Synthesis of a new high-pressure phase of manganese dioxide

Both amorphous and crystalline (rutile) forms of MnO₂ have been subjected to loading pressures of 220 and 250 kbar and heated by a laser to approximately 1000°C in a diamond-anvil press. In all runs, X-ray diffraction study of the quenched sample reveals a mixture of pyrolusite (the rutile form of MnO₂) plus an unknown phase. This phase has been tentatively indexed on the basis of a large cubic cell with lattice parameter a₀ = 9.868 ± 0.011Å. Shock-wave data for MnO₂ up to 1300 kbar indicate that any phase transformation must involve only a small volume change. If the high-pressure phase is the cubic phase of this paper, then the latter has 36 formula units per unit cell, implying a zero-pressure volume change of 3.2% from the rutile to cubic phase. The cubic phase may provide an alternative model for the high-pressure phase of oxides having the rutile structure.