Local structure investigation of β-Ni(OH)₂ under pressure using combined Raman and Ni K-edge extended x-ray absorption fine structure studies

We present a combined high pressure Ni K-edge x-ray absorption and Raman spectroscopic investigation of β-Ni(OH)2. Extended x-ray absorption fine structure analysis reveals a progressive shrinking of the Ni-O and Ni-Ni bond distances with pressure. As pressure is increased, Raman data show a continuous hardening of the phonon modes associated with the vibration of Ni-O sublattice, whereas a systematic frequency softening is displayed by the hydroxyl (O-H) symmetric stretching mode. This contrasting behavior implies a gradual development of a lattice instability with pressure in nickel hydroxide. Under pressure hydrogen bonds become increasingly unstable mainly due to the displacement of H atoms around the Ni-O axis inducing a local-lattice instability may explain the structural, Raman and x-ray absorption results.