Piezoelectric materials convert mechanical stress into electrical charges and conversely. They are key functional materials which are pivotal to “smart” technologies including biomedical devices, ultrasensitive sensing, sonars and equipment for scientific research such as the atomic force microscope (AFM). For decades, the most widely used piezoelectric has been a solid solution between PbTiO3 and PbZrO3 perovskites, commonly called PZT. However, due to the toxicity of lead, the use of PZT-based materials is more and more restricted in many countries. Despite intense investigations carried out to find lead-free alternatives [1][2], the piezoelectric performances of PZT remain unsurpassed.

Recently, it has been shown that Ba(Ti0.8Zr0.2)O3-(Ba0.7Ca0.3)TiO3 solid solution (in short BCTZ) is a very promising material as it exhibits a very large piezoelectric response [3]. It has been recognized that the phase diagram and instabilities of BCTZ and thus the origin of the outstanding features of BCTZ are different from those of PZT. In particular, the role of oxygen polyhedral rotations on the mechanism of polarization appears important but remains hardly understood. In-depth investigations are expected to provide a better understanding of how to optimize the electromechanical response of BCTZ and related compounds and more generally guide us toward the design of new optimized lead-free piezoelectrics.

The objective of this PhD project is to achieve better understanding of the microscopic mechanisms able to produce a large electromechanical response in BCTZ and related materials, in order to guide the rational design of new lead-free piezoelectric compounds.

The experimental work will focus on growth and characterization of large, cm-size single crystals (see figure) in the BaZrO3-BaTiO3-CaTiO3 system [4]. In particular, challenging rich-BaZrO3 and rich-CaTiO3 compositions of the phase diagram will be explored through their single crystal growth. This approach promises a crucial advance compared to previous investigations on polycrystalline ceramics, where the intrinsic properties remain often obscured by grain boundaries and secondary phases. Moreover, large-sized single crystal will allow their structural characterization by synchrotron, neutron and light scattering techniques.  The investigation of crystals cut along desired crystallographic axes will allow identifying axis of high piezoelectric response. Improving the understanding of both the structural and physical properties will provide the necessary input for advanced modeling of these single crystals. (interaction with PhD project 2015-07-MC).

We are looking for motivated applicants appreciating scientific challenges, with excellent and appropriate skills in the fields of crystal growth, solid-state physics, materials science and physical chemistry.

[1] Properties of epitaxial films made of relaxor ferroelectrics. S. Prosandeev, D. Wang and L. Bellaiche, Phys. Rev. Lett. 111, 247602 (2013).
[2] Thermotropic phase boundaries in classic ferroelectrics. T.T.A. Lummen et al., Nature Comm. 5, 3172 (2014).
[3] Large Piezoelectric Effect in Pb-Free Ceramics. W. Liu and X. Ren, Phys. Rev. Lett. 103, 257602 (2009)
[4] Continuous cross-over from ferroelectric to relaxor state and piezoelectric properties of  BaTiO3-BaZrO3-CaTiO3 single crystals, F. Benabdallah, P. Veber, M. Prakasam, O. Viraphong, K. Shimamura , M. Maglione, J. Appl. Phys. 115, 144102 (2014)

photo: BCTZ single crystal grown at ICMCB Bordeaux

Project Partners

Institute for Condensed Matter Chemistry Bordeaux (ICMCB), France

growth of single crystals of BCTZ using top-seeded solution growth. Growth of targeted compositions will be followed by cutting of single crystals along suitable crystallographic directions; and by chemical, morphological and crystallographic characterization. Investigation of dielectric, piezoelectric and pyroelectric properties

Luxembourg Institute for Science and Technology

advanced characterization of the local and average crystal structure, determination of the phase diagram by using Raman spectroscopy, X-ray diffraction, diffuse scattering and absorption using synchrotron radiation. Pair distribution functions will be investigated at the ILL neutron source, for understanding the local structure namely in terms of octahedral tilts

Industry Partner: FEE GmbH, Germany

Forschungsinstitut für mineralische und metallische Werkstoffe-Edelsteine/Edelmetalle where the crystal growth of large sized single crystals will be carried out under industry conditions, as well as their orientation and cutting.

Host Country (employment): Luxembourg

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