Periodic DFT Study of the Structural and Electronic Properties of Bulk CoAl 2 O 4 Spinel F. Tielens,* ,² M. Calatayud, ² R. Franco, ‡ J. M. Recio, ‡ J. Pe ´ rez-Ramı ´rez, § and C. Minot ² Laboratoire de Chimie The ´ orique, UniVersite ´ Pierre et Marie Curie, Paris VI, Case 137, 4 Place Jussieu, F-75252 Paris Cedex 05, France, Departamento de Quı ´mica, Fı ´sica y Analı ´tica, UniVersidad de OViedo, E-33006 OViedo, Spain, and Laboratory for Heterogeneous Catalysis, Catalan Institution of Research and AdVanced Studies (ICREA) and Institute of Chemical Research of Catalonia (ICIQ), AVenue Paı ¨sos Catalans s/n, E-43007 Tarragona, Spain ReceiVed: June 22, 2005; In Final Form: NoVember 4, 2005 In this study, structural and electronic properties of CoAl 2 O 4 spinel are investigated for the first time by means of quantum chemical computational tools. Coupling supercell periodic calculations under the density functional theory formalism with a nonempirical quasi-harmonic Debye model, we examine the influence of temperature on the relative stability of several cation distributions of Co 2+ and Al 3+ over tetrahedral and octahedral interstices of the oxygen sublattice. Our simulations are able to reproduce the experimentally observed trend: (i) the normal spinel is calculated to be the stable structure at static and low-temperature conditions, and (ii) as the temperature increases, the preference of structures with Al 3+ at tetrahedral sites (and Co 2+ at octahedral sites) is found to progress following an asymptotic conduct. The effects of the cation distributions on geometrical variations of electronic and magnetic properties of CoAl 2 O 4 can be interpreted as dominated by the local behavior of Co 2+ at octahedral sites. Introduction The spinels AB 2 X 4 constitute one of the most interesting and important families of crystalline compounds, with applications in many different areas as magnetic materials, ceramics, and catalysis. 1 In the stoichiometric formula AB 2 X 4 , equivalent to (AX)(B 2 X 3 ), the A indicates a divalent cation, B a trivalent cation, and X a divalent anion. Different compounds have this structure, mainly oxides and sulfides but also selenides and tellurides. 2-4 The A and B cations can occupy two different sites in the structure, octahedral (O h ) and tetrahedral (T d ). Studies of cation distributions in spinels are of considerable interest in solid-state chemistry for a better understanding of correlation between structure and properties such as color, diffusivity, magnetic behavior, conductivity, and catalytic activity etc., which are well-known to be dependent on the relative O h and T d occupancy by metals. 5-7 The cation distribution of A and B cations over tetrahedral and octahedral sites in spinels is affected by the combination and nature of the two cations and depends strongly on pressure and temperature. The cation distribution can be unequivocally characterized by the so-called degree of inversion x. This parameter is defined as the fraction of the divalent metal cations in octahedral sites In this formula, the parentheses and square brackets denote the tetrahedral and octahedral surroundings, respectively. Normal spinels have x ) 0, while inverse spinels (x ) 1) are found with divalent cations occupying octahedral sites and trivalent ones occupying tetrahedral and octahedral sites in equal proportion. When the degree of inversion is equal to 2/3, the spinel is called random since the di- and trivalent cations are randomly distributed among both coordinations, the number of trivalent cations being double than that of divalent cations in both tetrahedral and octahedral sites. Typical normal spinels at room conditions are MgAl 2 O 4 , FeAl 2 O 4 , ZnAl 2 O 4 , and FeCr 2 O 4 , and typical inverse spinels are Fe 3 O 4 , MgFe 2 O 4 , and MnFe 2 O 4 . 8 Although it is possible to find in nature and to synthesize spinels in a more or less wide range of cation distributions, forcing normal spinels to be inverse is sometimes experimentally difficult or even impossible. Temperature has been the variable most widely used in this respect. Usually, an increasing of temperature yields to a continuous increase of the degree of inversion parameter. Sometimes a sudden finite change of x appears in a narrow range of temperature that is associated with an order-disorder phase transition of a second-order type, the prototypical MgAl 2 O 4 spinel being one of the best examples showing this behavior. 9 In this paper, we focus on the CoAl 2 O 4 spinel. This spinel is also known as Thenard’s (Dresden) blue and has been exten- sively used since 1802 as pigment and more recently in various catalytic applications such as NO x reduction 10,11 and Fischer- Tropsch synthesis. 12,13 Despite the interest generated, many aspects regarding geometric and electronic structure of both the bulk and the surfaces are not well-understood or even unknown. At room temperature, CoAl 2 O 4 is an almost normal spinel 14 (i.e., most of Co 2+ ions are situated at the tetrahedral sites and most Al 3+ ions occupy octahedral sites in the slightly distorted cubic close packed oxygen sublattice). At high temperatures, part of the Co 2+ and the Al 3+ ions may interchange their positions. The temperature dependence of the cation distribution in CoAl 2 O 4 has been studied from X-ray experiments on quenched samples 14-16 and from in situ optical transmission measure- ments. 17,18 The main result is that the degree of inversion * To whom correspondence should be addressed. Phone: 33-1-44-27- 96-60; fax: 33-1-44-27-41-17; e-mail: tielens@lct.jussieu.fr. ² Universite ´ Pierre et Marie Curie. ‡ Universidad de Oviedo. § ICREA and ICIQ. (Co 1-x Al x )[Co x Al 2-x ]O 4 (1) 988 J. Phys. Chem. B 2006, 110, 988-995 10.1021/jp053375l CCC: $33.50 © 2006 American Chemical Society Published on Web 12/17/2005