jp9015883_si_003.pdf (756.67 kB)
The First Crystallographic and Spectroscopic Characterization of a 3d-Metal Borohydride: Mn(BH4)2
journal contribution
posted on 2009-05-21, 00:00 authored by Radovan Černý, Nicolas Penin, Hans Hagemann, Yaroslav FilinchukThe first crystal structure of a 3d-metal borohydride
is presented. Solvent-free homoleptic manganese borohydride Mn(BH4)2 forms at ambient conditions in ball-milled mixtures
of alkali metal borohydrides and MnCl2. It crystallizes
in the trigonal crystal system with the space group symmetry P3112 and is stable from 90 to 450 K, where the
compound melts. Thermal expansion of Mn(BH4)2 between 90 and 400 K is highly anisotropic and strongly nonuniform.
The structure of Mn(BH4)2 shows interesting
similarity to α-Mg(BH4)2: the two structures
are made of similar layers L with the composition M4(BH4)10 per cell. The layers are
stacked along the c-axis, and rotated by 120°
by the 31 axis in Mn(BH4)2 and by
60° by the 61 axis in α-Mg(BH4)2. Three identical layers are stacked along one unit cell vector c in Mn(BH4)2, while six layers are
stacked in α-Mg(BH4)2. In Mn(BH4)2 the layers L are connected directly, and share atoms.
In α-Mg(BH4)2 the layers L are intercalated
by a thin layer L′, which contains one Mg atom per layer per
cell. The layer L is chiral, and both borohydrides crystallize in
chiral space groups. Similar to α-Mg(BH4)2, the structure of Mn(BH4)2 is not densely
packed and contains isolated voids with the estimated volume of 21
Å3 each, which occupy in total 6% of the space. The
resemblance between Mn(BH4)2 and α-Mg(BH4)2 is also reflected in their Raman and infrared
spectra.