Pushing the Limits of Powder Diffraction: Determination of the Crystal and Local Structures of Highly Complex Perovskite Materials
The amount of structural information that can be obtained from powder diffraction measurements has increased tremendously in recent years. This is a result of both the rapid improvement in the quality of data obtainable as well as advances in the methods used to analyze this data. This talk will illustrate what has become possible by giving examples where cutting edge methodologies, such as algorithms for ab-initio structure solution and local structure analysis using the Pair Distribution Function (PDF) method, have been used to gain important structural insights into perovskite materials which have multiferroic and clean energy applications. The first part of this talk will discuss the structure solutions of several A3BX6 double perovskite compounds which display unusual patterns of non-cooperative octahedral tilting.
These exceptionally complex structures, some of which are polar, could only be solved using a combination of electron, X-ray, and neutron powder diffraction. This talk will also discuss the structures of a series of AA'BB'O6 perovskites which have the unusual combination of layered ordering of the A-site cations and rock-salt ordering of the B-site cations. Some of these compounds are found to crystallize in the polar P21 space group, which in combination with their complex magnetic structures makes these potential multiferroic materials. Other AA'BB'O6 perovskites form complex nano-scale superstructures as a result of compositional modulation of the A-site cation occupancies in combination with twinning of the octahedral tilt systems.
Lastly, Reverse Monte Carlo (RMC) modeling of PDF data has been used to model the local structures of several perovskite phases which have simple average structures but much more complex local structures. Some of these compounds have multiple cations on the B-sites whose individual coordination environments cannot be ascertained from the average structure but can be resolved by RMC modeling of the PDF data. Other oxygen deficient phases have been studied which are known to be ionic conductors with possible application in solid oxide fuel cells. In these compounds the vacancies induce very large local distortions which produce local structures which are drastically different from the average structures. Determination of these local structures reveals the true ionic conduction pathways in these materials.