VOLUME 56, NUMBER21 PHYSICAL REVIEW LETTERS 26 MAY 1986 H and the W(001) Surface Reconstructions: Local Bonding to Surface States M. Weinert Department of Physics, Brookhaven National Laboratory, Upton, New York 11973 A. J. Freeman Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60201 and S. Ohnishi NEC Corporation, 1-1 Miyazaki 4-chome, Miyamae-ku, Kawasaki 213, Japan (Received 7 February 1986) The structural properties of the W(001)/?(l x D-2H system are predicted by use of all-electron total-energy calculations. Both the vibrational frequency (133 meV) and the work-function change (0.95 eV) are in excellent agreement with experiment at the calculated equilibrium H-W interlayer spacing of 1.12 A. A proposed electronic driving mechanism, which includes local bonding effects and depends on the character of the £2 surface-state wave functions, explains the various observed reconstructions of the clean-W(OOl) and H/W(001) systems, including symmetries and coverage dependence. PACS numbers: 73.20 - r , 68.35.Rh, 7L45.Nt The W(001) surface is one of the best studied in surface science because of the wealth of different physical phenomena that it exhibits. 1 Of particular in- terest has been the temperature-induced phase transi- tion of the clean surface and the unusual phase dia- gram versus hydrogen coverage. By now it is rather well established that the so-called Debe-King model 2 [cf. Fig. 1(a)] describes the low-temperature phase of the clean surface, but there is still controversy con- cerning the origin of the reconstruction: The phase transition from the high-temperature p (1 x 1) to the low-temperature c(2x2) plmg structure was first ex- plained 3,4 in terms of Fermi-surface nesting in which states of ±k near the new zone boundary mix, but photoemission experiments 5 have demonstrated that the surface-state coupling is too small to drive the transition. On the other hand, frozen-phonon total- energy local-density calculations 6 have shown that the Debe-King model is energetically favored and that upon reconstruction there is a substantial splitting and decrease of the density of states at the Fermi level. (a) (b) FIG. 1. Schematic of the (a) plmg and (b) clmm W(001) surface reconstructions. Filled circles show the position of the displaced atoms upon reconstruction. These investigations, however, did not determine the underlying mechanism which drives the transition. In contrast to the clean surface, hydrogen (which adsorbs in the bridge site at all coverages) dramatically changes the e(2x2) plmg reconstruction: Up to about half a monolayer coverage (0 = 0.5), the surface is again reconstructed but into a c(2x2) clmm structure [Fig. Kb)]; at saturation coverage (0=2), the surface re- turns to the p (1 x 1) structure. 7,8 A number of authors have calculated various aspects of the electronic structure of the clean 9 "* 11 and hy- drogen-covered 12 surfaces, with particular emphasis on the location of the surface states. In this paper we consider theoretically the equilibrium properties of H on the W(001) surface and the origin of the phase transitions observed for both the clean surface and the hydrogen-saturated W(001)p(lx 1)-2H surface. We make predictions for (i) the H position above the W surface (which favors the LEED, 13 rather than the high-resolution electron-energy-loss spectroscopy 14 value) and (ii) the vibrational frequency of the sym- metric stretch mode and the work-function change on hydrogen adsorption (both in excellent agreement with experiment). More importantly, from comparisons of the clean and hydrogen-covered surfaces, we are able to identify the mechanism driving the surface recon- struction. On this basis, we propose a model that na- turally accounts for the various reconstructions, in- cluding the symmetry of the clean surface and the symmetry switching of the reconstruction with the ad- sorption of hydrogen. The surfaces were modeled by a seven-layer ideal W(001) film with an additional layer on each side con- sisting of two bridge-bonded atoms per surface unit © 1986 The American Physical Society 2295