Chasing H- in Rare-earth Metal Oxyhydride Thin Films
D. Chaykina (TU Delft - ChemE/Materials for Energy Conversion and Storage)
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Abstract
Rare-earth metal oxyhydride thin films show a photochromic effect, where their transparency decreases (reversibly) with exposure to light with energy greater than its optical band gap. The precise underlying mechanism behind this effect is unknown, but is investigated in this thesis by using techniques such as muon spin relaxation, or materials science methods (aliovalent doping, changing the RE-cation, or altering the O:H ratio of the film). Rare-earth metal oxyhydrides have also been reported as hydride-ion conductors in their bulk form (powder pellets). Since some theories about photochromism involve diffusion, there was a suspicion that these two properties are related. Herein, ion mobility is addressed by electrochemical impedance spectroscopy, along with the other aforementioned methods. In summary, this thesis asserts that (1) photochromism does not involve long-range ion mobility and (2) some of the thin films made here are dominated by electronic rather than ionic mobility. An alternative idea for photochromism is given, where neutral hydrogen (H0) is formed alongside the reduced RE-cation (RE2+), and no mobility is required to prolong this darkened state. The local composition of the film under illumination, therefore, may be a H-deficient phase which is optically dark and highly conductive.