Isothermal Compression and High Pressure Phase Transformation of Nickel Oxide (Bunsenite)

Nickel oxide (NiO, Bunsenite) has been used as an analogue modeling composition in relation to other monoxide as MgO, FeO of earth interior. Crystal structure change of NiO from NaCl type fcc to rhombohedral was discovered by Rooksby in 1943 at low temperature183 ℃. We recently carried out experiments under high pressure to 12.0 GPa using diamond anvil cell with liquid pressure transmitting medium and direct compression to 29 GPa. Our experimental results show the isothermal compression of NiO under pressure from 1.22 GPa to 8.87 GPa, lattice parameters and molal volume. At zero pressure: a0=0.418 24 nm; Vm=11.009 910-3 nm3; density, 6.780 2 g/cm3 and at 8.87 GPa; a=0.411 39 nm; Vm=10.481 910-3 nm3; density, 7.125 4 g/cm3. All data show a small deviation from Drickamer and S. K. Agrawals results. The diffraction lines (111), (220) of NiO suddenly splitting until pressure loading up to 12 GPa at hydrostatic compression. Phase change of NiO from cubic to rhombohedral. The same results obtained by direct compression at pressure to 5.8 GPa and up to 29 GPa. Lattice parameter of rhomb, new phase a=0.296 70 nm; c=0.700 00 nm; Vm=10.708 410-3 nm3; density, 6.972 6 g/cm3, this date agree with Rooksby's a=0.296 60 nm. The magnetic properties of NiO vary with pressure change Neel temperature of NiO 250 ℃ (523 K), decreases, when pressure increases, whereas below Neel temperature NiO changed in antiferromagnetic and rhombohedral crystal structure. Calculation bulk modulus of NiO by Birch-Manghnahan equation best fits to the p-V data and assuming K0' values of 4, give K0=166.83 GPa. Molal volume vs bulk modulus shows the compressibility of NiO is approaching to MgO.