Growth of stable ultra-high density films of hcp lead on Ru(1010)

Growth of Pb on $\text{Ru}\text{(10}\text{1}\text{0)}$ has been investigated at 140 and 300 K by LEED and Auger spectroscopy. The interfacial contact layer evolves through a series of ordered Pb phases “split (2×1)”, c(3×2), c(4×2) - in which the Pb atoms assume their “normal” size. Beyond the monolayer point a c(2×4) phase appears, corresponding to development of compressed quasihexagonal lead overlayer. This phase is followed by a most unusual structure: a (0001)-oriented multilayer film of hexagonally close-packed lead in which the interatomic spacing is much smaller (16%) than that in fcc bulk lead. Furthermore, this spacing ($\text{3.02 ± 0.06}\text{A}\text{̊}$) is ∼ 9% less than that characteristics of the high pressure hcp phase of bulk lead and therefore corresponds to the most dense form of metallic Pb known. The ultra-high density phase was stable at 300 K and was still observable after heating to ∼ 600 K. No transitions to “normal” hcp or fcc structures occurred over the whole coverage range studied - up to 30 monolayers. These ultra-dense Pb films are also unreactive towards oxygen. In contrast to this, normal density, oxidisable, fcc Pb films are formed other fcc substrates, indicating that the structural and electronic properties of the ultra dense Pb films depend critically on the specific structure of the Ru(10$\text{1}$0) substrate.