Pressure-induced amorphization of crystalline silica

Polymorphic phase transition mechanism of compressed coesite

“…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 (the structural parameters are shown in Supplementary Table 2).…”

“…With the split peaks at different 2y (or Q) values, the integrated 1D diffraction spectra become very broad and weak and resemble the amorphization phenomenon reported in the literature 3 . A new set of diffraction peaks from a denser silica phase emerge at 32 GPa and coexist with the existing metastable phases.…”

“…Its discovery and its transition at higher pressures to stishovite 2 with silicon in octahedral coordination (SiO 6 ) demonstrated the importance of the pressure variable for understanding the deep Earth, and in effect, ushered in the new era of high-pressure mineral physics. Subsequent experiments at higher pressures and high temperatures produced the equilibrium conditions [3][4][5][6][7][8][9][10] , and revealed a number of stable post-stishovite phases, including octahedrally coordinated silica with the CaCl 2 and a-PbO 2 structures and eventually to the pyrite structure [11][12][13][14][15][16][17] . The tetrahedron-to-octahedron transition in pure silica also exemplifies the universal phenomena in all silicate minerals throughout the mantle [18][19][20] .…”

“…High pressure and low temperature create conditions that favour the denser packing from 4-to 5-to 6-coordination while hindering the true equilibrium and preserving metastable intermediate states 3,21,22 . Energy-dispersive X-ray diffraction study of polycrystalline coesite showed the disappearance of sharp crystalline peaks indicating that above 30 GPa silica collapses to an amorphous phase and is metastable up to above 40 GPa at which point an octahedrally ordered phase appears 3,6 . However, the possibility of the formation of a fully disordered phase was reputed 23 for lack of long-range atomic diffusion, advocating that a portion of crystal ordering may still be present in the amorphous phase.…”

The pressure-amorphized state in zirconium tungstate: a precursor to decomposition

“…Although it is widely believed that the phenomenon of pressure-induced amorphization (PIA) arises due to kinetic hindrance of equilibrium phase transitions, the structure of the equilibrium high-pressure phase, which the compound should have ideally evolved to, has remained speculative or unknown in most cases. Only in a few systems such as ice [1], quartz [2] and tetra-cyano-ethylene [8] can the high-pressure phase be identified from further measurements [9] and from computer simulations [10]. In this context, in addition to characterization of the p-amorphized state using x-ray diffraction and Raman spectroscopy, measurements of additional structural properties/features of this state such as its volume can be helpful in the identification of the equilibrium phase among different high-pressure polymorphs.…”