Incoherent quasielastic neutron scattering study of NH3 reorientations in various phases of Ni(NH3)6 (NO3)2

QNS measurements carried out for polycrystalline [Ni(NH3)6] (NO3)2 revealed NH3 reorientations about Ni-O axes occurring in all phases. In the high temperature phase these reorientations, having correlation time of an order of pico-second, occur via an activation process with a barrier of ca. 1.5 kcal/mole. Below 247 K there exist two phases, a metastable one and a stable one. NH3 reorientations in the metastable phase occur via activation over a barrier of ca. 0.5 kcal/mole. These reorientations in the stable phase occur probably via a reorientation-phonon coupling in view of an apparent zero activation barrier. A perfect agreement between the QNS and the calorimetric measurements is pointed out. (author)