Vacancy ordering and electronic structure of γ-Fe2O3

J. MARTÍNEZ; R. C. MERCADER; E. L. PELTZER Y BLANCÁ

Workshop; V Workshop on novel methods for electronic structure calculations; 2013

It is widely accepted that ferrimagnetic γ-Fe2O3 has a spinel structure in which 1/6 of the 16 octahedral iron sites are vacancies. Although there is consensus about this, maghemite structure seems to have three possible unit cells: two cubic ones, Fd-3m and P4132, and a tetrahedral one, P43212. Several experimental studies point to the three different space groups (227, 213 and 96 respectively) and it seems that the structure depends on two factors: the preparation method and the crystal size. However, the ordering of vacancies in any of those structures is still an unsolved issue. Currently, this point is quite difficult to tackle with computational modeling because supercells with at least three primitive cells are needed to represent an integer number of vacancies, which implies a big number of atoms for calculation. Moreover, the study of all possible configurations of vacancies implies an extremely large number of calculations to be performed. To our knowledge, at present only one work has been done for the space group P4132 using ab-initio calculations to determine the more stable configuration of vacancies. In this work, toward studying the magnetic response of maghemite nanoparticles to be used in the Fischer Tropsch synthesis, we have used the Fd-3m structure to study the vacancy ordering, analyzing all non-equivalent configurations. We have chosen to describe the systems by density functional theory, using a well-established combination of plane waves and pseudo-potentials with the LDA+U method. To simplify the problem, we have used the triclinic primitive cell, which contains ¼ of atoms from the unit cell; we built a 1×1×3 supercell to obtain an integer number of vacancies. Using the SOD (Site Occupancy Disorder) software we determined all the 22 possible and non-equivalent configurations for vacancies.  references 1.      Pecharromán C., González Carreno T., Iglesias J.E., ?The Infrared Dielectric Properties of Maghemite, g-Fe2O3, from Reflectance Measurement on Pressed Powders?; Phys. Cehm. Minerals 22 (1995) 21-29. 2.      Jørgensen J.E., Mosegaard L., Thomsen L.E., Jensen T.R., and Hanson J.C., ?Formation of g-Fe2O3 nanoparticles and vacancy ordering: An in situ X-ray powder diffraction study?; J. Solid State Chem. 180 (2007) 180-185. 3.      Braun P.B., "A structure in Spinels"; Nature, 170 (1952) 1123. 4.      R. Grau-Crespo, S. Hamad, C.R.A. Catlow and N. H. de Leeuw, ?Symmetry-adapted configurational modelling of fractional site occupancy in solids?; J. of Physics: Condensed Matter 19 (227), 256201. 5.      P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P. Seitsonen, A. Smogunov, P. Umari, and R. M. Wentzcovitch, ?Quantum Espresso: a modular and open-source software project for quantum simulations of materials?; J. of Physics: Cond. Matter 21, (2009) 395502.