Light metal borohydrides: going beyond crystal structures

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

2. MBH4 Aims and Methods • Aims • Obtain various phases of M(BH4)nM = Li, Na, Ca… • Study their structure and transformations • Understand their stability and find ways to influence it • Methods • Synchrotron diffraction: on powders and single-crystals • Varied temperature and pressure (diamond anvil cells) • Crystal-chemical and phenomenological analyses Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

3. LiBH4 Discrepancies: experiments vs. theory • LT phase: experiments find BH4 strongly deformed, theory – ideally tetrahedral • 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 todetect andevaluate the disorder of the BH4 unit Grown in-situ BH4 disorder stabilizes the P63mc structure

4. LiBH4 Powder diffraction with X-rays Good powder average is essential. Use 2D detectors! Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

5. LiBH4 Geometry of the BH4 group: revised Consistent results, including those from powder diffraction! Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

6. LiBH4 Cell parameters vs. temperature Really bad agreement with theory Large anharmonicity, negative volume jump Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

7. MBH4 Geometry of the BH4 group • Reliable geometry of the BH4 • Synchrotron: 2D detectors • Neutrons: isotopically substituted samples, like 7Li11BD4 • Corrections for the B-H bond distances • From the X-ray diffraction: to be elongated by 0.08 Å • All experimental: libration correction ~0.03 Å at 200 K, ~0.10 Åat 500 K ! different libration correction for D-atoms (neutrons) • Ideal tetrahedral, unless: • There are really good reasons for the distortion: polarizing cation like Al3+ • Distortion is accurately detected from an experiment or theory Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

8. LiBH4 High-pressure data:~2.4 GPa,  = 0.70140 Å Moderate P (up to 20 GPa) but excellent data Actual intensity at the strongest peak is over 3.2107 counts, reaching 108 counts at high angles. Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

9. LiBH4 Ab-initio structure solution from HP data 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 BH4, shortest Li…H Find a true symmetry (Platon): P1  Ama2 in the doubled cell, a ~ 5.30 and c ~ 6.45 Å Refinement (Fullprof) No preferred orientation, this time… Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

10. LiBH4 Short H…H contacts: BH4 deformation Post-experimantal DFT calculations: Confirmed! Including positions of H-atoms A novel coordination of BH4: prediction of this structure from theory was doomed to failure Destabilized Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

11. LiBH4 High pressure: equations of state V0 = 54.43(8) Å3 B0 = 14.4(5) GPa V0 = 49.5(1) Å3 B0 = 23.23(9) GPa B'0 = 3.51(15) V0 = 47.3(9) Å3B0 = 26(3)GPa Cubic phase at higher pressures Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

12. LiBH4 Four phases now (!) The new dense structure is quenchable (T > 200K), andmay show better H-storage properties if stabilized at ambient conditions by a chemical substitution Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

13. NaBH4 Phase transitions with T Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

14. High-pressure: new phase NaBH4 • 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 – ? Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

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, 27Aug 2008

16. NaBH4 Importance of the texture & H-atoms Rietveld refinement • No texture: RB increases from 7.9% to 45% • No H-atoms: RB increases from 7.9% to 17%

17. NaBH4 New phase at 11.2 GPa: BaSO4-type structure a-axes approximately aligned with the compression direction The first case were the simultaneous « solution » of a structure and of a texture was essential for success Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

18. Ca(BH4)2 Centro vs. non-centro Fddd Model from Miwa et al., 2006 F2dd Our model Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

19. Ca(BH4)2 Phase transitions with T Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

20. Geometry of the BH4…M interaction • Coordination numbers for the metal atoms: • Be and Al atoms: 3 – trigonal-planar • Mg: 4 – deformed tetrahedral • Li: 4 – tetrahedral, 6 – octahedral (high P) • Na, K, Ca: 6 – octahedral, and even 7 for K No simple rule • Coordination numbers for the BH4 group: • MBH4: 4 – tetrahedral; 6 – octahedral • M(BH4)2:3 – T-shaped for Ca; 2 – linear for Be and Mg • Be(BH4)2 and Al(BH4)3:1 – terminal ligand • unusual: 4– square-planar (HP LiBH4); square-pyramidal 5 in LiK(BH4)2 Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

21. LiBH4 P-T diagram heated pressure cells Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

22. LiBH4 P-T phase diagram Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

23. LiBH4 Transformation mechanisms in the Hexagonal and Cubic Branches P63/mmc (Z=2) Fm-3m (Z=1) F1u=G4- X5- A2u=G2- I4mm (Z=1) P63mc (Z=2) M2- Cmcm (Z=2) Pnma (Z=4) Ama2 (Z=2) Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

24. D=4a2b2-g2 a2>0, b2>0, g>0, D>0 a2<0, b2<0, g>2(a2b2)1/2>0, D>0 a2<0, b2<0, g<-2(a2b2)1/2<0, D>0 Yu.M.Gufan and E.S.Larin, Sov.Phys.Solid State, v.22, 270 (1979) LiBH4 Phenomenological phase diagram F(h,x)=a1h2+a2h4+a3h6+b1x2+b2x4+b3x6+gh2x2 Phase 0: h0=0, x0=0 (parent P63/mmc or Fm-3m); Phase A: hA0 xA=0 (P63mc/I4mm); Phase B: hB=0, xB 0 (Pnma/Cmcm); Phase C:hC 0, xC 0 (Pmc21/Ama2). Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

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

26. Geometry of the BH4…M interaction • Formation of stable (BH4-M)n fragments • Common BH4…Mcoordination via tetrahedral edges Mg Mg • Possible formation of partly covalent BH4…M bonds! molecular orbitals like in diborane B2H6 • Directional BH4…M interaction defines anion-centered complexes Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

27. Anion-centered complexes First proposed for Li4(NH2)(BH4)3 Filinchuk et al., Inorg. Chem., 2006 BH4 Tetrahedral coordination NH2 Saddle-like coordination Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

28. Chemically destabilized borohydrides? • Mixed-cation borohydrides: LiK(BH4)2 and LiSc(BH4)4 • Mixed-anion borohydrides: Li-BH4-NH2,Na-BH4-AlH4, Mg-BH4-AlH4 All are ordered and too stable • Partial substitution of the BH4 anion:Li-BH4-Cl–allows to tune properties • Partial substitution in the BH4 anion:M-BH3F – similar to Na3AlH6-xFx– unlikely • Amidoboranes:M-BH3NH2 – not prepared yet from MBH4 Mosegaard et al., J. Phys. Chem. C, 2008 Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008

29. Summary • New methodology • We can see hydrogen atoms by X-rays: BH4 disorder and deformation • Structure solution: try to lower symmetry (P1 !) and use texture (DACs) • New information • New forms of borohydrides • Microscopic mechanisms of phase transformations: P- & T-dependence of the order parameters • Crystal chemistry of BH4…M interaction: directionality, cation-anion layers • New ideas • Stabilization of high-pressure forms by chemical substitutions (« chemical pressure ») – in-situ studies of the reactions • Anion-centered complexes, partly covalent BH4…Mbonding Y. Filinchuk IUCr XXI, Osaka, 27Aug 2008