H sites undetermined since 1950s when initial work

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H-sites undetermined since 1950’s when initial work began Model the hydrogen storage reaction LiNH 2 + LiH Li 2 NH + H 2 two stable compounds react to form a third + H 2 reversible (280°C, 1 bar H 2 ) 6.5 mass% theoretical H 2 capacity Ima2 (46), Li 2 NH, 32 atom a = 7.12 Å, b = 10.07 Å, c = 7.09 Å * J. F. Herbst, L.G. Hector, Jr., Phys. Rev. B 72 (2005) 125120 . * M. Balogh et al., J. Alloys Compd . 420 (2006) . * VASP Code is used for all DFT calculations. See: Density Functional Theory: Structure Determination and Discrimination
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Ima2 Crystal Structure – from neutron powder diffraction at GM and NIST* (now confirmed at 50 K) 2 (deg rees ) 10152025303540455055606570 Intensity 0 20 40 60 80 100 Li-disordered cubic Fd-3m (partially occupied Li 32e sites) 2 (deg rees ) 10152025303540455055606570 Intensity 0 20 40 60 80 100 orthorhombic Ima2 2 (deg rees ) 10152025303540455055606570 Intensity 0 20 40 60 80 100 VASP-relaxed Ima2 * M. Balogh, et al. J. Alloys Compd. 420 (2006) 326. Ima2 structure also, agrees with exp. vibrational data! Density Functional Theory: Structure Determination and Discrimination
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Li 2 NH Structure Space Group E el E el ZPE E 0 E 0 F 298 F 298 F 298 1 (Structur e V in the next slide) Ima2 a -1698.84 0.00 46.38 -1652.46 0.00 -7.03 -1659.49 0.00 2 Pnma * -1699.93 -1.09 46.62 -1653.31 -0.85 -6.57 -1659.88 -0.39 3 C2/m * -1701.22 -2.38 46.32 -1654.90 -2.44 -5.56 -1660.46 -0.97 4 Pnma b -1701.82 -2.98 47.06 -1654.76 -2.30 -6.16 -1660.92 -1.43 5 Pbca c -1703.37 -4.53 46.51 -1656.86 -4.40 -6.47 -1663.33 -3.84 DFT-computed energetics of various Li 2 NH crystal structures in the GGA PW91. All energies in kJ/mol-f.u. (f.u. formula unit). F 298 = F 298 - F 298 (Ima2) G. Hector, Jr, J.F. Herbst, J. Physics: Condens. Matter 20(6) (2008) 064229,1-11. F. Herbst, L.G. Hector, Jr., Phys. Rev. B 72 (2005) 125120 . . Magyari-Kope, et al., Phys. Rev. B 73 (2006) 220101 . Mueller and G. Ceder, Phys. Rev. B. 74 (2006) 134104 . Density Functional Theory: Structure Determination and Discrimination*
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Noreus & Werner J. Less-Comm. Met. 97 (1984) 215 C2/m (No. 12) Z = 4 Zolliker, Yvon, Jorgensen, & Rotella Inorg. Chem. 25 (1986) 3590 C2/c (No. 15) Z = 8 LTI LTII Mg Ni H * J.F. Herbst, L.G. Hector, Jr., Phys. Rev. B 79 (2009) 155113. Density Functional Theory: Structure Determination and Discrimination for Mg 2 NiH 4 *
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Phonon Spectrum for LTII [GGA (PW91)] 48 1x2x2 (224 atom) supercells no soft modes; total phonon DoS shows no imaginary excursions E el (LTII) = – 23.88 eV Density Functional Theory: Structure Determination and Discrimination for Mg 2 NiH 4 *
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Phonon Spectrum for LTI [GGA (PW91)] 42 2x2x2 (224 atom) supercells 9 soft modes at high symmetry points: capture & optimize each – all metals except lower Z-pt monoclinic C2/m structure E el (lower Z-pt C2/m structure) = – 23.62 eV/Mg 2 NiH 4 E el (LTI) = – 23.13 eV E el (LTII) = – 23.88 eV lower Z-pt C2/m structure Z = 8 Density Functional Theory: Structure Determination and Discrimination for Mg 2 NiH 4 *
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Follow Soft Mode: P-1 Structure Low Energy Structure from Path Starting at Z- point c b a 84 2x2x2 (224 atom) supercells triclinic P-1 (No. 2) Z =4 NiH 4 tetrahedra irregular, flattened E el (LTII) = – 23.88 eV E el (P-1) = – 23.76 eV Density Functional Theory: Structure Determination and Discrimination for Mg 2 NiH 4 *
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DFT structure determination: useful (e.g. for thermochemical calculations) in the instance that a structure is not available in the experimental literature).
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