In recent years mixed halide perovskites MAPbI3-xBrx have become an aspirant for the application in tandem solar cells due to the tunability of their bandgap.[1] However, halide migration in the perovskite particles and across particle interfaces under working conditions, i.e., illumination and elevated temperatures, results in heterogeneous compositions and thus in a deterioration of optoelectronic performance.[2] Tuning the morphology of perovskite particles, as well as the use of “passivation” additives is believed to control ion migration and thus increase the stability of mixed halide compositions, but exact mechanisms are still unclear.
We use in-situ X-ray diffraction in combination with solid-state NMR ….

39 Min.

Ruddlesden-Popper (RP) compounds (general formula (AX)(ABX3)n) are a current hot topic in materials research as they possess a high flexibility regarding their anion composition. This makes them an ideal platform for experiments which aim on tuning of physical properties for example by substitution of oxygen by fluorine (O2-/2F-) yielding the corresponding oxyfluorides. The targeted synthesis of such new RP-oxyfluorides is a challenging task as most of them are metastable and therefore suitable synthesis conditions are mostly found by numerous trial and error experiments. Understanding the formation chemistry of oxyfluorides is therefore crucial for optimizing reaction conditions and gaining access to new metastable products.
For the fluorination of La2NiO4 with PVDF laboratory in situ XRD experiments were established as highly useful tool for obtaining optimized reaction conditions. These investigations were additionally combined with in situ neutron powder diffraction to reveal a complex fluorination pathway involving four reaction intermediates. This talk is about the application of such in situ XRD fluorination experiments for the preparation of new oxyfluorides (La2NiO2.5F3,1 La2CoO3F3, La2(Ni,Cu)O3F22,3,4) as well as their less stable reaction intermediates.

46 Min.

Polyoxometallates are known to form a plethora of large polyanions, the number of which even grows with the introduction of heteroatoms. They are frequently synthesized under solvothermal conditions, but with suitable precursors even at room temperature reactions can be carried out. Under such conditions the growth of single crystals is not always possible and structure solution from powder diffraction data remains the only way to determine the crystal structure. Herein, the challenging case of the arsenato polyoxovanadate [Ni(en)3]2[V12As8O40(H2O)]x4H2O is presented, where both lab and synchrotron based powder diffraction data were employed for structure solution and refinement.

35 min.