Hydrogen distribution around Eu2+ and Gd3+ probes in samarium hydrides

Dilute 153Eu and 155Gd probe atoms formed by radioactive decay of 153Sm and 155Eu, respectively, in SmH2+x (0≤x≤1) between 4.1 and 20 K have been studied by Moessbauer spectroscopy. The results demonstrate that the local hydrogen environment in SmH2+x determines the europium valency. Eu2+ formed in stoichiometric SmH2 at low temperatures experiences local cubic symmetry. In the SmH2+x series, more than three octahedral hydrogen neighbours force europium into the trivalent state. At elevated temperatures, however, Eu2+ repels a hydrogen from its nearest-neighbour shell. This is evidenced by the observation of a well-defined quadrupole interaction at Gd3+ formed from Eu2+ in SmH2 at low temperatures. SmH2 is found to order magnetically at 13±2 K. (orig.)