Pressure Induced Hydrogen Order-Disorder Transition in β-Ni(OH)2

Abstract

Here we address the challenging problem of pressure induced hydrogen sublattice disordering in layered hydroxides using synchrotron X-ray powder diffraction (XRPD) and first-principles calculations based on density functional theory (DFT) using β-Ni(OH)2 as a model system. XRPD data demonstrate anomalous behaviors in some Bragg peaks. The shift in (003) and (013) peaks with pressure is large as compared to that of other Bragg peaks and these two peaks are not discernible above 5 GPa, similar to the characteristic observed in β-Co(OH)2. The underlying changes within the structure were probed using DFT calculations on a √3 × √3 × 1 supercell of β-Ni(OH)2. Structural relaxation yields a hydrogen disordered structure as the ground state for β-Ni(OH)2 at pressures above 5 GPa. The disordered model also describes well the high pressure XRPD data. The hydrogen sublattice largely affects the electronic density of the (003) and (013) lattice planes thus permitting the development of a one-to-one correspondence of the anomalous behavior of these Bragg peaks with the hydrogen sublattice disordering. Further evidence of a possible structural rearrangement was obtained by the pressure dependence of the unit-cell volume (P vs V) as well as the ratio of the out-of-plane to in-plane lattice parameter (c/a). With the availability of a large number of data points with smaller pressure steps, the above two quantities are shown to behave differently below and above 2.5 GPa. The P vs V data are well described by a second order Birch-Murnaghan equation-of-state, whereas for c/a, a clear change in trend is observed for data above 2.5 GPa. The pressure response of β-Ni(OH)2 is similar to that of Co(OH)2 reported in our recent high pressure study, and therefore pressure induced hydrogen order-disorder transition may be a general characteristic of layered hydroxides.