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1 Light metal borohydrides: going beyond crystal structures Yaroslav Filinchuk Dmitry Chernyshov Vladimir Dmitriev SNBL @ ESRF www.filinchuk.comwww.snbl.eu

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2 Aims and Methods Methods Synchrotron diffraction: on powders and single-crystals Varied temperature and pressure (diamond anvil cells) Crystal-chemical and phenomenological analyses MBH 4 Aims Obtain various phases of M(BH 4 ) n M = Li, Na, Ca… Study their structure and transformations Understand their stability and find ways to influence it Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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3 Discrepancies: experiments vs. theory LiBH 4 LT phase LT phase: experiments find BH 4 strongly deformed, theory – ideally tetrahedral HT phase HT phase: experiments find it hexagonal, theory says the hexagonal is unstable Single-crystal data: Contribution of H-atoms to X-ray diffraction intensities is sufficient to accurately localize hydrogen atoms, and to detect and evaluate the disorder of the BH 4 unit BH 4 disorder stabilizes the P6 3 mc structure Grown in-situ

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4 Powder diffraction with X-rays LiBH 4 Good powder average is essential. Use 2D detectors! Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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5 Geometry of the BH 4 group: revised LiBH 4 Consistent results, including those from powder diffraction! TechniqueT (K)Sp. gr.B-H (Å)H-B-H (°)Ref. Synchrotron diffraction on single crystals 225Pnma1.104(11)-1.131(15)108.8(9)-109.9(7)this work 535P6 3 mc1.05(6)-1.11(9)108(3)-111(3)this work Neutron powder diffraction on 7 Li 11 BD 4 3.5Pnma1.208(3)-1.225(6)107.2(3)-111.7(4)Hartman 2007 360Pnma1.184(16)-1.217(15)105.4(5)-111.8(8)Hartman 2007 400P6 3 mc0.96(8)-1.02(3)106(3)-112(3)Hartman 2007 Synchrotron powder diffraction 90Pnma1.108(8)-1.151(8)104.9(5)-112.4(8)this work 293Pnma1.04(2)-1.28(1)85.1(9)-120.1(9)Soulié 2002 293Pnma1.28(3)-1.44(3)60(1)-131(1)Züttel 2003 293Pnma1.118(7)-1.151(11)98.7(9)-113(1)this work 382P6 3 mcfixed to 1.10fixed to 109.5this work 408P6 3 mc1.27(2)-1.29(2)106.4(2)-112.4(9)Soulié 2002 Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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6 Cell parameters vs. temperature LiBH 4 Large anharmonicity, negative volume jump Really bad agreement with theory Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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7 Geometry of the BH 4 group MBH 4 Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Reliable geometry of the BH 4 2D detectors Synchrotron: 2D detectors isotopically substituted Neutrons: isotopically substituted samples, like 7 Li 11 BD 4 Corrections for the B-H bond distances 0.08 Å From the X-ray diffraction: to be elongated by 0.08 Å libration correction All experimental: libration correction ~0.03 Å at 200 K, ~0.10 Å at 500 K ! ! different libration correction for D-atoms (neutrons) Ideal tetrahedral, unless: There are really good reasons for the distortion: polarizing cation like Al 3+ Distortion is accurately detected from an experiment or theory

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8 LiBH 4 High-pressure data : ~2.4 GPa, = 0.70140 Å Actual intensity at the strongest peak is over 3.2 10 7 counts, reaching 10 8 counts at high angles. Moderate P (up to 20 GPa) excellent but excellent data Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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9 LiBH 4 Ab-initio structure solution from HP data No preferred orientation, this time… Indexing (Dicvol04): Primitive tetragonal cell, a ~ 3.75 and c ~ 6.45 Å Structure solution by global optimization in direct space (FOX): No solution in all space groups P1, add all chemical info: tetrahedral BH 4, shortest Li…H Find a true symmetry (Platon): P1 Ama2 in the doubled cell, a ~ 5.30 a ~ 5.30 and c ~ 6.45 Å Refinement (Fullprof) Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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10 LiBH 4 Short H…H contacts: BH 4 deformation Post-experimantal DFT calculations:Confirmed! Including positions of H-atoms A novel coordination of BH 4 : prediction of this structure from theory was doomed to failure Destabilized Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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11 LiBH 4 High pressure: equations of state V 0 = 54.43(8) Å 3 B 0 = 14.4(5) GPa V 0 = 49.5(1) Å 3 B 0 = 23.23(9) GPa B' 0 = 3.51(15) V 0 = 47.3(9) Å 3 B 0 = 26(3) GPa Cubic phase at higher pressures Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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12 LiBH 4 Four phases now (!) The new dense structure is quenchable (T > 200K), and may show better H-storage properties if stabilized at ambient conditions by a chemical substitution Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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13 NaBH 4 Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Phase transitions with T

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14 High-pressure: new phase Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Not related to the cubic & tetragonal phases Indexed in Pnma Failed to solve the structure by all means Lower symmetry – test in P1 Preferred orientation – ? NaBH 4

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15 Diffraction geometry Compression direction coincides with a direction of the incident X-ray beam Under these conditions, the texture is not detectable even from the 2D (area detector) data Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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16 Importance of the texture & H-atoms No texture:R B increases from 7.9% to 45% No H-atoms:R B increases from 7.9% to 17% Rietveld refinement NaBH 4

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17 New phase at 11.2 GPa: BaSO 4 -type structure a-axes approximately aligned with the compression direction NaBH 4 The first case were the simultaneous « solution » of a structure and of a texture was essential for success Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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18 Centro vs. non-centro Ca(BH 4 ) 2 Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Fddd Model from Miwa et al., 2006 F2dd Our model

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19 Phase transitions with T Ca(BH 4 ) 2 Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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20 Geometry of the BH 4 …M interaction Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Coordination numbers for the BH 4 group: 46 MBH 4 : 4 – tetrahedral; 6 – octahedral 32 M(BH 4 ) 2 : 3 – T-shaped for Ca; 2 – linear for Be and Mg 1 Be(BH 4 ) 2 and Al(BH 4 ) 3 : 1 – terminal ligand 45 unusual: 4 – square-planar (HP LiBH 4 ); square-pyramidal 5 in LiK(BH 4 ) 2 Coordination numbers for the metal atoms: 3 Be and Al atoms: 3 – trigonal-planar 4 Mg: 4 – deformed tetrahedral 46 Li: 4 – tetrahedral, 6 – octahedral (high P) 67 Na, K, Ca: 6 – octahedral, and even 7 for K No simple rule

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21 LiBH 4 P-T diagram Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 heated pressure cells

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22 LiBH 4 P-T phase diagram Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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23 P6 3 /mmc (Z=2) P6 3 mc (Z=2) Pnma (Z=4) A 2u = 2- M 2- Fm-3m (Z=1) Ama2 (Z=2) Cmcm (Z=2) I4mm (Z=1) F 1u = 4- X 5- LiBH 4 Transformation mechanisms in the Hexagonal and Cubic Branches Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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24 F( )=a 1 2 +a 2 4 +a 3 6 +b 1 2 +b 2 4 +b 3 6 + 2 2 Phase 0: 0 =0, 0 =0 (parent P6 3 /mmc or Fm-3m); Phase A: A 0 A =0 (P6 3 mc / I4mm); Phase B: B =0, B 0 (Pnma / Cmcm); Phase C: C 0, C 0 (Pmc2 1 / Ama2). Phase 0: 0 =0, 0 =0 (parent P6 3 /mmc or Fm-3m); Phase A: A 0 A =0 (P6 3 mc / I4mm); Phase B: B =0, B 0 (Pnma / Cmcm); Phase C: C 0, C 0 (Pmc2 1 / Ama2). =4a 2 b 2 - 2 a 2 >0, b 2 >0, >0, a 2 2(a 2 b 2 ) 1/2 >0, a 2 <0, b 2 <0, <-2(a 2 b 2 ) 1/2 <0, Yu.M.Gufan and E.S.Larin, Sov.Phys.Solid State, v.22, 270 (1979) LiBH 4 Phenomenological phase diagram Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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25 All phases contain layers, which are deformed or reshuffled Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Reconstructivetransition

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26 Geometry of the BH 4 …M interaction Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Formation of stable (BH 4 -M) n fragments Common BH 4 …M coordination via tetrahedral edges Mg anion-centered complexes Directional BH 4 …M interaction defines anion-centered complexes ! Possible formation of partly covalent BH 4 …M bonds ! molecular orbitals like in diborane B 2 H 6

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27 Anion-centered complexes Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 First proposed for Li 4 (NH 2 )(BH 4 ) 3 Filinchuk et al., Inorg. Chem., 2006 BH 4 Tetrahedral coordination NH 2 Saddle-like coordination

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28 Chemically destabilized borohydrides? Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008 Mosegaard et al., J. Phys. Chem. C, 2008 Mixed-cation borohydrides: LiK(BH 4 ) 2 and LiSc(BH 4 ) 4 Mixed-anion borohydrides: Li-BH 4 -NH 2, Na-BH 4 -AlH 4, Mg-BH 4 -AlH 4 Li-BH 4 -Cl Partial substitution of the BH 4 anion: Li-BH 4 -Cl – allows to tune properties All are ordered and too stable M-BH 3 F Partial substitution in the BH 4 anion: M-BH 3 F – similar to Na 3 AlH 6-x F x – unlikely M-BH 3 NH 2 Amidoboranes: M-BH 3 NH 2 – not prepared yet from MBH 4

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29 Summary New methodology We can see hydrogen atoms by X-rays: BH 4 disorder and deformation Structure solution: try to lower symmetry (P1 !) and use texture (DACs) New ideas Stabilization of high-pressure forms by chemical substitutions (« chemical pressure ») – in-situ studies of the reactions Anion-centered complexes, partly covalent BH 4 …M bonding New information New forms of borohydrides Microscopic mechanisms of phase transformations: P- & T-dependence of the order parameters Crystal chemistry of BH 4 …M interaction: directionality, cation-anion layers Y. Filinchuk IUCr XXI, Osaka, 27 Aug 2008

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