A new polymorphic phase transition
Researchers using the HP-CAT 16-ID-B and 16-BM-D beamlines at the Advanced Photon Source have discovered a new polymorphic phase transition mechanism of coesite to post-stishovite.
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.
Q.Y. Hu, J.F. Shu, A. Cadien, Y. Meng, W.G. Yang, H.W. Sheng and H.K. Mao, “Polymorphic Phase Transition Mechanism of Compressed Coesite,” Nature Communications 6, Article Number: 6630, DOI: 10.1038/ncomms7630, Published March 20, 2015.