PHASE-DIAGRAM FOR HYDROGEN ADSORPTION ON RH(311)

Hydrogen adsorption on rhodium (311) leads to a sequence of ordered superstructures: (1 x 3)-H, (1 x 2)-H, (1 x 3)-2H and (1 x 2)-2H(rec). A complete phase diagram of this adsorption system was derived from the development of LEED beam profiles and integral intensities as a function of coverage and temperature. All homogeneous phases are separated by coexistence regions, indicating first-order phase transitions with changing coverage. An order-disorder transition with changing temperature can only be observed for the (1 x 3)-H phase at coverages THETA less-than-or-equal-to 0.33 (critical temperature T(c) = 185 +/- 5 K). In contrast, at coverages slightly above THETA = 0.33 an order-order transition from (1 x 3) to (1 x 2) order occurs. For the other phases thermal desorption prevents a study of phase transitions with temperature.