Hydrogen-induced localization of Ce-4f electrons in mixed-valent CeNiIn: Observation of heavy-fermion and magnetically ordered states

Hydriding studies have been carried out on the mixed-valent compound CeNiIn having a Kondo temperature TK ∼ 100 K. Two distinct hydride phases, CeNiInHy, with y = 0.3–0.6 and 1.6 are obtained which retain the hexagonal structure of the parent compound. The higher hydride phase with a composition of y = 1.6 is relatively unstable and it transforms into the lower hydride phase over a short period of time. Magnetization measurements indicate valence transformation of cerium from mixed-valent for the parent compound to Ce3+ for the hydrides. While the higher hydride composition CeNiInH1.6 shows ferromagnetic ordering below ≅7 K, the lower hydride phase with a composition CeNiInH0.6 shows heavy fermion behaviour. Thus, Ce valence changes from mixed-valent to normal trivalence via a heavy-fermion like state as a function of hydrogen concentration.