Prediction of a hexagonal SiO ₂ phase affecting stabilities of MgSiO ₃ and CaSiO ₃ at multimegabar pressures

Abstract

Ultrahigh-pressure phase relationship of SiO 2 silica in multimegabar pressure condition is still quite unclear. Here, we report a theoretical prediction on a previously uncharacterized stable structure of silica with an unexpected hexagonal Fe 2 P-type form. This phase, more stable than the cotunnite-type structure, a previously postulated postpyrite phase, was discovered to stabilize at 640 GPa through a careful structure search by means of ab initio density functional computations over various structure models. This is the first evidential result of the pressure-induced phase transition to the Fe 2 P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO 9 tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO 2 on the stability of MgSiO 3 and CaSiO 3 at multimegabar pressures. A postperovskite phase of MgSiO 3 breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe 2 P-type SiO 2 and CsCl (B2)-type MgO. CaSiO 3 perovskite, on the other hand, directly dissociates into SiO 2 and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO 2 , MgSiO 3 , and CaSiO 3 indicate that the Fe 2 P-type SiO 2 could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.ab initio density functional simulation | multimegabar crystalline phase | silicon dioxide | super-Earth mantle E xploration of new high-pressure phases of silica has always attracted substantial interests in geophysics, materials science, and condensed matter physics because of its broad fundamental applicability (1-10). Low-pressure polymorphs of silica such as quartz have Si tetrahedrally coordinated by O, whereas in stishovite-(a rutile-type SiO 2 phase), CaCl 2 -, α-PbO 2 -, and pyrite (FeS 2 )-type phases, which form subsequently from ∼10 to ∼250 GPa, the coordination number (CoN) of Si increases to 6. Recent ab initio theoretical calculations have predicted a phase transition from the pyrite-to the cotunnite (α-PbCl 2 )-type structure around 700 GPa accompanied with further increase in CoN to 9 (11, 12). Although these studies showed some new aspects of SiO 2 at the pressures where experiments of solid-solid phase change are nearly impossible, they adopted the cotunnite structure to SiO 2 simply based on the knowledge of high-pressure behaviors of some analog compounds (13-16) with no attention to other candidates.Recent improvements in detection methods allow for the discovery of low-mass terrestrial exoplanets with 1-10 times Earth's mass (M ⊕ ), so-called "super-Earths" (17-20). Because pressure conditions of their deep mantles, as well as those of the cores of some gas giants, are located in a sub-terapascal range (21, 22), ultrahigh-pressure and temperature stabilities of SiO 2 and some other silicates such as MgSiO 3 and CaSiO 3 , major components in the Earth's...

Extremely long transition phase of thermal convection in the mantle of massive super-Earths

“…The dependence is the same as the one in our previous studies Miyagoshi et al 2014;; α decreases by a factor of 10 from the surface to the bottom boundary. This profile is estimated for MgO in a former ab initio calculation (Tsuchiya and Tsuchiya 2011) and is consistent with the previous laboratory measurements under high pressure (Chopelas and Boehler 1992).…”

Polymorphic phase transition mechanism of compressed coesite

“…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] .…”

Prediction of 10-fold coordinated TiO 2 and SiO 2 structures at multimegabar pressures

“…The central pressures of such planets can reach 30 TPa, which is extreme enough to compress electronic shells and involve core electrons in bonding. The phase stability and properties of systems at extreme pressures is mostly the realm of theory (3)(4)(5)(6)(7)(8)(9)(10)(11), although recent advances in diamond anvil cell techniques have seen the achievement of pressures as high as 640 GPa (12), and high-energy lasers now permit X-ray diffraction measurements at pressures approaching 1 TPa (13)(14)(15). A recent ramped compression experiment on diamond reached about 5 TPa, although diffraction measurements were not attempted (16).…”

“…Symmetry constraints prevent some structural relaxations, but searching with symmetry constraints is very useful because it often leads to the identification of low-enthalpy structures. In our searching, we have used numbers of formula units per cell of 1,2,3,4,6,8,12, and 16. …”