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     COLLOQUIUM OF THE COMPUTATIONAL MATERIALS SCIENCE CENTER
       AND THE DEPARTMENT OF COMPUTATIONAL AND DATA SCIENCES
                         (CSI 898-Sec 001)
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          New transitory phases of silica under high pressure

                          Qingyang Hu
Geophysical Laboratory, Carnegie Institution of Washington, Washington DC

Silica is one of the most-abundant natural compounds and a major component 
of the Earth’s crust and mantle. Its various high-pressure forms make it 
an often-used study subject for scientists interested in the transition 
between different chemical phases under extreme conditions, such as those 
mimicking the deep Earth. Compressing single crystal coesite SiO2 under 
hydrostatic pressures of 26~53 gigapascal at room temperature, we discover 
a new polymorphic phase transition mechanism of coesite to post-stishovite, 
by means of single-crystal synchrotron x-ray diffraction experiment and 
first-principles computational modeling. 

The transition features the formation of multiple previously unknown triclinic 
phases of SiO2 on the transition pathway as structural intermediates. 
Coexistence of the low-symmetry phases results in extensive splitting of the 
original coesite X-ray diffraction peaks that appear as dramatic peak 
broadening and weakening, resembling an amorphous material. This work 
provides new insights into the structural transition of SiO2 crystal under 
high pressures, and clarifies the issue of the pressure-induced amorphization 
of coesite, which has often been cited as an archetypal example of the 
amorphization phenomena in general.

                           November 2, 2015
                               4:30 pm
                    Exploratory Hall, room 3301 
                          Fairfax Campus

Refreshments will be served at 4:15 PM.
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Find the schedule at http://www.cmasc.gmu.edu/seminars.htm