Pressure Induced Hydrogen Order–Disorder Transition\nin β‑Ni(OH)₂

Abstract

Here\nwe address the challenging problem of pressure induced hydrogen\nsublattice disordering in layered hydroxides using synchrotron X-ray\npowder diffraction (XRPD) and first-principles calculations based\non density functional theory (DFT) using β-Ni­(OH)₂ as a model system. XRPD data demonstrate anomalous behaviors in\nsome Bragg peaks. The shift in (003) and (013) peaks with pressure\nis large as compared to that of other Bragg peaks and these two peaks\nare not discernible above 5 GPa, similar to the characteristic observed\nin β-Co­(OH)₂. The underlying changes within the structure\nwere probed using DFT calculations on a √3 × √3\n× 1 supercell of β-Ni­(OH)₂. Structural relaxation\nyields a hydrogen disordered structure as the ground state for β-Ni­(OH)₂ at pressures above 5 GPa. The disordered model also describes\nwell the high pressure XRPD data. The hydrogen sublattice largely\naffects the electronic density of the (003) and (013) lattice planes\nthus permitting the development of a one-to-one correspondence of\nthe anomalous behavior of these Bragg peaks with the hydrogen sublattice\ndisordering. Further evidence of a possible structural rearrangement\nwas obtained by the pressure dependence of the unit-cell volume (<i>P</i> vs <i>V</i>) as well as the ratio of the out-of-plane\nto in-plane lattice parameter (<i>c</i>/<i>a</i>). With the availability of a large number of data points with smaller\npressure steps, the above two quantities are shown to behave differently\nbelow and above 2.5 GPa. The <i>P</i> vs <i>V</i> data are well described by a second order Birch–Murnaghan\nequation-of-state, whereas for <i>c</i>/<i>a</i>, a clear change in trend is observed for data above 2.5 GPa. The\npressure response of β-Ni­(OH)₂ is similar to that\nof Co­(OH)₂ reported in our recent high pressure study,\nand therefore pressure induced hydrogen order–disorder transition\nmay be a general characteristic of layered hydroxides.