Surface-Mediated Thin Terbium Hydride Film Formation

Abstract

This work was performed to study the correlation between the surface and bulk phenomena that occur during H(2) interaction with terbium, leading to three-hydride TbH(x) (x approximately 3) formation. This reaction is accompanied by the transition of the original metal into a semiconductor. It was found that thin films are particularly useful for such studies. Measurements of work function changes DeltaPhi(H/Tb) were chosen to illustrate the surface phenomena, and the relative electrical resistance R(H/Tb)/R(0) and light transparency T(H/Tb)/T(0) correspond to the bulk properties. Additionally, BET experiments were performed to determine the influence of three-hydride formation on the area of a thin Tb film. It was observed at 298 K that a precursor state of the adsorbate arose at the beginning of the reaction, when (H/Tb < 0.1), decreasing the work function by DeltaPhi = 12 mV. A higher uptake of hydrogen caused an increased work function, followed by DeltaPhi transients. This has been interpreted as local hydride formation on the surface and its expansion into the bulk, until a concentration of H/Tb approximately 3 was reached. TbH(x) (x approximately 3) formation resulted in DeltaPhi = approximately 200 mV and an increase in the thin film area by a factor of approximately 3. These phenomena were accompanied by characteristic changes in the bulk properties. The light-reflecting thin Tb film was transformed into a transparent hydride, with an approximately 23-fold increase in R/R(0). At 78 K, only a small amount of hydrogen (H/Tb = 0.13) was consumed, leading to DeltaPhi = -23 mV. This uptake is stable up to 100 K. Increasing the temperature above this value resulted in the additional large absorption of hydrogen. This could suggest the formation of a low-temperature surface phase of the hydride.