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Seminarium Fizyki Politechniki Wrocławskiej

11:15 poniedziałek, 20-01-25
PWr, bud. A1, sala 322

A Journey Through Dielectric Relaxation in Hybrid Systems: Facts, Myths and New Concepts

dr Andrzej Nowok

Katedra Fizyki Doświadczalnej, Wydział Podstawowych Problemów Techniki, Politechnika Wrocławska

The interaction of electromagnetic (EM) radiation with hybrid organic-inorganic compounds gives rise to quantized transitions between the electronic, vibrational and rotational molecular energy states, as broadly described by photoluminescence, UV/visible, infrared or Raman studies. However, when one asks, ‘what happens when EM radiation in the frequency range of 10⁻⁶ to 10¹² Hz is applied to these systems’, the answer is often tentative and incomplete. The current consensus is that such wavelengths cause the polar building blocks of the material to reorient (particularly when the crystal structure is disordered), leading to the polarization of the material, dispersion of the real dielectric permittivity, and the occurrence of dielectric absorption. Although this description appears simple and elegant, its exact understanding has been beset with apparently conflicting observations and conceptual difficulties. For example, while the unit cell volume of hybrid materials changes substantially with temperature, the energy barrier associated with the field-assisted reorientational dynamics remains independent of temperature. Furthermore, dielectric relaxation shows minimal sensitivity to external pressure although hybrid perovskites and their analogues are known for their remarkable softness, characterized by low Young’s moduli and high compressibility. In fact, changes in relaxation dynamics for a given compound do not correlate with alterations in its unit-cell volume or hydrogen bonds length even if all the unit-cell parameters evolve similarly under pressure and temperature. Finally, recent high-pressure dielectric studies indicate that relaxation dynamics are not directly linked to phase transition conditions, challenging conventional assumptions. These complex and sometimes contradictory findings underscore the need to rethink the dielectric response in hybrid organic-inorganic compounds. I will discuss these challenges, explore new concepts, and propose a more comprehensive framework for understanding dielectric relaxation in these materials.

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