Studies on the structural stability of Co2P2O7 under pressure

The crystal structural evolution of Co₂P₂O₇ was studied by using in situ high pressure angle dispersive x-ray diffraction with synchrotron radiation. The results demonstrate that the α phase of Co₂P₂O₇ goes through a partially irreversible structural transformation to β phase under pressure. The pressure is conductive to reduce the longest Cosbnd O bond length of the α phase, and then more uniform Cosbnd O bonds and regular hexagonal arrangement of CoO₆ octahedra of the β phase are favored. According to the Birch-Murnaghan equation, the fitted bulk modulus B₀ is 158.1(±5.6) GPa for α phase and 276.5(±6.5) GPa for β phase. Furthermore, the first-principles calculations show that these two phases of Co₂P₂O₇ have almost equal total energies, and also have similar band structures and spin-polarized density of states at their ground states. This may be the reason why these two phases of Co₂P₂O₇ can coexist in the pressure released state. It is found that the band gap energies decrease with increasing pressure for both phases.