Schottky Defects – Lead(IV) Oxide Fluorite

Schottky defects occur when a pair of oppositely charged ions leave the lattice structure. In the case of lead(IV) oxide, lead has a chage of +4 and the oxide ions have a charge of 2-, this means to balance charge for every 1 lead ion that is lost two oxide ions must also leave. This results in the overall density of the lattice being reduced. Due to the now missing counter ion, the surrounding ions move away from each other resulting in an increase in volume.

The departing ions leave vacant sites behind (represented by the grey and red areas on the 2D diagram) and as the lower button labelled ‘Show Conduction‘ demonstrates, this can lead to ions being able to move through the structure. This is especially important in the development of solid electrolytes such as LISICON and NAISCON.

Schottky defects can also occur in rock salt type structures and this again results in ionic conductivity. The rutile structure of lead(IV) oxide, used in lead acid batteries, also exhibits vacancy defects.

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A. R. West, Solid State Chemistry, John Wiley & Sons Ltd, 2014.