INSTITUTO DE FISICA LA PLATA
Unidad Ejecutora - UE
congresos y reuniones científicas
Vacancy ordering and electronic structure of gamma-Fe2O3
J. MARTÍNEZ; R. C. MERCADER; E. PELTZER Y BLANCÁ
Workshop; V Workshop on novel methods for electronic structure calculations; 2013
Facultad de Química - Universidad de la República
It is widely accepted that ferrimagnetic gamma-Fe2O3 has a spinel structure with 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 Fd-3m, P4132 and a tetrahedral 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 superscells 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 pseudopotentials with the LDA+U method. To simplify the problem, we 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 not equivalent configurations for vacancies.