Valence induction

The average valance of the Ti component can be pushed toward 4.0 by valence induction, simply by increasing the amount of Li present relative to that of titanium, to give a formula Li1+xTi2 x04. This is equivalent to acceptor doping of the phase. The additional Li+ ions enter the octahedral sites, as the tetrahedral sites are already occupied, to a maximum of Li [Li (nTi nJCL and the acceptor doping is balanced by holes. In order to write formal defect equations for this situation, it is convenient... 

Aliovalent additives are often called donor dopants, when they tend to provide electrons and enhance intrinsic n-type semiconducting behavior, or acceptor dopants, when they tend to give a population of mobile holes and enhance /j-typc semiconducting behavior. The process of creating electronic defects in a crystal in this way is called valence induction. 

This process of creating electronic defects is called valence induction, and it increases the conductivity range of NiO tremendously. Indeed, at high Li concentrations, the conductivity approaches that of a metal (although it still exhibits semiconductor behaviour in that its conductivity increases with temperature). 

Valence inductivity, and the manner in which it is affected by catalyst preparation variables, is obviously a factor which must be considered in all laboratories which utilize supported oxide catalysts. 

This phenomenon, called valence inductivity, appears to be of fairly general occurrence among transition group ions of appropriate radius and charge. Examples of the effect in supported oxides of nickel and of iron will be described later. The effect is of obvious interest in connection with supported and promoted catalysts. 

The key to valence induction is that the cation of the host structure can take two valence states, one of which is regarded as the host cation plus a weakly bound hole or electron. 

The perovskites BaCeOj and BaZrOj are insulating solids when prepared in air. These are converted to oxygen-deficient phases by doping the B-sites with fixed valence trivalent ions, such as In ". Charge balance is not possible by valence induction and instead is achieved by the introduction of a population of oxygen vacancies. The presence of two substituents is then balanced by the formation of one oxygen vacancy. The formulae of the products are BaCej MPj or BaZrj M 03 2> example, with defect populations of 2In VJJ. 

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