Speaker
Mr
Ilia Pankin
(Southern Federal University, IRC "Smart materials")
Description
The material under study is manganese borohydride Mn(BH4)2. The unique thermodynamics and kinetic properties in combination with high volumetric and gravimetric hydrogen densities make this compound a very promising candidate to solid state hydrogen storage. Local atomic structure of Mn(BH4)2 upon heating was completely studied by in-situ temperature-dependent measurements of X-ray powder diffraction patterns and Mn K-edge XANES (X-ray Absorption Near Edge Structure) at SNBL-01b beamline of ERSF. Diffraction patterns indicate amorphisation during heating the sample above 110 oC. XANES Mn K-edge spectra also undergo significant changes at the temperature from 120 oC to 160o C. TGA analysis reveals drastic weight reduction (up to 10 mas %) of Mn(BH4)2 at the same temperature range. It was concluded that temperature induced phase transition which is accompanied by sample amorphisation process as well as abundant hydrogen release were observed.
We have performed ab-inito structure prediction using pseudopotential approaches within DFT approximation (VASP 5.3 code). Structural models with different contents of hydrogen atoms per unit cell were considered. Cell shape, cell volume and atomic position were relaxed. The simulation indicates collapse of porous structure and decrease in the interatomic distances Mn-B and Mn-Mn provided by significant reduction of cell volume. To find possible stable structures of dense Mn-B phase we have applied evolutionary algorithms as implemented in the USPEX code. Several low-energy candidates were selected for further analysis. Owing to amorphisation process we expect that the sample after heating has unhomogeneous structure with a nanodomain features corresponding to various crystalline atomic ordering. XANES Mn K-edge spectra were calculated for the lowest energy structural models, predicted by means of simulations. Numerical analysis of the discrepancy between experimental and theoretical XANES spectra was performed in order to verify the MnxBy local structure observed in the experiment.
Primary author
Mr
Ilia Pankin
(Southern Federal University, IRC "Smart materials")
Co-authors
Dr
Alexander Guda
(Southern Federal University)
Prof.
Alexander Soldatov
(Southern Federal University, Smart Materials)
Dr
Aram Bugaev
(Southern Federal University, Smart Materials)
Dr
Kirill Lomachenko
(Southern Federal University, Smart materials)
Dr
Vladimir Dmitriev
(ESRF)
Prof.
Yaroslav Filinchuk
(Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain)
