Hydrogen Absorption in Metal Hydrides : Transmission of light in relation to hydrogen concentration and site occupancy of ultrathin vanadium films

Sammanfattning: In this study the effect of hydrogenation on the optical properties in the wavelength range 400-1023 nm of an ultrathin iron-vanadium superlattice is investigated. Specifically, mea- surements of transmission are performed under different states of hydrogenation, along with measurements of absolute hydrogen concentration and hydrogen site occupancy. The trans- mission measurements are used to construct pressure-concentration isotherms. Isotherms and transmission data are in turn correlated to concentration and hydrogen occupancy.  The results show a wavelength dependent decrease in transmission with hydrogenation. The decrease is greatest around 550 nm, and the wavelength of maximum decrease shifts to higher wavelengths with increasing hydrogen pressure. The non-uniform decrease will make the use of transmission as a measurement of hydrogen concentration dependent on the wavelength of the probing light.  15N resonant NRA is used to perform direct, real-space measurement of absolute hydro- gen concentration. The achieved concentrations are 0.092, 0.38 0.40 H/V. Comparing the concentrations and corresponding transmissions to the location of the plateau region in the transmission based isotherms, it appears that the system is in a single phase at 0.38 and 0.40 H/V, and in a mixed phase at 0.092 H/V. Using a combination of resonant NRA and RBS, while exploiting crystal lattice ion channeling, indirect measurements of hydrogen site occupancy are performed. At all investigated concentrations the system does not display tetrahedral site occupancy, but it remains uncertain whether the occupancy is octahedral or some dislocated octahedral-tetrahedral intermediate.  The relation of hydrogen concentration and optical transmission is investigated via a linear regression analysis. The data points generally deviate by more than one standard deviation from the fitted lines, and lie outside of the error estimation. These deviations might indicate that a linear model is inappropriate, where one possible explanation could be that the mapping from transmission to concentration is dependent on the phase of the system.