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Geometric Analysis and Formability of the Cubic A2BX6 Vacancy Ordered Double Perovskite Structure

preprint
submitted on 02.07.2020 and posted on 03.07.2020 by Anjie Cheng, Chenyang Lyu, Tianyi Shi, Ziheng Wang, Robert Palgrave

A geometric analysis of the cubic A2BX6 structure commonly formed by metal halides is presented. Using the ‘hard sphere’ approximation, where the ions are represented by spheres of a fixed radius, we derive four limiting models that each constrain the distances between constituent ions in different ways. We compare the lattice parameters predicted by these four models with experimental data from the Inorganic Crystal Structure Database (ICSD). For the fluorides, the maintenance of the AX bond length at the sum of the A and X radii gives the best approximation of the lattice parameter, leading to structures with widely separated BX6 octahedra. For the heavier halides, a balance between forming an A site cavity of the correct size, and maintaining suitable anion-anion distances determines the lattice parameter. It is found that in many A2BX6 compounds of heavier halides, the neighbouring octahedra show very significant anion-anion overlap, meaning that the commonly used description of these materials of having isolated BX6 octahedra is misleading. We use the geometric models to derive formability criteria for vacancy ordered double perovskites.

History

Email Address of Submitting Author

r.palgrave@ucl.ac.uk

Institution

University College London

Country

United Kingdom

ORCID For Submitting Author

0000-0003-4522-2486

Declaration of Conflict of Interest

No conflict of interest

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