Low‐Temperature Tunneling of Hydrogen in Nb(OH)x and Nb(NH)x.

Hydrogen interstitials in Nb are trapped by O or N impurity atoms below ∼ 160 K. The trapped hydrogen occupies two nearest‐neighbor tetrahedral sites. It represents, therefore, a model system for experimental studies of the tunneling dynamics of a particle in a double‐well potential. The tunneling dynamics of the trapped hydrogen was investigated by neutron spectroscopy in the temperature range between 0.05 and 160 K. The experiments demonstrate the transition from a low‐temperature coherent tunneling ( T ≦ 10 K) to an incoherent jump diffusion at elevated temperatures ( T ≧ 10 K). Up to ∼ 60 K, both coherent tunneling and jump diffusion is controlled by a nonadiabatic coupling of the hydrogen to conduction electrons (Kondo's coupling constant K = 0.0055 ± 0.005). Above ∼ 60 K, the jump diffusion of the hydrogen is increasingly dominated by an interaction with phonons.