In situ observation of the phase transition behavior of shocked baddeleyite

Abstract:

Baddeleyite (ZrO₂) is used to infer shock pressures and understand the impact history of planetary bodies. Previous researches on natural shocked-baddeleyite in the earth’s rock have observed grain refinement and the orthogonally related crystallographic orientations of the baddeleyite grains, which infer the occurrence of phase transition. Although the phase transition behavior of baddeleyite at high pressures has been intensively investigated under hydrostatic conditions, there is little information on dynamic response of the phase transition under shock loading conditions. Therefore, it is required to clarify the phase transition behavior of baddeleyite under shock compression in order to utilized as a shock barometer. Here, we performed in situ X-ray diffraction measurements on shock-loaded baddeleyite on nanosecond time scale using shock-driving laser and synchrotron X-ray.

The experiments were performed at NW14A beamline of the Photon Factory Advanced Ring (PF-AR), KEK, Japan. Nd:glass laser with the wavelength of 1064 nm, pulse energy of 16 J, and pulse width of 12 ns was used for shock-driving source, while the synchrotron X-ray pulse was used as a probe source. The time-resolved XRD data at 6.5, 12.4, 15.4, 32.0, and 1000 ns after shock compression were collected. Shock pressure was estimated by VISAR (velocity interferometer system for any reflector) measurement.

The peak shock pressure was estimated to 15 GPa from VISAR data. Time-resolved XRD data clearly showed that the shock compression caused a phase transition from monoclinic phase (ambient phase) to orthorhombic-I phase (high-pressure phase). No other phase was observed in this study. The phase transition occurred during shock compression, and the orthorhombic-I phase immediately returned back to the monoclinic phase during subsequent release. At 6.5 ns, the compression of the monoclinic phase was 3.3 GPa, and the orthorhombic-I phase was observed at the same time. The maximum compression of the monoclinic phase was 3.6 GPa at 12.4 ns. Since no further compression of the monoclinic phase was observed and considering the volume fraction of the orthorhombic-I phase, the phase transition was assumed to be almost complete, not partial. The phase transition between the monoclinic and orthorhombic-I ZrO₂ during shock was similar to those under static compression condition. This study refined the phase transition behavior of baddeleyite under shock conditions up to 15 GPa, and the phase transition boundary was determined from direct observations. These findings improve the understanding of the shock history of planetary bodies.