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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

KFM 27: Polar oxides: Lithium niobate and lithium tantalate

KFM 27.3: Vortrag

Donnerstag, 21. März 2024, 15:40–16:00, TC 010

Ab initio investigation of the ferroelectric phase transition in LiNbO₃ and LiTaO₃ — •Alexander Kapp, Wafaa Al Nachwati, and Simone Sanna — Institute for Theoretical Physics, Justus Liebig University, Giessen, Germany

Lithium niobate (LN, LiNbO₃) and Lithium tantalate (LT, LiTaO₃) are synthetic, isomorph solids. Both crystallize in a trigonal lattice, belonging to the space group R3c for the ferroelectric state and R3c for the paraelectric states. These materials display several favorable properties such as ferroelectricity, piezoelectricity and the Pockels effect, just to name some examples. For this reason, these crystals are used in many applications e.g., in the telecommunication market for mobile telephones or optical modulators, and also for laser uses like Q-switching or frequency doubling. In this work, we model the transition from the paraelectric to the ferroelectric state and determine the Curie temperature in both crystals using ab initio molecular dynamics, as implemented in the Vienna Ab initio Simulation Package (VASP). Within this approach, atoms in the supercell can move according to existing forces, estimated by density functional theory (DFT). In our calculations, we take into account the expansion of cell volume with increasing temperature. Moreover, very large supercells are used to reduce finite-size effects. Within this approach, we obtain transition temperatures in close agreement with the measured values.

Keywords: lithium niobate; lithium tantalate; phase transition; molecular dynamics