Molybdenum layered structure

Molybdenum disulfide has a layered structure. Each layer is a sandwich consisting of between two layers of ions (Fig. 9.5). The sulfur ions form trigonal prisms and half of the prisms contain a molybdenum ion in the middle. The chemical reactivity of M0S2 is associated with the edges of the sandwich, whereas the basal planes are much less reactive. The edges form the sites where gases adsorb and where the catalytic activity resides. 

Molybdenum trioxide has a layered structure with orthorhombic symetry [16] (a=3.963, b=ll855, c=3.696 A), this structure consists of double layer sheets parallel to the (010) cleavage plane. The building unit is a distorted M0O6 octahedron, with Mo-0 distances 1.67, 1.73, 1.95 (twice), 2.25 and 2.33 A (Fig.l)... 

Molybdenum sulfide (M0S2) has an unusual layer structure in which sulfide atoms in a given upper layer sandwiching the Mo atoms are located directly above the S atoms in the lower layer, but alternate MoS2 layers are offset as in hep. Thus, the structure is A(Mo)A, B(Mo)B, A(Mo)A, B(Mo)B There is little cohesion between successive M0S2 sheets, re-... 

Fig. 8. Crystal structure of M0S2. (a) Side view of a single-layer S-Mo S slab of M0S2. The molybdenum atoms (dark) are coordinated to six sulfur atoms (bright) in a trigonal prismatic coordination, (b) Within each layer, the sulfur lattice (and the molybdenum lattice) are hexagonal ly arranged with inplane interatomic distances tZs s or i/m., no equal to 3.15A. (c) Illustration of the 2II-M0S2 stacking sequence of successive layers in bulk M0S2. The distance between the molybdenum layers is 6.15 A.

X-Ray diffraction showed that the molybdenum disulfide powder used in this experiment has a hexagonal layer structure. A remarkable feature of such layer compounds is that the powder is composed of small single crystal particles. In view of these facts, an interesting question arises as to whether... 

Considerable progress has been made in studying tribofilms in the last decade. A number of important surface and thin film techniques have been developed in recent years, which are contributing to a better understanding of at least some tribochemical processes of boundary lubrication. In recent years, X-ray absorption near edge structure XANES spectroscopy, a powerful technique for tribofilm characterization, was used to identify a layered structure (surface and bulk) of tribofilms. The chemistry of tribofilms generated by the combination of zinc dialkyldithiophosphate (ZDDP) and molybdenum dialkyldithiocarbamate (MoDTC) has been examined. 

The postulated existence of the oxysulfide MoOS can be ruled unlikely on the basis of the structures of Mo02 und MoS2. Molybdenum dioxide has a three-dimensional structure with distorted octahedral coordination (d2sp3 hybridization) about the molybdenum atoms12while MoS2 has a layer structure with a trigonal prismatic coordination about the molybdenum atoms (d4sp hydridization). Consequently, any existence of a MoOS species may be expected to be only transient. 

Molybdenum trioxide is a white solid at room temperature but becomes yellow when hot and melts at 795°C to a deep yellow liquid. It is the anhydride of molybdic acid, but it does not form hydrates directly, although these are known (see later). One of its two polymorphs, the stable a-form, has a rare type of layer structure in which each molybdenum atom is surrounded by a distorted octahedron of oxygen atoms. 

However, pure niobium disulphide has a hexagonal structure in which the metal atoms in each layer are located directly above or below those in adjacent layers. Jamison showed that with this configuration niobium disulphide is not a good lubricant. He postulated that when niobium disulphide behaves as a good lubricant, additionai niobium atoms intercalated into the structure will have resulted in a change in eiectron bonding to favour the molybdenum disulphide structure. 

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