EXAFS study of the Jahn-Teller distortion in layered nickel oxyhydroxide

The local environment of Ni-III ions in the HNiO2 oxyhydroxide has been studied by Ni-K edge EXAFS and compared to those of various layered nickelates. The Ni-III site in the beta(III)-HNiO2 phase exhibits a Jahn-Teller distortion, as in LiNiO2 or NaNiO2, characterized by four Ni-O distances at 1.87 Angstrom and two long ones at 2.03 Angstrom. A very strong hydrogen bonding network frustrates the cooperative Jahn-Teller distortion observed in NaNiO2 and leads to the occurrence of four short Ni-Ni distances at 2.82 Angstrom and two long ones at 3.03 Angstrom. Such a distance distribution may explain the very pronounced broadening of the X-ray diffraction lines. In the gamma-oxyhydroxide containing Ni-III and Ni-IV ions, the Jahn-Teller distortion of Ni sites observed in ANiO(2) (A = H, Li, Na) phases has disappeared as a result of the Ni-III/Ni-IV hopping. (C) 1997 Elsevier Science B.V