i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 7 1 1 e2 7 1 2
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Discussion
Reply to “A comment on ‘Prediction of crystal structure, lattice
dynamical, and mechanical properties of CaB2H2’
by Vajeeston et al., Int J Hydrogen Energy, 36 (2011)
10149e10158”
P. Vajeeston*, P. Ravindran, H. Fjellvåg
Department of Chemistry, Center for Materials Sciences and Nanotechnology, University of Oslo, P.O. Box 1033 Blindern,
N-0315 Oslo, Norway
article info
Article history:
Received 6 October 2011
Accepted 24 October 2011
Available online 17 November 2011
Dear Editor,
We would like to bring the following points to your kind
attention.
The main motivation of Ref. [1] is to find the crystal structure and its structural stability of the chosen chemical
composition CaB2H2 and we have not studied/proposed the
reaction pathways or possible decomposition pathways of
Ca(BH4)2. According to Ref. [2] CaB2Hx was an intermediate
product of two-step decomposition of Ca(BH4)2. Also, it is well
known that the isolectronic systems of CaB2H2 such as SrAl2H2,
SrGa2H2, and BaGa2H2 are well stable phases and their properties are well characterized [3,4]. So, it is worthwhile to
investigate the structural stability and properties of CaB2H2. It
should be noted that it might be possible to form several phases like CaBH5, CaB2H2 and CaB2, etc. depending upon the
preparatory condition. But existence of such compound is
unknown in the literature. Most of the compounds crystallize
at some point during their production process. Recently
Frankcombe studied (Ref. [5]) the structural relationship
between the several structures proposed in the literature for
the a- and b-phases of Ca(BH4)2 using the density functional
theory calculation. In addition to that in Ref. [5] the author
demonstrated that the experimentally proposed CaB2Hx
(assumed CaB2H2) structure with space group Pnma is structurally unstable as well as it was too high in energy to be
a Ca(BH4)2 dehydrogenation intermediate. But the author has
not put much effort to find the correct ground state crystal
structure of the experimentally proposed chemical composition CaB2H2. Further, though the stability of CaB2H2 is high in
the isolated condition, its stability will differ during the
Ca(BH4)2 decomposition condition due to the fact that finite
temperature, size of the CaB2H2 particles, chemical environment caused by decomposed products, and hydrogen partial
pressure (hence the energetics of the diagram of Frankcombe
will change) and this is completely ignored in Ref. [5]. So,
DOI of original article: 10.1016/j.ijhydene.2011.10.121.
* Corresponding author. Tel.: þ47 22855613.
E-mail address: ponniahv@kjemi.uio.no (P. Vajeeston).
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2011.10.097
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 7 1 1 e2 7 1 2
without knowing the energetics of the reaction products at the
conditions mentioned above it is not possible to correctly
predict the decomposition path of Ca(BH4)2. Moreover, apart
from the view of formation of CaB2H2 as an intermediate in
Ca(BH4)2 decomposition reaction, the detailed investigation
about stability, thermodynamical, and mechanical properties
of CaB2H2 will be of interest in view of understanding the
properties of new hydride that is not yet studied
experimentally.
Zhang et al. [6] in their recent publication studied the
possible decomposition paths of Ca(BH4)2 and Mg(BH4)2 where
they try to find the ground state crystal structure of CaB2Hn
(n ¼ 2, 4, 6) complexes using PEGS simulations along with DFT
calculations. For the CaB2H2 and CaB2H4 crystal the predicted
low-energy structures are thermodynamically unstable.
Similarly, for the CaB2H2 phase the experimentally proposed
crystal structure as well as the predicted low-energy structure
in Ref. [6] are metallic, which is unexpected due to the highly
ionic character of these phases. In Ref. [1] we have systematically studied the structural stability of the CaB2H2 using ICSD
data base based structure search approach. Among the
considered 22 potentially applicable AB2X2-structure models
for the CaB2H2 chemical composition, a NiNa2O2 derived
atomic arrangement was found to have lowest total energy
(for more details see Ref. [1]). Similarly, it is so far believed that
CaB2 might be stabilize in MgB2-type structure and several
publications are also made assuming this structure as ground
state structure to study the super conductivity, lattice
dynamics, etc. On the other hand we have predicted a much
more energetically stable configuration for CaB2 from our
hybrid data base approach (ICSD followed by simulating
annealing and genetic algorithm approach). According to our
approach the CaB2 crystallize in Cmmm space group and the
considered phases (CaB2H2 and CaB2) are dynamically as well
as mechanically stable. According to our electronic structure
calculation CaB2 is a metallic and CaB2H2 is a non-metallic
phase.
Regarding the decomposition reaction energetics it is very
hard to assign a set of specific reactions especially for the
borides. Simple stoichiometric arguments suggest a number of
potential accessible decomposition reactions. Up-to-date only
few of them are considered. It should be recalled that there are
many groups around the world assume different reaction
pathways (for example see Refs. [5e9]), but none of the
theoretically predicted intermediate complexes/compounds/
species are not fitted with the experimental observations. This
clearly indicates that there is more room for the new species. If
one believes that the calculated formation energy for the isolated bulk phase is valid to explain the energetics of possible
decomposition paths of Ca(BH4)2, as noted by Frankcombe in his
comment, the CaB2H2 phase may not involve in the decomposition path of Ca(BH4)2. From our study we have noted that CaB2
phase also not involved in the decomposition path of Ca(BH4)2
because the formation energy of CaB2 is almost same as the
reaction energy of Ca þ 2B þ 4H2 reaction. In Ref. [6] the authors
identified a new phase CaB2H6 which forms a low-energy
intermediate (44.04 kJ/mol H2) and its formation energy practically degenerated that of CaB12H12. This reaction energy was not
taken into account by Frankcombe’s in the comment and hence
the conclusion presented in the comment may change when
this phase is also taken into account. Finally, in all calculations
reported so far the reaction energies are calculated only for the
isolated pure (without any defects) and periodic systems. But, in
reality, the compounds are mixed with more than one phase/
compound (when it forms as an intermediate), finite size and
the stoichiometry is also not well defined under the decomposition reaction condition. In such situation it is very hard to
define the intermediate species, especially in borohydrides
where the decomposition mechanism become much more
complicated than that in alanates and other hydride family.
references
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Hydrogen Energy 2011;36:10149.
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[4] Björling T, Norèus D, Häussermann U. J Am Chem Soc 2006;
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[5] Frankcombe TJ. J Phys Chem C 2010;114:9503.
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82:174107.
[7] Miwa K, Aoki M, Noritake T, Ohba N, Nakamori Y, Towata S,
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