In cathode-ray tubes, high current densities at relatively low operating temperatures can be achieved with dispenser cathodes containing scandium oxide. The surface structure and precise emission mechanism of the scandate cathode are not known. The surface layers of such a cathode consist of the four elements Sc, Ba, O, and W. We have studied Sc, Ba, and O overlayers and various combinations of these elements deposited on the W(001) surface, under ultrahigh vacuum conditions. We have performed in situ measurements of the work function changes accompanying the surface treatments, and correlate these with the observed surface structures. High resolution medium-energy ion scattering was used to determine the compositional depth distributions and the geometrical structures at this model cathode surface. Auger electron spectroscopy peak height ratios show that W(001), with all three overlayer elements present, can be considered representative for real scandate cathodes. The observed ultra-low work function of the W(001)/Sc,Ba,O model system explains the high current densities on real scandate cathodes.