Resistivity and hydrogen uptake measurements in evaporated Mg films at 350 K

Resistivity measurements were performed during hydriding-decomposition cycles of UHV-prepared Mg films. The hydrogen exposures were made in situ at p(H2) = 0.6 torr and 350 K. To obtain measurable uptake rates at these conditions a thin layer of Pd (approximate to 7 nm) was deposited on top of the typically 380 nm thick Mg films. Mg and Pd film thicknesses and hydrogen uptakes were determined by the frequency shift of a quartz crystal microbalance (QCM). Both the QCM and the quartz substrate onto which the Mg resistor film was deposited were mounted on the same sample holder. The p, T conditions were chosen so that hydrogen was distributed nearly homogeneously through the sample. The total hydrogen uptakes were in most cases kept small (<40 at.% H) to minimize the morphology changes. The resistivity vs. H concentration relation is, within experimental uncertainty, linear up to x = (H/Mg) approximate to 0.4 according to the formula rho(x) = rho(0)(1 + c . x), c = 1.08 +/- 0.25. The relatively large uncertainty is caused by a spread in the uptake data and by the irreversible resistance changes during hydriding-decomposition cycles. Both these uncertainties could be attributed to morphological changes induced by the volume expansion-contraction accompanying the cycles. Exploratory measurements were also made at much larger H uptakes. These measurements reveal a rapid reversible resistivity increase at (H/Mg) approximate to 1.2, which can be qualitatively attributed to the metal-insulator transition expected for the Mg-H-x system when the hydride phase becomes continuous. The measurements and their quantitative interpretation were complicated by the morphological changes of the sample(s) caused by volume expansion-contraction.