Np–O phases

9 phases (tier A 4, tier C 1, liquid 2, gas 2); 8 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 · Np–O system page · Markdown · field reference

Phases of the Np–O system (9)
Phase	Structure	Basis	Stability (calculated)	Literature values	TDB models
TETRAG_AD phase:np-o:ad-tetrag	beta-Np · A_d · tP4 · P4/nmm (129)	tier A: standard phase name	stable in 1 of 1 calculated diagram(s): 555.03–855.95 K, x(O) 0–0	0	1
BCC_A2 phase:np-o:bcc-a2	W · A2 · cI2 · Im-3m (229)	tier A: standard phase name	stable in 1 of 1 calculated diagram(s): 855.95–916.84 K, x(O) 0–0	1	1
GAS phase:np-o:gas	gas	gas	stable in 1 of 1 calculated diagram(s): 301.03–2500 K, x(O) 0.99993–1	2	1
ION phase:np-o:ion	liquid	liquid	stable in 1 of 1 calculated diagram(s): 916.84–2500 K, x(O) 0–0.61213	0	1
LIQUID phase:np-o:liquid	liquid	liquid	not stable in 1 calculated diagram(s)	6	1
NP2O5 phase:np-o:np2o5	Np2O5 · mP28 · P2/c (13)	tier C: assessor-stated (Hallstedt 2025 crystallographic table)	stable in 1 of 1 calculated diagram(s): 300–702.28 K, x(O) 0.71429–0.71429	11	1
NPO2_C1 phase:np-o:npo2-c1	CaF2 · C1 · cF12 · Fm-3m (225)	tier A: standard phase name	stable in 1 of 1 calculated diagram(s): 300–2500 K, x(O) 0.65532–0.66667	14	1
O2GAS phase:np-o:o2gas	gas	gas	stable in 1 of 1 calculated diagram(s): 300–1187.28 K, x(O) 1–1	0	1
ORTHO_AC phase:np-o:ortho-ac	alpha-Np · A_c · oP8 · Pnma (62)	tier A: standard phase name	stable in 1 of 1 calculated diagram(s): 300–555.03 K, x(O) 0–0	3	1

TETRAG_AD (Np–O)

Id: phase:np-o:ad-tetrag; status assigned
Structure: beta-Np · A_d · tP4 · P4/nmm (129); type st:a-d-beta-np-tp4; tier A: standard phase name; confidence high
Stoichiometry: Np 1.0000
Stability: stable in 1 of 1 calculated diagram(s): 555.03–855.95 K, x(O) 0–0
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:np-o:ad-tetrag
tdb: tdb:NpO-16Smi.tdb:TETRAG_AD

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

NpO-16Smi.tdb: TETRAG_AD, type code %, sha256 f83f0e3f59b5ca95…

57: PHASE TETRAG_AD % 1 1 !
58: CONST TETRAG_AD : NP : !

Example references (1 of 1; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase

BCC_A2 (Np–O)

Id: phase:np-o:bcc-a2; status assigned
Structure: W · A2 · cI2 · Im-3m (229); type st:a2-w-ci2; tier A: standard phase name; confidence high
Stoichiometry: Np 1.0000
Stability: stable in 1 of 1 calculated diagram(s): 855.95–916.84 K, x(O) 0–0
Literature: 1 values (1 accepted) from 1 papers
Interactive: phase registry entry

Names (3): corpus: corpus:phase:np-o:bcc-a2
paper: paper:10.1016/j.jct.2016.07.040:γ-Np
tdb: tdb:NpO-16Smi.tdb:BCC_A2

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

NpO-16Smi.tdb: BCC_A2, type code %B, sha256 f83f0e3f59b5ca95…

47: PHASE BCC_A2 %B 2 1 3 !
48: CONST BCC_A2 : NP : VA : !

Example references (1 of 1; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase; reports values for this phase

GAS (Np–O)

Id: phase:np-o:gas; status assigned
Structure: gas; type st:gas; gas; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 301.03–2500 K, x(O) 0.99993–1
Literature: 2 values (2 accepted) from 1 papers
Interactive: phase registry entry

Names (4): corpus: corpus:phase:np-o:gas
paper: paper:10.1016/j.jct.2016.07.040:G, paper:10.1016/j.jct.2016.07.040:gas
tdb: tdb:NpO-16Smi.tdb:GAS

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

NpO-16Smi.tdb: GAS, type code %, sha256 f83f0e3f59b5ca95…

73: PHASE GAS:G % 1 1 !
74: CONST GAS:G : NP NPO NPO2 O O2 O3 : !

Example references (2 of 2; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase; reports values for this phase
doi:10.1016/j.jnucmat.2011.05.014 P. Gotcu-Freis, J.-Y. Colle, C. Guéneau, N. Dupin, B. Sundman, R.J.M. Konings, A thermodynamic study of the Pu–Am–O system, Journal of Nuclear Materials 414, 408-421 (2011).
cited as the source of values for this phase

ION (Np–O)

Id: phase:np-o:ion; status assigned
Structure: liquid; type st:liquid; liquid; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 916.84–2500 K, x(O) 0–0.61213
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:np-o:ion
tdb: tdb:NpO-16Smi.tdb:ION

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

NpO-16Smi.tdb: ION, type code %, sha256 f83f0e3f59b5ca95…

39: PHASE ION:Y % 2 1 1 !
40: CONST ION:Y : NP+4 : O-2 VA O : !

Example references (1 of 1; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase

LIQUID (Np–O)

Id: phase:np-o:liquid; status assigned
Structure: liquid; type st:liquid; liquid; confidence high
Stoichiometry: Np 1.0000
Stability: not stable in 1 calculated diagram(s)
Literature: 6 values (6 accepted) from 1 papers
Interactive: phase registry entry

Names (5): corpus: corpus:phase:np-o:liquid
paper: paper:10.1016/j.jct.2016.07.040:L, paper:10.1016/j.jct.2016.07.040:L1, paper:10.1016/j.jct.2016.07.040:liquid
tdb: tdb:NpO-16Smi.tdb:LIQUID

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

NpO-16Smi.tdb: LIQUID, type code %, sha256 f83f0e3f59b5ca95…

42: PHASE LIQUID:L % 1 1 !
43: CONST LIQUID:L : NP : !

Example references (3 of 3; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase; reports values for this phase
doi:10.1063/1.4721655 R. Böhler, M. J. Welland, F. De Bruycker, K. Boboridis, A. Janssen, R. Eloirdi et al., Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements, Journal of Applied Physics 111 (2012).
cited as the source of values for this phase
doi:10.1063/1.4825256 Rudy J. M. Konings, Ondrej Beneš, Attila Kovács, Dario Manara, David Sedmidubský, Lev Gorokhov et al., The Thermodynamic Properties of the f-Elements and their Compounds. Part 2. The Lanthanide and Actinide Oxides, Journal of Physical and Chemical Reference Data 43 (2014).
cited as the source of values for this phase

NP2O5 (Np–O)

Id: phase:np-o:np2o5; status assigned
Structure: Np2O5 · mP28 · P2/c (13); type st:np2o5-mp28; 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 (mp_oqmd_space_groups_differ, no_structure_token); structure as stated in the SGTE crystallographic table for this phase name and system
Stoichiometry: Np 0.2857, O 0.7143
Stability: stable in 1 of 1 calculated diagram(s): 300–702.28 K, x(O) 0.71429–0.71429
Literature: 11 values (11 accepted) from 1 papers
Interactive: phase registry entry

Names (4): corpus: corpus:phase:np-o:np2o5
paper: paper:10.1016/j.jct.2016.07.040:Np2O5, paper:10.1016/j.jct.2016.07.040:Np2O5(cr)
tdb: tdb:NpO-16Smi.tdb:NP2O5

TDB models of NP2O5 (1): PHASE and CONSTITUENT statements as written

NpO-16Smi.tdb: NP2O5, type code %, sha256 f83f0e3f59b5ca95…

67: PHASE NP2O5 % 2 2 5 !
68: CONST NP2O5 : NP : O : !

Example references (3 of 3; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase; reports values for this phase
doi:10.1063/1.4825256 Rudy J. M. Konings, Ondrej Beneš, Attila Kovács, Dario Manara, David Sedmidubský, Lev Gorokhov et al., The Thermodynamic Properties of the f-Elements and their Compounds. Part 2. The Lanthanide and Actinide Oxides, Journal of Physical and Chemical Reference Data 43 (2014).
cited as the source of values for this phase
doi:10.1524/ract.1994.6667.s1.109 L. Merli, J. Fuger, Thermochemistry of a Few Neptunium and Neodymium Oxides and Hydroxides, Radiochimica Acta 66-67, 109-114 (1994).
cited as the source of values for this phase

NPO2_C1 (Np–O)

Id: phase:np-o:npo2-c1; status assigned
Structure: CaF2 · C1 · cF12 · Fm-3m (225); type st:c1-caf2-cf12; tier A: standard phase name; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 300–2500 K, x(O) 0.65532–0.66667
Literature: 14 values (14 accepted) from 1 papers
Interactive: phase registry entry

Names (7): corpus: corpus:phase:np-o:npo2-c1
paper: paper:10.1016/j.jct.2016.07.040:NpO1.90, paper:10.1016/j.jct.2016.07.040:NpO2, paper:10.1016/j.jct.2016.07.040:NpO2(cr), paper:10.1016/j.jct.2016.07.040:NpO2(g), paper:10.1016/j.jct.2016.07.040:NpO2-x
tdb: tdb:NpO-16Smi.tdb:NPO2_C1

TDB models of NPO2_C1 (1): PHASE and CONSTITUENT statements as written

NpO-16Smi.tdb: NPO2_C1, type code %, sha256 f83f0e3f59b5ca95…

62: PHASE NPO2_C1:I % 2 1 2 !
63: CONST NPO2_C1:I : NP+3 NP+4 : O-2 VA : !

Example references (5 of 5; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase; reports values for this phase
doi:10.1063/1.4825256 Rudy J. M. Konings, Ondrej Beneš, Attila Kovács, Dario Manara, David Sedmidubský, Lev Gorokhov et al., The Thermodynamic Properties of the f-Elements and their Compounds. Part 2. The Lanthanide and Actinide Oxides, Journal of Physical and Chemical Reference Data 43 (2014).
cited as the source of values for this phase
doi:10.1021/je60039a029 Elmer J. Huber, Charles E. Holley, Enthalpy of formation of neptunium dioxide, J. Chem. Eng. Data 13, 545-546 (1968).
cited as the source of values for this phase
doi:10.1063/1.1698923 Edgar F. Westrum, J. B. Hatcher, Darrell W. Osborne, The Entropy and Low Temperature Heat Capacity of Neptunium Dioxide, The Journal of Chemical Physics 21, 419-423 (1953).
cited as the source of values for this phase
doi:10.1063/1.4721655 R. Böhler, M. J. Welland, F. De Bruycker, K. Boboridis, A. Janssen, R. Eloirdi et al., Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements, Journal of Applied Physics 111 (2012).
cited as the source of values for this phase

O2GAS (Np–O)

Id: phase:np-o:o2gas; status assigned
Structure: gas; type st:gas; gas; confidence high
Stability: stable in 1 of 1 calculated diagram(s): 300–1187.28 K, x(O) 1–1
Literature: 0 values (0 accepted) from 0 papers
Interactive: phase registry entry

Names (2): corpus: corpus:phase:np-o:o2gas
tdb: tdb:NpO-16Smi.tdb:O2GAS

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

NpO-16Smi.tdb: O2GAS, type code %, sha256 f83f0e3f59b5ca95…

70: PHASE O2GAS % 1 1 !
71: CONST O2GAS : O2 :!

Example references (1 of 1; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase

ORTHO_AC (Np–O)

Id: phase:np-o:ortho-ac; status assigned
Structure: alpha-Np · A_c · oP8 · Pnma (62); type st:a-c-alpha-np-op8; tier A: standard phase name; confidence high
Stoichiometry: Np 1.0000
Stability: stable in 1 of 1 calculated diagram(s): 300–555.03 K, x(O) 0–0
Literature: 3 values (3 accepted) from 1 papers
Interactive: phase registry entry

Names (3): corpus: corpus:phase:np-o:ortho-ac
paper: paper:10.1016/j.jct.2016.07.040:α-Np(cr)
tdb: tdb:NpO-16Smi.tdb:ORTHO_AC

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

NpO-16Smi.tdb: ORTHO_AC, type code %, sha256 f83f0e3f59b5ca95…

52: PHASE ORTHO_AC % 1 1 !
53: CONST ORTHO_AC : NP : !

Example references (3 of 3; assessments first):

doi:10.1016/j.jct.2016.07.040 A.L. Smith, J.-Y. Colle, O. Beneš, R.J.M. Konings, B. Sundman, C. Guéneau, Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling, The Journal of Chemical Thermodynamics 103, 257-275 (2016).
assessment that models this phase; reports values for this phase
doi:10.1063/1.3474238 Rudy J. M. Konings, Ondrej Beneš, The Thermodynamic Properties of the f-Elements and Their Compounds. I. The Lanthanide and Actinide Metals, Journal of Physical and Chemical Reference Data 39 (2010).
cited as the source of values for this phase
doi:10.1098/rspa.1970.0118 J. A. Lee, Kurt Alfred Georg Mendelssohn, P. W. Sutcliffe, Specific heats of plutonium and neptunium, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences 317, 303-317 (1970).
cited as the source of values for 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}
}