous crystallization method. They were grown from a mixture of PbF2 (17.164 g, 70 mmol), V2O5 (3.1829 g, 17.5 mmol) and NH4HF2 (3.9928 g, 5 mmol). The mixture of the raw materials was put in a platinum crucible, and the crucible was gradually heated to 600 C and kept for 20 h to ensure that the solution mixed homogeneously, then it was crystallized spontaneously in the crucible mouth. When the crystal growth is completed, and followed by cooling to 500 C at the rate of 2 C/h. Finally, the temperature was cooled to room temperature at a rate of 10 C/h. Needle-shaped yellow crystals are obtained (Fig. 2). 2.4. Single crystal X-ray diffraction A single crystal of Pb10V6O25 with dimensions 0.23 mm 0.10 mm 0.08 mm was selected under microscope, and then glued on the end of a glass fiber for single crystal X-ray determination study. The diffraction data were collected at room temperature on a Bruker Smart APEX II single crystal diffractometer equipped with a 4K CCD-detector (graphite Mo Ka radiation, l ¼ 0.71073 Å). The reduction of data were carried out with the Bruker Suite software package [37]. The numerical absorption corrections were performed with the SADABS program and integrated with the SAINT program [38]. All calculations were performed with programs from the SHELXTL crystallographic software package [39]. The structures were solved by direct methods, and all of the atoms were refined using full-matrix least-squares techniques with anisotropic thermal parameters and finally converged for Fo2 2s(Fo2). The structures were examined using the Adsym subroutine of PLATON [40], and no additional symmetry could be applied to the models. Crystallographic data and structural refinements for the title compound are summarized in Table 1. The final refined atomic positions and isotropic thermal parameters of each atom are summarized in Table S1. Selected bond distances and
Fig. 2. Photograph of the Pb10V6O25 crystals.
Table 1 Crystal data and structure refinement for Pb10V6O25. Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions
Volume Z, Calculated density Absorption coefficient F(000) Crystal size Theta range for data collection Limiting indices Reflections collected/unique Completeness to theta ¼ 25.05 Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)]a R indices (all data)a Extinction coefficient Largest diff. peak and hole a
Pb10V6O25 2777.54 273(2) K 0.71073 Å hexagonal, P63/m a ¼ 10.1045(6) Å b ¼ 10.1045(6) Å c ¼ 7.3503(6) Å g ¼ 120 649.93(8) Å3 1, 7.097 mg/m3 66.625 mm1 1158 0.23 mm 0.10 mm 0.08 mm 4.03e25.05 12 h 12, 12 k 10, 8 l 8 4127/416 [R(int) ¼ 0.1246] 99.50% Full-matrix least-squares on F2 416/6/41 1.141 R1 ¼ 0.0481, wR2 ¼ 0.1294 R1 ¼ 0.0491, wR2 ¼ 0.1303 0.0131(15) 5.559 and 2.798 e/Å3
R1 ¼ SjjFoj jFcjj/SjFoj and wR2 ¼ [Sw(F2o F2c )2/Sw F4o]1/2 for F2o > 2s(F2o).
angles are given in Table S2 in the Supporting Information. 2.5. Energy-dispersive spectroscopy Elemental analysis was performed on the single crystals using a HITACHI SU8010 scanning electron microscope (SEM) with energydispersive spectroscopy (EDS) capabilities. The crystals were mounted on carbon tape and analyzed using a 20 kV accelerating voltage and an accumulation time of 1 min. As a qualitative measure, EDS confirmed the presence of each reported element in the title compound. 2.6. Thermal analyses Fig. 1. Experimental and calculated XRD patterns of Pb10V6O25.
Thermogravimetric analysis (TGA) and differential scanning
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calorimetry (DSC) were conducted on a HITACHI STA 7300 thermal analyzer instrument. The crystal samples (5e10 mg) were enclosed in a Al2O3 crucible and heated from 30 to 1000 C at a rate of 10 C/ min under argon atmosphere. 2.7. Infrared spectroscopy Infrared spectroscopy (IR) for Pb10V6O25 was recorded on a SHIMADZU IRAffinity-1 Fourier transform infrared spectrometer with a resolution of 2 cm1 in the range of 400e4000 cm1, with the sample contained in a KBr matrix.
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The geometry optimization was converged when the residual forces on the atoms were less than 0.03 eV/Å, the displacements of atoms were less than 0.001 Å, and the energy change was less than 1.0 105 eV/atom. The electronic structures and optical properties were calculated using optimized geometry of Pb10V6O25. The other calculation parameters and convergent criteria were the default values of the CASTEP code.
3. Results and discussion 3.1. Crystal structure description
2.8. UVeVisible(Vis)Near-infrared (NIR) diffuse reflectance spectroscopy The UVeViseNIR diffuse reflectance spectrum of the Pb10V6O25 powder sample was measured with a Shimadzu SolidSpec3700DUV Spectrophotometer at room temperature in the measurement range from 190 to 2600 nm. Reflectance spectrum was converted to absorbance using the Kubelka-Munk function,
FðRÞ ¼
ð1 RÞ2 K ¼ S 2R
where R is the reflectance, K is the absorption, and S is the scattering [41]. 2.9. Theoretical calculations The electronic structures and the optical properties of Pb10V6O25 were calculated using the CASTEP package [42,43]. As shown in Fig. 3 and the CIF file, a static positional disorder was found in O4, whose atomic occupancy is about 0.27 in each site. After carefully investigate the geometry of Pb10V6O25, the authors believe the O4 atoms may be either siting in a site or in b site and name them as PbVO-a and PbVO-b respectively (shown in Fig. 3, PbVO is the abbreviation of Pb10V6O25). Therefore the authors made detailed investigation on the electronic structures and optical properties of PbVO-a and PbVO-b. During the calculation, the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional was adopted [44]. Under the ultrasoft pseudopotential, the following orbital electrons were treated as valence electrons: Pb: 5d106s26p2, V: 3s23p63d34s2, O: 2s22p4. The kinetic energy cutoff of 380 eV was chosen, and the numerical integration of the Brillouin zone was performed using a 3 3 4 Monkhorst-Pack kpoint sampling. The cell parameters and the atomic coordinates of all the atoms were optimized during the geometry optimization.
Pb10V6O25 crystallizes in hexagonal system, centrosymmetric space group P63/m with lattice constants a ¼ 10.1045(6) Å, c ¼ 7.3503(6) Å, g ¼ 120 , and Z ¼ 1. The asymmetric unit contains two different crystallographic positions lead atoms, one vanadium atom, four different oxygen atoms. The O(4) atom site occupation factor (S.O.F) is only 0.27, especially. The structure is very similar to the well-known apatite type Ba5V3O12F (Fig. 4b). Compared with Ba5V3O12F, O(4) replaces the F atom, so Pb10V6O25 formular can be written as 2(Pb5V3O12)O. The whole crystal structure of Pb10V6O25 is show in Fig. 5. In the structure, one Pb(1) coordinated with nine O atoms form a Pb(1)O9 polyhedron (Fig. 5a), and then it connect with six VO4 tetrahedra (Fig. 5b), which are three corner- and three edge-sharing with O atoms to form a Pb(1)V6O24 group (Fig. 5c). In the ab plane, the Pb(1)V6O24 groups are connected by sharing bridging O corners to generate hexagonal star channels and form the two-dimensional (2D) infinite [Pb(1)V6O24]∞ layers (Fig. 5d). Furthermore, the Pb(1)V6O24 groups are further linked each other by face-sharing with three O(3) atoms along the c axis (Fig. S1b) and form a 3D framework (Fig. 5e). The Pb(1)V6O24 groups construct hexagonal star channels, where the Pb(2)3O(4) groups are filled (Fig. 5f). The Pb(1) atom is nine-folded coordinating to oxygen atoms with the Pb(1)eO bond lengths in the range 2.451(9)e2.954(17) Å, while Pb(2) atom is eight-folded coordinating to oxygen atoms (Fig. S1a in the Supporting Information) with the Pb(2)eO bond lengths in the range 2.375(15)e2.970(13) Å. Each vanadium atom is coordinated with four O atoms (Fig. 5b) forming a VO4 tetrahedron with V-O bond lengths ranging from 1.707(10) to 1.748(14) Å, and these values similar to those reported previously in other compounds [45e47]. The bond valence sums (BVS) [48,49]of all atoms in Pb10V6O25 were calculated and listed in Table 2. The bond valence calculation results for Pb2þ, V5þ, and O2 are 1.979 and 1.981, 5.005 and 2.00e2.06, respectively. These valence sums agree well with the expected oxidation states.
Fig. 3. The model used for calculations.
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Fig. 4. Cell structures of Pb10V6O25 and Ba5V3O12F.
Fig. 5. Crystal structure of Pb10V6O25.
Table 2 Bond valence analysis of Pb10V6O25. Atom
Pb1
Pb2
V1
O1 O2 O3 O4 P Cations
0.103/0.103/0.103 0.4/0.4/0.4 0.156/0.157/0.157 e 1.979
0.298/0.298/0.233/0.233 0.098 0.491 0.165/0.165 1.981
1.297/1.297 1.159 1.252 e 5.005
P Anions 1.931 2.057 2.056 1.83
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ratio of Pb: V: O is about 9.98: 6.00: 25.12, which is approximately equal to the theoretical 10: 6: 25 ratio of Pb10V6O25, verifying the validity of the crystal structure. 3.3. Thermal analyses
Fig. 6. The TG/DSC curve of Pb10V6O25.
TGA and DSC measurements were carried out with polycrystalline samples of Pb10V6O25 (shown in Fig. 6). The DSC curve exhibits an endothermic peak at 887 C and there is no obvious weight loss from 30 to 1000 C from the TG curve. In order to verify whether Pb10V6O25 melts congruently, 3 g powder of Pb10V6O25 was heated to 1000 C and held at this temperature for 24 h, then cooled to room temperature at a rate of 20 C/h. Analysis of the powder XRD patterns of the sample revealed that the diffraction patterns were different from the initial Pb10V6O25 powder (Fig. S3 in the Supporting information). The experimental results showed that the compound has decomposed into Pb3(VO4)2 and Pb4V2O9, which demonstrates that Pb10V6O25 is an incongruent melting compound. 3.4. Vibrational spectroscopy The coordination of vanadium in Pb10V6O25 was confirmed by the IR spectrum. As shown in Fig. S4, the absorption bands at 818 and 733 cm1 may be attributed to VeO vibrations. This assignment was consistent with the vanadates previously reported [50e52]. The IR spectrum further confirms the existence of the VO4 function groups, which are consistent with the result obtained from the single-crystal X-ray structural analysis. 3.5. UVeViseNIR diffuse reflectance spectrum
Fig. 7. UVeVis-IR diffuse-reflectance spectra of Pb10V6O25.
The UVeViseNIR diffuse reflectance spectrum collected for the reported compound is shown in Fig. 7. The results indicate that its cutoff edge is about 434 nm and experimental band gap is 2.86 eV. Its band gap should be due to both of the distortions resulting from the VO4 groups and the degree of V (3d) orbital involved in the conduction bands. It suggests the potential applications of this crystal for visible light range. 3.6. The calculated electronic structures and optical properties
3.2. EDS analysis EDS was performed to verify atomic composition of the title compound. The EDS spectrum (Fig. S2) indicates that the molar
Using the method described above, the band structures along high symmetry points of the first Brillouin zone of PbVO-a and PbVO-b have been obtained. As shown in Fig. 8, the obtained bandgaps of PbVO-a and PbVO-b are 2.438 and 2.439 eV,
Fig. 8. The obtained bandgaps of PbVO-a (a) and PbVO-b (b).
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its high temperature solution by spontaneous crystallization. The structure consists of infinite 3D matrix Pb(1)V6O24 with hexagonal star tunnels along the c axis, where Pb(2)3O(4) groups are filled. Diffuse reflectance spectrum proves that Pb10V6O25 possesses an experimental band gap of 2.86 eV, and thermal analyses and powder XRD patterns verify it melting incongruently. Meanwhile, combining experiments with theoretical calculations, Pb-O and V-O polyhedra may responsible for the optical properties of the title compound. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 51462033, 51562036), Natural Science Foundation of Xinjiang Uygur Autonomous Region of China (No. 2015211C251), Funded by Scientific Research Program of the Higher Education Institution of XinJiang (XJEDU2016S086) Appendix A. Supplementary data Fig. 9. The projected density of states (PDOS) of Pb10V6O25.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2017.09.036. References
Fig. 10. The refractive indices and birefringence of Pb10V6O25.
respectively, which are slightly smaller than the experimental values (2.86 eV). The underestimation of bandgaps is relation with the derivative discontinuity of the exchange-correlation energy [53e55] and we note that both compounds own similar electronic structures. We took PbVO-b as an example and the projected density of states (PDOS) was also obtained. As shown in Fig. 9, the states at the top of the valence band (from 10 eV to Fermi level) are mainly from the O-p, Pb-sp, and V-d states. The Pb-sp mixed, O-p and Vd states are also found at the bottom of the conduction bands. The states around the Fermi level indicate the refractive indices of Pb10V6O25 may have relation with the Pb-O and V-O polyhedra. Based on the electronic structures, the refractive indices and birefringence of Pb10V6O25 are also obtained (shown in Fig. 10). As shown in Fig. 10, the refractive indices of ordinary light (no) are larger than that of the extraordinary light (ne), indicating the Pb10V6O25 compound is negative uniaxial crystal. The obtained birefringence is about 0.036e0.033 at 450e1200 nm. 4. Conclusion The apatite-type lead vanadate Pb10V6O25 has been grown from
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Pb10V6O25: A new lead vanadate with apatite structure Zhizhong Zhang a, Xiaoyu Dong b, Zhaohui Chen a, *, Yunjing Shi b, Qun Jing a, ** a b
College of Chemistry and Chemical Engineering, School of Physics and Technology, Xinjiang University, 666 Shengli Road, Urumqi 830046, China Engineering Department of Chemistry and Environment, Xinjiang Institute of Engineering, 236 Nanchang Road, Urumqi 830091, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2017 Received in revised form 12 September 2017 Accepted 13 September 2017 Available online 18 September 2017
The apatite-type crystal Pb10V6O25 has been grown from high temperature solution by spontaneous crystallization. It crystallizes in the hexagonal space group P63/m with lattice parameters a ¼ 10.1045(6) Å, c ¼ 7.3503(6) Å and Z ¼ 1. The structure of Pb10V6O25 is composed of Pb(1)O9 polyhedra and VO4 tetrahedra, which form infinite Pb(1)V6O24 three-dimension (3D) framework with hexagonal star tunnels along the c axis, where Pb(2)3O(4) groups are filled. The band gap for Pb10V6O25 is calculated to be 2.86 eV, from the UVeViseNIR diffuse reflectance spectrum. First-principles calculations are performed to elucidate the electronic structure and optical properties. Thermal behavior and vibration spectroscopy of Pb10V6O25 are also reported in this work. © 2017 Elsevier B.V. All rights reserved.
Keywords: Pb10V6O25 Crystal structure Optical properties Theoretical calculation
1. Introduction Vanadium-containing compounds have been investigated for a variety of applications, including as magnetic materials [1e4], catalysts [5e8], cathode materials [9e12], ion-exchange materials [13,14] birefringent materials [15,16], and nonlinear-optical materials [17e19]etc. The diverse applications attribute to the origination of various coordination environments and oxidation states found in vanadates. In general, vanadium may exist as V0 (metallic), V2þ, V3þ, V4þ and V5þ depending on the synthesis procedure and chemical environment [20]. Therefore, it can form tetrahedron, pentahedron, triangular pyramids, and octahedron, and has many interesting properties [21]. In recent years, researchers have synthesized a series of vanadium oxide-based compounds with apatite structures, such as A5(VO4)3F (A ¼ Ca, Sr, Ba) [22e24], A5(VO4)3Cl (A ¼ Ca, Sr, Ba) [25,26], Pb5(VO4)3Cl [27]and Ca5(VO4)3OH [28]. Apatites are important materials with applications in medical prostheses, environmental remediation and catalysis [29e34]. In the aspect of optical applications, their robust physical properties coupled with various coordination environments have made them suitable vanadate hosts for optically characteristics [35,36].
In order to further investigate the crystal structures with physical properties, we have extended the study on the synthesis of vanadium-containing compounds with apatite structure. Single crystal of the title compound is obtained firstly during our attempt to explore new compounds in the PbeVeO system. In this paper, the synthesis, crystal structure, vibrational spectroscopy, UVeViseNIR diffuse reflectance spectrum and theoretical calculations of the title compound are reported. 2. Experimental section 2.1. Synthesis Polycrystalline samples of Pb10V6O25 were synthesized by solidstate reaction method. A stoichiometric mixture of PbO (99.5%, Shanghai Shanpu Chemical Co., Ltd), and V2O5 (99.5%, Shanghai Shanpu Chemical Co., Ltd) was thoroughly ground and put into a corundum crucible. The sample was first preheated at 300 C for 10 h and then heated at 500 C for 24 h. Afterwards it was sintered at 600 C and kept at this temperature for 72 h with several intermediate mixings and grindings, the pure Pb10V6O25 were obtained. 2.2. Powder X-ray diffraction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Chen), [email protected] (Q. Jing). http://dx.doi.org/10.1016/j.molstruc.2017.09.036 0022-2860/© 2017 Elsevier B.V. All rights reserved.
The powder X-ray diffraction (XRD) data were collected at room temperature in the angular range of 2q ¼ 10e70 with a step width of 0.02 and a fixed counting time of 1 s/step using an automated Bruker D2 X-ray diffractometer with Cu Ka radiation (l ¼ 1.5418 Å).
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Fig. 1 shows the observed powder XRD patterns of Pb10V6O25 together with those calculated from the single crystal data for comparison. It is clear that the observed XRD patterns are in good agreement with the theoretical ones, confirming the pure powder of this compound. 2.3. Single crystal growth Single crystals of Pb10V6O25 were grown by the spontaneous crystallization method. They were grown from a mixture of PbF2 (17.164 g, 70 mmol), V2O5 (3.1829 g, 17.5 mmol) and NH4HF2 (3.9928 g, 5 mmol). The mixture of the raw materials was put in a platinum crucible, and the crucible was gradually heated to 600 C and kept for 20 h to ensure that the solution mixed homogeneously, then it was crystallized spontaneously in the crucible mouth. When the crystal growth is completed, and followed by cooling to 500 C at the rate of 2 C/h. Finally, the temperature was cooled to room temperature at a rate of 10 C/h. Needle-shaped yellow crystals are obtained (Fig. 2). 2.4. Single crystal X-ray diffraction A single crystal of Pb10V6O25 with dimensions 0.23 mm 0.10 mm 0.08 mm was selected under microscope, and then glued on the end of a glass fiber for single crystal X-ray determination study. The diffraction data were collected at room temperature on a Bruker Smart APEX II single crystal diffractometer equipped with a 4K CCD-detector (graphite Mo Ka radiation, l ¼ 0.71073 Å). The reduction of data were carried out with the Bruker Suite software package [37]. The numerical absorption corrections were performed with the SADABS program and integrated with the SAINT program [38]. All calculations were performed with programs from the SHELXTL crystallographic software package [39]. The structures were solved by direct methods, and all of the atoms were refined using full-matrix least-squares techniques with anisotropic thermal parameters and finally converged for Fo2 2s(Fo2). The structures were examined using the Adsym subroutine of PLATON [40], and no additional symmetry could be applied to the models. Crystallographic data and structural refinements for the title compound are summarized in Table 1. The final refined atomic positions and isotropic thermal parameters of each atom are summarized in Table S1. Selected bond distances and
Fig. 2. Photograph of the Pb10V6O25 crystals.
Table 1 Crystal data and structure refinement for Pb10V6O25. Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions
Volume Z, Calculated density Absorption coefficient F(000) Crystal size Theta range for data collection Limiting indices Reflections collected/unique Completeness to theta ¼ 25.05 Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)]a R indices (all data)a Extinction coefficient Largest diff. peak and hole a
Pb10V6O25 2777.54 273(2) K 0.71073 Å hexagonal, P63/m a ¼ 10.1045(6) Å b ¼ 10.1045(6) Å c ¼ 7.3503(6) Å g ¼ 120 649.93(8) Å3 1, 7.097 mg/m3 66.625 mm1 1158 0.23 mm 0.10 mm 0.08 mm 4.03e25.05 12 h 12, 12 k 10, 8 l 8 4127/416 [R(int) ¼ 0.1246] 99.50% Full-matrix least-squares on F2 416/6/41 1.141 R1 ¼ 0.0481, wR2 ¼ 0.1294 R1 ¼ 0.0491, wR2 ¼ 0.1303 0.0131(15) 5.559 and 2.798 e/Å3
R1 ¼ SjjFoj jFcjj/SjFoj and wR2 ¼ [Sw(F2o F2c )2/Sw F4o]1/2 for F2o > 2s(F2o).
angles are given in Table S2 in the Supporting Information. 2.5. Energy-dispersive spectroscopy Elemental analysis was performed on the single crystals using a HITACHI SU8010 scanning electron microscope (SEM) with energydispersive spectroscopy (EDS) capabilities. The crystals were mounted on carbon tape and analyzed using a 20 kV accelerating voltage and an accumulation time of 1 min. As a qualitative measure, EDS confirmed the presence of each reported element in the title compound. 2.6. Thermal analyses Fig. 1. Experimental and calculated XRD patterns of Pb10V6O25.
Thermogravimetric analysis (TGA) and differential scanning
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calorimetry (DSC) were conducted on a HITACHI STA 7300 thermal analyzer instrument. The crystal samples (5e10 mg) were enclosed in a Al2O3 crucible and heated from 30 to 1000 C at a rate of 10 C/ min under argon atmosphere. 2.7. Infrared spectroscopy Infrared spectroscopy (IR) for Pb10V6O25 was recorded on a SHIMADZU IRAffinity-1 Fourier transform infrared spectrometer with a resolution of 2 cm1 in the range of 400e4000 cm1, with the sample contained in a KBr matrix.
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The geometry optimization was converged when the residual forces on the atoms were less than 0.03 eV/Å, the displacements of atoms were less than 0.001 Å, and the energy change was less than 1.0 105 eV/atom. The electronic structures and optical properties were calculated using optimized geometry of Pb10V6O25. The other calculation parameters and convergent criteria were the default values of the CASTEP code.
3. Results and discussion 3.1. Crystal structure description
2.8. UVeVisible(Vis)Near-infrared (NIR) diffuse reflectance spectroscopy The UVeViseNIR diffuse reflectance spectrum of the Pb10V6O25 powder sample was measured with a Shimadzu SolidSpec3700DUV Spectrophotometer at room temperature in the measurement range from 190 to 2600 nm. Reflectance spectrum was converted to absorbance using the Kubelka-Munk function,
FðRÞ ¼
ð1 RÞ2 K ¼ S 2R
where R is the reflectance, K is the absorption, and S is the scattering [41]. 2.9. Theoretical calculations The electronic structures and the optical properties of Pb10V6O25 were calculated using the CASTEP package [42,43]. As shown in Fig. 3 and the CIF file, a static positional disorder was found in O4, whose atomic occupancy is about 0.27 in each site. After carefully investigate the geometry of Pb10V6O25, the authors believe the O4 atoms may be either siting in a site or in b site and name them as PbVO-a and PbVO-b respectively (shown in Fig. 3, PbVO is the abbreviation of Pb10V6O25). Therefore the authors made detailed investigation on the electronic structures and optical properties of PbVO-a and PbVO-b. During the calculation, the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional was adopted [44]. Under the ultrasoft pseudopotential, the following orbital electrons were treated as valence electrons: Pb: 5d106s26p2, V: 3s23p63d34s2, O: 2s22p4. The kinetic energy cutoff of 380 eV was chosen, and the numerical integration of the Brillouin zone was performed using a 3 3 4 Monkhorst-Pack kpoint sampling. The cell parameters and the atomic coordinates of all the atoms were optimized during the geometry optimization.
Pb10V6O25 crystallizes in hexagonal system, centrosymmetric space group P63/m with lattice constants a ¼ 10.1045(6) Å, c ¼ 7.3503(6) Å, g ¼ 120 , and Z ¼ 1. The asymmetric unit contains two different crystallographic positions lead atoms, one vanadium atom, four different oxygen atoms. The O(4) atom site occupation factor (S.O.F) is only 0.27, especially. The structure is very similar to the well-known apatite type Ba5V3O12F (Fig. 4b). Compared with Ba5V3O12F, O(4) replaces the F atom, so Pb10V6O25 formular can be written as 2(Pb5V3O12)O. The whole crystal structure of Pb10V6O25 is show in Fig. 5. In the structure, one Pb(1) coordinated with nine O atoms form a Pb(1)O9 polyhedron (Fig. 5a), and then it connect with six VO4 tetrahedra (Fig. 5b), which are three corner- and three edge-sharing with O atoms to form a Pb(1)V6O24 group (Fig. 5c). In the ab plane, the Pb(1)V6O24 groups are connected by sharing bridging O corners to generate hexagonal star channels and form the two-dimensional (2D) infinite [Pb(1)V6O24]∞ layers (Fig. 5d). Furthermore, the Pb(1)V6O24 groups are further linked each other by face-sharing with three O(3) atoms along the c axis (Fig. S1b) and form a 3D framework (Fig. 5e). The Pb(1)V6O24 groups construct hexagonal star channels, where the Pb(2)3O(4) groups are filled (Fig. 5f). The Pb(1) atom is nine-folded coordinating to oxygen atoms with the Pb(1)eO bond lengths in the range 2.451(9)e2.954(17) Å, while Pb(2) atom is eight-folded coordinating to oxygen atoms (Fig. S1a in the Supporting Information) with the Pb(2)eO bond lengths in the range 2.375(15)e2.970(13) Å. Each vanadium atom is coordinated with four O atoms (Fig. 5b) forming a VO4 tetrahedron with V-O bond lengths ranging from 1.707(10) to 1.748(14) Å, and these values similar to those reported previously in other compounds [45e47]. The bond valence sums (BVS) [48,49]of all atoms in Pb10V6O25 were calculated and listed in Table 2. The bond valence calculation results for Pb2þ, V5þ, and O2 are 1.979 and 1.981, 5.005 and 2.00e2.06, respectively. These valence sums agree well with the expected oxidation states.
Fig. 3. The model used for calculations.
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Fig. 4. Cell structures of Pb10V6O25 and Ba5V3O12F.
Fig. 5. Crystal structure of Pb10V6O25.
Table 2 Bond valence analysis of Pb10V6O25. Atom
Pb1
Pb2
V1
O1 O2 O3 O4 P Cations
0.103/0.103/0.103 0.4/0.4/0.4 0.156/0.157/0.157 e 1.979
0.298/0.298/0.233/0.233 0.098 0.491 0.165/0.165 1.981
1.297/1.297 1.159 1.252 e 5.005
P Anions 1.931 2.057 2.056 1.83
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ratio of Pb: V: O is about 9.98: 6.00: 25.12, which is approximately equal to the theoretical 10: 6: 25 ratio of Pb10V6O25, verifying the validity of the crystal structure. 3.3. Thermal analyses
Fig. 6. The TG/DSC curve of Pb10V6O25.
TGA and DSC measurements were carried out with polycrystalline samples of Pb10V6O25 (shown in Fig. 6). The DSC curve exhibits an endothermic peak at 887 C and there is no obvious weight loss from 30 to 1000 C from the TG curve. In order to verify whether Pb10V6O25 melts congruently, 3 g powder of Pb10V6O25 was heated to 1000 C and held at this temperature for 24 h, then cooled to room temperature at a rate of 20 C/h. Analysis of the powder XRD patterns of the sample revealed that the diffraction patterns were different from the initial Pb10V6O25 powder (Fig. S3 in the Supporting information). The experimental results showed that the compound has decomposed into Pb3(VO4)2 and Pb4V2O9, which demonstrates that Pb10V6O25 is an incongruent melting compound. 3.4. Vibrational spectroscopy The coordination of vanadium in Pb10V6O25 was confirmed by the IR spectrum. As shown in Fig. S4, the absorption bands at 818 and 733 cm1 may be attributed to VeO vibrations. This assignment was consistent with the vanadates previously reported [50e52]. The IR spectrum further confirms the existence of the VO4 function groups, which are consistent with the result obtained from the single-crystal X-ray structural analysis. 3.5. UVeViseNIR diffuse reflectance spectrum
Fig. 7. UVeVis-IR diffuse-reflectance spectra of Pb10V6O25.
The UVeViseNIR diffuse reflectance spectrum collected for the reported compound is shown in Fig. 7. The results indicate that its cutoff edge is about 434 nm and experimental band gap is 2.86 eV. Its band gap should be due to both of the distortions resulting from the VO4 groups and the degree of V (3d) orbital involved in the conduction bands. It suggests the potential applications of this crystal for visible light range. 3.6. The calculated electronic structures and optical properties
3.2. EDS analysis EDS was performed to verify atomic composition of the title compound. The EDS spectrum (Fig. S2) indicates that the molar
Using the method described above, the band structures along high symmetry points of the first Brillouin zone of PbVO-a and PbVO-b have been obtained. As shown in Fig. 8, the obtained bandgaps of PbVO-a and PbVO-b are 2.438 and 2.439 eV,
Fig. 8. The obtained bandgaps of PbVO-a (a) and PbVO-b (b).
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its high temperature solution by spontaneous crystallization. The structure consists of infinite 3D matrix Pb(1)V6O24 with hexagonal star tunnels along the c axis, where Pb(2)3O(4) groups are filled. Diffuse reflectance spectrum proves that Pb10V6O25 possesses an experimental band gap of 2.86 eV, and thermal analyses and powder XRD patterns verify it melting incongruently. Meanwhile, combining experiments with theoretical calculations, Pb-O and V-O polyhedra may responsible for the optical properties of the title compound. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 51462033, 51562036), Natural Science Foundation of Xinjiang Uygur Autonomous Region of China (No. 2015211C251), Funded by Scientific Research Program of the Higher Education Institution of XinJiang (XJEDU2016S086) Appendix A. Supplementary data Fig. 9. The projected density of states (PDOS) of Pb10V6O25.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2017.09.036. References
Fig. 10. The refractive indices and birefringence of Pb10V6O25.
respectively, which are slightly smaller than the experimental values (2.86 eV). The underestimation of bandgaps is relation with the derivative discontinuity of the exchange-correlation energy [53e55] and we note that both compounds own similar electronic structures. We took PbVO-b as an example and the projected density of states (PDOS) was also obtained. As shown in Fig. 9, the states at the top of the valence band (from 10 eV to Fermi level) are mainly from the O-p, Pb-sp, and V-d states. The Pb-sp mixed, O-p and Vd states are also found at the bottom of the conduction bands. The states around the Fermi level indicate the refractive indices of Pb10V6O25 may have relation with the Pb-O and V-O polyhedra. Based on the electronic structures, the refractive indices and birefringence of Pb10V6O25 are also obtained (shown in Fig. 10). As shown in Fig. 10, the refractive indices of ordinary light (no) are larger than that of the extraordinary light (ne), indicating the Pb10V6O25 compound is negative uniaxial crystal. The obtained birefringence is about 0.036e0.033 at 450e1200 nm. 4. Conclusion The apatite-type lead vanadate Pb10V6O25 has been grown from
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