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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

KFM 6: Poster

KFM 6.4: Poster

Dienstag, 28. März 2023, 17:00–19:00, P3

Composition dependent optical properties of LiNb_1−xTa_xO₃ solid solutions — •Felix Bernhardt, Florian Pfeiffer, and Simone Sanna — Justus-Liebig-Universität, Gießen, Germany

Lithium niobate (LN) and lithium tantalate (LT) are ferroelectric crystals with a wide range of applications, extending from piezoelectric sensors [1] to integrated photonics [2]. Their structural similarities enable their combination to LiNb_1−xTa_xO₃ (LNT) solid solutions. As the optical absorption edge of these alloys depend on their composition, one can determine the crystal composition non destructively, e.g. by optical spectroscopy.

Here, we use special quasi-random structures (SQS) to simulate LNT crystals with different compositions. These structures mimic an ideal random alloy, even when periodic boundary conditions are employed.

We calculate the first order dielectric tensor of LNT for different Ta concentrations by using density functional theory (DFT) and the indepent particle approximation (IPA). Furthermore, we employ the Bethe-Salpeter equation (BSE) on top of quasi-particle calculations in GW-approximation for pure LN and LT to model excitonic effects.

Our calculations show a clear dependence on the absorption edge with respect to the Nb/Ta ratio of the LNT crystals. The inclusion of quasi-particle effects significantly shifts the absorption edge to higher energies. Our results are in good agreement to experimental data.

[1] M. Xu et al., ACS Appl. Mater. Interfaces, 9, 40, (2017) [2] W. Sohler et al., Optics & Photonics News, 19, 1, (2008) [3] A. van de Walle et al., Calphad, 42, 13-18 (2013)