Segregation of deuterium and hydrogen on surfaces of palladium deuteride and hydride at low temperatures

The distribution of deuterium (hydrogen) between the surface and the bulk of palladium deuteride (hydride) within the interval of temperatures 78-160 K was studied by simultaneously measuring the work function changes and Da (H-2) pressure in the process of thin PdDy (0.005 < y < 0.855) and PdHx(0.005 x < 0.900) film formation. It has been found that at 78 K the concentration of the positively polarized, atomic deuterium (hydrogen) adspecies on the outer surface of palladium deuteride (hydride) approaches 1.9 (1.5), while the atomic ratio D/Pd (H/Pd) in the bulk does not exceed 1. The work function strongly decreases (by similar to 2400 mV for PdD0.785) Surface segregation of deuterium predominates that of hydrogen. Above 120 K an increase of the work function is observed in the process of PdDy, (PdHx) formation. This is interpreted as deuterium (hydrogen) positioning below the surface image plane within the first layer of Pd atoms. Under these conditions the surface concentration of deuterium (hydrogen) adatoms does not differ significantly from that in the bulk, approaching 1.