Pressure-Induced Structural Transitions in Cristobalite Silica Molecular dynamics simulations, based on a new charge-transfer multiple-coordination state.

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Pressure-Induced Structural Transitions in Cristobalite Silica Molecular dynamics simulations, based on a new charge-transfer multiple-coordination state potential, were used to elucidate the high-pressure behavior of cristobalite silica. Upon densification of the material, structural units eventually convert from SiO 4 tetrahedra to SiO 6 octahedra. This transformation occurs in two stages: first, oxygen rearranges into a hcp sub-lattice; second, silicon atoms jump into the newly formed interstices of this dense oxygen sub- structure to yield stishovite. Further compression produces the post-stishovite phase. The configuration intermediate to the two transformation stages corresponds to the X-I phase, which has been observed experimentally, but whose structure has been a mystery until now. Our simulations revealed the nature of the X-I phase and its significance for the densification process. John Kieffer, PI, University of Michigan, DMR Jay D. Bass, Co-PI, University of Illinois 16 GPa (C222 1 ) a’ = 6.7 Å b’ = 5.7 Å c’ = 6.1 Å c’ a’ 20 GPa (P ) a = 4.2 Å b = 4.2 Å c = 6.0 Å 0 GPa (P ) a = 4.9 Å b = 4.9 Å c = 5.4 Å 25 GPa (P42/mnm) a = 4.2 Å b = 4.2 Å c = 2.8 Å 14 GPa (P ) 40 GPa (Pnnm) a = 3.7 Å b = 4.5 Å c = 2.8 Å X-I phase  -cristobalite stishovitepost-stishovite

Pressure-Induced Amorphization of Quartz Silica John Kieffer, PI, University of Michigan, DMR Jay D. Bass, Co-PI, University of Illinois (a)  -quartz at 0 GPa (b) High-P quartz at 25 GPa (c) Ideal high-P quartz with bcc anion sub-lattice Molecular dynamics simulations based on a new charge-transfer multiple- coordination state potential have been used to elucidate the high-pressure behavior of quartz silica. Upon densification the structure eventually amorphizes (d), with silicon existing in mostly 6-coordinated states. This process occurs in two stages: first, the oxygen rearranges (b), approaching a dense bcc sub-lattice (c); second, before actually achieving configuration (c), silicon atoms jump into the newly formed interstices. Because these displacements occur in an uncoordinated fashion, silicon ends up occupying random positions to yield an amorphous structure (d). (d) Amorphized quartz at 30 GPa

Polyamorphism, Structural Transitions, and Related Phenomena Education: Five undergraduate (Ranjeet Rao, Surair Bashir, Vashist Vasanthakumar, Peter Gullekson, and Nicole Tucker) and two graduate students (Liping Huang and Jason Nicholas) contributed to this work. Ranjeet is now a graduate student at the University of Illinois and recipient of an NSF Graduate Fellowship. Surair (an African-American female), Vashist, Peter, and Nicole are undergraduates at Michigan. Jason has graduated with an M.S. degree in May ‘03, and is now a Ph.D. student at U.C. Berkeley. John Kieffer, PI, University of Michigan, DMR Jay D. Bass, Co-PI, University of Illinois Liping received her Ph.D. in May ‘04. As the 2003 recipient of the Norbert J. Kreidl Award from the ACerS, she was invited to give a featured lecture on her simulation work during the Norbert J. Kreidl Memorial Conference, June 23-26, 2004, in Slovakia. Liping is now a postdoctoral research associate in my group and is still involved in the project. Outreach: The PI is co-organizer and participant in the Materials Camp held annually at Michigan. 20 high school science teachers spend a week learning about materials science, with the goal of using this information in their curricula and teach high school students about the role of materials in technology.