A combined synchrotron diffraction and first-principles investigation on structural properties of Co(OH)2under pressure up to 7 GPa

Abstract

An H–H repulsion induced structural anomaly in Co(OH)2 at pressure around 2.5 GPa is reported using systematic synchrotron X-ray powder diffraction and density functional theory (DFT) based first-principles calculations. The diffraction pattern evidences anomalous pressure dependence of Bragg peaks (003) and (013) compared to others indicating a much stronger change in the hydrogen sublattice with pressure. Analysis of the lattice parameters reveals a subtle but clear change in the trend of the out-of-plane to in-plane lattice constants' ratio above 2.5 GPa. Two independent pressure runs using two different pressure transmitting media yielded similar results, thus proving the observations to be independent of the minor variations in the quasi-hydrostatic conditions during the pressure run. DFT calculations were performed on an extended unit cell of Co(OH)2 which enables the description of hydrogen in either ordered or disordered state. Calculations showed that only the hydrogen disordered structure is energetically favourable at high pressures. We attribute the anomalous behaviour of the (003) and (013) Bragg peaks above 2.5 GPa to the charge delocalization along hydrogen sublattice coming due to the hydrogen ordered-disordered transition, in agreement with DFT results showing the H disordered state to be energetically more favourable at higher pressure. A second-order Brich-Murnaghan equation of state (EoS) can well describe the low- and high-pressure regime separately with a 20% higher bulk modulus of the latter, further supporting the subtle transition around 2.5 GPa. Present results also complement the previous Raman spectroscopic study on Co(OH)2 which reported anomalous behaviour in the (2) mode around 2.5 GPa.