An ab initio LAPW study of the α and β phases of bulk molybdenum trioxide, MoO3
Introduction
Transition metal oxides are well known for their diverse structural, physical and chemical properties [1], [2], [3]. These materials exist in many crystallographic forms with stoichiometries differing only slightly from each other and transition metal ions exhibiting various oxidation states. Among these oxides, molybdenum oxides represent an important class of system which are widely studied and used in many technological applications. In particular, molybdenum trioxide MoO3 is an important heterogeneous catalyst [2], [4], [5], electrochromic component [6] and active material in secondary lithium batteries [7], [8], [9], [10], [11].
Molybdenum trioxide can exist in two crystalline polymorphs form, the thermodynamically stable orthorhombic α-MoO3 [12], and the metastable monoclinic β-MoO3 phase [13], [14], [15], [16]. α-MoO3 crystallizes with lattice constants a = 13.855 Å, b = 3.696 Å and c = 3.963 Å. It has a layer structure in which each layer is built up of MoO6 octahedron at two levels, connected along y-axis by common edges and corners, so as to form zigzag rows and along z-axis by common corners only (see Fig. 1). Moreover each layer exhibits, in the y-axis direction, oxygen atoms which are common for three different octahedron. Each octahedra also shares, along z-axis, two oxygen atoms with two neighboring octahedron. Besides, for each MoO6 octahedra there is only one oxygen atom which is doubly bounded to the molybdenum atom (MoO). Therefore, three kinds of structurally different lattice oxygens exist, i.e., terminal oxygen (singly coordinated, O1), asymmetric bridging oxygen (doubly coordinated, O2, ), and symmetric bridging oxygen (triply coordinated, O3, ). The MoO3 layers are parallel to the (1 0 0) crystal plane, such as, the inter layer interaction is weak, and hence the (1 0 0) plane is the most exposed and thermodynamically most stable, where only O atoms are exposed on the surface. β-MoO3 is similar to WO3 and is related to the three-dimensional ReO3 structure [13], which consists in corner-connected octahedra network, as shown in Fig. 2. It crystallizes with lattice constants a = 7.122 Å, b = 5.366 Å, c = 5.566 Å, and β = 92.01. The β → α transformation is both exothermic and photochromic, with yellow β-MoO3 converting to the white α phase above 400 °C at moderate heating rates [13]. The relatively high-transformation temperature implies that β-MoO3 ought to have a fair measure of kinetic stability at or near room temperature. Nevertheless, X-ray diffraction measurements have shown that α-MoO3 can be stabilized in the ReO3 structure (β-MoO3) by partially substituting molybdenum by tungsten [14].
Many applications of the molybdenum trioxide are prepared in thin film form [17]. A micro-Raman spectroscopy characterization [18] has shown that the films are almost composed by the β phase or (and) splashed species which consist of a mixture of the α and β phases. In addition, β phase has been a subject of deeper spectroscopic study [19]. In this study, the authors have shown that this photochromic material is promising in the areas of color displays and camouflage applications. Despite these applications and the more general importance of the β-MoO3, both experimental and theoretical studies are still lacking. In contrast, α-MoO3 has been the subject of many experimental [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] and theoretical [20], [21] studies with respect to their structural and electronic properties (Note that the amount of theoretical work still few compare to the experimental one). In order to fill this gap, we initiated the present study. In doing so, we aim in particular to investigate the forces involved in the bonding between metal and oxygen and to understand why the β structure is metastable.
In this paper, both MoO3 phases are studied using a periodical boundary condition ab initio LAPW method. In Section 2, we describe briefly computational details and Section 3 presents results and discussion. Finally, we summarize our conclusions in Section 4.
Section snippets
Computational details
The detailed geometries of both MoO3 phase are obtained, in the present calculations, from total energy optimization, where the experimental structure for the orthorhombic structure is used as a starting point for the α phase [12]. The monoclinic phase has much more degrees of freedom than the α phase, and hence requires much more computing power for the structural optimization. We have therefore, chosen as a model system, the pseudo cubic structure (defect perovskite or ReO3 like structure)
Geometric structure
The computed lattice constants for orthorhombic MoO3 are listed in Table 1, together with data from experimental analysis. A comparison shows that the calculated constants are larger than 4% in respect to the experiment. Error on the distances between molybdenum atoms and the different oxygen atoms is less than 0.03 Å compared to the experimental values. This confirms the good agreement of our theoretical structure with experiment. Results from the structure optimization of cubic MoO3 are also
Conclusion
This theoretical study provides a detailed picture of the geometric and electronic structure of both phase of the trioxide molybdenum. Geometry optimizations of the orthorhombic MoO3 based on LAPW calculations yield to lattice parameter values in agreement with experiment, the Mo–O bond lengths are very close to experimental values. Charge distribution results confirm the mixed ionic and covalent character of bonding in α-MoO3. The symmetrically bridging oxygens exhibit more ionic feature while
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