Synthesis and Characterization of New Nickel Phosphates, ANi₄(PO₄)₃ (A=K, Rb)

Experimental

Synthesis.

KNi4(PO4)3: KNi4(PO4)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: RbNi4(PO4)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 ANi4(PO4)3 (A=K, Rb) were determined by single crystal X-ray diffr-action methods. Preliminary examination and data collection were performed with Mo Kα1 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 ANi4(PO4)3 (A=K, Rb) were corrected for absorption using the multi-scan method.11 In case of KNi4(PO4)3, the final cycle of refinement perform-ed on Fo2 with 1183 unique reflections afforded residuals wR2 = 0.059 and the conventional R index based on the reflections having Fo2 > 2σ (Fo2) is 0.024. For RbNi4(PO4)3, the final cycle of refinement performed on Fo2 with 1202 unique reflections afforded residuals wR2=0.091 and the conventional R index based on the reflections having Fo2 > 2σ (Fo2) 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/Å3. No unusual trends were found in the goodness of fit as a function of Fo, 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. BaSO4 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

Table 1.Crystal data and structure refinement for ANi4(PO4)3

Table 2.Atomic coordinates, equivalent isotropic displacement parameters and bond valence sums (BVSs) for KNi4(PO4)3

Table 3.aUeq is defined as one third of the trace of the orthogonalized Uij tensor.

 

Result and Discussion

Crystal Structure. The structural studies of ANi4(PO4)3 (A=K, Rb) demonstrate the existence of another members of the AM4(PO4)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 AM4(PO4)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. Ni3O6 octahedra form a one-dimensional chain along the c-axis by sharing edges and these chains are linked via Ni2O6 octahedra to form the two-dimensional layer parallel to the ac plane. The edge-sharing trigonal bipyramidal Ni1O5 acts as a bridge to connect the layers and finally the tetrahedral PO4 link the Ni polyhedra to complete the three-dimensional framework, ∞3[Ni4(PO4)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 NaNi4(PO4)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 ions13 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 PO4 tetrahedra.14 According to the bond valence calculations,15 the global instability indices, Gii for KNi4(PO4)3 and RbNi4(PO4)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+] [Ni2+]4- [P5+]3[O2−]12.

Table 4.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 Ni2+ ions are around 2.78 eV for KNi4(PO4)3 and 2.68 eV for RbNi4(PO4)3. Usually Ni2+ ions with octa-hedral coordinations with oxygen atoms show greenish colors. According to the investigation by Rossman et al., bright yellow oxides containing Ni2+ ions are found when the Ni2+ 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.

Figure 1.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.

Figure 2.Basic polyhedral units around Ni atoms. Atom color codes as in Figure 1. (a) Ni1O5 trigonal bipyramid (b) Ni2O6 and Ni3O6 octahedra.

Figure 3.Solid-state UV/Vis absorption spectra of KNi4(PO4)3.