Pressure-stabilized divalent ozonide CaO3 and its impact on Earth’s oxygen cycles

Abstract

High pressure can drastically alter chemical bonding and produce exotic compounds that defy conventional wisdom. Especially significant are compounds pertaining to oxygen cycles inside Earth, which hold key to understanding major geological events that impact the environment essential to life on Earth. Here we report the discovery of pressure-stabilized divalent ozonide CaO3 crystal that exhibits intriguing bonding and oxidation states with profound geological implications. Our computational study identifies a crystalline phase of CaO3 by reaction of CaO and O2 at high pressure and high temperature conditions; ensuing experiments synthesize this rare compound under compression in a diamond anvil cell with laser heating. High-pressure x-ray diffraction data show that CaO3 crystal forms at 35 GPa and persists down to 20 GPa on decompression. Analysis of charge states reveals a formal oxidation state of −2 for ozone anions in CaO3. These findings unravel the ozonide chemistry at high pressure and offer insights for elucidating prominent seismic anomalies and oxygen cycles in Earth’s interior. We further predict multiple reactions producing CaO3 by geologically abundant mineral precursors at various depths in Earth’s mantle.

The Structure and Elasticity of CaO3 Under High Pressure by First-Principles Simulation

“…Under 0-10 GPa, the lattice vibration produces a negative value (frequency less than 0) in the Brillouin region (Figure 3A) which means the CaO 3 structure is not stable (Gonze, 1997), but it is stable in the range of 10-100 GPa (Figure 3B). These results are consistent with the previous result (Wang et al, 2020) that the CaO 3 may exist stably in the mantle transition zone.…”

“…CaO 3 belongs to the tetragonal system, and the space group is p-421 m (Wang et al, 2020). In this study, first-principles calculations are performed using the density functional theory (Hohenberg and Kohn, 1964;Kohn and Sham, 1965) with the plane wave pseudopotential.…”

“…The CaO 3 density increases linearly with the increased pressure (Earth's depth) (Figure 4). The CaO 3 is dynamically stable at 20-50 GPa and may exist in the mantle transition zone and the lower mantle (Wang et al, 2020). Therefore, the densities of several main mineral phases in this area are also listed in Figure 4, including wadsleyite (Inoue et al, 1998), ringwoodite (Inoue et al, 1998), CaSiO 3 perovskite (Karki and Crain, 1998), and MgSiO 3 perovskite (Karki et al, 1997) under high pressure and density of the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981).…”

“…Recently, Wang et al (2020) proposed an alternative mechanism, that is, CaO 3 may decompose into CaO and O 2 at 20 GPa, resulting in the change of the material composition at this depth to explain seismic wave velocity anomalies near 660 km depth in the Earth's mantle. CaO 3 is a newly discovered material that may exist in the mantle transition zone, and CaO 3 may form at 35 GPa and existence under reduced pressure to 20 GPa.…”

“…CaO 3 is a newly discovered material that may exist in the mantle transition zone, and CaO 3 may form at 35 GPa and existence under reduced pressure to 20 GPa. Once reaching the transition zone at depths of less than 500 km, CaO 3 would decompose to provide a sporadic source of extra O 2 that would work its way up toward the surface of the Earth to complete the oxygen cycle (Wang et al, 2020). Therefore, this new mineral and reaction phenomenon may affect the composition and structure of the mantle transition zone and the lower mantle.…”