THE SURFACE STRUCTURES OF Pd(100)-(1×1) AND c(2×2)-K

Abstract

The surface structure of the Pd (100)- c (2×2)-K phase formed by adsorption of K at room temperature has been determined by quantitative analysis of low-energy electron-diffraction (LEED) intensity-energy measurements. K atoms occupy four-fold hollow sites on a slightly perturbed substrate. The vertical distance between the K layer and the first Pd layer is determined to be 2.13±0.06 Å, which corresponds to an effective hard-sphere radius of 1.83 Å for the adsorbed K atoms. The second Pd layer is rumpled with a splitting of 0.04 Å between two bilayers. An analysis of LEED intensities measured for the clean Pd(100) surface confirms previous reports of an expansion of the first interlayer spacing. Adsorption of K in the c (2×2) structure results in a reduction of this expansion from 5% to 1%.

Surface structure of K/Pd(100) and the effect of H coadsorption: Density functional theory calculations

“…We find that K atoms adsorb preferentially at the hollow site with the K adsorption heights 2.44 Å for p(2 · 2) and 2.50 Å for c ( good agreement with previous experiment [10] and theory [11]. The calculated substrate structure for the c(2 · 2) phase, however, disagrees with the previous LEED structure [10]. We suggest as the origin of this disagreement a possibility of hydrogen contamination of the surface sample used in the LEED study.…”

“…The LEED result of a larger d 12 for K/Pd(1 0 0) reminds us of a previous dispute about Dd 12 of the clean Pd(1 0 0) surface: While DFT studies [18][19][20] predicted top-layer contractions of Dd 12 $ À 1%, LEED studies [10,21,22] reported relatively large expansions of Dd 12 = 2.5-4.6%.…”

“…When compared with the LEED result for the K/Pd(1 0 0)-c(2 · 2) surface [10], the hollow-site adsorption and K adsorption height agree well, but the substrate structure deviates beyond the experimental error ranges. The calculated top-layer contraction Dd 12 = À0.3%, although somewhat reduced from Dd 12 = À1.5% of the clean Pd(1 0 0) surface, is still insufficient to explain the measured expansion Dd 12 = +1.3%.…”

“…We suggest as the origin of this disagreement a possibility of hydrogen contamination of the surface sample used in the LEED study. Our calculations demonstrate that H coadsorptions on K/Pd(1 0 0)-c(2 · 2) modify the substrate structure to be compared better with the LEED data [10]. The amount of hydrogen atoms remaining on the surface sample [10] can be estimated as 0.1-0.4 ML.…”

“…1 ML is defined as one K atom per 1 · 1 surface unit cell. The atomic structure of the c(2 · 2) surface was reported by a LEED study of Burchhardt et al [10]: K atoms occupy the hollow site with the K adsorption height 2.54 Å , and the Pd(1 0 0) substrate undergoes an expansion by +1.3% of the first interlayer spacing and a rumpling of 0.04 Å in the second layer. There is no experimental report for the p(2 · 2) structure.…”