Preparation and characterization of clean, single-crystalline YHx films (0<= x <= 2.9) on W(110)

Yttrium can be loaded with hydrogen up to high concentrations causing drama tic structural and electronic changes of the host lattice. We report on the preparation of clean, single-crystalline YHx films (0 less than or equal t o x less than or equal to 2.9). The films have been characterized in situ c ombining angle-resolved photoelectron spectroscopy (ARPES) and low energy e lectron diffraction. Direct Y dihydride growth, i.e., Y evaporation under a H-2 partial pressures of approximate to 5 x 10(-6) mbar at 500 K on W(110) , is the most convenient starting point for the preparation of clean single -crystalline Y hydride films covering H concentrations from the "clean meta l" (x approximate to 0) up to the lower boundary of the pure trihydride pha se (x approximate to 2.9). Upon annealing Y dihydride films the desired H c oncentration can be adjusted within the alpha-phase or the (alpha+beta) two -phase regime. On the other hand, the extension of our photoelectron spectr ometer with an homemade ultrahigh vacuum (UHV) compatible hydrogenation sys tem allows to induce the transition from Y dihydride to Y trihydride within a few minutes. The hydrogenation system combines a high-pressure reaction cell with hydrogen permeation through a Pd-24%Ag tube. The overall design i s such that the sample never gets in contact with non-UHV compartments. For direct Y dihydride growth on W(110) two equally populated face-centered-cu bic(lll) domains rotated by 180 degrees with respect to each other are obse rved. In the alpha- and gamma-phase the Y atoms form a hexagonal-close-pack ed(0001) oriented lattice. Furthermore, the previously established model fo r in situ I-I concentration estimation in Y [J. Hayoz et al., Phys: Rev. B 58, R4270 (1998)] is extended successfully from the alpha to beta to the be ta to gamma-phase transition. Ultraviolet photoemission spectroscopy data u nequivocally reveal the opening of a gap extending as far as 1 eV below E-F for normal electron emission upon the phase-transformation from Y dihydrid e to Y trihydride. It also appears that the H absorption rate strongly depe nds on the H-2 purity. Our experimental results demonstrate the capability of this setup for in situ preparation and investigations on the geometrical and electronic structure of Y hydride films and, more generally, rare-eart h hydride films using ARPES. (C) 2000 American Vacuum Society. [S0734-2101( 00)01805-4].