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Temperature-dependent crystal structure alterations in the brownmillerite-type material Ba2In2O5 play a fundamental role in its applications: i) photocatalytic CO2 conversion; ii) oxygen transport membranes; and iii) proton conduction. This is connected to a reversible uptake of up an equimolar amount of water. In this study, in situ X-ray and neutron diffraction were combined with Raman spectroscopy and solid-state nuclear magnetic resonance experiments to unravel the effects of Cr doping and water content on the crystal structure transitions of Ba2In2O5(H2O)x over a wide temperature range (10 K ≤ T ≤ 1573 K, x < 1). A mixture of isolated and correlated protons was identified, leading to a highly dynamic situation for the protons. Hence, localisation of the protons by diffraction techniques was not possible. Cr doping led to an overall higher degree of disorder and stabilisation of the tetragonal polymorph, even at 10 K. In contrast, a further disordering at high temperatures, leading to a cubic polymorph, was found at 1123 K. Cr doping in Ba2In2O5 resulted in severe structural changes and provides a powerful way to adjust its physical properties to the respective application.

We acknowledge the ILL for the provision of beam-time at the D2B and D20 diffractometers and the assistance by Jan Bernauer, Aasir Rashid, and Dipl.-Ing. Claudia Fasel (TU Darmstadt) during the collection of FTIR spectra + thermogravimetric measurements. The German Federal Ministry of Education and Research is acknowledged for funding the NexPlas project (project number: 03SF0618B). This work was funded by the Deutsche Forschungsgemeinschaft (Grant INST 268/379/1 FUGG). We acknowledge the support by the Deutsche Forschungsgemeinschaft (DFG—German research foundation) and the Open Access Publication Fund of the Technical University of Darmstadt covering the APC.