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

2. 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

3. 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

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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

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

14. 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

15. 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

16. 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

17. 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

18. 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

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

20. 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

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

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

23. 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

24. 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

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

26. 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

27. 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

28. 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

29. 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