Multiple pathways in pressure-induced phase transition of coesite

Abstract

High-pressure single-crystal X-ray diffraction method with precise control of hydrostatic conditions, typically with helium or neon as the pressure-transmitting medium, has significantly changed our view on what happens with low-density silica phases under pressure. Coesite is a prototype material for pressure-induced amorphization. However, it was found to transform into a high-pressure octahedral (HPO) phase, or coesite-II and coesite-III. Given that the pressure is believed to be hydrostatic in two recent experiments, the different transformation pathways are striking. Based on molecular dynamic simulations with an ab initio parameterized potential, we reproduced all of the above experiments in three transformation pathways, including the one leading to an HPO phase. This octahedral phase has an oxygen hcp sublattice featuring 2 × 2 zigzag octahedral edge-sharing chains, however with some broken points (i.e., point defects). It transforms into α-PbO phase when it is relaxed under further compression. We show that the HPO phase forms through a continuous rearrangement of the oxygen sublattice toward hcp arrangement. The high-pressure amorphous phases can be described by an fcc and hcp sublattice mixture.

Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts

“…5 , inset). In fact, the very large differences in enthalpy between phases stable at corresponding conditions (stishovite, CaCl 2 -type, seifertite) and coesite-IV, V indicate that using modern algorithms of crystal structure predictions or molecular dynamic simulations 18 , 30 , 31 , it would be highly difficult (if possible at all) to envisage their existence. On the other hand, the observed enthalpy difference is similar to the values calculated for carbon polymorphs, for example, such as diamond and C 60 32 .…”

“…On the contrary, based on a combination of X-ray diffraction and ab initio metadynamics simulation, Hu et al 17 have proposed a transition between 26 and 53 GPa from coesite-I to post stishovite (i.e., built of only SiO 6 octahedra) through a series of triclinic intermediate phases featuring both SiO 4 tetrahedra and SiO 6 octahedra. Further first-principles calculations made by Liu et al 18 suggest that post-stishovite phase should eventually transform to the one with α-PbO 2 structure. So far, the structures of coesite-III and other possible high-pressure polymorphs of coesite remain unknown calling for further investigations.…”

Lattice dynamics : From fundamental research to practical applications

“…But when the thermodynamical stability is concerned in the analysis, we see that coesite-IV and cosite-V have approximately 0.4 eV/ion higher enthalpy than the competing phases stishovite and seifertite, see Fig. 6.10, making their crystal structures extremely difficult, or even impossible, to predict using modern crystal structure prediction algorithms and MD simulations [74][75][76]. Due to this large difference in enthalpy, coesite-IV and cosite-V should be considered as metastable phases of silica, that could 76.5989 8 Table 6.1: Experimental reported (exp.)…”

Computational searches for crystal structures of dioxides of group 14 elements (CO2, SiO2, GeO2) under ultrahigh pressure

“…Previous theoretical studies have successfully reproduced properties of known crystal phases and proposed unidentied crystal structures of AO 2 (A ¼ C, Si, or Ge) under ultrahigh pressure. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] These theoretical predictions were made by stochastic trials including genetic algorithms, or by geometry optimization or MD simulation starting from a known structure or an intuitive initial guess. Since a stable structure in an ultrahigh pressure environment might be completely different from that under normal pressure, especially in the case of CO 2 , it is not easy to comprehensively collect all polymorphs of AO 2 .…”