A TYC Soirée on Multiferroic Materials

Thursday 19th April
Time: 4pm
Venue: Lecture Room G20, Royal School of Mines, Imperial College London
Contact: Ms Hafiza Bibi
Tel: 02075947252

Designing Improper Ferroelectricity the proper way - from group theory and first principles calculations 

Dr Nicholas Bristowe, University of Kent, UK

The classification of distortions in functional materials is an important part of the process of understanding their structure-property relationship. Perovskites (ABX3) are among the most studied systems, which is in part due to the many functional properties that they exhibit, but also due to their richness in structural distortions and phase transitions. More formally, the degrees of freedom in an aristotype “parent” structure, may be defined as transforming as irreducible representations (irreps) of the parent space group (and setting). One particularly valuable aspect of classifying these distortions in the formal language of irreps is to understand physical phenomena that can arise due to secondary order parameters which feature at linear order in the Landau-style free energy potential. This process is particularly valuable when understanding improper ferroelectricity where third order terms in the free energy expansion are invariably the key to understanding the resulting polarisation. We seek here to generalise a recipe for designing magnetoelectric multiferroics in perovskites. These recipes are based on symmetry arguments alone, but illustrated and quantified through first principles calculations, and we use as the ingredients both structural and magnetic degrees of freedom, which we classify in terms of transforming as irreps of the parent space group.  


New functionalities from gradient couplings: Flexoelectricity and more

Dr Massimiliano Stengel, Institut de Ciencia de Materials de Barcelona (CSIC), Spain

Flexoelectricity, the coupling between an inhomogeneous deformation and the electrical polarization, has emerged a "hot" topic in modern materials science due to its cross-cutting relevance to many phenomena of fundamental and technological interest. Understanding the intriguing physics that governs its behaviour at the nanoscale is crucial to harnessing the potential of strain gradients in practical applications, and such a progress requires a substantial support from theory. Due to the inherent breakdown of lattice periodicity that a strain gradient entails, however, first-principles calculations of flexoelectricity remain technically challenging at several levels. In this talk, I will discuss the recent methodological developments that have made such calculations feasible, and their application to realistic materials systems. In particular, I will focus on manifestations of the flexoelectric effect in SrTiO3, either at the level of the bulk crystal, surfaces or ferroelastic twin boundaries. More generally, I will emphasize the opportunities that spatial gradients (e.g. a nonuniform polarization, strain or magnetization field) offer for materials design, by enabling functionalities that would be otherwise forbidden in a uniform crystalline phase.


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