{ "note": "Hydrogen number densities. rho_H in H atoms per cubic angstrom; 1 kg H per m^3 = 5.9753e-04 H per A^3 (N_A / 1.00784 g/mol). pressure_GPa 0.0001 means 1 atm; 0 means a zero-pressure calculation or equation-of-state value. Built 2026-10-06; see research/gap-hydrogen-density-census.md for status labels and caveats.", "superhydrides": [ { "compound": "SH3", "pressure_GPa": 220, "volume_A3_per_fu": 12.98, "h_per_fu": 3, "rho_H": 0.2311, "metallic": true, "source": "Errea et al. arXiv:1502.02832 (PBE, static): V = 13.3334 A^3 at 200 GPa and 12.4925 A^3 at 250 GPa; 220 GPa interpolated two ways (their quadratic E(V) fit gives 12.99; Vinet curve fitted to the Papaconstantopoulos a(P) table and anchored at 250 GPa gives 12.97)", "status": "INTERPOLATED from FETCHED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 386.8, "kind": "eta_matched", "eta_H_eV_per_A2": 10.1, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 12.96, 12.99 ], "h_eV_A": 43.7, "source_tags": [ "S1", "S2", "S3" ] }, { "compound": "SH3", "pressure_GPa": 250, "volume_A3_per_fu": 12.492, "h_per_fu": 3, "rho_H": 0.2401, "metallic": true, "source": "Errea et al. arXiv:1502.02832, Fig. 5 caption and Table VI (84.3035 a.u.^3; PBE, pressure without zero-point energy)", "status": "FETCHED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 401.9, "kind": "eta_matched", "eta_H_eV_per_A2": 10.7, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "h_eV_A": 44.6, "source_tags": [ "S1", "S2" ] }, { "compound": "SH3", "pressure_GPa": 280, "volume_A3_per_fu": 12.08, "h_per_fu": 3, "rho_H": 0.2483, "metallic": true, "source": "Extrapolated from Errea et al. 250 GPa volume with the compression ratio V(280)/V(250) = 0.966 of a Vinet or Birch-Murnaghan fit to the Papaconstantopoulos et al. a(P) table (arXiv:1501.03950, Table III); linear extension of Errea's P(V) fit gives 11.96", "status": "EXTRAPOLATED from FETCHED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 415.6, "kind": "eta_matched", "eta_H_eV_per_A2": 10.7, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 11.96, 12.12 ], "h_eV_A": 43.1, "source_tags": [ "S1", "S2", "S3" ] }, { "compound": "LaH10", "pressure_GPa": 250, "volume_A3_per_fu": 28.345, "h_per_fu": 10, "rho_H": 0.3528, "metallic": true, "source": "Wang, Yi, Cho arXiv:1907.07820 Fig. S1: a = 4.904, 4.820, 4.748 A at 220, 260, 300 GPa (PBE; second pseudopotential 4.901, 4.818, 4.745); quadratic interpolation gives a = 4.840 A. Kruglov et al. arXiv:1810.01113 print 28.5 A^3 at 250 GPa", "status": "INTERPOLATED from FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 590.4, "kind": "eta_matched", "eta_H_eV_per_A2": 8.9, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 28.31, 28.5 ], "h_eV_A": 25.2, "source_tags": [ "S1", "S5", "S6" ] }, { "compound": "LaH10", "pressure_GPa": 300, "volume_A3_per_fu": 26.759, "h_per_fu": 10, "rho_H": 0.3737, "metallic": true, "source": "Wang, Yi, Cho arXiv:1907.07820 Fig. S1: a = 4.748 A (4.745 with the second pseudopotential). Liu et al. PNAS 2017 Table S2: a = 4.78 A (27.30 A^3)", "status": "FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 625.4, "kind": "eta_matched", "eta_H_eV_per_A2": 9.2, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 26.71, 27.3 ], "h_eV_A": 24.6, "source_tags": [ "S1", "S5", "S7", "S31" ] }, { "compound": "CaH6", "pressure_GPa": 150, "volume_A3_per_fu": 21.456, "h_per_fu": 6, "rho_H": 0.2796, "metallic": true, "source": "a = 3.501 A quoted by Banacky and Noga arXiv:2106.10732 for the Wang et al. 2012 structure. Checks: H-H = 1.24 A at 150 GPa in Wang et al. arXiv:1203.0263 gives a = 3.507 A; Jeon et al. arXiv:2111.10797 Fig. 1(c) reads 3.50 A", "status": "SECONDARY (two FETCHED cross-checks)", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 468.0, "kind": "eta_matched", "eta_H_eV_per_A2": 6.7, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 21.44, 21.57 ], "h_eV_A": 24.0, "source_tags": [ "S1", "S8", "S9", "S10" ] }, { "compound": "CaH6", "pressure_GPa": 200, "volume_A3_per_fu": 19.652, "h_per_fu": 6, "rho_H": 0.3053, "metallic": true, "source": "Jeon et al. arXiv:2111.10797 Fig. 1(c), a read from the plot as 3.40 A (PBE)", "status": "DIGITISED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 511.0, "kind": "eta_matched", "eta_H_eV_per_A2": 7.7, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "h_eV_A": 25.2, "source_tags": [ "S1", "S9" ] }, { "compound": "CaH6", "pressure_GPa": 250, "volume_A3_per_fu": 18.297, "h_per_fu": 6, "rho_H": 0.3279, "metallic": true, "source": "Jeon et al. arXiv:2111.10797 Fig. 1(c), a read from the plot as 3.32 A (PBE)", "status": "DIGITISED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 548.8, "kind": "eta_matched", "eta_H_eV_per_A2": 8.2, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "h_eV_A": 25.0, "source_tags": [ "S1", "S9" ] }, { "compound": "CaH6", "pressure_GPa": 300, "volume_A3_per_fu": 17.23, "h_per_fu": 6, "rho_H": 0.3482, "metallic": true, "source": "Vinet and Birch-Murnaghan fits to six points read from Jeon et al. arXiv:2111.10797 Fig. 1(c) (120 to 250 GPa), extended to 300 GPa; both give 17.23 A^3 (a = 3.254 A); a power law on the last four points gives 17.27", "status": "EXTRAPOLATED from DIGITISED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 582.8, "kind": "eta_matched", "eta_H_eV_per_A2": 8.8, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 17.0, 17.5 ], "h_eV_A": 25.3, "source_tags": [ "S1", "S9" ] }, { "compound": "MgH6", "pressure_GPa": 300, "volume_A3_per_fu": 15.055, "h_per_fu": 6, "rho_H": 0.3985, "metallic": true, "source": "a = 3.111 A with H-H = 1.1 A at 300 GPa, quoted by Banacky and Noga arXiv:2106.10732 from Feng et al. RSC Adv. 5, 59292 (2015); the RSC page returned 403", "status": "SECONDARY", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 667.0, "kind": "eta_matched", "eta_H_eV_per_A2": 13.5, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "h_eV_A": 33.9, "source_tags": [ "S1", "S10" ] }, { "compound": "MgH6", "pressure_GPa": 400, "volume_A3_per_fu": null, "h_per_fu": 6, "rho_H": null, "metallic": true, "source": "no lattice constant found at 400 GPa", "status": "NOT FOUND", "conditions": "Im-3m", "kind": "eta_matched", "eta_H_eV_per_A2": 14.8, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "source_tags": [ "S1" ] }, { "compound": "YH10", "pressure_GPa": 300, "volume_A3_per_fu": 24.334, "h_per_fu": 10, "rho_H": 0.4109, "metallic": true, "source": "Liu et al. PNAS 2017 Table S2 (PMC5502634): a = 4.6 A at 300 GPa. Check: Heil et al. arXiv:1901.04001 give d(H-H) = 1.03 and 1.11 A at 300 GPa, which for the LaH10-type cell means a = 4.60 A", "status": "FETCHED (two significant figures)", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 687.7, "kind": "eta_matched", "eta_H_eV_per_A2": 13.4, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 23.9, 24.7 ], "h_eV_A": 32.6, "source_tags": [ "S1", "S7", "S11" ] }, { "compound": "YH10", "pressure_GPa": 400, "volume_A3_per_fu": 22.1, "h_per_fu": 10, "rho_H": 0.4525, "metallic": true, "source": "No printed value found. Estimated from the 300 GPa volume with a Murnaghan step (K about 950 GPa, K' = 3) and from the trend of the P-V panel in Heil et al. arXiv:1901.04001 Fig. 5(e), which stops at 350 GPa", "status": "ESTIMATE", "conditions": "Fm-3m", "kg_H_per_m3": 757.3, "kind": "eta_matched", "eta_H_eV_per_A2": 14.2, "eta_source": "eta_H: Quan, Ghosh, Pickett, PRB 100, 184505 (2019), arXiv:1906.02695, Tables I-III", "volume_range_A3": [ 21.6, 22.6 ], "h_eV_A": 31.4, "source_tags": [ "S1", "S11" ] }, { "compound": "LaH10", "pressure_GPa": 250, "volume_A3_per_fu": 28.345, "h_per_fu": 10, "rho_H": 0.3528, "metallic": true, "source": "Same volume as the 250 GPa row above (Wang, Yi, Cho; Kruglov et al.)", "status": "INTERPOLATED from FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 590.4, "kind": "eta_matched_hutcheon", "eta_H_eV_per_A2": 5.59, "eta_source": "eta_H: Hutcheon, Shipley, Needs, PRB 101, 144505 (2020), arXiv:2001.09852 (rigid muffin-tin, ELK); the paper prints no volumes", "h_eV_A": 15.8, "source_tags": [ "S12", "S5", "S6" ] }, { "compound": "YH10", "pressure_GPa": 400, "volume_A3_per_fu": 22.1, "h_per_fu": 10, "rho_H": 0.4525, "metallic": true, "source": "Same estimate as the 400 GPa row above", "status": "ESTIMATE", "conditions": "Fm-3m", "kg_H_per_m3": 757.3, "kind": "eta_matched_hutcheon", "eta_H_eV_per_A2": 11.46, "eta_source": "eta_H: Hutcheon, Shipley, Needs, PRB 101, 144505 (2020), arXiv:2001.09852 (rigid muffin-tin, ELK); the paper prints no volumes", "h_eV_A": 25.3, "source_tags": [ "S12" ] }, { "compound": "H3S", "pressure_GPa": 150, "volume_A3_per_fu": 14.737, "h_per_fu": 3, "rho_H": 0.2036, "metallic": true, "source": "Einaga et al. Nat. Phys. 12, 835 (2016), arXiv:1509.03156: bcc a = 3.089(1) A at 150 GPa, room temperature (XRD; S sublattice)", "status": "FETCHED", "conditions": "measured, 300 K", "kg_H_per_m3": 340.7, "kind": "measured", "source_tags": [ "S4" ] }, { "compound": "D3S", "pressure_GPa": 173, "volume_A3_per_fu": 14.142, "h_per_fu": 3, "rho_H": 0.2121, "metallic": true, "source": "Einaga et al. arXiv:1509.03156: bcc a = 3.0468(2) A at 173 GPa, room temperature", "status": "FETCHED", "conditions": "measured, 300 K", "kg_H_per_m3": 355.0, "kind": "measured", "source_tags": [ "S4" ] }, { "compound": "H3S", "pressure_GPa": 200, "volume_A3_per_fu": 13.333, "h_per_fu": 3, "rho_H": 0.225, "metallic": true, "source": "Errea et al. arXiv:1502.02832, Fig. 5 caption (89.9822 a.u.^3; PBE, static)", "status": "FETCHED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 376.5, "kind": "calculated", "source_tags": [ "S2" ] }, { "compound": "H3S", "pressure_GPa": 210, "volume_A3_per_fu": 13.012, "h_per_fu": 3, "rho_H": 0.2306, "metallic": true, "source": "Papaconstantopoulos et al. arXiv:1501.03950 Table III: a = 5.6 a.u. at 210 GPa (LAPW); value already used in gap-ambient-hopfield-eta.md H52", "status": "FETCHED", "conditions": "Im-3m, LAPW volume", "kg_H_per_m3": 385.9, "kind": "calculated", "source_tags": [ "S3", "S32" ] }, { "compound": "LaH10", "pressure_GPa": 150, "volume_A3_per_fu": 33.2, "h_per_fu": 10, "rho_H": 0.3012, "metallic": true, "source": "Drozdov et al. Nature 569, 528 (2019), arXiv:1812.01561: Fm-3m a = 5.1019(5) A, V = 132.80(4) A^3 at 150 GPa; authors estimate LaH9.6 from the volume, which would give 0.289", "status": "FETCHED", "conditions": "measured", "kg_H_per_m3": 504.1, "kind": "measured", "source_tags": [ "S14" ] }, { "compound": "LaH10", "pressure_GPa": 172, "volume_A3_per_fu": 32.975, "h_per_fu": 10, "rho_H": 0.3033, "metallic": true, "source": "131.9 A^3 at 172 GPa for the earlier experiment (Geballe et al. 2018), as quoted in Drozdov et al. arXiv:1812.01561", "status": "SECONDARY", "conditions": "measured", "kg_H_per_m3": 507.5, "kind": "measured", "source_tags": [ "S14" ] }, { "compound": "LaH10", "pressure_GPa": 150, "volume_A3_per_fu": 32.774, "h_per_fu": 10, "rho_H": 0.3051, "metallic": true, "source": "Kruglov et al. arXiv:1810.01113 structure table: Fm-3m a = 5.08 A at 150 GPa (DFT)", "status": "FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 510.6, "kind": "calculated", "source_tags": [ "S6" ] }, { "compound": "LaH10", "pressure_GPa": 200, "volume_A3_per_fu": 30.4, "h_per_fu": 10, "rho_H": 0.3289, "metallic": true, "source": "Kruglov et al. arXiv:1810.01113: calculated volume 30.4 A^3 at 200 GPa", "status": "FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 550.5, "kind": "calculated", "source_tags": [ "S6" ] }, { "compound": "CaH6", "pressure_GPa": 190, "volume_A3_per_fu": 20.035, "h_per_fu": 6, "rho_H": 0.2995, "metallic": true, "source": "Ma et al. PRL 128, 167001 (2022), arXiv:2103.16282 (ar5iv HTML): Im-3m a = 3.422 A, V = 40.07 A^3 at 190 GPa; calculated 40.09 A^3", "status": "FETCHED", "conditions": "measured", "kg_H_per_m3": 501.2, "kind": "measured", "source_tags": [ "S15" ] }, { "compound": "CaH6", "pressure_GPa": 172, "volume_A3_per_fu": 20.61, "h_per_fu": 6, "rho_H": 0.2911, "metallic": true, "source": "Equation-of-state fit to the Jeon et al. arXiv:2111.10797 Fig. 1(c) points, evaluated at 172 GPa (the same fit returns 19.99 A^3 at 190 GPa against 20.04 measured)", "status": "INTERPOLATED from DIGITISED", "conditions": "Im-3m, DFT volume", "kg_H_per_m3": 487.2, "kind": "calculated", "source_tags": [ "S9" ] }, { "compound": "YH6", "pressure_GPa": 201, "volume_A3_per_fu": 21.3, "h_per_fu": 6, "rho_H": 0.2817, "metallic": true, "source": "Kong et al. Nat. Commun. 12, 5075 (2021), arXiv:1909.10482, sample 2: Im-3m a = 3.492(1) A, V = 42.6(1) A^3 at 201 GPa on the H2 vibron scale (238 GPa on the diamond scale)", "status": "FETCHED", "conditions": "measured, pressure on the H2 vibron scale", "kg_H_per_m3": 471.4, "kind": "measured", "source_tags": [ "S16" ] }, { "compound": "YH9", "pressure_GPa": 201, "volume_A3_per_fu": 26.15, "h_per_fu": 9, "rho_H": 0.3442, "metallic": true, "source": "Kong et al. arXiv:1909.10482, sample 2 after laser heating: P63/mmc a = 3.406 A, c = 5.210 A, V = 52.3 A^3 (Z = 2)", "status": "FETCHED", "conditions": "measured, pressure on the H2 vibron scale", "kg_H_per_m3": 576.0, "kind": "measured", "source_tags": [ "S16" ] }, { "compound": "YH6", "pressure_GPa": 237, "volume_A3_per_fu": 20.65, "h_per_fu": 6, "rho_H": 0.2906, "metallic": true, "source": "Kong et al. arXiv:1909.10482, sample 1: Im-3m a = 3.457(1) A, V = 41.3(1) A^3 at 237 GPa (H2 vibron scale)", "status": "FETCHED", "conditions": "measured, pressure on the H2 vibron scale", "kg_H_per_m3": 486.3, "kind": "measured", "source_tags": [ "S16" ] }, { "compound": "YH9", "pressure_GPa": 237, "volume_A3_per_fu": 25.25, "h_per_fu": 9, "rho_H": 0.3564, "metallic": true, "source": "Kong et al. arXiv:1909.10482, sample 1: P63/mmc a = 3.364(1) A, c = 5.153(1) A, V = 50.5(1) A^3", "status": "FETCHED", "conditions": "measured, pressure on the H2 vibron scale", "kg_H_per_m3": 596.5, "kind": "measured", "source_tags": [ "S16" ] }, { "compound": "YH6", "pressure_GPa": 166, "volume_A3_per_fu": 22.903, "h_per_fu": 6, "rho_H": 0.262, "metallic": true, "source": "a = 3.578 A at 166 GPa (Troyan et al. experiment) as quoted by Banacky and Noga arXiv:2106.10732", "status": "SECONDARY", "conditions": "measured", "kg_H_per_m3": 438.4, "kind": "measured", "source_tags": [ "S10" ] }, { "compound": "LaBeH8", "pressure_GPa": 80, "volume_A3_per_fu": 40.4, "h_per_fu": 8, "rho_H": 0.198, "metallic": true, "source": "Zhang et al. PRL 128, 047001 (2022), arXiv:2106.09879: Fm-3m a = 5.643 A at 50 GPa and 5.17 A at 150 GPa (DFT); Murnaghan interpolation gives 40.2 A^3 at 80 GPa. A search snippet of a 2026 RSC supplement gave a = 5.46 A (40.6 A^3) at 80 GPa; the file returned 404. The synthesis paper (Song et al. PRL 130, 266001) was not opened", "status": "INTERPOLATED from FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 331.4, "kind": "calculated", "volume_range_A3": [ 40.2, 40.7 ], "source_tags": [ "S17", "S31" ] }, { "compound": "LaBeH8", "pressure_GPa": 50, "volume_A3_per_fu": 44.923, "h_per_fu": 8, "rho_H": 0.1781, "metallic": true, "source": "Zhang et al. arXiv:2106.09879 structure table", "status": "FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 298.0, "kind": "calculated", "source_tags": [ "S17" ] }, { "compound": "LaBeH8", "pressure_GPa": 150, "volume_A3_per_fu": 34.547, "h_per_fu": 8, "rho_H": 0.2316, "metallic": true, "source": "Zhang et al. arXiv:2106.09879 Table 1 (DFT lattice parameter)", "status": "FETCHED", "conditions": "Fm-3m, DFT volume", "kg_H_per_m3": 387.5, "kind": "calculated", "source_tags": [ "S17" ] } ], "ambient": [ { "compound": "VH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 19.477, "h_per_fu": 2, "rho_H": 0.1027, "metallic": true, "source": "COD 1538936 (Mueller and Weymann, J. Less-Common Met. 119, 115 (1986)): Fm-3m a = 4.271 A, Z = 4", "status": "FETCHED", "conditions": "CaF2 type; dihydride plateau 2 to 3 atm near 313 K (SECONDARY), so it needs a small H2 overpressure at room temperature", "kg_H_per_m3": 171.8, "source_tags": [ "S23", "S29", "S31" ] }, { "compound": "NH4BH4", "pressure_GPa": 0.0001, "volume_A3_per_fu": 84.944, "h_per_fu": 8, "rho_H": 0.0942, "metallic": false, "source": "Filinchuk, Chernyshov, Dmitriev arXiv:1003.5378 Table 1, citing Karkamkar et al. Chem. Mater. 21, 4356 (2009): Fm-3m a = 6.978 A", "status": "FETCHED (review table)", "conditions": "decomposes to NH3BH3 and H2 within hours at room temperature (SECONDARY)", "kg_H_per_m3": 157.6, "source_tags": [ "S24", "S31" ] }, { "compound": "TiH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 21.758, "h_per_fu": 2, "rho_H": 0.0919, "metallic": true, "source": "COD 1532053 (Renaudin, Aghajanyan, Yvon, J. Alloys Compd. 356, 120 (2003)): TiD2 Fm-3m a = 4.4316 A, Z = 4", "status": "FETCHED", "conditions": "deuteride cell", "kg_H_per_m3": 153.8, "source_tags": [ "S23" ] }, { "compound": "NH3BH3", "pressure_GPa": 0.0001, "volume_A3_per_fu": 65.73, "h_per_fu": 6, "rho_H": 0.0913, "metallic": false, "source": "COD 4116439 (Klooster et al., J. Am. Chem. Soc. 121, 6337 (1999)): Pmn21 a = 5.395, b = 4.887, c = 4.986 A, V = 131.46 A^3, Z = 2, 200 K", "status": "FETCHED", "conditions": "orthorhombic phase, 200 K", "kg_H_per_m3": 152.8, "source_tags": [ "S23" ] }, { "compound": "Mg2FeH6", "pressure_GPa": 0.0001, "volume_A3_per_fu": 66.462, "h_per_fu": 6, "rho_H": 0.0903, "metallic": false, "source": "COD 4344502 (Didisheim et al., Inorg. Chem. 23, 1953 (1984)): Mg2FeD6 Fm-3m a = 6.430 A, Z = 4", "status": "FETCHED", "conditions": "deuteride cell", "kg_H_per_m3": 151.1, "source_tags": [ "S23" ] }, { "compound": "TiH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 22.254, "h_per_fu": 2, "rho_H": 0.0899, "metallic": true, "source": "COD 4031666 (Nersesyan, Russ. J. Inorg. Chem. 29, 495 (1984)): Fm-3m a = 4.465 A, Z = 4", "status": "FETCHED", "conditions": "hydride cell, combustion-synthesised sample", "kg_H_per_m3": 150.4, "source_tags": [ "S23" ] }, { "compound": "Mg2FeH6", "pressure_GPa": 0.0001, "volume_A3_per_fu": 66.866, "h_per_fu": 6, "rho_H": 0.0897, "metallic": false, "source": "a = 6.443 A for the hydride (search summary of DFT papers quoting Didisheim et al.); consistent with the 2.74 g/cm^3 on the Wikipedia page, which gives 149.9 kg/m^3", "status": "SECONDARY", "conditions": "hydride cell", "kg_H_per_m3": 150.2, "source_tags": [ "S28", "S31" ] }, { "compound": "N2H4BH3", "pressure_GPa": 0.0001, "volume_A3_per_fu": 78.171, "h_per_fu": 7, "rho_H": 0.0895, "metallic": false, "source": "COD 1502676 (Mebs et al., J. Phys. Chem. A 114, 10185 (2010)): Pbcn V = 625.37 A^3, Z = 8, 95 K", "status": "FETCHED", "conditions": "hydrazine borane, 95 K", "kg_H_per_m3": 149.9, "source_tags": [ "S23" ] }, { "compound": "AlH3 (alpha)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 33.727, "h_per_fu": 3, "rho_H": 0.0889, "metallic": false, "source": "COD 4343801 (Turley and Rinn, Inorg. Chem. 8, 17 (1969)): R-3c a = 4.4493, c = 11.8037 A, Z = 6", "status": "FETCHED", "conditions": "metastable at 1 atm", "kg_H_per_m3": 148.9, "source_tags": [ "S23" ] }, { "compound": "NH3 (solid)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 33.91, "h_per_fu": 3, "rho_H": 0.0885, "metallic": false, "source": "COD 2310927 (Olovsson and Templeton, Acta Cryst. 12, 832 (1959)): P213 a = 5.138 A, Z = 4", "status": "FETCHED", "conditions": "about 171 K (temperature from MEMORY; not in the COD entry)", "kg_H_per_m3": 148.1, "source_tags": [ "S23" ] }, { "compound": "polyethylene (C2H4 unit)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 46.223, "h_per_fu": 4, "rho_H": 0.0865, "metallic": false, "source": "Bunn cell a = 7.40, b = 4.93, c = 2.534 A, Pnam, two C2H4 per cell (search summary). The GAMGI tutorial page gives 7.48, 2.55, 4.97 A, i.e. 47.40 A^3 and 0.0844", "status": "SECONDARY", "conditions": "fully crystalline, room temperature; commercial material is partly amorphous", "kg_H_per_m3": 144.8, "source_tags": [ "S30", "S31" ] }, { "compound": "NH3BH3", "pressure_GPa": 0.0001, "volume_A3_per_fu": 69.701, "h_per_fu": 6, "rho_H": 0.0861, "metallic": false, "source": "COD 4124654 (Hughes, J. Am. Chem. Soc. 78, 502 (1956)): I4mm a = 5.255, c = 5.048 A, Z = 2", "status": "FETCHED", "conditions": "tetragonal phase, room temperature", "kg_H_per_m3": 144.1, "source_tags": [ "S23" ] }, { "compound": "alpha-UH3", "pressure_GPa": 0.0001, "volume_A3_per_fu": 35.531, "h_per_fu": 3, "rho_H": 0.0844, "metallic": true, "source": "COD 1538598 (Johnson, Leider, Biel, J. Nucl. Mater. 60, 231 (1976)): Pm-3n a = 4.142 A, Z = 2", "status": "FETCHED", "conditions": "minority low-temperature form (MEMORY)", "kg_H_per_m3": 141.3, "source_tags": [ "S23" ] }, { "compound": "NbH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 23.752, "h_per_fu": 2, "rho_H": 0.0842, "metallic": true, "source": "COD 9009022 (Wyckoff, Crystal Structures, 1963): Fm-3m a = 4.563 A, Z = 4", "status": "FETCHED", "conditions": "dissociation pressure above 1 atm at room temperature (MEMORY)", "kg_H_per_m3": 140.9, "source_tags": [ "S23" ] }, { "compound": "BeH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 24.265, "h_per_fu": 2, "rho_H": 0.0824, "metallic": false, "source": "COD 1540992 (Smith et al., Solid State Commun. 67, 491 (1988)): Ibam a = 9.082, b = 4.160, c = 7.707 A, Z = 12", "status": "FETCHED", "kg_H_per_m3": 137.9, "source_tags": [ "S23" ] }, { "compound": "beta-UH3", "pressure_GPa": 0.0001, "volume_A3_per_fu": 36.65, "h_per_fu": 3, "rho_H": 0.0819, "metallic": true, "source": "COD 1008630 (Bartscher et al., Solid State Commun. 53, 423 (1985)): a = 6.643 A, Z = 8", "status": "FETCHED", "kg_H_per_m3": 137.0, "source_tags": [ "S23" ] }, { "compound": "BaReH9", "pressure_GPa": 0.0001, "volume_A3_per_fu": 112.843, "h_per_fu": 9, "rho_H": 0.0798, "metallic": false, "source": "a = 5.287, c = 9.323 A (Stetson, Yvon, Fischer, Inorg. Chem. 33, 4598 (1994)) as quoted by Singh arXiv:cond-mat/0608686; P63/mmc, Z = 2; calculated band gap 3.58 eV in the same paper", "status": "SECONDARY", "kg_H_per_m3": 133.5, "source_tags": [ "S25" ] }, { "compound": "Th4H15", "pressure_GPa": 0.0001, "volume_A3_per_fu": 189.014, "h_per_fu": 15, "rho_H": 0.0794, "metallic": true, "source": "COD 2311027 (Zachariasen, Acta Cryst. 6, 393 (1953)): I-43d a = 9.11 A, Z = 4", "status": "FETCHED", "kg_H_per_m3": 132.8, "source_tags": [ "S23" ] }, { "compound": "YH3", "pressure_GPa": 0.0001, "volume_A3_per_fu": 38.291, "h_per_fu": 3, "rho_H": 0.0783, "metallic": false, "source": "COD 1527835 (Miron et al., Kristallografiya 17, 404 (1972)): YD3 P-3c1 a = 6.34, c = 6.60 A, Z = 6", "status": "FETCHED", "conditions": "deuteride cell", "kg_H_per_m3": 131.1, "source_tags": [ "S23" ] }, { "compound": "CH4 (solid)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 51.085, "h_per_fu": 4, "rho_H": 0.0783, "metallic": false, "source": "COD 5900000 (Strukturbericht 1, 613): Fm-3m a = 5.89 A, Z = 4", "status": "FETCHED", "conditions": "COD lists 93 K, which is above the melting point; the density matches the usual 20 K value", "kg_H_per_m3": 131.0, "source_tags": [ "S23" ] }, { "compound": "Mg2CoH5", "pressure_GPa": 0.0001, "volume_A3_per_fu": 65.66, "h_per_fu": 5, "rho_H": 0.0761, "metallic": false, "source": "a = 4.463, c = 6.593 A, V = 131.32 A^3, P4/nmm (Zolliker et al., Inorg. Chem. 24, 4177 (1985)) as tabulated by Matar arXiv:1212.0698; Materials Project mp-642660 relaxes to 127.1 A^3", "status": "SECONDARY", "kg_H_per_m3": 127.4, "source_tags": [ "S26", "S27" ] }, { "compound": "HfH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 26.377, "h_per_fu": 2, "rho_H": 0.0758, "metallic": true, "source": "COD 2310961 (Sidhu, Acta Cryst. 7, 447 (1954)): tetragonal a = 3.478, c = 4.361 A, Z = 2", "status": "FETCHED", "kg_H_per_m3": 126.9, "source_tags": [ "S23" ] }, { "compound": "Be(BH4)2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 105.506, "h_per_fu": 8, "rho_H": 0.0758, "metallic": false, "source": "COD 4343675 (Marynick and Lipscomb, Inorg. Chem. 11, 820 (1972)): I41cd a = 13.62, c = 9.10 A, Z = 16", "status": "FETCHED", "kg_H_per_m3": 126.9, "source_tags": [ "S23" ] }, { "compound": "LiBH4", "pressure_GPa": 0.0001, "volume_A3_per_fu": 54.171, "h_per_fu": 4, "rho_H": 0.0738, "metallic": false, "source": "Soulie et al. 2002 cell a = 7.17858, b = 4.43686, c = 6.80321 A (Pnma, Z = 4) from the Filinchuk et al. arXiv:1003.5378 table; COD 1504402 gives 213.5 A^3 at 225 K (0.0749)", "status": "FETCHED (review table)", "conditions": "room temperature", "kg_H_per_m3": 123.6, "source_tags": [ "S23", "S24" ] }, { "compound": "ScH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 27.358, "h_per_fu": 2, "rho_H": 0.0731, "metallic": true, "source": "COD 9009038 (Wyckoff, Crystal Structures, 1963): Fm-3m a = 4.783 A, Z = 4", "status": "FETCHED", "kg_H_per_m3": 122.3, "source_tags": [ "S23" ] }, { "compound": "ZrH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 27.569, "h_per_fu": 2, "rho_H": 0.0725, "metallic": true, "source": "COD 1540309 and 2310940 (Rundle, Shull, Wollan, Acta Cryst. 5, 22 (1952)): I4/mmm a = 3.52, c = 4.45 A, Z = 2", "status": "FETCHED", "kg_H_per_m3": 121.4, "source_tags": [ "S23" ] }, { "compound": "Al(BH4)3 (alpha, solid)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 165.755, "h_per_fu": 12, "rho_H": 0.0724, "metallic": false, "source": "Aldridge et al. 1997 cell (C2/c a = 21.917, b = 5.986, c = 21.787 A, beta = 111.90 deg, 150 K) from the Filinchuk et al. arXiv:1003.5378 table; Z = 16 is my inference from the molecular volume of the beta phase", "status": "FETCHED (review table), Z inferred", "conditions": "150 K; liquid at room temperature", "kg_H_per_m3": 121.2, "source_tags": [ "S24" ] }, { "compound": "Al(BH4)3 (beta, solid)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 171.422, "h_per_fu": 12, "rho_H": 0.07, "metallic": false, "source": "Aldridge et al. 1997 cell (Pna21 a = 18.021, b = 6.138, c = 6.199 A, 195 K) from the Filinchuk et al. table; Z = 4 confirmed by Materials Project mp-569787 (64 sites, 694.9 A^3 calculated)", "status": "FETCHED (review table)", "conditions": "195 K; liquid at room temperature", "kg_H_per_m3": 117.2, "source_tags": [ "S24", "S27" ] }, { "compound": "Mg(BH4)2 (alpha)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 114.677, "h_per_fu": 8, "rho_H": 0.0698, "metallic": false, "source": "COD 4000985 (Filinchuk et al., Chem. Mater. 21, 925 (2009)): P6122 a = 10.354, c = 37.055 A, Z = 30, 100 K; COD 2102921 gives 3434.8 A^3 at 293 K", "status": "FETCHED", "conditions": "100 K", "kg_H_per_m3": 116.7, "source_tags": [ "S23" ] }, { "compound": "LaNi5H6", "pressure_GPa": 0.0001, "volume_A3_per_fu": null, "h_per_fu": 6, "rho_H": 0.0687, "metallic": true, "source": "115 kg/m^3 as stated by Zuttel (2003); no cell data fetched", "status": "FETCHED (density statement only)", "kg_H_per_m3": 115.0, "source_tags": [ "S22" ] }, { "compound": "LaH3", "pressure_GPa": 0.0001, "volume_A3_per_fu": 43.904, "h_per_fu": 3, "rho_H": 0.0683, "metallic": false, "source": "a = 5.6 A (experimental value as quoted in a search summary of arXiv:2302.05625); Materials Project mp-1018144 relaxes to 43.27 A^3 (0.0693)", "status": "SECONDARY", "kg_H_per_m3": 114.4, "source_tags": [ "S27", "S31" ] }, { "compound": "MgH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 30.811, "h_per_fu": 2, "rho_H": 0.0649, "metallic": false, "source": "COD 4124639 (Ellinger et al., J. Am. Chem. Soc. 77, 2647 (1955)): P42/mnm a = 4.5168, c = 3.0205 A, Z = 2", "status": "FETCHED", "kg_H_per_m3": 108.6, "source_tags": [ "S23" ] }, { "compound": "H2O (ice Ih)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 32.067, "h_per_fu": 2, "rho_H": 0.0624, "metallic": false, "source": "COD 9015208 (Fortes et al., J. Chem. Phys. 120, 11376 (2004)): P63/mmc a = 4.4975, c = 7.3224 A, Z = 4", "status": "FETCHED", "conditions": "low temperature (not listed in the COD entry)", "kg_H_per_m3": 104.4, "source_tags": [ "S23" ] }, { "compound": "PdH (x = 1 limit)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 17.042, "h_per_fu": 1, "rho_H": 0.0587, "metallic": true, "source": "COD 1539199 (Schirber and Morosin, Phys. Rev. B 12, 117 (1975)): Fm-3m a = 4.085 A, Z = 4", "status": "FETCHED", "conditions": "x near 1 needs high H2 pressure or low-temperature loading; at 1 atm and 300 K x is about 0.6 to 0.7 (MEMORY)", "kg_H_per_m3": 98.2, "eta_H_eV_per_A2": 0.42, "eta_source": "gap-ambient-hopfield-eta.md H7 to H10 (0.41 to 0.42 eV/A^2)", "h_eV_A": 7.2, "source_tags": [ "S23", "S32" ] }, { "compound": "LiH", "pressure_GPa": 0.0001, "volume_A3_per_fu": 17.05, "h_per_fu": 1, "rho_H": 0.0587, "metallic": false, "source": "COD 1010895 (Zintl and Harder, Z. Phys. Chem. B 28, 478 (1935)): Fm-3m a = 4.085 A, Z = 4", "status": "FETCHED", "kg_H_per_m3": 98.2, "source_tags": [ "S23" ] }, { "compound": "Mg2NiH4", "pressure_GPa": 0.0001, "volume_A3_per_fu": 68.438, "h_per_fu": 4, "rho_H": 0.0584, "metallic": false, "source": "COD 1008269 (Soubeyroux et al., Mater. Res. Bull. 19, 1119 (1984)): monoclinic low-temperature form, V = 547.5 A^3, Z = 8", "status": "FETCHED", "kg_H_per_m3": 97.8, "source_tags": [ "S23" ] }, { "compound": "NaAlH4", "pressure_GPa": 0.0001, "volume_A3_per_fu": 71.51, "h_per_fu": 4, "rho_H": 0.0559, "metallic": false, "source": "COD 4031694 (Bel'skii et al., Russ. J. Inorg. Chem. 28, 1528 (1983)): I41/a a = 5.021, c = 11.346 A, Z = 4", "status": "FETCHED", "kg_H_per_m3": 93.6, "source_tags": [ "S23" ] }, { "compound": "H2 (solid)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 38.196, "h_per_fu": 2, "rho_H": 0.0524, "metallic": false, "source": "COD 9013073 (Ishmaev et al., Zh. Eksp. Teor. Fiz. 84, 394 (1983)): hcp para-H2 a = 3.782, c = 6.167 A, two molecules per cell, 4.2 K, no applied pressure", "status": "FETCHED", "conditions": "4.2 K", "kg_H_per_m3": 87.6, "source_tags": [ "S23" ] }, { "compound": "LaH2", "pressure_GPa": 0.0001, "volume_A3_per_fu": 45.282, "h_per_fu": 2, "rho_H": 0.0442, "metallic": true, "source": "COD 1538926 (Mueller, Knappe, Greis, Z. Phys. Chem. 114, 45 (1979)): Fm-3m a = 5.658 A, Z = 4", "status": "FETCHED", "kg_H_per_m3": 73.9, "source_tags": [ "S23" ] }, { "compound": "H2 (liquid)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 47.276, "h_per_fu": 2, "rho_H": 0.0423, "metallic": false, "source": "70.8 kg/m^3 at -253 C (Zuttel 2003); NIST gives 70.85 at the normal boiling point (MEMORY)", "status": "FETCHED (density statement)", "conditions": "20.3 K, 1 atm", "kg_H_per_m3": 70.8, "source_tags": [ "S22" ] }, { "compound": "CrH2 (composition disputed)", "pressure_GPa": 0.0001, "volume_A3_per_fu": 14.384, "h_per_fu": 2, "rho_H": 0.139, "metallic": true, "source": "COD 4124683 (Snavely and Vaughan, J. Am. Chem. Soc. 71, 313 (1949)): fcc a = 3.8605 A, Z = 4, entered as CrH2. Later work puts electrolytic fcc chromium hydride near CrH (MEMORY), which would give 0.0695", "status": "FETCHED cell, composition not accepted", "conditions": "not counted in the ranking", "kg_H_per_m3": 232.7, "source_tags": [ "S23" ] } ], "candidates": [ { "compound": "Mg2IrH6", "pressure_GPa": 0, "volume_A3_per_fu": 73.686, "h_per_fu": 6, "rho_H": 0.0814, "metallic": true, "source": "a = 6.655 A (PBE), Dolui et al. arXiv:2310.07562, as recorded in gap-ambient-hopfield-eta.md H24; not re-opened", "status": "FETCHED in existing note", "conditions": "hypothetical at 1 atm", "kg_H_per_m3": 136.3, "eta_H_eV_per_A2": 3.7, "eta_source": "digitised values 2.3 to 5.2 eV/A^2 in gap-ambient-hopfield-eta.md H23", "h_eV_A": 45.5, "source_tags": [ "S32" ] }, { "compound": "Mg2RhH6", "pressure_GPa": 0, "volume_A3_per_fu": 71.255, "h_per_fu": 6, "rho_H": 0.0842, "metallic": true, "source": "Wu et al. arXiv:2608.15745: V0 = 285.02 A^3 (a = 6.5809 A, Z = 4) from the fitted equation of state; made above 40 GPa", "status": "FETCHED", "conditions": "zero-pressure volume from the measured equation of state", "kg_H_per_m3": 140.9, "eta_H_eV_per_A2": 2.7, "eta_source": "digitised, gap-ambient-hopfield-eta.md H21", "h_eV_A": 32.1, "source_tags": [ "S19", "S32" ] }, { "compound": "Mg2RhH6", "pressure_GPa": 39, "volume_A3_per_fu": 55.903, "h_per_fu": 6, "rho_H": 0.1073, "metallic": true, "source": "Wu et al. arXiv:2608.15745: a = 6.0697(1) A, V = 223.61(6) A^3 at 38.97 GPa", "status": "FETCHED", "conditions": "measured", "kg_H_per_m3": 179.6, "source_tags": [ "S19" ] }, { "compound": "Li2AgH6", "pressure_GPa": 0, "volume_A3_per_fu": 69.787, "h_per_fu": 6, "rho_H": 0.086, "metallic": true, "source": "Gao et al. arXiv:2502.18281 supplement: primitive cell a = 4.6213 A, 60 deg (PBEsol)", "status": "FETCHED", "conditions": "hypothetical, 0.319 eV/atom above the hull", "kg_H_per_m3": 143.9, "eta_H_eV_per_A2": 2.91, "eta_source": "upper bound M_H lambda omega_2^2, gap-ambient-hopfield-eta.md H26", "h_eV_A": 33.8, "source_tags": [ "S20", "S32" ] }, { "compound": "Li2AuH6", "pressure_GPa": 0, "volume_A3_per_fu": 73.884, "h_per_fu": 6, "rho_H": 0.0812, "metallic": true, "source": "Primitive-cell a = 4.71 A from a search snippet; the paper holding it was not identified. Ouyang et al. arXiv:2501.12222 and the Gao et al. supplement print no lattice constant", "status": "SECONDARY (unopened source)", "conditions": "hypothetical", "kg_H_per_m3": 135.9, "eta_H_eV_per_A2": 3.62, "eta_source": "upper bound M_H lambda omega_2^2, gap-ambient-hopfield-eta.md H28", "h_eV_A": 44.6, "source_tags": [ "S20", "S32", "S31" ] }, { "compound": "RbPH3", "pressure_GPa": 0, "volume_A3_per_fu": 75.827, "h_per_fu": 3, "rho_H": 0.0396, "metallic": true, "source": "Dangic et al. arXiv:2411.03822 supplement: R3m a = 4.233 A, 90.777 deg (static); SSCHA a = 4.297 A, 91.071 deg gives 79.30 A^3 and 0.0378", "status": "FETCHED", "conditions": "hypothetical", "kg_H_per_m3": 66.2, "source_tags": [ "S21" ] }, { "compound": "PdH4 (cubic)", "pressure_GPa": 0, "volume_A3_per_fu": 34.009, "h_per_fu": 4, "rho_H": 0.1176, "metallic": true, "source": "a = 4.082 A, space group 229, two formula units per conventional cell (Alexandria agm071021503), as recorded in gap-gao-pareto-data-and-pdh4.md F42; not re-opened", "status": "FETCHED in existing note", "conditions": "hypothetical, 319 meV/atom above the hull", "kg_H_per_m3": 196.8, "source_tags": [ "S32" ] }, { "compound": "atomic metallic hydrogen (I41/amd)", "pressure_GPa": 500, "volume_A3_per_fu": 1.144, "h_per_fu": 1, "rho_H": 0.8743, "metallic": true, "source": "McMahon and Ceperley, PRB 84, 144515 (2011), arXiv:1106.5526 Table I: a = 2.299 a.u., c/a = 2.545, r_s = 1.226 a.u. at 500 GPa", "status": "FETCHED", "conditions": "calculated", "kg_H_per_m3": 1463.1, "source_tags": [ "S18" ] } ] }