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Seminars

05.07.2012. :: Seminar Centra za fiziku cvrstog stanja i nove materijale - Emil Bozin


U cetvrtak 5.jula 2012, sa pocetkom u 12 casova u citaonici IF/a, odrzace se seminar pod naslovom

Detekcija dinamickih feroelektricnih nanodomena u gvozdje telurid termoelektriku iz merenja neutronskim rasejanjem

Dr Emil Bozin


Наслов и абстракт на енглеском су:

Detection of Dynamic Ferroelectric Nanodomains in Thermoelectric Lead Telluride by Neutron Scattering Probes

Local symmetry breaking in complex materials is emerging as an important contributor to materials properties but is inherently difficult to study. It was a surprise to find such a symmetry breaking in the therмoelectric (TE) material PbTe that has a simple rock-salt average crystal structure. Our neutron total scattering and atomic pair distribution function (PDF) analysis reveals the existence of a novel paraelectric state at and above room temperature [1]. However, on cooling the structural dipoles do not order, but disappear resulting in an undistorted rock-salt ground-state. I will discuss these PDF results and results of our follow-up study of the lattice dynamics using inelastic neutron scattering (INS) [2], which shows that the lattice dynamics are responsive to the local symmetry broken phase observed by PDF, giving not only key insights into the behavior of PbTe, but also revealing INS as a powerful tool for studying local structure. The new result of the INS study is the observation of the unexpected appearance on warming of a new zone center phonon branch in PbTe. In a harmonic solid the number of phonon branches is strictly determined by the contents and symmetry of the unit cell. The appearance of the new mode appears to confirm a crossover to a dynamic lower symmetry structure with increasing temperature while no structural transition is seen crystallographically. The observation resembles relaxor ferroelectricity, however, since there are no inhomogeneous dopants in pure PbTe, this anomalous behavior is an intrinsic response of the system. The results cannot be explained within the framework of conventional phase transition theories such as soft-mode theory and challenges our basic understanding of the physics of materials. We suggest that new thermoelectrics should be sought among materials that, like PbTe [1], are close to a ferroelectric instability.

[1] E.S. Bozin et aI., Science v330, pp1660 (2010).
[2] K.M.0. Jensen et aI., arXiv:1203.5943 (2012).
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