HiPSEC

Shock recovery studies show that micro-turbulent mass transport in shock-generated melts and ultrafast growth and formation of high pressure phase at P-T-t conditions of 26 GPa, 2000-3000 K, 700 ns. This study has significant implications to understanding of dramatic meteorite impact processes in early solar terrestrial system, by Tschauner et al. Proceedings of the National Academy of Sciences, vol. 106(33), 13691-13695, August 2009.

Figure: Images and composition mapping of shocked samples. EMP wavelength-dispersive X-ray mapping of magnesium (A), silicon (B), and iron (C) concentration in section of the shocked sample. (D) EMP backscattered electron (BSE) image of sample. Melted areas are clearly indicated by the fine scale dispersion of metal melt droplets (bright spots).

Prof. Chen's research group has unveiled that the wurtzite-type boron nitride (w-BN) has a greater indentation strength than diamond, currently the hardest known material in the world. The study also shows that another material, lonsdaleite (also known as hexagonal diamond), should be even stronger than w-BN and 58 percent stronger than diamond, setting a new record. This analysis represents the first case where a material exceeds diamond in strength under the same loading conditions. In addition, by showing the underlying atomistic mechanism that can strengthen some materials, this work may provide new approaches for designing "superhard" materials, by Pan, et al., Physical Review Letters vol. 102(5), 055503, February 2009.

Our recent studies have revealed that high pressure compels outer shell electrons to pair in higher energy levels and forces an electronic transition at 44 GPa in FeCO3. The elastic contribution of neighboring clusters to the pressure-induced spin pairing of Fe2+ d-electrons is demonstrated by the hysteresis of formation of spin-like domains in the FeCO3. High-resolution structural analysis allowed the observation of fine structural rearrangements accompanying the electronic phase transition, such as the lengthening of the bond distance of the triangular CO3 unit bridging adjacent octahedra, and the sudden reduction of the amplitudes of thermal vibrations. These studies are published by Lavina et al., in Geophysical Research Letters, vol. 36, L23306, December 2009, Physical Review B (condensed matter and materials physics), vol. 82, 064110, August 2010, and High Pressure Research: An International Journal, vol. 30(2), 224-229, June 2010.

Figure: The spin transition in FeCO3 determines a gain in absorption in the visible range, as shown by the photos taken at different pressures. The transition occurs trough the formation of spin like-domains evidenced by the splitting of all diffraction peaks.

We have observed high pressure induced structural phase transition from Cmmm to Pbnm in the iron based FeSe superconductor around 1.6 GPa and near Tc, through high resolution high-pressure/low-temperature synchrotron x-ray diffraction. Another new high pressure induced structural phase transition of I4mm--Cm21--P1 has been identified in NH3BH3, a highly promising potential hydrogen storage compound, through combined x-ray, neutron, and DFT investigations. These two recent high pressure experimental studies on "energy-relevant materials" are published by Kumar et al., in Journal of Physical Chemistry B, vol. 114, 12597-12606 September 2010, and Chemical Physics Letters vol. 495, 203-207 2010 respectively.

Figures: Low temperature high pressure x-ray diffraction of FeSe and phonon density of state of FeSe at high pressure.