A-Site-Deficient Titanates, Niobates and Tantalates

A number of perovskite phases are able to support considerable populations of A-site vacancies provided that charge neutrality is maintained. The best known of these are the (3,4) perovskites La2 3Ti03, which has a vacancy population on 1/3 of 

Nul represents the perfect stracture with no defects present at all. The defects formed from this ideal state must be balanced in terms of charge and atom numbers as the equation shows. 

Additional La can also be accommodated in the stmcture to partially occupy the remaining A-site vacancies. In this case the substitution may be balanced by the creation of further TP or oxygen ion vacancies. For example, the phase Laj TiO has a nominal formula La5,gTi 53 Ti 02gg2- 

A number of species populate the A-sites to give ordered structures of the cubic AA B Oj2 type (Section 2.1.3). If the A or ions are replaced with cations of higher charge, the stmcture still persists, but now the charge excess is corrected by a vacancy population on the A-sites. For example, La CUjTi Ou, Bi CUjTl Ojj and Yj CUjTi Ojj all have 1/3 of the A-sites vacant, while in CUjTijTa Ojj, all of the A-sites are vacant. 

This latter phase can take in further Ta content to form the perovskite CUj Ta O,, in which not only are the A-sites vacant but also a considerable percentage of the A -sites. The composition varies widely, from Cu to Cu jTa Ojj j,. 

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