Polarization Decorrelation Regions in Perovskite Relaxors

Relaxors are very exciting, rather than relaxing, materials. In fact, since their discovery in Russia in the 60s, researchers all over the world have struggled to understand how relaxor behaviour originates in materials at the atomic scale, in order to be able to control this phenomenon. But what is relaxor behaviour? Imagine to have a solid in your hand that changes colour sharply when heated up at, say, 37C. Relaxors would rather change colour gradually over a certain temperature range across 37C. Most strikingly, the temperature range of colour change would shift depending on the speed with which you shake your hand. Now, if instead of colour change we take dielectric permittivity (the ability of a material to become polarised under an electric field) and we consider as speed the frequency of an applied alternate electric field, the definition of relaxor is complete. One important class of relaxor materials is based on BaTiO 3 perovskites. It is generally understood that in these materials relaxor behaviour occurs when the titanium atoms are partly replaced by different atoms. In pure BaTiO 3 and at room temperature, Ti is slightly displaced from the centre of the unit cell. This is at the basis of electrical polarisation in these materials, which leads to effects like ferroelectricity and piezoelectricity (the material deforms when subjected to an electric field). Relaxor behaviour originates when electrical polarisation is disrupted by replacing Ti with another atom. The mechanism by which this happens is however still unclear. In the POLDERs project, we are going to uncover these aspects. Our previous research shows that there is a difference if the atom substituting Ti has the same charge or not. In particular, this should have an influence on the size and polarity of the regions disrupting electrical polarisation, which we call POLDERs (POLarization DEcorrelation Regions). Similar to atomic scale LEGO blocks, in BaTiO 3 we will replace Ti, at the same time, with foreign atoms with different characteristics (such as Zr or Nb, with same or higher charge than Ti, respectively), with the aim to induce and control POLDERs size. Measurements of the atomic scale structure, including atomic displacements and lattice vibrations, and of the macroscopic dielectric properties coupled with multiscale modelling will allow us to discover the influence of POLDERs on the origin of relaxor behaviour. This will provide access to finely tuning relaxor properties on purpose, which is a game-changer for numerous applications including microwave transducers and filters for telecommunications, and energy storage capacitors.

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