Ultrafast x-ray diffraction of high-pressure phases in dynamically compressed TiO₂

We investigate the high-pressure polymorphism of TiO2 under laser-shock compression from 54(5) to 137(7) GPa using in situ femtosecond x-ray diffraction. Our results provide experimental evidence of the Pca21-type distorted fluorite structure formed from polycrystalline TiO2 dynamically compressed to 54(5) GPa. Upon higher compression, we observe the direct formation of the ninefold coordinated Fe2P-type phase at 68(4) and 78(3) GPa in polycrystalline and [001]-oriented TiO2, respectively. This represents an unprecedented 100 GPa reduction in the shock synthesis pressure of the Fe2P-type structure relative to quasihydrostatic loading conditions. On pressure release, the Fe2P-type phase transforms to the α-PbO2 structure and, at later times, reverts to rutile. Thus, the rutile → Fe2P and α-PbO2 → rutile transformations are both observed to occur on nanosecond timescales. Our results highlight the unique ability of high-strain-rate uniaxial compression to synthesize novel high-pressure phases and also indicate the importance of in situ atomic-level probes in developing pressure-temperature phase diagrams.