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Synthesis and Characterization of New Nickel Phosphates, ANi<sub>4</sub>(PO<sub>4</sub>)<sub>3</sub> (A
Synthesis and Characterization of New Nickel Phosphates, ANi4(PO4)3 (A
Bulletin of the Korean Chemical Society. 2014. Apr, 35(4): 1225-1228
Copyright © 2014, Korea Chemical Society
  • Received : December 04, 2013
  • Accepted : December 20, 2013
  • Published : April 20, 2014
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Yoonmi Im
Pilsoo Kim
Hoseop Yun

Abstract
Keywords
Experimental
Synthesis.
KNi4(PO4)3: KNi 4 (PO 4 ) 3 was prepared by the reaction of elements with the use of the reactive halide-flux technique. A combination of the pure elements, Ni powder (Alfa Aesar 99.8%), S powder(Sigma-Aldrich) and P powder (Sigma-Aldrich 99%) were mixed in a fused silica tube in molar ratio of Ni:P:S=4:5:6 and then KCl (Alfa Aesar 99%) was added. The mass ratio of the reactants and the halide was 1:3. The tube was evacuated to 0.133 Pa, sealed, and heated gradually (30 K/h) to 1023 K, where it was kept for 72 h. The tube was cooled to room temperature at the rate of 6 K/h.
RbNi4(PO4)3: RbNi 4 (PO 4 ) 3 was prepared by the reaction of elements with the use of the reactive halide-flux techni-que. A combination of the pure elements, Ni powder (Alfa Aesar 99.8%), Se powder (Sigma-Aldrich) and P powder (Sigma-Aldrich 99%) were mixed in a fused silica tube in molar ratio of Ni:P:Se=3:4:8 and then RbCl (Alfa Aesar 99%) was added. The mass ratio of the reactants and the halide was 1:2. The tube was evacuated to 0.133 Pa, sealed, and heated gradually (20 K/h) to 923 K, where it was kept for 72 h. The tube was cooled to room temperature at the rate of 12 K/h.
In both cases, the excess halide was removed with distilled water and yellow needle-shaped crystals were obtained. The role of chalcogens in the reactions is not clear but it is helpful to obtain the product as single crystals. The crystals are stable in air and water. A qualitative X-ray fluorescence analysis of the crystals indicated the presence of K or Rb, Ni, and P. The compositions of the compounds were determined by single-crystal X-ray diffraction.
Crystallographic Studies. The structures of ANi 4 (PO 4 ) 3 (A=K, Rb) were determined by single crystal X-ray diffr-action methods. Preliminary examination and data collection were performed with Mo Kα 1 radiation (λ = 0.71073 Å) on a RIGAKU R-ASXIS RAPID diffractometer. The cell con-stants and an orientation matrix were determined from least-squares, using the setting angles in the range 3.0° < θ < 27.5°. The crystallographic details are described in Table 1 . Intensity data were collected with the ω scan technique.
The intensity statistics and systematic absences are con-sistent with the orthorhombic space group, Pnnm . The initial positions for all atoms were obtained by using direct methods of the SHELXS-86 program. 10 The structure was refined by full-matrix least-squares techniques with the use of the SHELXL-97 program. 10 The data for ANi 4 (PO 4 ) 3 (A=K, Rb) were corrected for absorption using the multi-scan method. 11 In case of KNi 4 (PO 4 ) 3 , the final cycle of refinement perform-ed on F o 2 with 1183 unique reflections afforded residuals wR2 = 0.059 and the conventional R index based on the reflections having F o 2 > 2σ (F o 2 ) is 0.024. For RbNi 4 (PO 4 ) 3 , the final cycle of refinement performed on F o 2 with 1202 unique reflections afforded residuals wR2=0.091 and the conventional R index based on the reflections having F o 2 > 2σ (F o 2 ) is 0.035.
A difference Fourier synthesis calculated with phases based on the final parameters shows no peak heights greater than 1.11 and 1.91 e/Å 3 . No unusual trends were found in the goodness of fit as a function of F o , sinθ/λ and Miller indices. Final values of the atomic coordinates and equivalent iso-tropic displacement parameters are given in Tables 2 , 3 . An-isotropic displacement parameters and complete tabulations on the X-ray studies can be found in CIF format in the Supporting Information Section.
Solid-State UV/Vis Spectroscopy. Optical diffuse reflec-tance measurements of the powdered sample were perform-ed at room temperature using a Shimadzu UV-2400 PC spectrophotometer operating in the range of 200-800 nm. BaSO 4 powder was used as reference material. The absorp-tion data were calculated from the diffuse reflectance data with the use of the Kubelka-Munk relation. 12
Crystal data and structure refinement for ANi4(PO4)3
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Crystal data and structure refinement for ANi4(PO4)3
Atomic coordinates, equivalent isotropic displacement parameters and bond valence sums (BVSs) for KNi4(PO4)3
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Atomic coordinates, equivalent isotropic displacement parameters and bond valence sums (BVSs) for KNi4(PO4)3
Atomic coordinates, equivalent isotropic displacement parameters and bond valence sums (BVSs) for RbNi4(PO4)3
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aUeq is defined as one third of the trace of the orthogonalized Uij tensor.
Result and Discussion
Crystal Structure. The structural studies of ANi 4 (PO 4 ) 3 (A=K, Rb) demonstrate the existence of another members of the AM 4 (PO 4 ) 3 family (A=Alkali metal, M=Co, Fe, Mg, Mn, Ni). 1-9 Selected bond distances and angles can be found in Table 4 and the Supporting Information Section, respect-ively. The title compounds are isostructural with AM 4 (PO 4 ) 3 and the detailed descriptions of this structural type have been given previously. 1-9 A view down the a-axis, given in Figure 1 shows the three-dimensional framework structure and tunnels, where the alkali metal cations are located. There are three crystallographically independent Ni atoms and two types of Ni coordination are found in this structure ( Figure 2 ). The Ni1 is coordinated by five O atoms in a trigonal bipyramidal fashion and the Ni2 and Ni3 are surrounded by six O atoms in the distorted octahedral symmetry. The P atom is coordinated to four O atoms to form the regular tetrahedron. Ni3O 6 octahedra form a one-dimensional chain along the c-axis by sharing edges and these chains are linked via Ni2O 6 octahedra to form the two-dimensional layer parallel to the ac plane. The edge-sharing trigonal bipyramidal Ni1O 5 acts as a bridge to connect the layers and finally the tetrahedral PO 4 link the Ni polyhedra to complete the three-dimensional framework, 3 [Ni 4 (PO 4 ) 3 ] . As a result, an empty hexagonal channel along the a-axis is formed. The free diameters of the channels are about 4.8 Å, which is similar to that of NaNi 4 (PO 4 ) 3 . The alkali metal cations, K + or Rb + reside in this channel through the electrostatic Coulombic interaction.
The Ni-O distances ranging from 1.949(3) to 2.320(2) Å are consistent with the sum of the ionic radii of each ions 13 except the Ni2-O7 and Ni3-O8. The P-O distances ranging from 1.496(3) to 1.607(3) Å appear to be typical for the PO 4 tetrahedra. 14 According to the bond valence calculations, 15 the global instability indices, Gii for KNi 4 (PO 4 ) 3 and RbNi 4 (PO 4 ) 3 are 0.0779 and 0.0994 v.u, respectively, which are typical of the unstrained structures. 16 The charge balance of the title compounds can be described by [A + ] [Ni 2+ ] 4 - [P 5+ ] 3 [O 2− ] 12 .
Bond lengths [Å] for ANi4(PO4)3
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Bond lengths [Å] for ANi4(PO4)3
Solid-State UV/Vis Spectroscopy. UV/Vis absorption spectral data show that absorption peaks of crystal field splittings of the Ni 2+ ions are around 2.78 eV for KNi 4 (PO 4 ) 3 and 2.68 eV for RbNi 4 (PO 4 ) 3 . Usually Ni 2+ ions with octa-hedral coordinations with oxygen atoms show greenish colors. According to the investigation by Rossman et al ., bright yellow oxides containing Ni 2+ ions are found when the Ni 2+ ions enter sites significantly deviated from the regular octa-hedral symmetry. 17 Therefore, we believe that the electronic transitions localized mainly on the distorted Ni polyhedra are responsible for the colors observed in the title compounds.
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View of ANi4(PO4)3 down the a-axis showing the structure of the framework. Alkali metals, Ni, P, and O atoms are represented by green, blue, turquoise, and red spheres, respec-tively with arbitrary radii; NiO5, NiO6 polyhedra are drawn in sky blue and blue, respectively. PO4 tetrahedra are drawn in pink.
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Basic polyhedral units around Ni atoms. Atom color codes as in Figure 1. (a) Ni1O5 trigonal bipyramid (b) Ni2O6 and Ni3O6 octahedra.
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Solid-state UV/Vis absorption spectra of KNi4(PO4)3.
Acknowledgements
Supporting Information.Crystallographic data for the structures reported here have been deposited with FIZ (De-position No. CSD-427036 and CSD-427037 for KNi4(PO4)3and RbNi4(PO4)3, respectively). These data can be obtained free of charge from FIZ, D-76344, Eggenstein-Leopoldshafen, Germany, E-mail: crysdata@fiz-karlsruhe.de).
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