Breaking the aristotype: featurisation of polyhedral distortions in perovskite crystals

While traditional crystallographic representations of structure play an important role in materials science, they are unsuitable for efficient machine learning. A range of effective numerical descriptors have been developed for molecular and crystal structures. We are interested in a special case, where distortions emerge relative to an ideal high-symmetry parent structure. We demonstrate that irreducible representations form an efficient basis for the featurisation of polyhedral deformations with respect to such an aristotype. Applied to dataset of 552 octahedra in ABO3 perovskite-type materials, we use unsupervised machine learning with irreducible representation descriptors to identify four distinct classes of behaviour, associated with predominately corner, edge, face, and mixed connectivity between neighbouring octahedral units. Through this analysis, we identify SrCrO3 as a material with tuneable multiferroic behaviour. We further show, through supervised machine learning, that thermally activated structural distortions of CsPbI3 are well described by this approach.

Breaking the aristotype: featurisation of polyhedral distortions in perovskite crystals

While traditional crystallographic representations of structure play an important role in materials science, they are unsuitable for efficient machine learning. A range of effective numerical descriptors have been developed for molecular and crystal structures. We are interested in a special case, where distortions emerge relative to an ideal high-symmetry parent structure. We demonstrate that irreducible representations form an efficient basis for the featurisation of polyhedral deformations with respect to such an aristotype. Applied to dataset of 552 octahedra in ABO3 perovskite-type materials, we use unsupervised machine learning with irreducible representation descriptors to identify four distinct classes of behaviour, associated with predominately corner, edge, face, and mixed connectivity between neighbouring octahedral units. Through this analysis, we identify SrCrO3 as a material with tuneable multiferroic behaviour. We further show, through supervised machine learning, that thermally activated structural distortions of CsPbI3 are well described by this approach.

Electronic-structure evolution of SrFeO3-x during topotactic phase transformation

Oxygen-vacancy-induced topotactic phase transformation between the ABO2.5 brownmillerite structure and the ABO3 perovskite structure attracts ever-increasing attention due to the perspective applications in catalysis, clean energy field, and memristors. However, a detailed investigation of the electronic-structure evolution during the topotactic phase transformation for understanding the underlying mechanism is highly desired. In this work, multiple analytical methods were used to explore evolution of the electronic structure of SrFeO3-x thin films during the topotactic phase transformation. The results show that the increase in oxygen content produces a new unoccupied state of O 2p character near the Fermi energy, inducing the insulator-to-metal transition. More importantly, the hole states are more likely constrained to the dx2-y2 orbital than to the d3z2-r2 orbital. Our results reveal an unambiguous evolution of the electronic structure of SrFeO3-x films during topotactic phase transformation, which is crucial not only for fundamental understanding but also for perspective applications such as solid-state oxide fuel cells, catalysts, and memristor devices.

Tendencies in ABO3 Perovskite and SrF2, BaF2 and CaF2 Bulk and Surface F-Center Ab Initio Computations at High Symmetry Cubic Structure

We computed the atomic shift sizes of the closest adjacent atoms adjoining the (001) surface F-center at ABO3 perovskites. They are significantly larger than the atomic shift sizes of the closest adjacent atoms adjoining the bulk F-center. In the ABO3 perovskite matrixes, the electron charge is significantly stronger confined in the interior of the bulk oxygen vacancy than in the interior of the (001) surface oxygen vacancy. The formation energy of the oxygen vacancy on the (001) surface is smaller than in the bulk. This microscopic energy distinction stimulates the oxygen vacancy segregation from the perovskite bulk to their (001) surfaces. The (001) surface F-center created defect level is nearer to the (001) surface conduction band (CB) bottom as the bulk F-center created defect level. On the contrary, the SrF2, BaF2 and CaF2 bulk and surface F-center charge is almost perfectly confined to the interior of the fluorine vacancy. The shift sizes of atoms adjoining the bulk and surface F-centers in SrF2, CaF2 and BaF2 matrixes are microscopic as compared to the case of ABO3 perovskites.

Epitaxial growth of perovskite oxide films facilitated by oxygen vacancies

Single-crystal epitaxial films of technologically important and scientifically intriguing multifunctional ABO3 perovskite-type metal oxides are essential for advanced applications and understanding of these materials. In such films, a film-substrate misfit...

B-site Cation Inter-Diffusion in Yttrium Substituted Barium Zirconate

B-site cation inter-diffusion in the ABO3 perovskite yttrium-substituted barium zirconate (BZY) was studied at temperatures from 1100 to 1460 °C under reducing and oxidising conditions. The experiments followed two different...

Formation mechanism of Ruddlesden-Popper faults in compressive-strained ABO3 perovskite superlattices

Ruddlesden-Popper (RP) faults have emerged as a promising candidate for defect engineering in epitaxial ABO3 perovskites. Functionalities could be fine-tuned by incorporating RP faults into ABO3 thin films and superlattices....