Co–O phases

9 phases (tier A 4, tier C 1, liquid 2, gas 2); 7 stable in a calculated diagram. Structure types are assigned only when the stored evidence supports an assignment under the stated tier rules (tier A: standard phase name; tier B: unique Materials Project + OQMD on-hull match, DFT-supported; tier C: one structure stated by the assessment's crystallographic table, assessor-stated); other phases list candidates and are marked needs_review. Stability is from phase diagrams calculated with pycalphad from the SGTE binary collection (evidence class: model).

Data: interactive registry · full JSON · Co–O system page · Markdown · field reference

Phases of the Co–O system (9)
Phase	Structure	Basis	Stability (calculated)	Literature values	TDB models
HALITE_B1 phase:co-o:b1-halite	NaCl · B1 · cF8 · Fm-3m (225)	tier A: standard phase name	stable in 1 of 1 calculated diagram(s): 300–1916.89 K, x(O) 0.5–0.5	15	1
BCC_A2 phase:co-o:bcc-a2	W · A2 · cI2 · Im-3m (229)	tier A: standard phase name	not stable in 1 calculated diagram(s)	0	1
FCC_A1 phase:co-o:fcc-a1	Cu · A1 · cF4 · Fm-3m (225)	tier A: standard phase name	stable in 1 of 1 calculated diagram(s): 693.99–1767.99 K, x(O) 0–0.00123	6	1
GAS phase:co-o:gas	gas	gas	stable in 1 of 1 calculated diagram(s): 1524.39–2500 K, x(O) 0.99862–1	0	1
SPINEL_H11 phase:co-o:h11-spinel	Al2MgO4 · H1_1 · cF56 · Fd-3m (227)	tier C: assessor-stated (Hallstedt 2025 crystallographic table)	stable in 1 of 1 calculated diagram(s): 300–1185 K, x(O) 0.57143–0.57143	5	1
HCP_A3 phase:co-o:hcp-a3	Mg · A3 · hP2 · P6_3/mmc (194)	tier A: standard phase name	stable in 1 of 1 calculated diagram(s): 300–1726.24 K, x(O) 0–0.00052	0	1
ION phase:co-o:ion	liquid	liquid	stable in 1 of 1 calculated diagram(s): 1766.99–2500 K, x(O) 0–0.50213	0	1
LIQUID phase:co-o:liquid	liquid	liquid	not stable in 1 calculated diagram(s)	17	1
O2GAS phase:co-o:o2gas	gas	gas	stable in 1 of 1 calculated diagram(s): 300–1185 K, x(O) 1–1	0	1

HALITE_B1 (Co–O)

Id: phase:co-o:b1-halite; status assigned
Structure: NaCl · B1 · cF8 · Fm-3m (225); type st:b1-nacl-cf8; tier A: standard phase name; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 300–1916.89 K, x(O) 0.5–0.5
Literature: 15 values (15 accepted) from 1 papers
Interactive: phase registry entry

Names (3): corpus: corpus:phase:co-o:b1-halite
paper: paper:10.1361/105497103770330514:CoO
tdb: tdb:CoO-03Che-LB.tdb:HALITE_B1

TDB models of HALITE_B1 (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: HALITE_B1, type code %A, sha256 6da44079fa3d4ae1…

56: PHASE HALITE_B1:I %A 2 1 1 !
57: CONST HALITE_B1:I : CO+2 : O-2 : !

Example references (3 of 3; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase; reports values for this phase
doi:10.2320/matertrans1960.29.125 G. M. Kale, S. S. Pandit, K. T. Jacob, Thermodynamics of Cobalt (II, III) Oxide (Co3O4): Evidence of Phase Transition, Trans. JIM 29, 125-132 (1988).
cited as the source of values for this phase
doi:10.2320/jinstmet1952.41.8_803 Nobumasa Kemori, Iwao Katayama, Zensaku Kozuka, Measurements of Standard Molar Free Energies of Formation of NiO, Cu2O and CoO by EMF Method at High Temperatures, J.Japan Inst.Metals 41, 803-808 (1977).
cited as the source of values for this phase

BCC_A2 (Co–O)

Id: phase:co-o:bcc-a2; status assigned
Structure: W · A2 · cI2 · Im-3m (229); type st:a2-w-ci2; tier A: standard phase name; confidence high
Stability: not stable in 1 calculated diagram(s)
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:co-o:bcc-a2
tdb: tdb:CoO-03Che-LB.tdb:BCC_A2

TDB models of BCC_A2 (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: BCC_A2, type code %B, sha256 6da44079fa3d4ae1…

46: PHASE BCC_A2 %B 2 1 3 !
47: CONST BCC_A2 : CO : O VA% : !

Example references (1 of 1; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase

FCC_A1 (Co–O)

Id: phase:co-o:fcc-a1; status assigned
Structure: Cu · A1 · cF4 · Fm-3m (225); type st:a1-cu-cf4; tier A: standard phase name; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 693.99–1767.99 K, x(O) 0–0.00123
Literature: 6 values (6 accepted) from 1 papers
Interactive: phase registry entry

Names (3): corpus: corpus:phase:co-o:fcc-a1
paper: paper:10.1361/105497103770330514:Co(fcc)
tdb: tdb:CoO-03Che-LB.tdb:FCC_A1

TDB models of FCC_A1 (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: FCC_A1, type code %A, sha256 6da44079fa3d4ae1…

41: PHASE FCC_A1 %A 2 1 1 !
42: CONST FCC_A1 : CO : O VA% : !

Example references (2 of 2; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase; reports values for this phase
doi:10.2320/jinstmet1952.41.8_803 Nobumasa Kemori, Iwao Katayama, Zensaku Kozuka, Measurements of Standard Molar Free Energies of Formation of NiO, Cu2O and CoO by EMF Method at High Temperatures, J.Japan Inst.Metals 41, 803-808 (1977).
cited as the source of values for this phase

GAS (Co–O)

Id: phase:co-o:gas; status assigned
Structure: gas; type st:gas; gas; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 1524.39–2500 K, x(O) 0.99862–1
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:co-o:gas
tdb: tdb:CoO-03Che-LB.tdb:GAS

TDB models of GAS (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: GAS, type code %, sha256 6da44079fa3d4ae1…

67: PHASE GAS:G % 1 1 !
68: CONST GAS:G : CO COO CO2 O O2 O3 : !

Example references (1 of 1; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase

SPINEL_H11 (Co–O)

Id: phase:co-o:h11-spinel; status assigned
Structure: Al2MgO4 · H1_1 · cF56 · Fd-3m (227); type st:h1-1-al2mgo4-cf56; tier C: assessor-stated (Hallstedt 2025 crystallographic table); confidence assessor-stated
Structure source: B. Hallstedt, The SGTE collection of binary datasets, Calphad 89 (2025) 102833, supplementary crystallographic table (mmc2) doi:10.1016/j.calphad.2025.102833
Caveat: not certain from the name or DFT alone (unrecognised_name_tokens:SPINEL); structure as stated in the SGTE crystallographic table for this phase name and system
Caveat: partitioned order/disorder model in CoO-03Che-LB.tdb also describes the disordered state
Stability: stable in 1 of 1 calculated diagram(s): 300–1185 K, x(O) 0.57143–0.57143
Literature: 5 values (5 accepted) from 1 papers
Interactive: phase registry entry

Names (3): corpus: corpus:phase:co-o:h11-spinel
paper: paper:10.1361/105497103770330514:Co3O4
tdb: tdb:CoO-03Che-LB.tdb:SPINEL_H11

TDB models of SPINEL_H11 (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: SPINEL_H11, type code %, sha256 6da44079fa3d4ae1…

61: PHASE SPINEL_H11:I % 3 1 2 4 !
62: CONST SPINEL_H11:I : CO+2 CO+3 : CO+2 CO+3 : O-2 : !

Example references (2 of 2; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase; reports values for this phase
doi:10.2320/matertrans1960.29.125 G. M. Kale, S. S. Pandit, K. T. Jacob, Thermodynamics of Cobalt (II, III) Oxide (Co3O4): Evidence of Phase Transition, Trans. JIM 29, 125-132 (1988).
cited as the source of values for this phase

HCP_A3 (Co–O)

Id: phase:co-o:hcp-a3; status assigned
Structure: Mg · A3 · hP2 · P6_3/mmc (194); type st:a3-mg-hp2; tier A: standard phase name; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 300–1726.24 K, x(O) 0–0.00052
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:co-o:hcp-a3
tdb: tdb:CoO-03Che-LB.tdb:HCP_A3

TDB models of HCP_A3 (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: HCP_A3, type code %A, sha256 6da44079fa3d4ae1…

51: PHASE HCP_A3 %A 2 1 0.5 !
52: CONST HCP_A3 : CO : O VA% : !

Example references (1 of 1; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase

ION (Co–O)

Id: phase:co-o:ion; status assigned
Structure: liquid; type st:liquid; liquid; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 1766.99–2500 K, x(O) 0–0.50213
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:co-o:ion
tdb: tdb:CoO-03Che-LB.tdb:ION

TDB models of ION (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: ION, type code %, sha256 6da44079fa3d4ae1…

36: PHASE ION:Y % 2 1 1 !
37: CONST ION:Y : CO+2 CO+3 : O-2 VA : !

Example references (1 of 1; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase

LIQUID (Co–O)

Id: phase:co-o:liquid; status assigned
Structure: liquid; type st:liquid; liquid; confidence high
Stoichiometry: Co 1.0000
Stability: not stable in 1 calculated diagram(s)
Literature: 17 values (16 accepted) from 1 papers
Interactive: phase registry entry

Names (5): corpus: corpus:phase:co-o:liquid
paper: paper:10.1361/105497103770330514:L1, paper:10.1361/105497103770330514:L2, paper:10.1361/105497103770330514:liquid
tdb: tdb:CoO-03Che-LB.tdb:LIQUID

TDB models of LIQUID (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: LIQUID, type code %, sha256 6da44079fa3d4ae1…

33: PHASE LIQUID:L % 1 1 !
34: CONST LIQUID:L : CO : !

Example references (3 of 3; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase; reports values for this phase
doi:10.1002/zaac.19512650110 P. Asanti, E. J. Kohlmeyer, Über die thermischen Eigenschaften der Verbindungen von Kobalt mit Sauerstoff und Schwefel, Zeitschrift anorg allge chemie 265, 90-98 (1951).
cited as the source of values for this phase
doi:10.2320/jinstmet1952.41.8_803 Nobumasa Kemori, Iwao Katayama, Zensaku Kozuka, Measurements of Standard Molar Free Energies of Formation of NiO, Cu2O and CoO by EMF Method at High Temperatures, J.Japan Inst.Metals 41, 803-808 (1977).
cited as the source of values for this phase

O2GAS (Co–O)

Id: phase:co-o:o2gas; status assigned
Structure: gas; type st:gas; gas; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 300–1185 K, x(O) 1–1
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:co-o:o2gas
tdb: tdb:CoO-03Che-LB.tdb:O2GAS

TDB models of O2GAS (1): PHASE and CONSTITUENT statements as written

CoO-03Che-LB.tdb: O2GAS, type code %, sha256 6da44079fa3d4ae1…

64: PHASE O2GAS % 1 1 !
65: CONST O2GAS : O2 :!

Example references (1 of 1; assessments first):

doi:10.1361/105497103770330514 Ming Chen, Bengt Hallstedt, Ludwig J. Gauckler, Thermodynamic assessment of the Co-O system, JPE 24, 212-227 (2003).
assessment that models this phase

Please cite

R. Arróyave, J. Saal, D. Shin, ORCHID-Registry: phase registry of binary systems (version 2026.10; Version 1, machine-extracted), Zenodo (2026). doi:10.5281/zenodo.23735970
R. Arróyave, J. Saal, D. Shin, ORCHID data: umbrella record of the ORCHID datasets (version 2026.10; Version 1, machine-extracted), Zenodo (2026). doi:10.5281/zenodo.23736065

Also cite B. Hallstedt, The SGTE collection of binary datasets, Calphad 89 (2025) 102833, https://doi.org/10.1016/j.calphad.2025.102833 (CC BY 4.0), and the original assessment named in each file; A. Jain et al., The Materials Project: A materials genome approach to accelerating materials innovation, APL Materials 1 (2013) 011002, https://doi.org/10.1063/1.4812323 (CC BY 4.0); J. E. Saal et al., JOM 65 (2013) 1501, https://doi.org/10.1007/s11837-013-0755-4 and S. Kirklin et al., npj Comput. Mater. 1 (2015) 15010, https://doi.org/10.1038/npjcompumats.2015.10 (CC BY 4.0); the original source of every value you use (paper_citation / paper_doi, and cited_dois for values the paper cites from earlier work). Version 1, machine-extracted: values were read by language-model readers constrained to quote their source and checked automatically, and the release was then audited on the printed pages by adversarial audits, an independent second review of fit-facing records and a preliminary human review of a frozen sample. Flags such as accepted and ESPEI-ready record screening results, not certification; known issues are listed in the release notes.

@misc{orchid_phase_registry_2026,
  author    = {Arr{\'o}yave, Raymundo and Saal, James and Shin, Dongwon},
  title     = {{ORCHID-Registry: phase registry of binary systems}},
  version   = {2026.10},
  publisher = {Zenodo},
  year      = {2026},
  doi       = {10.5281/zenodo.23735970},
  url       = {https://doi.org/10.5281/zenodo.23735970},
  note      = {Version 1, machine-extracted}
}

@misc{orchid_data_2026,
  author    = {Arr{\'o}yave, Raymundo and Saal, James and Shin, Dongwon},
  title     = {{ORCHID data: umbrella record of the ORCHID datasets}},
  version   = {2026.10},
  publisher = {Zenodo},
  year      = {2026},
  doi       = {10.5281/zenodo.23736065},
  url       = {https://doi.org/10.5281/zenodo.23736065},
  note      = {Version 1, machine-extracted}
}