Dresden 2003 – scientific programme

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DS: Dünne Schichten

DS 23: Elektrische und optische Eigenschaften II

DS 23.9: Talk

Thursday, March 27, 2003, 17:15–17:30, GER/38

Variable reflectance Mg₂NiH_x thin films — •W Lohstroh, J Isidorsson, I A M E Giebels, S Enache, and R Griessen — FEW: Solid State Physics, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands

Thin films of Mg₂NiH_x show intriguing changes in their optical properties as the hydrogen content x in the film is varied. Pure Mg₂Ni is metallic and a shiny mirror. At high H concentrations the transparent semiconductor Mg₂NiH₄ is formed. However, in-between a sharp drop in reflection is observed but no transmission thus nearly 90% of the incoming light are absorbed and the film appears black. At that stage electrical resistivity still has metallic character. By changing the hydrogen concentration it is possible to switch reversibly between the three optical states. Electrolytic hydrogen loading combined with optical reflection and transmission measurements and x-ray diffraction with in-situ gas loading has been used to relate the optical properties to the structure and the hydrogen concentration in the films. The beginning of the two-phase region between the solid solution Mg₂NiH_0.3 and Mg₂NiH₄ coincides with the occurrence of the black state. The metal to semiconductor transition was further investigated using temperature dependent transport measurements.