# Alkaline earth metal peroxides

**Alkaline earth [metal peroxides](https://www.edgepedia.org/metal-peroxides)** are inorganic salts of the group 2 metals (beryllium, magnesium, calcium, strontium, barium, and radium) that contain the peroxide anion \( \mathrm{O_2^{2-}} \), in which two oxygen atoms are joined by a covalent single bond; IUPAC defines peroxides in the inorganic sense as salts of this anion.<sup>[1](https://goldbook.iupac.org/terms/view/P04510/html)</sup> Their general formula is \( \mathrm{XO_2} \), with the metal in the +2 oxidation state and each oxygen at −1. The family is defined by a steep stability trend down the group: beryllium peroxides are not known, magnesium peroxide is thermodynamically unstable at ambient pressure, while calcium, strontium, and barium form increasingly stable peroxides under ordinary conditions.<sup>[2](https://doi.org/10.1038/srep13582)</sup>

| Key fact | Detail |
|---|---|
| Definition | Salts of the peroxide anion \( \mathrm{O_2^{2-}} \), formula \( \mathrm{XO_2} \) for group 2 metals; each \( \mathrm{M^{2+}} \) ion pairs with one dumbbell-shaped \( \mathrm{O_2^{2-}} \) ion<sup>[1](https://goldbook.iupac.org/terms/view/P04510/html)</sup><sup> • </sup><sup>[3](https://www.sciencedirect.com/science/article/pii/S2214714425017490)</sup> |
| Stability trend | Increases down the group: beryllium peroxides are not known; Pa3 \( \mathrm{MgO_2} \) decomposes to MgO and \( \mathrm{O_2} \) at 650 K at ambient pressure; Ca, Sr, and Ba form increasingly stable peroxides<sup>[2](https://doi.org/10.1038/srep13582)</sup> |
| Structures | SrO2 and BaO2 adopt the CaC2-type tetragonal structure (I4/mmm) at ambient pressure, with O–O bond lengths of 1.483 Å (SrO2) and 1.493 Å (BaO2)<sup>[2](https://doi.org/10.1038/srep13582)</sup> |
| Standard synthesis | Precipitation of the octahydrates \( \mathrm{XO_2 \cdot 8H_2O} \) by adding \( \mathrm{H_2O_2} \) to aqueous ammoniacal \( \mathrm{Sr(NO_3)_2} \) or \( \mathrm{CaCl_2} \), followed by dehydration<sup>[4](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910013097.pdf)</sup> |
| Oxygen release | In water, \( \mathrm{CaO_2} \) hydrolyzes to \( \mathrm{Ca(OH)_2} \) and \( \mathrm{H_2O_2} \), which then decomposes to \( \mathrm{O_2} \); granulated \( \mathrm{CaO_2} \) can release oxygen over 5 to 7 months<sup>[5](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)</sup><sup> • </sup><sup>[6](https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-peroxide)</sup> |
| Principal uses | Calcium peroxide serves as a slow oxygen and hydrogen peroxide source in soil and groundwater remediation, gold leaching, seed coatings, and as a curing agent for polysulfide sealants<sup>[6](https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-peroxide)</sup><sup> • </sup><sup>[7](https://www.sciencedirect.com/science/article/pii/S0304389417303254)</sup> |
| High-pressure chemistry | \( \mathrm{MgO_2} \) becomes thermodynamically stable above about 96 GPa, where MgO and oxygen react at 2150 K to form tetragonal I4/mcm \( \mathrm{MgO_2} \)<sup>[2](https://doi.org/10.1038/srep13582)</sup> |

## Bonding and structures

The peroxide ion consists of two oxygen atoms joined by a covalent single bond, giving a dumbbell-shaped \( \mathrm{O_2^{2-}} \) unit; in calcium peroxide each \( \mathrm{Ca^{2+}} \) is octahedrally coordinated by six peroxide ions.<sup>[3](https://www.sciencedirect.com/science/article/pii/S2214714425017490)</sup> The O–O bond is weak compared with most chemical bonds, with bond-dissociation energies of roughly 20 to 50 kcal/mol cited for peroxide molecules generally, which underlies both the oxidizing power of these compounds and their tendency to release oxygen.<sup>[8](https://www.bnl.gov/esh/cms/pdf/peroxides.pdf)</sup>

**Crystal chemistry varies with cation size.** [Strontium](https://www.edgepedia.org/strontium) and barium peroxides crystallize at ambient pressure in the CaC2-type structure (space group I4/mmm), with two formula units per cell and the O–O bond parallel to the c axis; their O–O distances are 1.483 Å and 1.493 Å respectively.<sup>[2](https://doi.org/10.1038/srep13582)</sup> [Calcium peroxide](https://www.edgepedia.org/calcium-peroxide) synthesized at low temperature is orthorhombic, and density functional theory calculations find this structure thermodynamically stable against decomposition into CaO and half a molecule of \( \mathrm{O_2} \).<sup>[9](https://pubs.rsc.org/en/content/articlelanding/2013/ra/c3ra43617a)</sup> The octahydrate \( \mathrm{CaO_2 \cdot 8H_2O} \) is tetragonal, with lattice parameters a = 6.212830 Å and c = 11.0090 Å reported from a new x-ray powder diffraction pattern.<sup>[4](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910013097.pdf)</sup>

## The stability trend down the group

The defining chemical feature of this family is how stability changes with cation size. The small, highly charge-dense \( \mathrm{Mg^{2+}} \) ion polarizes the peroxide ion strongly, and pyrite-type (Pa3) \( \mathrm{MgO_2} \), though synthesizable at near-ambient conditions under high oxygen fugacity, decomposes to MgO and \( \mathrm{O_2} \) on heating to 650 K at ambient pressure.<sup>[2](https://doi.org/10.1038/srep13582)</sup> A genetic-algorithm structure search combined with first-principles calculations reached the same conclusion for \( \mathrm{MgO_2} \): its cubic pyrite structure is not stable against decomposition, whereas an orthorhombic calcium peroxide structure is.<sup>[9](https://pubs.rsc.org/en/content/articlelanding/2013/ra/c3ra43617a)</sup> [Beryllium](https://www.edgepedia.org/beryllium) peroxides are not known at all.<sup>[2](https://doi.org/10.1038/srep13582)</sup>

**Pressure reverses the trend for magnesium.** Using synchrotron x-ray diffraction in laser-heated diamond anvil cells, researchers showed that MgO and oxygen react at pressures above 96 GPa and T = 2150 K to form tetragonal I4/mcm \( \mathrm{MgO_2} \), with an O–O bond length of 1.454 Å at 96 GPa; [Raman spectroscopy](https://www.edgepedia.org/raman-spectroscopy) confirmed the peroxide ion in the recovered material.<sup>[2](https://doi.org/10.1038/srep13582)</sup> This matches ab initio predictions that I4/mcm \( \mathrm{MgO_2} \) becomes stable above 116 GPa at 0 K, and suggests magnesium peroxide could be an abundant mineral in highly oxidized terrestrial exoplanet interiors.<sup>[2](https://doi.org/10.1038/srep13582)</sup>

**Calcium peroxide gains stability under pressure too.** [Structure](https://www.edgepedia.org/structure) searches over 0 to 200 GPa found several low-enthalpy \( \mathrm{CaO_2} \) phases with C2/c, I4/mcm, and \( P2_1/c \) symmetries below 40 GPa, all thermodynamically stable against decomposition into CaO and \( \mathrm{O_2} \); stability increases with pressure and peaks at the CaO B1–B2 phase transition at 65 GPa, and phonon calculations indicate \( \mathrm{CaO_2} \) is a stable calcium oxide at mantle temperatures and pressures.<sup>[10](https://arxiv.org/abs/1502.03329)</sup>

## Synthesis

**Wet precipitation is the standard laboratory route** for calcium and strontium peroxides. Adding hydrogen peroxide to aqueous ammoniacal \( \mathrm{Sr(NO_3)_2} \) or \( \mathrm{CaCl_2} \) precipitates the octahydrates \( \mathrm{SrO_2 \cdot 8H_2O} \) and \( \mathrm{CaO_2 \cdot 8H_2O} \), which are then converted to the anhydrous peroxides by dehydration techniques.<sup>[4](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910013097.pdf)</sup> A typical octahydrate preparation dissolves 10 g of \( \mathrm{Sr(NO_3)_2} \) in 60 ml of 3% hydrogen peroxide and mixes the solution carefully with diluted ammonia.<sup>[11](https://research.chalmers.se/publication/177835/file/177835_Fulltext.pdf)</sup> For the calcium salt, one route dehydrates \( \mathrm{CaO_2 \cdot 8H_2O} \) in water at 50 °C and vacuum-dries the filtered precipitate at 100 °C; another reacts dilute hydrogen peroxide with a calcium oxide hydrate suspension or a calcium salt solution with ammonia and hydrogen peroxide.<sup>[5](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)</sup> Careful dehydration of the octahydrate, first under reduced pressure in a desiccator and then over \( \mathrm{P_2O_5} \) at 100 °C, yields the anhydrous peroxide.<sup>[12](https://www.sciencemadness.org/library/books/brauer_ocr.pdf)</sup>

**Direct oxidation works only for the heavier members.** Calcium peroxide, unlike \( \mathrm{SrO_2} \) and \( \mathrm{BaO_2} \), cannot be synthesized by direct reaction of molecular oxygen with the oxide.<sup>[5](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)</sup> [Barium peroxide](https://www.edgepedia.org/barium-peroxide), by contrast, is prepared by heating loose BaO at 500 °C in a stream of oxygen that has been thoroughly dried and freed of \( \mathrm{CO_2} \).<sup>[12](https://www.sciencemadness.org/library/books/brauer_ocr.pdf)</sup> To produce magnesium peroxide on a commercial scale, a light grade of magnesium oxide is blended with hydrogen peroxide, and the resulting slurry is dried, giving a white powder that holds roughly 25% \( \mathrm{MgO_2} \) and 7% active oxygen; the balance consists of magnesium oxide, hydroxide, and carbonate, while a hydroxide-based route can reach up to 60% \( \mathrm{MgO_2} \).<sup>[13](https://pubchem.ncbi.nlm.nih.gov/compound/Magnesium-dioxide)</sup> Nanostructured strontium peroxide with average particle sizes of 5 to 500 nm has been prepared from a strontium source, an alkali, and hydrogen peroxide in mixed organic solvent–water media.<sup>[14](https://patents.google.com/patent/CN119873756A/en)</sup>

## Reactions and oxygen release

In contact with water, the peroxides hydrolyze. For calcium peroxide the sequence is:

\[ \mathrm{CaO_2(s) + 2H_2O \rightarrow Ca(OH)_2 + H_2O_2} \]

\[ \mathrm{2H_2O_2 \rightarrow O_2 + 2H_2O} \]

so the solid acts as a two-step oxygen source, first forming hydrogen peroxide and then liberating molecular oxygen.<sup>[5](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)</sup> In strongly alkaline conditions (pH 12–13) the hydrogen peroxide intermediate disproportionates so quickly that no measurable \( \mathrm{H_2O_2} \) is released and the conversion to \( \mathrm{O_2} \) is direct.<sup>[5](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)</sup> [Magnesium](https://www.edgepedia.org/magnesium) peroxide behaves similarly: water gradually decomposes it with liberation of oxygen, and dilute acids convert it to hydrogen peroxide, while strong heating drives off all peroxide oxygen.<sup>[13](https://pubchem.ncbi.nlm.nih.gov/compound/Magnesium-dioxide)</sup> With dilute sulfuric acid, barium peroxide historically served this way as a route to hydrogen peroxide solutions, the basis of the nineteenth-century Brin process.<sup>[15](https://ndl.ethernet.edu.et/bitstream/123456789/35551/1/2.pdf)</sup>

The release rate depends on solubility and purity. Because \( \mathrm{Ca(OH)_2} \) is more soluble than \( \mathrm{Mg(OH)_2} \), \( \mathrm{CaO_2} \) achieves higher hydrogen peroxide release rates and capacities than \( \mathrm{MgO_2} \), and commercial \( \mathrm{CaO_2} \) also reaches higher purity, giving it a higher oxygen production capacity.<sup>[3](https://www.sciencedirect.com/science/article/pii/S2214714425017490)</sup><sup> • </sup><sup>[5](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)</sup> Calcium peroxide is preferred over sodium percarbonate when slower oxygen release is needed.<sup>[5](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)</sup>

## Applications

**Environmental remediation dominates modern use.** Calcium peroxide's stability makes it an effective solid phase for supplying \( \mathrm{H_2O_2} \) and \( \mathrm{O_2} \) to aerobic biodegradation and chemical degradation of contaminants in water and soil, and commercial products such as PermeOx Plus and Cool-Ox have been used in field projects.<sup>[7](https://www.sciencedirect.com/science/article/pii/S0304389417303254)</sup> Granulated \( \mathrm{CaO_2} \) has been proposed for raising oxygen levels in lakes, releasing oxygen over 5 to 7 months in laboratory and field tests.<sup>[6](https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-peroxide)</sup>

**Agricultural and industrial uses are long established.** Calcium peroxide serves as a seed disinfectant and oxygen-supplying seed coating; rice seedling emergence from flooded soils increased with coating rates of 0 to 40% by weight of a 60% \( \mathrm{CaO_2} \) product as planting depth increased from 0 to 2.5 cm.<sup>[6](https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-peroxide)</sup> Other documented uses include curing polysulfide sealants in double-glazing window units, increasing gold recovery while reducing cyanide consumption at Australian gold mines, dough conditioning, bleaching of oils, dentifrices, and as a hardening accelerator for sulfide rubber.<sup>[6](https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-peroxide)</sup> Magnesium peroxide is used as a bleaching and oxidizing agent and as a sustained-release oxygen agent for treating petroleum contamination in subsurface environments.<sup>[13](https://pubchem.ncbi.nlm.nih.gov/compound/Magnesium-dioxide)</sup><sup> • </sup><sup>[16](https://patents.google.com/patent/JP6364260B2/en)</sup>

**Handling reflects the oxidizing character.** The Joint FAO/WHO Expert Committee on Food Additives evaluated calcium peroxide in 1965 with the caution that it is a powerful oxidizing substance, and contact with the powder can severely irritate and burn skin and eyes.<sup>[6](https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-peroxide)</sup>

## References

1. [Peroxides, IUPAC Compendium of Chemical Terminology (Gold Book), P04510](https://goldbook.iupac.org/terms/view/P04510/html)
2. [S. S. Lobanov et al. (2015). Stable magnesium peroxide at high pressure. Scientific Reports 5, 13582.](https://doi.org/10.1038/srep13582)
3. [A review on calcium peroxide: preparation raw materials and methods, modifications, environmental applications, and future prospects. ScienceDirect.](https://www.sciencedirect.com/science/article/pii/S2214714425017490)
4. [W. H. Philipp and P. A. Kraft (1989). Synthesis and thermal properties of strontium and calcium peroxides. NASA TM-103725.](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910013097.pdf)
5. [Controlled chemical synthesis of CaO2 particles coated with polyethylene glycol: characterization of crystallite size and oxygen release kinetics. RSC Advances (2018).](https://pubs.rsc.org/en/content/articlehtml/2018/ra/c7ra08758f)
6. [Calcium peroxide, CaO2, CID 14779. PubChem, with HSDB and JECFA data.](https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-peroxide)
7. [Application of calcium peroxide in water and soil treatment: A review. Journal of Hazardous Materials 337 (2017) 163–177.](https://www.sciencedirect.com/science/article/pii/S0304389417303254)
8. [Peroxides and how to deal with them. Brookhaven National Laboratory safety guidance.](https://www.bnl.gov/esh/cms/pdf/peroxides.pdf)
9. [Structures and stabilities of alkaline earth metal peroxides XO2 (X = Ca, Be, Mg) studied by a genetic algorithm. RSC Advances (2013).](https://pubs.rsc.org/en/content/articlelanding/2013/ra/c3ra43617a)
10. [Calcium peroxide from ambient to high pressures. arXiv:1502.03329.](https://arxiv.org/abs/1502.03329)
11. [The formation and structure of peroxy compounds of group IIa and IIb elements. Chalmers research publication.](https://research.chalmers.se/publication/177835/file/177835_Fulltext.pdf)
12. [Handbook of Preparative Inorganic Chemistry, 2nd ed. (Brauer).](https://www.sciencemadness.org/library/books/brauer_ocr.pdf)
13. [Magnesium peroxide, MgO2, CID 61745. PubChem, with HSDB and Kirk-Othmer data.](https://pubchem.ncbi.nlm.nih.gov/compound/Magnesium-dioxide)
14. [CN119873756A: Strontium peroxide nano material and preparation method and application thereof. Google Patents.](https://patents.google.com/patent/CN119873756A/en)
15. [Encyclopedic Dictionary of Named Processes in Chemical Technology, 4th ed. (Brin process entry).](https://ndl.ethernet.edu.et/bitstream/123456789/35551/1/2.pdf)
16. [JP6364260B2: Method for producing magnesium peroxide granules. Google Patents.](https://patents.google.com/patent/JP6364260B2/en)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Elements and inorganic substances › Oxides and oxygen compounds › Metal oxides and hydroxides › Main-group and alkali-metal oxides › Alkaline earth metal peroxides*

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