Modeling the Effective Size of Vacancies in Aliovalently Doped Perovskites

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Student Presentation

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Rick Ubic


Point defects like vacancies can have a profound effect on the structure of perovskite ceramics, but the exact mechanisms involved are unclear. While a few theoretical models exist for some perovskites, none are particularly accurate or suited to the impure or doped ceramics used in commercial devices. A new empirical approach is presented here. A predictive model for the pseudocubic lattice constant of such perovskites based solely on published ionic radii data has been developed. This model shows that vacancies are not zero-dimensional defects in these materials but have an effective size due to both bond relaxation and mutual repulsion of coordinating oxygen ions. For some compositions this size can be effectively smaller than the host cations – even negative - corresponding to the relaxation of coordinating oxygen ions towards the vacant site. For other compositions the size is larger due to electrostatic repulsion of oxygen ions. An empirical model has been derived for this behavior, according to which an intermediate composition has been engineered such that the effective vacancy size is independent of vacancy concentration. These models more consistently predict both pseudocubic lattice constant and cell volume for [(Ca,Sr,Pb)1-3xLn2x]TiO3 (Ln = Y, La, Ce, Nd) perovskites than existing methods.

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