Defining chemical expansion: the choice of units for the stoichiometric expansion coefficient

Chemical expansion refers to the spatial dilation of a material that occurs upon changes in its composition. When this dilation is caused by a gradual, iso-structural increase in the lattice parameter with composition, it is related to the composition change by the stoichiometric expansion coefficient. In this work, three different approaches to defining the stoichiometric expansion coefficient (alpha(S)) are discussed. While all three definitions of aS given here are legitimate, we show that there are advantages to selecting certain ones for comparison across different crystal structures. Examples are provided for changes in oxygen content in fluorite, perovskite, and Ruddlesden-Popper (K2NiF4) phase materials used in solid oxide fuel cells.