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

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.

P ressure and temperature are key thermodynamic variables that prominently influence material structure and properties. Diverse high-pressure and high-temperature (HPHT) conditions simulated in computation and generated in laboratory-based experimental devices offer exciting opportunities for new material discovery and exploration of otherwise inaccessible deep-Earth environments. Recent years have seen the advent and rapid advance of computational structure search and characterization of pressure-stabilized compounds with unusual stoichiometries, such as Na-Cl 1 , Xe-Fe 2 , Xe-O 3 , and La-H 4 series that do not exist at ambient conditions, and several of these compounds have already been experimentally synthesized 5,6 . Also notable are recent experimental and theoretical studies that have led to the discovery of unconventional iron oxides with unusual oxidation states 7 in FeO 2 (ref. 8 ), Fe 2 O 3 (ref. 9 ), and Fe 5 O 6 (ref. 10 ), opening avenues for making and exploring iron oxides with peculiar properties like unusual chemical valence and bonding interactions. Such results also restore the redox equilibria inside Earth and place oxygen reservoirs at greater depths than previously thought. These findings offer insights for elucidating large-scale geological activities that may have influenced events related to the origin of life on Earth.
Calcium and oxygen are two of the most abundant elements widely distributed in Earth's mantle 11 . In accordance with their respective valence electron counts, calcium and oxygen are expected to preferably form CaO 12 , which is present throughout the mantle. The oxidation state of O in CaO is O 2− at ambient conditions. Meanwhile, calcium peroxide 13,14 also can stabilize at ambient or high pressures 15 . In general, oxygen species with oxidation states higher than −2 can be synthesized in superoxide (O 2 − ), peroxide (O 2 2− ), and ozonide (O 3 −) compounds and play prominent roles in oxidation chemistry 16 . Among these compounds, ionic ozonides are regarded as a species with unusual reaction processes and properties, and are scarce due to their high reactivity, thermodynamic instability, and extreme sensitivity to moisture in ambient environments 17 .
In this work, we report on the discovery from a combined theoretical and experimental study a high-pressure phase of CaO 3 containing unusual divalent ozone anions that shed light on ozonide chemistry at extreme conditions, and the results offer insights for understanding deep-Earth chemical reactions that are relevant to oxygen cycles inside our planet.

Results and discussion
Stable Ca-O compounds at high pressure. For insights to help find new calcium oxide compounds, we have employed unbiased crystal structure search techniques as implemented in CALYPSO code 18,19 , which has been successful in resolving crystal structures of a large number and variety of materials at high pressure 20 . Here, we explore calcium oxides in the oxygen-rich regime, seeking compounds that do not exist under ambient conditions. Studies of mantle rocks have shown that oxygen fugacity of the upper mantle is relatively high 21 , thus connecting the present work to prominent geological topics concerning oxidation states of minerals and oxygen storage and cycles inside Earth. We have performed structure searches on Ca m O n (m = 1, 2 and n = 2, 3, 4) with maximum simulation cells up to four formula units (f.u.) at each composition, and this procedure identifies two stable Ca-O compounds, a CaO 2 phase at 30 GPa and an unusual stoichiometric CaO 3  We characterize the newly identified calcium ozonide by examining its synthesis routes and structural, bonding, and electronic properties. The CaO 3 phase crystalizes 22 in a tetragonal BaS 3 -type structure 23 (space group P-42 1 m, 2 f.u. per cell) in a wide range of pressures and exhibits a distinct configuration containing isolated V-shaped O 3 units and edge-sharing CaO 8 cuboid (Fig. 1a). We compare to some well-established compounds on key structural and bonding characters of the crystalline CaO 3 at 30 GPa, which is inside its stability field.  Formation enthalpy calculations reveal that CaO 3 is energetically favorable relative to decomposition into CaO 12 and solid O 2 (ref. 24 ) above 27.2 GPa via the reaction: To account for thermal effects, we further examine vibrational contributions and entropic effects for the relevant phases 25 , and construct the finite-temperature phase diagram of CaO 3 . Calculated zero-point energy values at 30 GPa for CaO, solid O 2 , and CaO 3 are 0.13, 0.22, and 0.32 eV/f.u., respectively, resulting in a minimal difference between the reactants and products for the above reaction of only −0.03 eV/f.u., which has only a minor impact on the threshold pressure for CaO 3 decomposing into CaO and O 2 , reducing it from 27.2 to 25.3 GPa (Fig. 1b). The threshold pressure for the stability of The powder X-ray diffraction (PXRD) patterns around the heating spot were collected are shown in Fig. 2b. We observe two distinct Bragg peaks at 10.6°and 11.4°and several small peaks from the raw 2D diffraction images and integrated PXRD patterns that do not correspond to CaO, CaO 2 , or any known calcium oxides. Meanwhile, the measured XRD pattern can be indexed by the predicted tetragonal BaS 3 -type structure of CaO 3 , together with CaO 4 , unreacted CaO, and oxygen, due to the mixed feature of obtained phases. The observed peaks at 10.6°a nd 11.4°in the XRD pattern correspond to the (200) Table 1 (Fig. 3b) show that electrons in the antibonding 2b 1 orbital dictate properties of O 3 anions. Alkali-metal ozonides 29 containing [O 3 ] − belong to a small group of chemical species hosting unpaired p-electrons that produce a paramagnetic state. In stark contrast, divalent ozonide anion [O 3 ] −2 has a closedshell configuration (Fig. 3b) with each O 3 unit containing 20 electrons in 10 orbitals with no unpaired p-electron, leading to nonmagnetic characteristics (Fig. 3c). To illustrate this point, we have constructed a model system of hypothetical Ca 0 O 3 where all Ca atoms were removed from the BaS 3 -type structure, and this model system exhibits partially unoccupied bonding states of the O 2p orbital (Fig. 3d), which become fully occupied once Ca was incorporated into the crystal lattice due to charge transfer from Ca to O, leading to the non-magnetic insulating state in CaO 3 . The newly discovered calcium ozonide is expected to have major implications for geoscience. In this context, we have examined additional viable routes producing CaO 3 involving several minerals abundant in Earth's mantle as reactants: with the structures of pertinent materials employed in calculating the reaction enthalpies are presented in Supplementary Table 1. Similar to the reaction shown in Eq. (1), the reactions in Eqs. (2) and (3) occur in oxygen-saturated environments and the reactants and products attain equilibrium at pressures of 20 GPa (Fig. 4a) and 40 GPa (Fig. 4b), respectively, corresponding to conditions near the top of the lower mantle, where previous studies reveal that oxygen fugacity is likely inhomogeneous with some regions containing relatively high oxygen content 21 , thus conducive to these reactions in forming CaO 3 . Our calculations show that the reactions described in Eqs. (1) and (2) produce CaO 3 at~20 GPa, which corresponds to pressures at the boundary of Earth's upper and lower mantle. Previous studies revealed that several minerals such as CaCO 3 (ref. 32 ), MgCO 3 (ref. 33 ), and CO 2 (ref. 34 ) can dissociate and produce oxygen in this pressure range, offering an abundant source of O 2 for these proposed reactions inside Earth's mantle. The resulting compound CaO 3 , which was not previously considered, provides an alternative mechanism to explain seismic anomalies near 660 km depth in Earth's mantle where pressure is 20 GPa 35,36 . The reaction route indicated in Eq. (4) describes the formation of CaO 3 and FeOOH by FeO 2 and CaO under H 2 O-saturated conditions and the equilibrium pressure of this reaction is about 90 GPa (Fig. 4c), corresponding to deep lower mantle conditions.  Recently, a pyrite FeO 2 phase stabilized at high pressure (76 GPa) and temperature (1800 K) was proposed 8 to exist in Earth's lower mantle below 1800 km. Our calculations show that once CaO and H 2 O are thrusted to deeper than 1800 km, they can react with FeO 2 and produce CaO 3 + FeOOH. Due to the higher density of FeOOH compared to that of the mantle 37 (Fig. 4d), FeOOH would sink towards the core 38 , while the lighter CaO 3 would ascend by mantle dynamic processes. 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 Earth to complete the oxygen cycle. Oxygen fugacity and oxidation states of minerals in geological environments play pivotal roles in deciding planetary chemical and physical dynamics, and such key information can be determined through mineral equilibria 39 . Quantification of oxygen fugacity depends sensitively on the content and stability of mineral assemblages at the pressures and temperatures in Earth's interior. Our discovery of divalent ozonide CaO 3 introduces a new ingredient to buffer oxygen fugacity and influence redox equilibria of Earth's mantle, providing crucial insights into the redox state of the largely inaccessible deeper mantle. Furthermore, our results highlight CaO as a reducing agent to react with free oxygen at high pressures, suggesting a natural reservoir for O 2 storage in Earth's mantle and providing a possible resolution to the missing O 2 paradox before the Great Oxidation Event 40 . The present findings also raise exciting prospects of synthesizing CaO 3 via additional avenues, such as those listed in Eqs. (2)-(4), in the laboratory setting for a more indepth understanding of these reactions and their roles in influencing important geological events. The discovery of crystalline divalent calcium ozonide is expected to stimulate further experimental and theoretical exploration for further insights into this compound and the associated intriguing bonding characters that hold great promise for probing exotic properties that have great fundamental significance and implications for practical processes in chemistry and geoscience.
We have conducted a joint computational and experimental exploration of calcium oxides at high pressure, aiming to probe unusual stoichiometry, structural form, and oxidation states. Our study leads to a discovery of CaO 3 , expanding both the calcium oxide family and ionic ozonide family of compounds. This rare crystalline ozonide is computationally predicted and then experimentally synthesized via reaction of solid CaO and O 2 at HPHT conditions in a DAC assisted by laser heating. Remarkably, a charge analysis indicates that the O 3 unit in CaO 3 carries a formal oxidation state of −2. These findings enrich fundamental understanding of bonding interactions between calcium and oxygen, highlighting novel ozonide chemistry at high pressure, and the reported results have major implications for elucidating prominent seismic anomalies and oxygen cycle processes in Earth's mantle.

Methods
Experimental procedures. High-purity CaO (Alfa, 99.95%) powder or Ca piece (Alfa, 99%) were compressed into thin plates of 50 μm × 50 μm × 15 μm dimensions and loaded in a DAC with a culet of 300 μm. The sample chamber has a 100 μm diameter hole drilled in a pre-indented rhenium or steel gasket (38 μm thickness). The DAC was placed in a sealed container immersed in liquid nitrogen. O 2 gas (99.999%) was piped into the container. Liquefied O 2 infused into the sample chamber as the pressure medium and precursor. The samples were pressurized to 35-40 GPa and heated up to~3100 K by an offline double-sided laser-heating (wavelength 1064 nm) system at HPSTAR and HPSynC of the Advanced Photon Source (APS), Argonne National Laboratory. Temperature was obtained from fitting the thermal radiation spectra to the Planck radiation function right after the reported chemical reaction has occurred in the DAC sample chamber. Laser spots at HPSTAR and HPSynC are approximately 20 μm in diameter. Pressure was calibrated by the fluorescence of ruby balls placed inside the sample chamber 41 . Optical absorption was monitored during and after the compression process. Synchrotron XRD data were also collected at 35-40 GPa and during the ensuing decompression process at the 13-BMC (λ = 0.4337 Å), GeoSoilEnviroCARS, Argonne National Laboratory and BL15U1 at Shanghai Synchrotron Radiation Facility (λ = 0.6199 Å). The Xray probing beam size was about 15 µm at the bending beamlines, and 5 µm at the undulator beamlines.
Ab initio calculations. Our structure prediction is performed using CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) methodology 18,42 as implemented in its same-name CALYPSO code 19 (CALYPSO code is free for academic use, by registering at http://www.calypso.cn.), which is based on a global minimization of free energy surfaces in conjunction with ab initio total-energy calculations. Structural optimization, electronic structure, and phonon calculations were performed in the framework of density functional theory within the generalized gradient approximation 43 as implemented in the VASP code 44 . The electron-ion interaction was described by the projector augmented-wave potentials 45 , with 3s 2 3p 6 4s 2 and 2s 2 2p 4 configurations treated as the valence electrons of Ca and O, respectively. The dynamic stability of the predicted new phases was verified by phonon calculations using the direct supercell method as implemented in the PHONOPY code 46 . Crystal structures were visualized with VESTA 47 .

Data availability
The authors declare that the main data supporting the findings of this study are contained within the paper and its associated Supplementary Information. All other relevant data are available from the corresponding authors upon reasonable request.