Crystallographic shear in molybdenum trioxide

Abstract

Thin specimens of molybdenum trioxide, after heating in the electron microscope, develop planar defects on {120} planes. The two beam theory of electron diffraction including absorption is used to interpret these defects as planes of discontinuity where neighbouring slabs of MoO 3 type structure are displaced relative to each other by the displacement vectors R = ±½ a +1/7 b . These ‘shear’ planes are chemical defects since oxygen atoms must be removed before the shear can occur. The density of shear planes is such that the non-stoichiometric range MoO 3- x (0 < x < 0.05) may be described as a disordered shear structure whereby the MoO 3 type matrix is interrupted by irregularly spaced shear planes whose density, area and orientation vary according to the exact composition of the specimen. The shear planes may be observed to nucleate and grow within domains of an intermediate phase. The crystal structure of the domains is studied using the displacement of bend extinction contours at the domain boundaries and dark field observations in addition to selected area diffraction. The composition MoO 2.9975 is estimated for the domain structure from the electron micrographs. A 7 x 7 MoO 3 superlattice of oxygen vacancies is deduced for the domain structure. These observations provide direct evidence of the nature, distribution and mechanism of production of the defects responsible for the non-stoichiometric range MoO 3- x (0 < x < 0.05). On the basis of our observations models for the thermal decomposition of molybdenum trioxide and for the production of the disordered shear structure are discussed. The latter is compared with three conflicting models recently proposed in the literature, without direct experimental evidence, for the mechanism of production of ordered and disordered shear structures in non-stoichiometric transition metal oxides.