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

The study investigates whether oxygen-rich calcium oxides with unconventional stoichiometries can become stable under high-pressure, high-temperature conditions relevant to Earth’s interior, and how such phases could influence deep-Earth redox processes and oxygen cycles. The context is the growing evidence that pressure dramatically changes chemical bonding and stabilizes unexpected compounds (e.g., unusual Na–Cl, Xe–Fe/O systems, and nonstandard iron oxides) that reshape our understanding of geochemical reservoirs and oxidation states at depth. Calcium and oxygen are abundant in the mantle and typically form CaO with O2− at ambient conditions, while higher-oxidation oxygen species occur in peroxides, superoxides, and rare ozonides that are highly reactive and unstable near ambient. The purpose here is to predict and synthesize an oxygen-rich calcium oxide, CaO3, characterize its structure, bonding, and electronic state, and assess its formation conditions and geoscience implications. The importance lies in revealing a divalent ozonide [O3]2− under pressure, introducing a new oxygen reservoir and redox buffer that may help explain seismic anomalies and aspects of Earth’s oxygen cycle.

Recent computational and experimental advances have uncovered pressure-stabilized compounds with unconventional stoichiometries (e.g., Na–Cl, Xe–Fe/Ni, Xe–O, La–H), some synthesized at extreme conditions. Unconventional iron oxides (FeO2, Fe2O3, Fe5O8) with unusual oxidation states have revised views on deep-Earth redox equilibria and oxygen storage. For calcium–oxygen chemistry, CaO is ubiquitous in the mantle and CaO2 (peroxide) can be stable from ambient to high pressure; superoxide and ozonide species are known in alkali/alkaline-earth systems but are scarce and unstable in ambient environments. Prior work indicates high oxygen fugacity in parts of the upper mantle, suggesting conditions conducive to oxygen-rich phases. These studies motivate searches for new Ca–O stoichiometries, particularly ozonides, under high pressure.

Computational: Unbiased crystal structure searches were conducted for Ca_mO_n (m = 1, 2; n = 2, 3, 4) with up to four formula units using the CALYPSO particle-swarm optimization method. First-principles calculations employed density functional theory within the generalized gradient approximation (PBE) using VASP with projector augmented-wave (PAW) potentials (valence: Ca 3s2 3p6 4s2, O 2s2 2p4). Total energies versus volume were fitted to the Birch–Murnaghan equation of state. Phonon dispersions were computed via the direct supercell method using PHONOPY to assess dynamical stability. Finite-temperature effects included vibrational free energies and zero-point energies at the harmonic level to build phase diagrams and reaction boundaries. Electron localization function (ELF) and Bader charge analyses were performed to resolve bonding and charge states; electronic density of states (DOS) and molecular orbital considerations were used to assess magnetism and band filling. Experimental: High-purity CaO powder or Ca metal pieces were loaded with liquefied O2 into diamond anvil cells (300 µm culets) with rhenium or steel gaskets (pre-indented to ~38 µm; 100 µm hole). Samples were compressed to 35–40 GPa and laser-heated to ~3100 K (1064 nm, double-sided; spot ~20 µm) at HPSTAR and HPSynC (APS). Temperature was determined by fitting thermal radiation spectra to the Planck function post-reaction; pressure was calibrated by ruby fluorescence. Raman spectra were collected during heating to detect new vibrational modes. Synchrotron X-ray diffraction was performed at APS 13-BMC (λ = 0.4337 Å) and SSRF BL15U1 (λ = 0.6199 Å) with beam sizes 15 µm (bending) and 5 µm (undulator). Powder XRD patterns were analyzed and indexed (including mixed-phase Rietveld fits) to identify CaO3 and coexisting phases; pressure–volume data were fitted to extract EOS parameters.

• Prediction and stability: Structure searches identified an oxygen-rich CaO3 phase stable against decomposition into CaO + solid O2 above ~27.2 GPa at 0 K; including zero-point energy reduces the threshold to ~25.3 GPa, increasing slightly with temperature (to ~26.5 GPa at 2000 K). Phonon calculations show dynamical stability from 20–50 GPa.
• Crystal structure: CaO3 adopts a tetragonal BaSnS3-type structure (space group P42/m, Z=2) featuring isolated V-shaped O3 units and edge-sharing CaO3 cuboids.
• Bond metrics at 30 GPa: O–O bond length = 1.44 Å; O–O–O angle = 114.57°; shortest Ca–O distance = 2.31 Å. Compared with KO3 (O–O 1.34 Å, angle 109.33°), CaO3 shows weaker O–O bonding and more ionic character.
• Experimental synthesis: CaO or Ca with O2 in a DAC at 35–40 GPa and ~3100 K yielded new Raman modes (≈767 and 1140 cm−1) consistent with CaO3 (O3 2− vibration near 767 cm−1) and a CaO4 superoxide signature near 1140 cm−1. Powder XRD at 35 GPa shows new Bragg peaks at 10.6° and 11.4°, indexed as (200) and (111) of tetragonal CaO3; refined lattice parameters for CaO3: a ≈ 4.67 Å, c ≈ 2.92 Å (theory: a = 4.87 Å, c = 2.98 Å). CaO3 signals persist upon decompression to at least 20 GPa.
• Equations of state: Experimental fits on decompression give B0 = 114(11) GPa, B0′ = 2.7 for CaO3 (and B0 = 103(9) GPa, B0′ = 3.9 for CaO). Theoretical B0 values: 99.8 GPa (B0′ = 4.0) for CaO3; 113.6 GPa (B0′ = 4.0) for CaO.
• Electronic structure and charges: ELF reveals covalent O–O bonding within O3 and ionic Ca–O interactions. Bader analysis at 30 GPa shows ~1.51 e transferred from Ca to each O3 unit (terminal O: −0.65 e each; bridge O: −0.21 e), indicating a formal [O3]2− anion—distinct from [O3]− in alkali ozonides. DOS and MO analysis indicate closed-shell, non-magnetic insulating behavior for CaO3.
• Thermodynamics and volume: The reaction CaO + O2 → CaO3 at 30 GPa exhibits a volume shrinkage ΔV/V = −7.96% (CaO: 23.48 Å3; O2: 15.86 Å3; CaO3: 36.20 Å3), making CaO3 formation PV-favored. MgO3 is not stabilized under similar pressures (positive formation enthalpy 1.45 eV/f.u.; smaller volume gain).
• Geoscience reaction pathways: Additional reactions forming CaO3 become favorable at mantle pressures: Ca(OH)2 + O2 → H2O + CaO3 near ~20 GPa; Ca(OH)2 + Al2O3 + O2 → 2AlOOH + CaO3 near ~40 GPa; 4FeO2 + CaO + H2O → 4FeOOH + CaO3 near ~90 GPa. CaO3 formation is plausible near the 660-km discontinuity (~20 GPa), potentially linked to oxygen released from carbonates and CO2 at depth.

The discovery of pressure-stabilized CaO3 with a divalent ozonide [O3]2− provides a new oxygen-bearing phase that can participate in and buffer redox equilibria in Earth’s mantle. The structural density gain and favorable PV term explain the stability of CaO3 at high pressures, while its persistence to 20 GPa suggests potential metastability near the transition zone. The non-magnetic, closed-shell electronic structure distinguishes CaO3 from alkali ozonides and supports its ionic character. Multiple plausible mantle reactions involving abundant minerals (CaO, Ca(OH)2, Al2O3, FeO2, H2O) can generate CaO3 across a wide depth range, offering mechanisms for transient oxygen storage and release. In particular, CaO3 formation near ~20 GPa provides an alternative explanation for seismic anomalies around the 660-km discontinuity by modulating mineral assemblages and densities. At greater depths (~90 GPa), reactions producing FeOOH (denser than mantle) and CaO3 suggest a deep oxygen cycle where FeOOH sinks while CaO3 ascends, eventually decomposing at shallower depths to release O2 and complete an oxygen circulation pathway.

A combined computational–experimental study predicts, synthesizes, and characterizes a high-pressure calcium ozonide, CaO3, crystallizing in a tetragonal BaSnS3-type structure with isolated O3 units. The compound is stabilized above ~25–27 GPa, is dynamically stable from 20–50 GPa, and exhibits a formal [O3]2− anion leading to closed-shell, non-magnetic insulating behavior. Raman and XRD measurements at 35–40 GPa confirm synthesis, and thermodynamic analysis highlights significant volume reduction driving stability. Proposed mantle reactions show CaO3 can form at multiple depths, suggesting new pathways for oxygen storage and release relevant to seismic anomalies and redox state in Earth’s interior. Future work should target laboratory synthesis via alternative mineral precursors, in situ characterization across broader P–T paths, exploration of kinetic barriers and metastability toward lower pressures, and detailed property measurements (transport, elasticity) to refine geophysical models.

• Stability field: CaO3 is stabilized only at high pressures; experimental signals persist to ~20 GPa but ambient-pressure stability is not demonstrated. The behavior upon full decompression and potential recovery at ambient conditions remain unknown.
• Mixed-phase synthesis: Experimental products include mixed phases (CaO3 with CaO4, unreacted CaO, and O2), which may limit precision in structural refinement and property determination.
• Indirect charge-state inference: The [O3]2− assignment relies on Bader charge analysis, ELF, and electronic structure calculations; direct experimental probes of oxidation state under pressure are challenging.
• Geoscience implications are based on thermodynamic calculations and assumed oxygen fugacity conditions; in situ confirmation within natural mantle assemblages and constraints on kinetics are outstanding.