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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

KFM 9: KFM Poster Session

KFM 9.11: Poster

Montag, 18. März 2024, 18:00–20:00, Poster E

Ab initio determination of the Curie temperature of stoichiometric LiNbO₃ and LiTaO₃ crystals — •Felix Bernhardt and Simone Sanna — Institut für theoretische Physik and Center for Materials Research, Justus-Liebig-Universität Gießen, Germany

Lithium niobate (LN) and lithium tantalate (LT) are ferroelectric crystals with a wide range of applications. As for some of them (i.e., in sensors) operation at high temperature is required, it is vital to understand their phase transition from the high-temperature, paraelectric phase to the low-temperature, ferroelectric phase. For low temperatures, LN and LT both exhibit a trigonal symmetry (R3c). The transition into a R3c symmetry at 1480K and 940K respectively is achieved mainly by a displacement of the central Li-ion in z-direction.
Here, the Curie-temperature of these two crystals is studied by employing the stochastic-self-consistent harmonic approximation (SSCHA). Within this method, the free energy of the crystal is minimized by optimizing the nuclear wave-function. Furthermore, the thermal expansion is taken into account by means of the quasi-harmonic approximation.
Inside these frameworks, phononic properties can be computed at a non perturbative level, including anharmonic effects. The comparison of phonon frequencies calculated within the common harmonic approximation implies non-negligible anharmonic contributions to both materials. By evaluating these refined phononic dispersions, we can determine the Curie-temperature by calculating and comparing the corresponding free energies of both phases.

Keywords: Lithium niobate, Lithium tantalate; ferroelectric phase transition; Ab-initio simulations