Np–O system

40 accepted literature values from 1 paper(s) (experimental 1, model 26, unspecified 13); 1 calculated phase diagram(s). 7 further extracted value(s) not accepted by the reader-agreement checks are only in the JSON/CSV.

Data: JSON · CSV · interactive explorer · interactive phase diagram · phases (9) · papers (bibliography) · Markdown

Accepted values by evidence class
Evidence class	Values
experimental	1
model	26
unspecified	13

Accepted values by property group
Property group	Values
invariant reaction	7
phase transition	1
formation	14
heat capacity / entropy / enthalpy	11
crystal structure	4
other property	3

Calculated phase diagrams

Binary phase diagrams calculated with pycalphad 0.11.2 from the SGTE binary collection (Hallstedt 2025, CC BY 4.0). Evidence class: model.

Assessments with a calculated phase diagram
TDB file	Assessment	SHA-256	Links
NpO-16Smi.tdb	A.L. Smith, J.-Y. Colle, O. Benes, R.J.M. Konings, B. Sundman, C. Gueneau, J. Chem. Thermodyn., 103, 257-75(2016). doi:10.1016/j.jct.2016.07.040	`f83f0e3f59b5ca9526ff567a1d28354abc1a3dcc0c99f14a83021650bd6be3b1`	viewer · TDB · JSON

Calculated three-phase invariants (model, mole fraction of the second component)
TDB file	T (K)	Phases at the invariant
NpO-16Smi.tdb	1116.83	ION (x(O) 5.500e-4) + ION#2 (x(O) 0.41177) + NPO2_C1 (x(O) 0.66642)
NpO-16Smi.tdb	916.84	BCC_A2 (x(O) 0) + ION (x(O) 0) + NPO2_C1 (x(O) 0.66664)
NpO-16Smi.tdb	916.84	BCC_A2 (x(O) 0) + ION (x(O) 0) + NPO2_C1 (x(O) 0.66664)
NpO-16Smi.tdb	855.95	TETRAG_AD (x(O) 0) + BCC_A2 (x(O) 0) + NPO2_C1 (x(O) 0.66666)
NpO-16Smi.tdb	702.28	NPO2_C1 (x(O) 0.66667) + NP2O5 (x(O) 0.71429) + GAS (x(O) 1)
NpO-16Smi.tdb	702.28	NPO2_C1 (x(O) 0.66667) + NP2O5 (x(O) 0.71429) + O2GAS (x(O) 1)
NpO-16Smi.tdb	555.03	TETRAG_AD (x(O) 0) + ORTHO_AC (x(O) 0) + NPO2_C1 (x(O) 0.66667)

Convex hulls: Materials Project DFT hull · OQMD DFT hull

Invariant reactions (literature)

Invariant reactions reported in the literature for the Np–O system (7)
T (K)	Reaction / phases	Compositions (mole fraction)	Class	Source	Location
3070	congruent melting: NpO2-x = L	NpO2-x: O 0.66667; L: O 0.66667	unspecified	[1] Smith 2016 cites Konings 2014 doi	Table 7, p. 10, row 7 report
3069.5	congruent melting: NpO1.90 = L	NpO1.90: O 0.6549; L: O 0.6549	model	[1] Smith 2016	Table 7, p. 10, row 5 report
2879	NpO2-x + G = L	NpO2-x: O 0.6633; L: O 0.672	model	[1] Smith 2016	Table 7, p. 10, row 6 report
1117	L1 + L2 = NpO2-x	L1: O 5.500e-4; NpO2-x: O 0.412	model	[1] Smith 2016	Table 7, p. 10, row 2 report
913	L1 = NpO2-x + γ-Np	NpO2-x: O 0.66667; γ-Np: O 0	model	[1] Smith 2016	Table 7, p. 10, row 1 report
700	Np2O5 = 2NpO2 + 1/2O2	Np2O5: O 0.71428; NpO2: Np 0.333333, O 0.666667	unspecified	[1] Smith 2016 cites Konings 2014 doi	Table 7, p. 10, row 4 report
700	Np2O5 = 2NpO2 + 1/2O2	Np2O5: O 0.71428; NpO2: Np 0.333333, O 0.666667	model	[1] Smith 2016	Table 7, p. 10, row 3 report

Formation enthalpies

Literature values in kJ/mol-atom where the per-atom basis is known. DFT columns: the lowest-energy entry at the same composition (Materials Project 2026.04.13, OQMD v1.8; different energy references, never averaged), from dft_parity.json.

Enthalpies of formation, Np–O system (14)
Phase	Compound	Value	T (K)	Class	MP DFT (kJ/mol-atom)	OQMD DFT (kJ/mol-atom)	Source	Location
Np(g)		470 kJ/mol (basis unstated) ± 5	298.15	model			[1] Smith 2016 cites Konings 2010 doi	Table 11, p. 14, row 1 report
Np2O5(cr)	Np2O5	-310.286 kJ/mol-atom	298.15	model	-332.73 (mp-aaabfope)	-307.97 (oqmd-113840)	[1] Smith 2016	Table 9, p. 12, row 3 report
Np2O5(cr)	Np2O5	-308.957 kJ/mol-atom ± 1.32857	298.15	unspecified	-332.73 (mp-aaabfope)	-307.97 (oqmd-113840)	[1] Smith 2016 cites Konings 2014 doi	Table 2, p. 3, row 7 report
Np2O5(cr)	Np2O5	-308.957 kJ/mol-atom ± 1.32857	298.15	model	-332.73 (mp-aaabfope)	-307.97 (oqmd-113840)	[1] Smith 2016 cites Konings 2014 doi	Table 9, p. 12, row 4 report
NpO(g)		-16.5 kJ/mol (basis unstated) ± 12.2	298.15	model			[1] Smith 2016	Table B.3, p. 17, row 4 report
NpO(g)		-13.3 kJ/mol (basis unstated) ± 11.5	298.15	model			[1] Smith 2016	Table B.3, p. 17, row 4 report
NpO(g)		-9.8 kJ/mol (basis unstated) ± 7.8	298.15	model			[1] Smith 2016	Table 11, p. 14, row 2 report
NpO(g)		-6.3 kJ/mol (basis unstated) ± 10.6	298.15	model			[1] Smith 2016	Table B.3, p. 17, row 3 report
NpO(g)		-2 kJ/mol (basis unstated) ± 11.4	298.15	model			[1] Smith 2016	Table B.3, p. 17, row 3 report
NpO2(cr)	NpO2	-359.5 kJ/mol-atom ± 0.9	298.15	model	-347.462 (mp-aaaaadwq)	-357.999 (oqmd-30883)	[1] Smith 2016 cites Konings 2014 doi	Table 9, p. 12, row 2 report
NpO2(cr)	NpO2	-359.5 kJ/mol-atom ± 0.9	298.15	unspecified	-347.462 (mp-aaaaadwq)	-357.999 (oqmd-30883)	[1] Smith 2016 cites Konings 2014 doi	Table 2, p. 3, row 5 report
NpO2(cr)	NpO2	-358.933 kJ/mol-atom	298.15	model	-347.462 (mp-aaaaadwq)	-357.999 (oqmd-30883)	[1] Smith 2016	Table 9, p. 12, row 1 report
NpO2(g)		-471.7 kJ/mol (basis unstated) ± 3.3	298.15	model			[1] Smith 2016	Table 11, p. 14, row 3 report
α-Np(cr)		0 kJ/mol-atom	298.15	unspecified			[1] Smith 2016 cites Konings 2010 doi	Table 2, p. 3, row 1 report

Other properties

Phase transition (1)

Phase transition, Np–O system
Property	Value	T (K)	Phases / composition	Class	Source	Location	Quote
Phase transition	3069.5 K	3069.5	congruent transformation	model	[1] Smith 2016	p. 10 report	In addition, the melting point of neptunium dioxide is calculated at T fus = 3069.5 K for congruently melting NpO1:90 , which is consistent with the recommended value at T fus 1⁄4 ð3070 62Þ K [14,22].

Heat capacity / entropy / enthalpy (11)

Heat capacity / entropy / enthalpy, Np–O system
Property	Value	T (K)	Phases / composition	Class	Source	Location
Entropy printed: S_m^o	197.72 unknown ± 0.1	298.15	Np(g) alloy: O 0	model	[1] Smith 2016 cites Konings 2010 doi	Table 11, p. 14, row 1 report
Entropy printed: S°m (298.15 K)	34.6286 J/mol-atom/K	298.15	Np2O5(cr) alloy: O 0.714286	model	[1] Smith 2016	Table 9, p. 12, row 3 report
Entropy printed: S°m(298.15 K)	26.5714 J/mol-atom/K ± 2.14286	298.15	Np2O5(cr) alloy: O 0.714286	unspecified	[1] Smith 2016 cites Konings 2014 doi	Table 2, p. 3, row 7 report
Entropy printed: S°m (298.15 K)	26.5714 J/mol-atom/K ± 2.14286	298.15	Np2O5(cr) alloy: O 0.714286	model	[1] Smith 2016 cites Konings 2014 doi	Table 9, p. 12, row 4 report
Entropy printed: S_m^o	253.06 unknown ± 4	298.15	NpO(g) alloy: O 0.5	model	[1] Smith 2016	Table 11, p. 14, row 2 report
Entropy printed: S°m (298.15 K)	26.7667 J/mol-atom/K ± 0.133333	298.15	NpO2(cr) alloy: O 0.666667	model	[1] Smith 2016 cites Konings 2014 doi	Table 9, p. 12, row 2 report
Entropy printed: S°m	26.7667 J/mol-atom/K ± 0.133333	298.15	NpO2(cr) alloy: O 0.666667	unspecified	[1] Smith 2016 cites Konings 2014 doi	Table 2, p. 3, row 5 report
Entropy printed: S°m (298.15 K)	26.7667 J/mol-atom/K	298.15	NpO2(cr) alloy: O 0.666667	model	[1] Smith 2016	Table 9, p. 12, row 1 report
Entropy printed: S_m^o	269.892 unknown ± 6	298.15	NpO2(g) alloy: O 0.666667	model	[1] Smith 2016	Table 11, p. 14, row 3 report
Entropy printed: S°m(298.15 K)	50.45 J/mol-atom/K ± 0.4	298.15	α-Np(cr) alloy: O 0	unspecified	[1] Smith 2016 cites Konings 2010 doi	Table 2, p. 3, row 1 report
Entropy printed: S°m	50.45 J/mol-atom/K ± 0.4	298.15	α-Np(cr) alloy: O 0	unspecified	[1] Smith 2016 cites Konings 2010 doi	Table 2, p. 3, row 1 report

Crystal structure (4)

Crystal structure, Np–O system
Property	Value	T (K)	Phases / composition	Class	Source	Location
Lattice a	4.188 Å	298.15	Np2O5 alloy: O 0.714286	unspecified	[1] Smith 2016 cites Lemire 2001	Table 1, p. 2, row 5 report
Lattice a	5.434 Å	298.15	NpO2 alloy: O 0.666667	unspecified	[1] Smith 2016 cites Lemire 2001	Table 1, p. 2, row 4 report
Lattice b	6.592 Å	298.15	Np2O5 alloy: O 0.714286	unspecified	[1] Smith 2016 cites Lemire 2001	Table 1, p. 2, row 5 report
Lattice c	4.09 Å	298.15	Np2O5 alloy: O 0.714286	unspecified	[1] Smith 2016 cites Lemire 2001	Table 1, p. 2, row 5 report

Other property (3)

Other property, Np–O system
Property	Value	T (K)	Phases / composition	Class	Source	Location	Quote
Other printed: O/Np	1.92 unspecified ± 0.011	2260		model	[1] Smith 2016	p. 15 report	Nevertheless, based on the present experimental results and CALPHAD model, we can estimate the composition at congruent vapourization at 2260 K as (O/Np) = (1.92 ± 0.011).
Other printed: fraction NpO2(g)	93.2 %	2260		model	[1] Smith 2016	p. 12 report	The calculated azeotropic vapour composition at 2260 K is: 93.2% NpO2(g), 6.1% NpO(g), and 0.7% O(g), which corresponds to an oxygen-to-metal ratio ðO=NpÞazeotropic = (1.945 ± 0.0031) at 2260 K.
Other printed: (O/Np)gas	1.939 unspecified	2260	gas	experimental	[1] Smith 2016 cites Gotcu-Freis 2011 doi	p. 4 report	The authors reported a vapour made of 93% NpO2(g), 6% NpO(g), and 1% Np(g) at 2260 K under vacuum, with an overall gaseous composition (O/Np)gas = 1.939 [11].

Sources

Reporting papers the values were read from (a value cited from another work names that work in its row).

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). doi:10.1016/j.jct.2016.07.040

Please cite

R. Arróyave, J. Saal, D. Shin, ORCHID-Values: thermodynamic and phase-equilibrium values read from the literature (version 2026.10; Version 1, machine-extracted), Zenodo (2026). doi:10.5281/zenodo.23735569
R. Arróyave, J. Saal, D. Shin, ORCHID-Diagrams: calculated binary phase diagrams (version 2026.10), Zenodo (2026). doi:10.5281/zenodo.23735859
R. Arróyave, J. Saal, D. Shin, ORCHID-Hulls: binary convex hulls (Materials Project, OQMD, assessed CALPHAD) (version 2026.10), Zenodo (2026). doi:10.5281/zenodo.23735891
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 the original source of every value you use (paper_citation / paper_doi, and cited_dois for values the paper cites from earlier work); 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). 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_literature_2026,
  author    = {Arr{\'o}yave, Raymundo and Saal, James and Shin, Dongwon},
  title     = {{ORCHID-Values: thermodynamic and phase-equilibrium values read from the literature}},
  version   = {2026.10},
  publisher = {Zenodo},
  year      = {2026},
  doi       = {10.5281/zenodo.23735569},
  url       = {https://doi.org/10.5281/zenodo.23735569},
  note      = {Version 1, machine-extracted}
}

@misc{orchid_phase_diagrams_2026,
  author    = {Arr{\'o}yave, Raymundo and Saal, James and Shin, Dongwon},
  title     = {{ORCHID-Diagrams: calculated binary phase diagrams}},
  version   = {2026.10},
  publisher = {Zenodo},
  year      = {2026},
  doi       = {10.5281/zenodo.23735859},
  url       = {https://doi.org/10.5281/zenodo.23735859}
}

@misc{orchid_hulls_2026,
  author    = {Arr{\'o}yave, Raymundo and Saal, James and Shin, Dongwon},
  title     = {{ORCHID-Hulls: binary convex hulls (Materials Project, OQMD, assessed CALPHAD)}},
  version   = {2026.10},
  publisher = {Zenodo},
  year      = {2026},
  doi       = {10.5281/zenodo.23735891},
  url       = {https://doi.org/10.5281/zenodo.23735891}
}

@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}
}