New Insight into the Vibrational Behavior of Nickel Hydroxide and Oxyhydroxide Using Inelastic Neutron Scattering, Far/Mid-Infrared and Raman Spectroscopies

The electrochemical properties of β-type nickel (oxy)hydroxides Ni(OH) 2 and NiO(OH) are known to depend significantly on their structural features. In this work, these features are investigated by means of inelastic neutron scattering and infrared and Raman spectroscopies. Our experiments probe both inter- and intramolecular (40−4000 cm -1 ) vibrational ranges and particular attention is paid to the low-frequency (40−300 cm -1 ) domain, where new features are observed. Namely, the presence of an IR active mode at around 130 cm -1, characteristic of the oxidized β-NiO(OH) phase, is observed for the first time. The effects of temperature (10−300 K) and deuteration are also measured in order to provide deeper insight into the nature of the vibrational states. A detailed comparison of all of the collected data allows us to propose a new assignment of the vibrational features observed in both β-Ni(OH) 2 and β-NiO(OH) and to quantify the proton dynamic relevance on the vibrational modes. We show that the interlayer interactions are mostly electrostatic and, hence, do not involve hydrogen bonding. We also point out that, in the oxidized β-phase, protons cannot be localized so that they can be considered as fully labile. To sum up, this investigation emphasizes that proton delocalization and crystalline disorder, which arise concomitantly upon oxidation, are the key factors that explain the evolution of the vibrational features observed between the oxidized and reduced β-type forms.