ORDERED STRUCTURES AND PHASE-DIAGRAM OF ATOMIC-HYDROGEN CHEMISORBED ON RUTHENIUM (001)

Chemisorbed hydrogen on Ru(001) was investigated with LEED and workfunction changes, DELTA-phi, both as a function of coverage and temperature in the temperature range above 15 K. Three ordered phases were found in addition to a (1 x 1) phase at saturation coverage: (squareroot 3 x squareroot 3)R30-degrees, 3 domains of p(2 x 1) and a (2 x 2)-3H structure at coverages theta = 0.33, 0.5 and 0.75, respectively, which all undergo order-disorder phase transitions. Their phase diagram was determined, too. No ordered phases exist below theta = 0.2. The critical temperatures of the superstructure phases are 74.5, 68.0 and 71.0 K, respectively. Clear order dependent contributions to DELTA-phi were found, which are attributed to admixtures of different configurations with different DELTA-phi in the disordered phases. From integrated LEED intensities of superstructure spots as a function of temperature scaling was tested. Whereas no scaling was found for the p(2 x 1) structure, scaling was observed for theta = 0.33 and 0.75, and exponents alpha of the specific heat were determined as 0.18 +/- 0.05, and 0.1 +/- 0.05, respectively. Whereas at theta = 0.33 the transition seems to be continuous, the situation for the (2 x 2)-3H phase cannot be uniquely identified from measurements of the exponent alpha alone.