Polymorphic phase transition mechanism of compressed coesite.

Q.Y. Hu,W.G. Yang,Y. Meng,H.-K. Mao,A. Cadien,H.W. Sheng,J.-F. Shu

doi:10.1038/ncomms7630

Abstract

Silicon dioxide is one of the most abundant natural compounds. Polymorphs of SiO₂ and their phase transitions have long been a focus of great interest and intense theoretical and experimental pursuits. Here, compressing single-crystal coesite SiO₂ under hydrostatic pressures of 26-53 GPa 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 modelling. 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 sheds light on the long-debated pressure-induced amorphization phenomenon of SiO₂, but also provides new insights into the densification mechanism of tetrahedrally bonded structures common in nature.

Highlights

Silicon dioxide is one of the most abundant natural compounds
Car-Parrinello molecular dynamics (MD) simulations[8] predicted a direct phase transition of coesite to an octahedral structure, by pushing the system along the softest mode in the lattice avoiding a collapse of crystalline symmetry
All diffraction spots collected on the area detector can be readily indexed with the monoclinic coesite structure (Fig. 1a)

Summary

Results

The diffraction pattern after splitting can be uniquely fit into four distinct but nearly isochoric structures (Fig. 1f) that each one has a unique orientation matrix and has a topotaxial relationship with the parent coesite crystal (Supplementary Table 1). We note that this phenomenon is in contrast to the reported formation of the intermediate amorphous phase[3]. These phases assume a reduced triclinic symmetry

GPa 1 GPa

Discussion

Methods