Crystal structures and photochromic properties of rare-earth metal hydride oxides

• Applicant: Professor Dr. Holger Kohlmann
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 564289361

Heteroanionic compounds like nitride oxides or oxide halogenides have gathered attention recently due to intriguing physical properties and potential applications (photocatalysists, thermoelectrics, luminescencence). Especially interesting and surprisingly little investigated are hydride oxides, which combine the rather hard oxide anion with the soft and polarizable hydride, H‒, ion. In thin films of oxygen containing rare-earth metal hydrides photochromism was observed, which makes them interesting for smart window applications. Due to inherent problems with characterizing thin films neither the chemical nature of the photochromic phases nor the mechanism of the reversible photodarkening are known. Recently, bulk phases for rare-earth metal hydride oxides were found, e. g. stoichiometric REHO and oxygen-rich Y2H0.50O2.75. The latter crystallizes in a tetragonally distorted, hydrogen stuffed bixbyite type structure and shows photochromic properties similar to the thin films and is thus likely to be (one of) the photochromic phase(s) therein. Bulk materials have distinct advantages over thin films, as for larger scale production, characterization, accuracy of crystal structures or the possibility for surface modifications for altering optical properties. This proposal is thus on new rare-earth metal hydride oxides as bulk phases and investigating their composition-structure-property relationships with a focus on photochromism. In situ investigations are extremely useful as a guide to effective synthesis planning, which will be used for the exploration of RE-H-O systems (RE = Y, Nd, Er, Ho) in two work packages. A1: Following the successful discovery of Y2H0.50O2.75 the reaction of REHO and REH2<=x<=3 in air (oxygen) will be followed by in situ X-ray and neutron diffraction, thermal analysis and when applicable Raman scattering; this will uncover intermediates and open up synthesis routes for the preparation of single phase bulk samples for the complete compositional space of H/O ratios. A2: Solid-state reactions between RE hydrides and oxides will be followed by in situ methods (see A1), those at high hydrostatic pressures in the GPa range at the large-volume press (LVP) beamlines at DESY and ESRF with in situ synchrotron diffraction up to 20 GPa and 2000 K. In work packages A1 and A2 discovered phases will be synthesized in bulk and characterized in two work packages. B1: Crystal structures will be solved by diffraction (X-ray, neutron, electron) methods and samples will be characterized chemically including investigation of their phase width in order to fully describe their chemical nature and structure. B2: Further spectroscopic techniques, especially for photochromism and its mechanism, will complement the characterization. This proposal thus aims at a more profound understanding of structure-property relationships in rare-earth metal hydride oxides and an assessment of their suitability as functional materials for smart windows.

• DFG Programme: Research Grants