The Ideal Crystal Structure of Cristobalite X-I: A Bridge in SiO2 Densification
On compression of α-cristobalite SiO2 to pressures above approximately 12 GPa, a new polymorph known as cristobalite X-I forms. The existence of cristobalite X-I has been known for several decades; however, consensus regarding its exact atomic arrangement has not yet been reached. The X-I phase constitutes an important step in the silica densification process, separating low-density tetrahedral framework phases from high-density octahedral polymorphs. It is the only nonquenchable high-density SiO2 phase, which reverts to the low-density form on decompression at ambient temperature. Recently, an experimental study proposed an octahedral model of SiO2 X-I with intrinsic structural defects involving partial Si site occupancies. In contrast, our new single-crystal synchrotron X-ray diffraction experiments have shown that the ideal structure of this phase should instead be described by a defect-free model, which does not require partial occupancies. Here, the structure of cristobalite X-I consists of octahedral chains with a 4-60°-2 zigzag chain geometry. This geometry has not been previously considered but is closely related to post-quartz, stishovite, and seifertite. In addition to the ideal, defect-free crystal structure, we also present a description of the defects that are most likely to form within the X-I phase. Density functional theory calculations support our observations, confirming the dynamic stability of the X-I geometry and reasonably reproducing the pressure of the phase transformation. The enthalpy of cristobalite X-I is higher than stishovite and seifertite, but X-I is favored as a high-pressure successor of cristobalite due to a unique transformation pathway. Elastic and lattice dynamical properties of the X-I phase show intermediate values between stable tetrahedral and octahedral polymorphs, confirming the bridge-role of this phase in SiO2 densification.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)
- Grant/Contract Number:
- AC02-06CH11357; NA0002006; FC52-08NA28554; EAR1440005; NA0001974; DMR1505817; FG02-99ER45775
- OSTI ID:
- 1464807
- Journal Information:
- Journal of Physical Chemistry. C, Vol. 122, Issue 30; ISSN 1932-7447
- Publisher:
- American Chemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- ENGLISH
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