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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.

Cobalt oxyhydroxide (HCoO2), which belongs to the heterogenite mineral family, exists in two hexagonal polytypes (2H) and rhombohedral (3R). Here, we investigate the high pressure behaviour of both polytypes of HCoO2 using state‐of‐the‐art first principles calculations‐based density functional theory. We have used both local density approximation (LDA) and generalised gradient approximation (GGA) for exchange correlation functional. For the calculation of electronic band structures, we have also used the on‐site Coulomb interaction U term with LDA calculations and found improvement in results. The obtained ground state properties for both polytypes agree well with experimental and previous theoretical data. The electronic band structure shows that both phases are semiconductor with an indirect band gap in the ran\ge of 2.01–2.06 eV. The pressure‐dependent phonon dispersion curves and elastic constants depict the instability of 2H and 3R phases around 35 and 40 GPa, respectively. The O‐H distance gradually decreases for both phases with pressure and results into the less compressibility in z‐direction. The theoretically calculated pressure‐dependent Raman spectra of 2H‐HCoO2 show the peak broadening nature of E2g mode with pressure whereas in the case of 3R‐HCoO2, the A1g mode disappears beyond 45 GPa pressure. This indicates the existence of phase transition beginning around this pressure, which is driven by the shear modulus. A detailed analysis of the mode compatibility in the case of 2H polytypes is also reported. The phonon mode evolution with pressure is also analysed.

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