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 pH2 = 0.6 torr and 350 K. To obtain measurable uptake rates at these conditions a thin layer of Pd (≈ 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) ≈ 0.4 according to the formula ρ(x) = ρ(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) ≈ 1.2, which can be qualitatively attributed to the metal—insulator transition expected for the Mg---Hz 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.Author Keywords: Magnesium hydride resistivity; Metal-insulator transition