Surface X-ray diffraction on clean and Cs-covered Ag(001)

Abstract

Abstract We have studied the structure of the clean and Cs covered Ag(001) surface, using surface X-ray diffraction. For the clean unreconstructed Ag(001) surface the analysis of the integer-order crystal truncation rods gives evidence for a compression of the first interlayer spacing relative to the bulk by Δd 12/d bulk = −0.8(8)%, whereas for the second interlayer spacing we obtain an expansion of Δd 23/d bulk = 1.0(8)%. For the first two Ag layers we observe enhanced thermal disorder as expressed by the isotropic mean-squared displacement amplitude, U = 0.011(1) Å2 at 340 K relative to the bulk value of 0.009 Å2 at this temperature. Our X-ray results for Ag(001) are in good agreement with previous experimental and theoretical results. The adsorption of 0.25 ML (1 ML = 1.25 × 1015 adatoms/cm2) Cs on the Ag(001) surface at 170 K leads to the formation of a c(2 × 4) superstructure. The Cs atoms are found to occupy fourfold hollow sites at d Cs = 2.49(20) Å above the Ag(001) surface thereby shifting the underlying Ag atoms laterally by 0.029(5) Å from their bulk positions. From the adsorption height we derive an effective Cs radius of 1.78(16) Å. Large anisotropic disorder is observed for the Cs adatoms. Within the harmonic approximation we derive mean-squared displacement amplitudes of U 11 = 0.08(1) Å2, U 22 = 0.10(1) Å2 and U 33 = 0.27(3) Å2. An alternative model, suggested by the very large value of U 33, was also tried, in which there are 15% of Ag vacancies in the top layer and a lower Cs site. This ‘unusual’ model is discussed in the context of current theories of alkali metal induced reconstructions.