SKM 2023 – wissenschaftliches Programm

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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

KFM 11: Crystal Structure Defects / Real Structure / Microstructure

KFM 11.4: Vortrag

Donnerstag, 30. März 2023, 15:00–15:20, POT 51

Disorder engineering for symmetry lowering in Prussian Blue analogues (PBAs) — •Yevheniia Kholina, Thomas Weber, and Arkadiy Simonov — ETH Zürich, Switzerland, Department of Materials, Laboratory for Multifunctional Ferroic Materials

A simple consequence of Neumann’s principle is that certain properties can exist in crystals with a certain symmetry. As a result, a lot of materials engineering is focused on designing particular symmetries, which allow the properties of interest. In oxide perovskites as well as their molecular analogues, control of crystal symmetry is achieved by octahedral tilts and cation ordering. In this work we show an alternative way of symmetry lowering, using disorder. We focus on Mn[Co] PBA with the chemical composition Mn[Co(CN)₆]_2/3. These crystals have 1/3 of Co(CN)₆ sites vacant. The structure of such PBAs is believed to be cubic with space group Fm3m. However, the optical measurements indicate that the crystal is twinned and an actual structure has tetragonal symmetry or lower. We collect single-crystal x-ray diffuse scattering from untwined crystal to probe the local structure and quantitatively characterise defect distribution, using 3D-ΔPDF analysis. The symmetry of the local structure is reduced to tetragonal by local vacancies ordering. The vacancies have stronger correlation along c direction. Such asymmetry is formed because the crystal is growing in this direction. This means the symmetry can be lowered by choosing a specific direction of the crystal growth. We believe this result can be applied to other disordered crystalline materials and be an effective way to lower the symmetry for properties design.