A Computational Study of the Symmetry of an Aluminophosphate Microporous Material. Incorporation of Iron Defects in Aluminum Tetrahedral Sites Jorge Gulı ´n-Gonza ´ lez* ,† and Carlos de las Pozas del Rı ´o ‡ Dpto. de Fı ´sica, Instituto Superior Polite ´ cnico Jose ´ A. Echeverrı ´a (ISPJAE), Calle 127 s/n. Apartado 6028, Habana 6, Marianao, La Habana, Cuba, and Division of Chemistry (CNIC), National Center for Scientific Research, P.O. Box 6990, La Habana, Cuba Received February 16, 2001. Revised Manuscript Received December 18, 2001 A computational study using classical Gibbs free energy minimization techniques of the AlPO 4 -5 unit cell symmetry is presented. The Gibbs free energy calculations were performed at temperatures up to 600 K. It was found that the orthorhombic Pcc2 and the hexagonal P6 structures are energetically favored with respect to the P6cc structure at temperatures up to 400 K. At T ) 500 K, the hexagonal P6cc structure has a very slightly lower energy than that obtained for the P6 and Pcc2 structures. However, the analysis of the vibrational modes reveals the existence of imaginary eigenvalues, which indicates that the space group P6cc does not describe correctly the unit cell of calcined AlPO 4 -5. Moreover, the incorporation of iron defects in tetrahedral aluminum sites of the AlPO 4 -5 unit cell was studied by minimization techniques. It seems probable that iron as Fe 3+ incorporates in the aluminum tetrahedral sites. Subsequently, several configurations of two and three Fe 3+ ions in the AlPO 4 -5 unit cell were studied. The analysis of the most stable configurations highlights the influence of two factors in the stability of FAPO-5 structure: the interaction between the Fe 3+ ions and the rigidity of the AlPO 4 -5 unit cell in the c direction. The combination of these factors leads to a low stability of the structures with all of the Fe 3+ ions in the same side of the unit cell and with the ions very close. 1. Introduction and Scope The problem of the symmetry of the AlPO 4 -5 unit cell has been the subject of a variety of studies since the synthesis of this material in 1982 1 . The AlPO 4 -5 pos- sesses a one-dimensional 12 membered channel, which is surrounded by four and six rings (see Figure 1). The 12-membered channel permits the adsorption of large molecules (e.g., hydrocarbons) and its use as catalysts. Initially the as-synthesized structure was refined in a hexagonal space group P6cc. 2,3 This space group gives a good description of the channel structure, but one of the oxygen sites has a rather large temperature factor and the Al-O-P angles involving this oxygen are close to 180°. Bennet et al. 2 attribute this fact to static disorder of the oxygen which is structurally distributed about three equivalent sites. Richardson et al. 4 in a neutron diffraction study of the calcined sample report that a higher hexagonal symmetry of P6/mcc was required to obtain a satisfactory refinement, but this model considers a random alternation of the Al and P on the tetrahedral sites, which is contrary to what was found in the experiment. The NMR experiment 5 sug- gests a lower symmetry than that reported from initial crystallographic study (P6cc). Other structure determinations alluded the possible alternatives to space group P6cc. Ohnishi et al. 6 have reported a reversible phase transition from hexagonal to orthorhombic symmetry in the presence of the tropine. Mora et al. 7 using high-resolution X-ray and neutron powder diffraction have found that between room-temperature and 363 K the AlPO 4 -5 structure is best described with the orthorhombic space group Pcc2. Recently, Klap et al. 8 have determined from single- crystal X-ray diffraction data that the crystal structure of AlPO 4 -5 is described as consisting of three types of microdomains, each exhibiting P6 symmetry. The do- mains are related by two sets of glide planes, and the average structure has P6cc symmetry. Many theoretical studies have suggested that the symmetry of AlPO 4 -5 is lower than P6cc. 9-11 The theoretical predictions of Henson et al. 10 confirm the * To whom correspondence should be addressed. E-mail: gulin@ electrica.ispjae.edu.cu. † Instituto Superior Polite ´cnico Jose ´ A. Echeverrı ´a (ISPJAE). ‡ National Center for Scientific Research. (1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (2) Bennett, J.; Cohen, J. P.; Flanigen, E. M.; Pluth, J. J.; Smith, J. V. Intrazeolite Chemistry; American Chemical Society: Washington, DC, 1983. (3) Qiu, S.; Pang, Q.; Kessler, H.; Guth, J. L. Zeolites 1989, 8, 440. (4) Richardson, J. W.; Pluth, J.; Smith, J. V. Acta Crystallogr. C. 1987, 43, 1469. (5) Peeters, M. P.; van de Ven, L.; de Haan, J. W.; van Hooff, J. H. J. Phys. Chem. 1993, 97, 9254. (6) Ohnishi, N.; Qiu, S.; Terasaki, O.; Kajitani, T.; Hiraga, K. Microporous Mater. 1992, 2, 73. (7) Mora, A. J.; Fitch, A. N.; Cole, M.; Goyal, R.; Jones, R. H.; Jobic, H.; Carr, S. W. J. Mater. Chem. 1996, 6, 1831. (8) Klap, G. J.; van Koningsveld; Graafsma, H.; Schreurs, A. M. M. Microporous Mesoporous Mater. 2000, 38, 403. (9) de Man, A. J. M.; Jacobs, W. P. J. H.; Gilson, J. P. W.; van Santen, R. A. Zeolites 1992, 12, 826. (10) Henson, N. J.; Cheetham, A. K.; Gale, J. D. Chem. Mater. 1996, 8, 664. 2817 Chem. Mater. 2002, 14, 2817-2825 10.1021/cm0101428 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/25/2002