5420
Inorg. Chem. 2000, 39, 5420-5421
Nickel Phosphate Based Zeotype, RbNiPO4 Paul F. Henry, Mark T. Weller,* and Robert W. Hughes Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, U.K.
ReceiVed June 27, 2000 Zeolite structures containing transition metal dopants, such as titanium, chromium, iron, cobalt, and nickel have been studied intensively recently because of their marked catalytic activities that are derived from the presence of a redox-active site.1,2 A further topic of considerable current interest is the development of framework structures containing high levels of magnetically active species, such as the first-row transition metals, that might exhibit cooperative effects in combination with porosity. For species such as cobalt(II) that facilely adopt tetrahedral coordination to oxygen, it is clear that the CoO4 unit may be included as part of the framework; indeed, many zeotype structures and other frameworks are known that are based on the oxocobaltate tetrahedron.3,4 For nickel(II), tetrahedral environments are much less stable and few structures containing the NiO4 unit have been reported. In the majority of nickel-containing complex oxides the nickel site is six-coordinate though five-coordinate species also occur, for example, in KNiPO4, which is formed from edge-linked phosphate tetrahedra and distorted NiO5 square pyramids.5 In terms of structures built from tetrahedra alone only one compound, leucite type, has previously been described as containing the NiO4 unit,6 but this material only contains a low proportion of nickel in the framework tetrahedral sites, 17%. For zeolites, the levels of nickel dopant achieved are very low, although the nickel(II) is believed to reside in the framework as a tetrahedral ion as shown by EXAFS experiments.7 In this communication we report the first zeotype structure (ABW-type) based on high levels of nickelate units. The material was synthesized using a solid-state route and structurally characterized using high-resolution powder X-ray and neutron diffraction. A polycrystalline sample of RbNiPO4 was synthesized through reaction of a gel derived from an aqueous solution of the component ions at 750 °C.8 The material was structurally characterized by Rietveld analysis, using GSAS,9 of powder X-ray data10 and powder neutron data,11 using a model taken from * To whom correspondence should be addressed. Fax: (+44) 2380 593592. E-mail:
[email protected]. (1) Walker, G. G.; Lapszewicz, J. A.; Foulds, G. A. Catal. Today 1994, 21, 519. (2) Lin, S. S.; Weng, H. S. Appl. Catal. 1993, A105, 289. (3) Chippindale, A. M.; Cowley, A. R.; Chen, J. S.; Gao, Q. M.; Xu, R. R. Acta Crystallogr. C 1999, 55, 845. (4) Feng, P. Y.; Bu, X. H.; Tolbert, S. H.; Stucky, G. D. J. Am. Chem. Soc. 1997, 119, 2497. (5) Fischer, P.; Lujan, M.; Kubel, F.; Schmid, H. Ferroelectrics 1994, 162, 37-44. (6) Bell, A. M. T.; Henderson, C. M. B. Acta Crystallogr. C 1996, 52, 21322139. (7) Thomas, J. M.; Xu, Y.; Catlow, C. R. A.; Couves, J. W. Chem. Mater. 1991, 3 (4), 667-672. (8) Stoichiometric quantities of Ni(NO3)2‚6H2O (Aldrich, 99.9%), (NH4)2HPO4 (Aldrich, 99%), and RbOH (Aldrich, 50 wt % solution in water) were dissolved in 2 M nitric acid. A total of 100 mL of ethanol was added to the solution and the pH raised by addition of conc. ammonia solution (0.880 ammonia) until precipitation occurred. The gel formed was then heated to dryness on a hot plate. Further decomposition of the ethanolic precursor was achieved by heating at 225 °C for 12 h. The resulting powder was ground thoroughly and annealed at 500 °C and then at 750 °C, each for 12 h. Unreacted starting material was then removed by washing with deionized water. A final 12 h anneal at 750 °C produced the end product.
Figure 1. RbNiPO4 structure viewed along the main channels (down [010]). For clarity, the structural data extracted with a single oxygen position along each Ni-O-P direction and large anisotropic thermal parameters (90% ellipsoids depicted) are shown.
CsZnPO4 polymorph II.12 UV-visible spectra were collected on a Perkin-Elmer Lambda19 spectrometer in the wavelength range 200-800 nm. The Kebulka-Munk correction was applied to the data. The structure essentially consists of an array of alternating regular phosphate tetrahedra and very strongly distorted nickelate tetrahedra in the ABW-type morphology (Figure 1).13 Rubidium ions occupying sites in the main eight-membered ring channels coordinate to framework oxygen atoms at distances between 2.78 and 2.97 Å. Static disorder, which is a common feature of the ABW structure,4 requires the structure to be refined with three of the oxygen atoms split over pairs of neighboring sites. It is possible to model the diffraction profile data using a simple ABW model and a single oxygen position for each phosphate-nickelate link, but it requires the use of very large anisotropic temperature (9) Larson, A. C.; Von Dreele, R. B. Generalised Structure Analysis System; Los Alamos National Laboratory: Los Alamos, NM, 1994. (10) Powder X-ray data: space group Pc21/n, a ) 9.286 Å, b ) 5.082 Å, c ) 8.935 Å, V ) 421.66 Å3, Z ) 4. The diffraction data were obtained on a Siemens D5000 diffractometer with graphite monochromated Cu KR1 radiation (λ ) 1.540 56 Å). Preliminary structure refinement of these data using the GSAS program9 showed that the data were compatible with an ABW-type structure but were very insensitive to the oxygen positions because of the dominance of the X-ray scattering by rubidium. (11) Powder neutron diffraction data: space group Pc21/n, a ) 9.28673(5) Å, b ) 5.08216(2) Å, c ) 8.93493(4) Å, V ) 421.698(7) Å3, Z ) 4, Rwp ) 5.14%, χ2 ) 9.28. Data were collected on the HRPD diffractometer, 1 m sample position, at the ISIS Spallation neutron source, Rutherford Appleton Laboratory, at room temperature over a period of 16 h. Rietveld refinement was carried out on the backscattering 168° detector bank. The structural model for the refinement was taken from CsZnPO4 polymorph II.12 A reasonable fit to the data was obtained using these positions, indicating the correct structure type, although refinement of three of the oxygen atom thermal factors, particularly in one anisotropic direction, produced high values indicative of some disorder in these atomic positions. The structural model was improved by splitting three oxygen sites into close pairs; refinement of the occupancy values for these site pairs produced a 2:1 site fraction, which is beyond the scope of this communication and will be described in detail elsewhere.15. (12) Blum, D.; Durif, A.; Averbuch-Pouchot, M. T. Ferroelectrics 1986, 69, 283. (13) Meier, W. M.; Olsen, D. H.; Baerlocher, Ch. Zeolites 1996, 17, 16-17.
10.1021/ic000712q CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000
Communications factors, perpendicular to the P-O-Ni direction, indicative of the structural disorder. For clarity the structure representation shown in Figure 1 uses the single central position for the oxygen atoms obtained using the model with large anisotropic temperature factors rather than the two refined sites that are approximately 0.4 Å from either side of this position for O2, O3, and O4.15 The Ni-O distances are refined to produce a distorted NiO4 tetrahedron with Ni-O distances in the range 1.92-1.98 Å with a 〈Ni-O〉 of 1.956 Å. These values are similar to but slightly larger than those found for this tetrahedral unit in leucite6 (〈Ni-O〉 ) 1.885 Å), but the refined nickel-oxygen distances in Cs2NiSi5O12 are rather shorter than those expected for a bond valence sum of +2 for nickel (Ni-O ) 1.910 Å). In addition, consideration of the RbNiPO4 structure shows that the distortion of the NiO4 tetrahedron, with the large O2-Ni-O4 angle of 149° and the configuration of these units relative to the phosphate groups, allows for the formation of an additional Ni-O4 interaction at around 2.3 Å. The number of such interactions will depend on the local occupancy of the various oxygen sites, but it is clear that such additional weak coordination to the nickel site can occur. It is not surprising that the structure that RbNiPO4 adopts allows an effective increase in the nickel coordination while formally belonging to the ABW class of compounds of the type MTT′O4. Ni2+ has a strong preference for five and six coordination to oxygen in complex oxides and is underbonded in the NiO4 tetrahedron. The Ni-O distances in RbNiPO4 are only slightly shorter than those derived for the NiO4 unit in nickel-doped zeolite phases, (14) Henry, P. F.; Hughes, E. M.; Weller, M. T. J. Chem. Soc., Dalton Trans. 2000, 555. (15) Henry, P. F.; Hughes, R. W.; Ward, S. C.; Weller, M. T. Manuscript in preparation.
Inorganic Chemistry, Vol. 39, No. 24, 2000 5421 where values of about 1.98 Å were found from EXAFS studies of the nickel atom environment.7 This value is in line with that expected for the late transition metals in tetrahedral coordination, e.g., CsCoPO4,14 where Co-O ) 1.917-1.953 Å, and CsZnPO4,12 where Zn-O ) 1.924-1.944 Å in ABW-type structures. We have also prepared CsNiPO4, which likewise adopts the ABW-type framework. However, distortions similar to those seen in RbNiPO4 are apparent in the structure refinement of powder X-ray and neutron diffraction data. The intense purple-blue color of the zeotype RbNiPO4 is also consistent with the presence of a distorted tetrahedral NiO4 unit. The absorption spectrum shows a broad envelope constructed from three transitions over the range 450-680 nm; tetrahedral nickel complexes normally demonstrate a strong absorption around 600650 nm attributable to a 3T1(F) to 3T1(P) transition with shoulders that can be assigned to spin-forbidden bands. The strong distortion of the nickel-oxygen tetrahedron coupled with the possible approach of a more distant additional oxygen producing a coordination geometry for some nickel sites closer to 5-fold would give such a complex envelope centered at 580 nm. Acknowledgment. This work was supported by the EPSRC (GR/K20955), and we thank R. M. Ibberson of the Rutherford Appleton Laboratories for assistance with the collection of the powder neutron diffraction data. Supporting Information Available: Atomic positional data obtained from the refinement of the neutron diffraction profile, key derived bond distances/angles, the final neutron Rietveld profile fit, and the UV/visible spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.
IC000712Q
