On the origin of a lattice expansion in palladium and PdAu vapour deposits on various substrates

Abstract

In order to gain more insight into the possible lattice expansion mechanism in vapour deposits of palladium, vapour deposits of palladium and of PdAu alloy particles and films were produced under well-controlled conditions, either in ultrahigh vacuum or in high vacuum, with and without the admission of H 2O, on amorphous films of carbon and SiO 2 as well as on single-crystal surfaces of NaCl and KBr. In all cases, lattice expansion values of up to 2.6% were measured, depending on the condition of degassing of the substrate and on the residual gas composition during deposition, especially with respect to water vapour. It was found that the lattice expansion was correlated with the presence of hydrogen on or in the substrate materials, as detected by secondary-ion mass spectroscopy (SIMS). X-ray photoelectron spectroscopy (XPS) analyses of a satellite of the main palladium peaks indicated a correlation with the concentration of hydrogen on the samples. Other impurities such as carbon or oxygen were excluded as being responsible for the lattice expansion by SIMS and XPS data. Furthermore, as no systematic difference in the lattice expansions was found for epitaxially grown deposits on alkali substrates and for non-oriented deposits on other substrate materials, the effect of pseudomorphism was shown not to be significant, too. Therefore the only remaining mechanism for lattice expansion is believed to be the formation of hydride, although the stabilization of this phase still needs to be clarified.