The search for hydrogen-storage materials has been extremelly vivid during the recent decades, with those having high gravimetric contents of hydrogen being at the focus of the research. Among them, borohydrides show record high gravimetric H content reaching 21 wt.%. To taylor their thermodynamic stability, multi-cation light metal borohydrides have been explored in recent years. Crystal structure of these compounds are very dificult to charactererize from experiment due to low crystallinity, presence of poorly scattering atoms and frequent substitutional disorder. On the theoretical grounds these crystalline compounds have been modelled using Density Functional Theory (DFT) [1]. Here we demonstrate that classical DFT functionals are insuffecient for their descritpion and that weak dispersive H...H interactions must be taken to account to reach qualititave agreement with experiment. We present our findings on an important case of LiSc(BH4)4 containing ca. 14.4 wt% H, for which we solve old-standing problem of identifying the most stable polymorphic α form [2] and also resolve the identity of the recently reported β form [3]. Our findigs are based on systematic DFT/DFT-D3 study combined with lattice dynamics in quasiharmonic approximation (direct phonon method) and utilizing the Zr(BH4)4 structure models with Sc → Zr substation and Li filing the interstiticals [4].
Acknowledgement: This research was carried out with the support of the Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw under grant no. ADVANCE++ (GA76-19). M.D. acknowledges the European Regional Development Fund, Research and Innovation Operational Programme, for project No. ITMS2014+: 313011W085 and Scientific Grant Agency of the Slovak Republic, grant No. VG 1/0223/19. The authors acknowledge the support from Polish National Science Centre under grant HYDRA no. 2014/15/B/ST5/05012.
References:
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[3] A. Starobrat, T. Jaroń, W. Grochala, Dalton Trans., 47, 4442 (2018).
[4] A. Starobrat, M. Derzsi, T. Jaroń, P. Malinowski and W. Grochala, available at https://arxiv.org (2020).
