Looking at hydrogen atoms with X-rays: comprehensive synchrotron diffraction study of LiBH4

Yaroslav Filinchuk, Dmitry Chernyshov

Swiss-Norwegian Beam Lines at ESRF, Grenoble, France.

Being considered as one of the most promising candidates for hydrogen storage, LiBH4 has become the subject of intensive theoretical and experimental investigations. Two LiBH4 polymorphs are known at ambient pressure, with a transition at ~380 K. Substantial theoretical and experimental efforts have been made to characterize their crystal structure. Certain discrepancies remained, however:

  • in the low-temperature (LT) phase, all theoretical studies showed nearly ideal tetrahedral geometry of the BH4 unit, while experiments described it as considerably distorted;
  • the high-temperature (HT) phase, reported from synchrotron powder diffraction data to be hexagonal [1], was found unstable by theory; a monoclinic structure has been suggested from ab initio calculations [2].

In order to resolve these discrepancies, both LiBH4 phases were studied by synchrotron diffraction on single crystals. It shows that in the LT polymorph the BH4 group has a geometry of a regular tetrahedron. The space group P63mc has been determined unambiguously for the HT phase. Anisotropic displacement ellipsoids, refined also for hydrogen atoms, reveal a libration-like smearing of the BH4 group, which is well approximated by a TLS model. The revealed disorder suggests that the unaccounted entropy is the reason why ab initio calculations have failed to evaluate correctly the stability of the P63mc structure.
A more accessible technique, synchrotron powder diffraction, was evaluated for its ability to provide accurate information on the positions of hydrogen atoms. Refinement of the LT structure from the integrated 2D diffraction images (MAR345 detector) resulted in a non-distorted BH4 geometry. We conclude that the previous powder diffraction studies done with 1D detectors suffer from a poor powder average. The HT phase remains hexagonal from the polymorphic transition at 381K up to decomposition at ~560K. Refinement of the TLS tensor showed that the libration-like disorder of the BH4 group is nearly isotropic, in agreement with the single crystal experiment.
In the presentation, B-H distances will be compared with those obtained from DFT calculations and from neutron powder diffraction study of triply isotopically substituted 7Li11BD4 [3]. Libration corrections and displacement of the electron cloud relative to an average nuclear position of an H-atom will be discussed. Positions of H-atoms and structural changes in LiBH4 were tracked by synchrotron powder diffraction from 80 to 400K with ~1K step. These results and results of combined Raman spectroscopy / powder diffraction study using a fast-readout CCD detector will also be presented.
Contrary to general belief that only neutron diffraction is capable to locate hydrogen, we show that for light hydrides the contribution of H-atoms to X-ray diffraction intensities is sufficient not only to accurately localize hydrogen atoms, but also to see and quantify the disorder of the BH4 unit.

[1] Soulié J.-P., Renaudin G., Cerný R., Yvon K., J. Alloys Compd., 2002, 346, 200.

[2] Lodziana Z., Vegge T., Phys. Rev. Lett., 2004, 93, 145501.

[3] Hartman M.R. et al. J. Solid State Chem., 2007, in press.
Keywords: synchrotron powder diffraction, hydrogen storage, analysis of disordered structures