Wadsley defects

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... 

An isolated CS plane is referred to as a Wadsley defect and a random array of CS planes is considered to constitute planar (extended) defects which are entirely different from point defects. It is obvious that when CS planes occur at regular intervals, the composition of the crystal is stoichiometric, whereas a random array of CS planes results in nonstoichiometric compositions. While we have invoked anion vacancies which are later annihilated in our description of CS plane formation, we must point out that vacancies are not essential precursors for the formation of CS planes. Accommodating anion-deficient nonstoichiometry through CS mechanism is a special feature restricted to d° metal oxides such as W03, Nb205 and TiOz which exhibit soft phonon modes. Soft phonon modes in metal oxides arise from soft metal-orxygen potentials which permit large cation relaxation. The latter... 

Wadsley defects -> defects in solids Wagner Carl... 

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). 

Such shearing can occur randomly or in an ordered fashion. A random distribution of such shearing planes are call Wadsley defects. They give a continuous range of non-stochiometric compositions. 

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... 

VO2 and Ti02) leads to the formation of extended defects, i.e. CS planes, rather than to point defects. In the early literature reports, the definition of CS involved the removal of a complete sheet of anion sites to form an extended CS plane defect (Wadsley 1964, Anderson and Hyde 1967). Consequently, the role played by true point defects in non-stoichiometric oxides was not obvious from these earlier reports and answer to this was sought by Anderson (1970, 1971), Anderson et al (1973) (1.13.2). However, although this definition of CS is a convenient one, the situation is not so straightforward as we now demonstrate. 

Chapter 2 describes non-stoichiometric compounds derived from extended defects . Research in this field showed significant growth towards the end of the 1960s (although important papers by Professor A. Magneli and Professor A. D, Wadsley were published in the 1950s), initiated by the proposition of the shear structure model by Professor J. S. Anderson and... 

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... 

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