A computational study of the symmetry of an aluminophosphate microporous material.: Incorporation of iron defects in aluminum tetrahedral sites

A computational study using classical Gibbs free energy minimization techniques of the AlPO4-5 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 AlPO4-5. Moreover, the incorporation of iron defects in tetrahedral aluminum sites of the AlPO4-5 unit cell was studied by minimization techniques. It seems probable that iron as Fe3+ incorporates in the aluminum tetrahedral sites. Subsequently, several configurations of two and three Fe3+ ions in the AlPO4-5 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 Fe3+ ions and the rigidity of the AlPO4-5 unit cell in the c direction. The combination of these factors leads to a low stability of the structures with all of the Fe3+ ions in the same side of the unit cell and with the ions very close.