Wadsley phases

These oxides contain d and d transition metal ions with a high formal valence state which are linked to oxygens by strong covalent bonds. Their framework is depicted as an array of distorted tetrahedra, pentahedra, or octahedra as in the Wadsley phases" belonging to the (V-0), (Mo-0), (V-Mo-0), (W-0) systems (44,46). 

An isolated cs plane or a random array of cs planes, known as the Wadsley defect, still gives rise to nonstoichiometry. Regularly recurring cs planes result in a homologous series of stoichiometric intermediate phases. Occurrence of such equidistant cs planes in a crystal indicates a cooperative mechanism. The formula of a cs phase depends on the cs plane index as well as the width of the parent slab between cs planes. If MO is the formula of the parent line phase, the formula of the homologous series of oxides resulting from cs may be represented as where n is the width (number of... 

It must be pointed out that defects in block structures giving rise to nonstoichiometry are possible. Both Wadsley defects and point defects are known in block structure phases. Nonstoichiometry is introduced by the insertion of rows or columns... 

When the defect interaction energy is much larger than the thermal energy, it can lead to an ordering of defects into superlattice structures and to the appearance of phases having ordered arrays of defects. Other interactions may also become important as the spin-dependent interactions between the d electrons in the Fj gS system (24). We shall not consider these order-disorder phenomena, since they are discussed by Wadsley (30). Some of the structural-consequences of ordering are considered below. 

The Binary Titanium Sulphides.—The titanium-sulphur system between the composition limits of TiS and TiS2 has long been regarded as a classical point-defect type of non-stoicheiometric phase system. The situation is described in standard texts, such as that of Wells, °° or the review by Wadsley, " and we will survey this historical... 

IR-11.6.1 Introduction Several special problems of nomenclature for non-stoichiometric phases have arisen with the improvements in the precision with which their structures can be determined. Thus, there are references to homologous series, non-commensurate and semi-commensurate structures, Vernier structures, crystallographic shear phases, Wadsley defects, chemical twinned phases, infinitely adaptive phases and modulated structures. Many of the phases that fall into these classes have no observable composition ranges although they have complex structures and formulae an example is Mo17047. These phases, despite their complex formulae, are essentially stoichiometric and possession of a complex formula must not be taken as an indication of a non-stoichiometric compound (cf. Section IR-11.1.2). 

It is already dear that the equilibrium behavior of materials having gross defects is not predicted by theories which are a simple extension of the classical point defect models. This is because strong interactions between defects in parent structures yield extended defects that become regular structural features of textured intermediate phases, which when ordered leave very low concentrations of point defects. The most studied of these structural features is the Wadsley defect resulting from crystallographic shear in typical... 

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