The role of vibrational coupling in understanding the thermodynamics of hydride formation

receptivetrucksMechanics

Oct 27, 2013 (3 years and 9 months ago)

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The
r
ole of
v
ibrational coupling
in
understanding
the thermodynamics
of hydride formation


Ashley C. Stowe
1
, Monika Hartl
2
, Alice Acatrinci
2
, Luke Daemen
2
, Donald
Anton
1
, Ragaiy Zidan
1


1
Energy Security
Directorate
, Savannah River National Laboratory, Aike
n,
SC 29808

2
Manuel Lujan, Jr. Neutron Scattering Center, Los Alamos National
Laboratory, Los Alamos, NM 87545


The search for materials with ever higher hydrogen storage
capacities

has
utilized the Edisonian approach of alloying or combining various known

hydrogen storage materials in the hopes of identifying new high capacity
compounds.

There have been some
notable

successes over the years
.

T
hese
new
materials

are categorized as

A
x
B
y

compounds,

in which a very stable
hydride
, A type,

is

alloyed with an u
nstable
, B type,

hydride
,

forming a
novel
material

with intermediate thermodynamics. LaNi
5
H
6
,

TiFe
H
2

and
Mg
2
NiH
6

are notable examples

of AB
5
, AB, and A
2
B compounds
respectively
. This
general
phenomenon
can be applied to complex metal
hydrides

as well.

Fo
r instance,

NaAlH
4

can be thought of as a combination
of very stable
NaH and

unstable

AlH
3
,
which

indeed
results in

thermodynamics
which
are intermediate to its
progenitors’,

resulting in a
metasable phase having useful hydrogen storage
thermodynamics
. Sim
ilarly

NaMgH
3

is the result of combining NaH and MgH
2

and should have
pro
p
erties intermediate to these well known species
. Incoherent inelastic
neutron scattering has been conducted to understand the phonon vibrations
of these complex metal hydrides, as t
hey relate to altering the hydrogenation
thermodynamics.

Comparison has been made with the neutron vibrational
spectrum both reagents (NaH and MgH
2

in the case of NaMgH
3
) in order to
further understand the nature of hydrogen bonding in the ternary materia
l.

The
incoherent inelastic neutron scattering (
IINS
)

measurements of NaM
g
H
3

have

also

revealed that the structure is
rhombohedral rather than the
published orthorhombic structure. These two unit cells are mathematically
identical; however, the rhombohed
ral structure yields vibrations which
reflect the observed
IINS

spectrum within a
n

experimental error.

Vibrational spectroscopy is a useful tool to probe the nature of the
interaction of hydrogen in these metastable hydrides which
may guide future
synthet
ic endeavors in the search for novel hydrogen storage materials.