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Zinc substituted MgH2 - a potential material forhydrogen storage applications
R. Varunaa a,b, H. Fjellvag c, P. Ravindran a,b,c,*
a Department of Physics, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, 610101, Indiab Simulation Center for Atomic and Nanoscale MATerials, Central University of Tamil Nadu, Thiruvarur, Tamil
Nadu, 610101, Indiac Center for Materials Science and Nanotechnology and Department of Chemistry, University of Oslo, Box 1033
Blindern, N-0315 Oslo, Norway
a r t i c l e i n f o
Article history:
Received 14 February 2019
Received in revised form
29 March 2019
Accepted 1 April 2019
Available online 28 April 2019
Keywords:
Hydrogen storage
Magnesium hydride
Zinc hydride
Density functional theory
Zn substituted MgH2
* Corresponding author.E-mail address: [email protected] (P. Ra
https://doi.org/10.1016/j.ijhydene.2019.04.0160360-3199/© 2019 Published by Elsevier Ltd o
a b s t r a c t
The search for efficient materials for onboard hydrogen storage applications is an emerging
research field. Using density functional calculations, we demonstrate Zn substituted MgH2
as a potential material for hydrogen storage. We predicted the ground state crystal
structure of ZnH2 which is found to be Pna21 (orthorhombic) structure with meta-stable
behavior. The structural phase stability and phase transition of Mg1�xZnxH2 systems
have been analyzed. The H site energy of Mg1�xZnxH2 systems is calculated to understand
the hydrogen desorption process. Our calculations suggest that Zn substitution reduces the
stability of MgH2, thereby it may reduce the decomposition temperature of MgH2. The band
structure and density of states calculations reveal that the Mg1�xZnxH2 systems are
insulators. The chemical bonding behavior of Mg1�xZnxH2 systems is established as iono-
covalent in nature. Moreover, Zn substitution in MgH2 induces disproportionate MgeH
bonds which could also contribute the reduction in the decomposition temperature as well
as H sorption kinetics.
© 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction
The worldwide energy demand coupled with shrinking fossil
fuel resources and environmental pollution are continuously
rising every year which have lead us to an increasing curiosity
to find alternative energy resources. Hydrogen is an ideal
chemical energy carrier that exhibits a high calorific value per
unit mass and is environmentally friendly [1e7]. However, the
main challenge is to store hydrogen safely and efficiently for
mobile and stationary applications. Based on this, many
researchers considered light weight magnesium hydride
vindran).
n behalf of Hydrogen En
(MgH2) for onboard hydrogen storage applications owing to its
high hydrogen storage capacity, low cost, and abundance. But
the practical use of MgH2 for hydrogen storage is hampered by
its high decomposition temperature and quite slow hydrogen
sorption kinetics [8e29]. The high decomposition temperature
in MgH2 is associated with strong ionic bonding. One of the
key solutions for this issue is by adding catalysts or
substituting other elements to make MgeH bond weaker. It is
demonstrated that by increasing the covalency in the hy-
drides, one can reduce their decomposition temperature [30].
Recently, it was reported that additive catalysts added MgH2
improve the hydrogen sorption kinetics and reduce the
ergy Publications LLC.
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 en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 6 13633
sorption temperature of MgH2 [31e38]. Based on these, we
have currently explored how solid solutions involving
meta-stable hydrides such as zinc hydride (ZnH2) with MgH2
can reduce the bond strength and enthalpy of formation and
hence, the decomposition temperature of MgH2.
Zinc hydride has been known for many years as a meta-
stable compound with a white color that will decompose
easily. So, its crystal structure is not yet identified experi-
mentally. Zinc hydride was first synthesized by Schlesinger
et al. in 1947 [39] via the reaction between dimethyl zinc and
lithium aluminium hydride. In 1951, Barbaras et al. [40] syn-
thesized zinc hydride using metal alkyls and lithium
aluminium hydride in ethyl ether solutions. Watkins and
Ashby [41] prepared ZnH2 using zinc halides and alkali metal
hydrides. Few theoretical studies have also been made on
electronic structure, equilibrium geometry, and harmonic
vibrational frequencies of molecular ZnH2 [42e45]. A linear
ZneH molecular structure has been predicted and the equi-
librium ZneH bond distances (range of 1.492e1.662 �A) are also
estimated by the above theoretical studies. From density
functional calculations, Koning et al. [46] reported that, as like
beryllium hydride andmagnesium hydride, molecular ZnH2 is
also possess highly associated hydrogen-bridged coordination
polymer behavior.
Later, Breckenridge et al. [47e49] studied the reaction of
ZnH2 by laser ablation of Zn under hydrogen atmosphere. The
infrared spectra of ZnH2 trapped in low-temperature matrices
have been studied by Greene et al. [50] and later byWang et al.
[51] Shayesteh et al. [52e54] synthesized gaseous ZnH2 mole-
cule by electrical discharge inside a high temperature furnace.
Huang et al. [55] studied the potential energy surface and
vibrational energy levels of molecular ZnH2 using ab-initio
calculations. A few years back, Panagopoulos et al. [56]
investigated the effect of cathodic hydrogen charging on the
structural and mechanical characteristics of zinc. Using X-ray
diffraction, the formation ofmetastable ZnH2 was detected on
the surface layers of zinc after cathodic hydrogen charging.
They have also observed that due to dislocation pinning
mechanisms, the ductility of zinc decreases with the increase
of cathodic hydrogen charging.
MgH2 is strongly ionicwith high thermal stability, whereas,
ZnH2 has noticeable covalency with poor stability and hence
there are practical problems with their use in hydrogen stor-
age applications. The possible ways to destabilize MgH2 is
substituting various elements at the cation or at the anion
sites [57,58]. In our recent studies [59], we attempted to
destabilize MgH2 by substituting fluorine at the anion site.
However, though one can substitute fluorine at H sites in
MgH2, the systembehaves highly stable and its decomposition
temperature increased. So in the present study, we attempted
to substitute Zn at the Mg site to destabilize MgH2. The reason
is that ZnH2 is meta-stable in nature and hence Zn substitu-
tion at Mg site in MgH2 is expected to improve the hydrogen
sorption properties and induce changes in the formation
enthalpy, H site energy, chemical bonding, and electronic
structure of MgH2. From theoretical and experimental studies,
it is reported that Zn substitution in MgH2 improve kinetics
and thermodynamic properties of MgH2 as follows. Chen et al.
[60] reported that stability of MgH2 cluster can be reduced by
substituting transition metals such as Sc, Ti, V, Cr, Mn, Fe, Co,
Ni, Cu, and Zn at a central atom in MgH2 cluster. So, they
concluded that the substitution of the above said transition
metals in MgH2 may improve the hydrogen sorption kinetics.
From experimental studies, Liang et al. [61] noted that
substituting Mg with Zn, Al, Ag, Ga, In or Cd reduces the
stability of MgH2.
Vegge et al. [62] calculated hydride formation energies of
magnesium-3d transitionmetal alloys and reported that there
is a gradual increase in the stability of the hydrides (from
MgSc to MgFe), which is followed by a rapid decrease in the
stability (from MgCo to MgZn). Zhou et al. [63] reported that
the heat of formation decreases when MgH2 is chemically
modified with 3d transition metals. They found that the
reduction of formation enthalpy increases in an order of Fe, Ti,
Co, Zn, Ni, V, Mn, Cu, and Cr. Tsuda et al. [64] presented the
catalytic activities of 3d transition metals M on MgeH disso-
ciation by attachingM on the top of Mg in aMgH2 cluster. They
reported that the catalytic activity of transition metal is
considerably low (except Sc and Ni) due to fully occupied or-
bitals of Zn which does not receive any electrons from MgH2.
Milanese et al. [65] studied the hydrogen sorption kinetics and
storage efficiencies of mechanically activated binarymixtures
ofMg (MgH2) with nine differentmetals (Al, Cu, Fe, Mn,Mo, Sn,
Ti, Zn, and Zr). They concluded that MgeZn mixture has good
reversibility with cyclic effect for hydrogen and also has high
storage efficiency with poor sorption kinetics.
Polanski et al. [66] described the influence of nano-sized
metal oxides such as Cr2O3, TiO2, Fe3O4, Fe2O3, In2O3, and
ZnO on MgH2 powder and found that these metal oxides
(except In2O3) improve the dehydrogenation properties. The
effect of partial substitution of Mg with 3d transition metals
on the formation enthalpy and electronic structure of MgH2
have been studied by Zeng et al. [67] Their results show that
Zn substitution lowers the formation enthalpy. Further, they
showed that the bonding nature of MgH2-3d systems are
governed by ionic as well as covalent bonding. Bhihi et al. [68]
reported that 6.25% of Zn substitution in MgH2 (Mg15ZnH32)
reduce the gravimetric density of hydrogen (6.97 wt%) and on
the other hand, it decreases the formation enthalpy
(�53.04 kJ/mol.H2) which is the advantageous properties for
hydrogen storage applications. They also reported that
increasing the % of Zn substitution affects the gravimetric
density and this can be overcome through a double substitu-
tion (transition metal and either Li or Al) on MgH2. It is inter-
esting to note that the crystal structure of ZnH2 is not yet
explored in any of the theoretical and experimental studies
so far.
Our main objective in the present study is to destabilize
MgH2 by reducing the strong ionic interaction betweenMg and
H so that it can be used for practical hydrogen storage appli-
cations. So, we focus on predicting the ground state crystal
structure of ZnH2 and analyzed the chemical bonding by
substituting Mg with Zn in MgH2. The important properties
like enthalpy of formation, enthalpy of mixing, and hydrogen
site energy of Mg1�xZnxH2 systems were calculated. The band
structure and density of states (DOS) of Mg1�xZnxH2 systems
were analyzed to understand the electronic ground state
by Zn substitution. The chemical bonding analyses of these
systems have been done through partial DOS, charge density,
electron localization function, Bader effective charge, Born
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 613634
effective charge, and crystal orbital Hamiltonian population
analyses.
It may be noted that zinc is having higher atomic weight
than magnesium and also each zinc atom replace one mag-
nesium atom in MgH2. Hence, the gravimetric density of
hydrogen will decrease with increases of Zn concentration in
MgH2. For example, 12.5% of Zn substituted MgH2, the gravi-
metric density of hydrogen is reduced from 7.66wt% to 6.41wt
%. For 50% Zn substituted MgH2, the gravimetric hydrogen
density is further reduced to 4.3 wt%. For ZnH2, the gravi-
metric density is much lower (2.99 wt %) compared to that of
MgH2. So, one can compromise the gravimetric density in
order to achieve appropriate decomposition temperature and
reaction kinetics.
Computational details
We have utilized the density functional theory (DFT) [69] with
plane-wave basis sets as implemented within the Vienna ab-
initio simulation package (VASP) [70] for all our calculations.
The projector augmented wave (PAW) [71] pseudo-potentials
were used. The generalized gradient approximation pro-
posed by Perdew, Burke, and Ernzrhof (GGA-PBE) [72] were
used to obtain accurate bond lengths and bond energy. The
ground state geometries were designated by minimizing
stresses and Hellmann�Feynman forces using the conjugate-
gradient algorithm until the force acting on all atomic sites
were less than 10�3 eV/�A. The total energy difference between
two consecutive iterations was set to 10�6 eV.
Our previous studies show that the plane wave cutoff of
350 eV is sufficient to accurately describe the structural pa-
rameters of MgH2 and hence we have used the same for all the
present calculations. The substitution of Zn in MgH2 have
been done using a supercell approach. We have used 1 � 1 x 2
supercell in order to achieve 25%, 50%, and 75% Zn substitu-
tion in P42/mnm phase of MgH2. For 12.5%, 37.5%, 62.5%, and
87.5% Zn substitution, we have constructed 2� 2� 1 super cell
of MgH2 in P42/mnm phase whereas 2 � 1 � 1 super cell for
Pna21 phase. Brillouin zone integrations were done using
Monkhorst-Pack k-point meshes [73]. A k-point grid of
11 � 8 � 11 was used for structural optimization of ground
state Pna21 structure of ZnH2 and the same density of k-point
was used for the substituted systems. For DOS calculations,
we have used the tetrahedron method with Bl€ochl correction
with higher k-point density. The band structures were drawn
along the high symmetry directions in the irreducible part of
the first Brillouin zone.
Results and discussions
Structural phase stability and ground state crystal structureof ZnH2
In order to identify the ground state crystal structure of ZnH2,
we have considered thirty seven potential AB2 types of
structural variants and substituted with Zn at the A site and H
at the B site. The involved structure types for our structural
phase study are SnF2 (C12/c1) [74], BaF2 (Fm3m) [75], BeH2
(Ibam) [76], MgH2 (P42/mnm) [77], BeF2 (P43n) [78], SrH2 (P63/
mmc) [79], BeF2 (P3121) [80], BeF2 (P6222) [81], SrH2 (Pnam) [82],
BaH2 (Pnma) [83], CaH2 (Pnma) [84], BeF2 (R3m) [78], ZnCl2 (P42/
nmc) [85], ZnCl2 (P121/n1) [86], ZnF2 (Pbcn) [87], ZnCl2 (Pna21)
[88], MgF2 (Pnnm) [89], TiH2 (I4/mmm) [90], ZnBr2 (I41/acd) [91],
ZnI2 (I41/acd) [92], ZnCl2 (I42d) [86], d-MgH2 (Pbc21) [93], MgH2
(Pbca) [93], BeCl2 (I43m) [78], CdBr2 (P63/mmc) [94], HgBr2(Cmc21) [95], HgBr2 (P3) [96], MgCl2 (P3m1) [97], ZnBr2 (I41/a)
[91], ZnCl2 (P42/n) [85], TiO2-anatase (I41/amd) [9], MgB2
(P6mmm) [98], TeAg2 (P21/c) [9], ε-BeH2 (P213) [99], g-SnF2(P41212) [100], b-SnF2 (P212121) [100], and b-MgH2 (Pa3) [9].
The structure type specifications found above refer to the
symmetry of the initial guess structures. From the chosen
structural starting points, full geometry optimization have
been carried out using force as well as stress minimization.
The calculated total energy per formula unit (f.u) and the cor-
responding equilibrium volume per f.u of all the 37 different
structures considered are tabulated in supporting information
Table S1. Analyzing the total energy values for the above 37
different structural types, we have found that the minimum
energy structure for the compound ZnH2 is Pna21 (ortho-
rhombic). It may be noted that ZnCl2 also possess the same
crystal structure as that of ZnH2 in the ground state. In order to
have a wide view of the competitive lower energy structures,
we have displayed the optimized energy/f.u versus volume/f.u
curve for 10 structural variants with the lowest energy in Fig. 1.
The calculated total energy as a function of volume has been
fitted to so-called the universal equation of state (UEOS) to
calculate the bulkmodulus (Bo) and its pressure derivative (Bo’).
The equilibrium volume for the ground state structure of ZnH2
(Pna21) was obtained as 38.84 Å3. From the UEOS fitting, the
calculated bulk modulus and its pressure derivative for ZnH2
are 7.408 GPa and 5.681, respectively. The lower bulk modulus
of ZnH2 indicates that it is comparatively soft material.
The calculated total energy versus volume relation in Fig. 1
shows that pressure-induced structural transitions occur in
ZnH2 upon increasing pressures as well as expanded volumes.
So, if ZnH2 in Pna21 phase is exposed to external pressures, it
transforms to P41212 (tetragonal) phase at 0.75 GPa. It may be
noted that MgH2 also posses this tetragonal structure at high
pressure. It is interesting to note from Fig. 1(b) that when we
expand the ZnH2 lattice, it transforms to an orthorhombic
structure with space group P212121. In general, when one
applies pressure, the systems usually transform from lower
symmetry structure to a high symmetry structure and the
present observation of orthothombic-to-tetragonal pressure
induced structural transition in ZnH2 is consistent with the
above viewpoint.
The calculated equilibrium volume, bulk modulus, and
pressure derivative of bulk modulus for ZnH2 in high pressure
tetragonal P41212 phase are 36.90 Å3, 7.057 GPa, and 6.726,
respectively. The bulk modulus and its pressure derivative for
the orthorhombic P212121 phase which is stable in the
extended volume are 7.891 GPa and 5.782, respectively. In
order to identify the transition pressure with an associated
volume jumps, we show a pressure-volume curve for ZnH2 in
Fig. 2. From Fig. 2, we found that the pressure induced phase
transition occurs at 0.75 GPa and the associated volume
change is 3.14%. So, this phase transition can be considered as
a first order phase transition. The transition pressure for the
Fig. 1 e (Color online) (a) Normal and (b) Enlarged view of the calculated unit cell volume versus total energy curves for ZnH2
in its ground state and other closer energy structures. The inset shows the energy versus volume range where
pressure-induced structural phase transitions happening.
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 en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 6 13635
Pna21 to P212121 phase transition occurred at �0.22 GPa with
an associated volume jump of 2.41%.
The optimized equilibrium structural parameters for MgH2
in its ground state and that for ZnH2 in its ground state
structure along with high pressure tetragonal as well as
expanded volume orthorhombic phase for ZnH2 were listed in
Fig. 2 e (Color online) The calculated pressure versus
volume curve for ZnH2.
Table 1. The calculated equilibrium structural parameters for
MgH2 are found to be in good agreement with corresponding
experimental data and that from previous DFT calculations.
The predicted ground state crystal structure of ZnH2 is visu-
alized in Fig. 3(a) alongwith stable phases at high pressure and
the expanded volume in Fig. 3(b) and 3(c), respectively. In the
ground state structure of ZnH2, each unit cell contains four Zn
atoms and eight H atoms where each Zn atom is tetrahedrally
coordinated with H atoms and each H atom is coordinated
with two Zn atoms as shown in Fig. 3(a). At the equilibrium
volume, the calculated average ZneH bond length is 1.674 �A
and the shortest HeH separation in ZnH2 is 2.687�A. In the high
pressure phase as well as the orthorhombic phase that
stabilize at an expanded volume of ZnH2 also has similar
coordinations for both H and Zn atoms (see Fig. 3(b) and 3(c)).
The calculated average ZneH bond length is 1.671 �A and the
shortest HeH bond length is 2.632 �A for the high pressure
phase at the phase transition point i.e at 0.75 GPa. The
calculated average ZneH bond length is 1.675 �A and the
shortest HeH bond length is 2.696 �A for the orthorhombic
phase in the phase transition point.
The ground state crystal structure of MgH2 along with
12.5% (Mg0.875Zn0.125H2), and 62.5% (Mg0.375Zn0.625H2) Zn
substituted MgH2 are plotted in supporting information Figs.
S1(a), S1(b), and S1(c), respectively. Fig. S1(a) represents the
ground state crystal structure of a-MgH2 in P42/mnm (tetrag-
onal) phase and each unit cell contains two Mg and four H
atoms. Each Mg atom is octahedrally coordinated to six H
atoms, whereas each H atom is coordinated to threeMg atoms
as shown in Fig. S1(a). At the equilibrium volume, the calcu-
lated average bond length between Mg and H is 1.942 �A and
minimumHeH bond length is 2.487�A. In 12.5% Zn substituted
Table 1 e The optimized equilibrium structural parameters at the ground state for MgH2 and ZnH2 and the structuralparameters for the high pressure tetragonal P41212 and the expanded volume orthorhombic P212121 phases in theircorresponding structural phase transition point.
Structure type Space group Unit cell(�A) Wyckoff position Volume of unit cell (�A3)
a b c
MgH2 P42/mnm 4.4928 4.4928 3.0037 Mg(2a): 0,0,0 60.63
(4.4982) (4.4982) (3.0075) H(4f):0.304,0.304,0 (0.304,0.304,0) (60.85) [77]
ZnH2 Pna21 4.8692 6.3559 4.7211 Zn(4a):0.059,0.125,0.377 146.11
H(4a): 0.046,0.099,0.024
H(4a): 0.111,0.658,0.972
ZnH2 (at 0.75 GPa) P41212 4.7624 4.7624 6.1654 Zn(4a):0.184,0.184,0 139.83
H(8b): 0.277,0.334,0.232
ZnH2 (at �0.22 GPa) P212121 4.8271 4.8258 7.3368 Zn(4a):0.499,0.248,0.125 170.91
H(4a): 0.749,0.102,0.998
H(4a): 0.648,0.493,0.254
Fig. 3 e (Color online) The crystal structures of ZnH2 in (a) Pna21 (ground state), (b) P41212 (high pressure), and (c) P212121(expanded lattice) phase.
Table 2 e The calculated enthalpy of formation (DHf) performula unit obtained as a function of Zn concentration inMgH2 for the ground state structure and estimatedbandgap (Eg) values of Mg1¡xZnxH2 in P42/mnm andPna21 phases.
Compound Calculated DHf (kJ/mol) Calculated Eg(eV)
P42/mnm Pna2
MgH2 �52.775 3.714 4.992
Mg0.875Zn0.125H2 �36.819 2.720 4.612
Mg0.75Zn0.25H2 �21.656 2.283 4.394
Mg0.625Zn0.375H2 �5.458 1.514 4.078
Mg0.5Zn0.5H2 2.639 0.711 4.060
Mg0.375Zn0.625H2 9.338 1.161 3.881
Mg0.25Zn0.75H2 14.167 0.822 3.810
Mg0.125Zn0.875H2 21.897 1.255 3.720
ZnH2 29.348 1.006 3.557
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 613636
MgH2, each cation is octahedrally surrounded by 6 H atoms
(see Fig. S1(b)) and for 62.5% Zn substituted MgH2, each cation
is surrounded by 4 H atoms as depicted in Fig. S1(c).
Phase stability, phase transition, and phase mixing by Znsubstitution in MgH2
In order to understand the structural phase stability of
Mg1�xZnxH2 systems, we have calculated the enthalpy of for-
mation (DHf) of these systems using the following equation,
DHf
�Mg1�xZnxH2
� ¼ E�Mg1�xZnxH2
�� ð1� xÞEðMgÞ � xEðZnÞ� EðH2Þ
(1)
where E(Mg1�xZnxH2), E(Mg), E(Zn), and E(H2) are the total
energy obtained for the optimized geometry of Mg1�xZnxH2,
hexagonal Mg, hexagonal Zn, and molecular H2, respectively.
The calculated DHf for all these systems in their ground state
structure are listed in Table 2. Fig. 4 represents the calculated
DHf of Mg1�xZnxH2 systems in both P42/mnm and Pna21structures as a function of Zn concentration. This figure
illustrates that one can reduce the stability of MgH2 by
substituting Zn at Mg site since the calculated absolute value
of DHf decrease with increase of Zn concentration in MgH2.
Hence the Zn substituted MgH2 systems are less stable
compared with pure MgH2.
The decreasing trend in stability with Zn substitution in
MgH2 can be understood as follows. In pure MgH2, the pres-
ence of strong ionic bonding betweenMg and Hmake it highly
stable. On the other hand, our bonding analyses discussed
Fig. 5 e The difference in the enthalpy of formation
between P42/mnm and Pna21 structure of Mg1¡xZnxH2 as a
function of Zn concentration.
Fig. 6 e The calculated enthalpy of mixing as a function of
Zn concentration in MgH2.
Fig. 4 e The calculated enthalpy of formation as a function
of Zn concentration in MgH2.
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 en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 6 13637
later (subsection Analysis of chemical bonding in MgH2 by Zn
substitution) show that Zn substitution reduces the ionicity by
introducing covalency into the systemand hence the enthalpy
of formation decrease with the increase of Zn concentration.
From the decreasing trend in structural stability with Zn
substitution in MgH2, one may expect that the dehydrogena-
tion enthalpy of Mg1�xZnxH2 also decreases with increasing
Zn concentration. As a result, one can reduce the decompo-
sition temperature which is one of the desirable properties for
MgH2 to use in hydrogen storage applications. The linear
variation between the enthalpy of formation and Zn concen-
tration implies that these systems obey Vegard's law.We have
calculated the difference between the enthalpy of formation
of P42/mnm and Pna21 structures of Mg1�xZnxH2 as a function
of Zn concentration to understand the phase stability and
phase transition point and found that the structural phase
transition from P42/mnm to Pna21 occurs at 40% Zn substitu-
tion in MgH2 (see Fig. 5).
The enthalpy of mixing (DHm) of Mg1�xZnxH2 has been
estimated using the following equation,
DHm
�Mg1�xZnxH2
� ¼ E�Mg1�xZnxH2
�� ½ð1� xÞEðMgH2Þþ ðxÞEðZnH2Þ� (2)
where E(Mg1�xZnxH2), E(MgH2), and E(ZnH2) are the total en-
ergies for Mg1�xZnxH2, MgH2, and ZnH2 in their optimized
ground state geometry. The DHm of Mg1�xZnxH2 systems as a
function of Zn concentration are shown in Fig 6. The calcu-
latedDHm of endmaterials (MgH2 and ZnH2) is zero since these
are ideal compounds. For the intermediate systems in
P42/mnm phase, the estimated DHm values are positive and
low, which gives hint that single phase of Mg1�xZnxH2 may
form at a reasonable temperature and other thermodynamic
conditions. On the other hand, for the intermediate systems in
Pna21 phase, the calculated DHm are negative. This implies
that these systems are stable and it is experimentally possible
to synthesize them. In our previous study [59], we have re-
ported that the enthalpy of mixing of fluorinated MgH2 is less
positive (below 4 kJ/mol) and hence the single phase of Mg2-H2�xFx may be synthesized experimentally [101]. The present
calculations are valid only at 0 K. In order to account for
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 613638
temperature effects to analyze phase mixing one should
go beyond first principle calculations as done by Pinatel
et al. [102].
The binary phase diagram of MgeZn system show that no
binary intermetallic compounds will form up to 50 atomic
percentage of Zn in Mg for the temperature up to 340 +C [103].
Our calculated enthalpy of formation as a function of Zn
substitution show that the optimum MgeZn composition
useful for H storage application with desirable H desorption
temperature lye within the composition range (i.e.
Mg0.5Zn0.5H2) and hence, onemay expect a robust reversible H
storage system from Zn substitutedMgH2 with desirable other
hydrogen storage properties.
Hydrogen site energy in Zn substituted MgH2
In order to interpret the influences of Zn substitution on the
stability of MgH2, we have calculated the H site energy
(H desorption enthalpy) [104e106]. The H site energy is the
energy needed to remove anH atom from its host lattice and is
calculated [107] by the following relation,
EdðHÞ ¼ EðAnH2n�1Þ þ�12
�EðH2Þ � EðAnH2nÞ (3)
The calculated H site energy versus composition for
Mg1�xZnxH2 in their corresponding ground state structure is
plotted in Fig. 7 where H desorption enthalpy is reduced from
115.69 kJ/mol (1.20 eV) to 87.29 kJ/mol (0.91 eV) when we
completely replace Mg with Zn in MgH2. The calculated H site
energy of MgH2 is relatively high which point out that the H
atoms are strongly bound to theMg. This is the reason for high
H desorption temperature since the desorption temperature
mainly involves the breaking of MgeH bonds. Besides, one can
expect poor reaction kinetics because the diffusion of
hydrogen atom through this highly stable hydride is expected
to be slow. Our calculated [59] H site energy forMgH2 is in good
agreement with corresponding value reported by Vajeeston
et al. [108] The calculated H site energy as a function of Zn
substitution in MgH2 varies in zig-zag manner as shown in
Fig. 7. The overall observation is that the H site energy de-
creases with respect to that of MgH2 by Zn substitution except
for the compositions Mg0.375Zn0.625H2 and Mg0.125Zn0.875H2.
Fig. 7 e The calculated H site energy as a function of Zn
concentration in MgH2.
Hence, one can expect that the Zn substitution reduces the
hydrogen desorption temperature of MgH2, thereby hydrogen
sorption reaction could be improved for Zn substituted MgH2.
For pristine ZnH2, the H site energy is very low (87.29 kJ/mol or
0.91 eV) as compared to that of MgH2 due to the meta-stable
nature of ZnH2 and hence, one could expect lower decompo-
sition temperature in ZnH2 over MgH2.
Electronic structure of Mg1�xZnxH2
The computed band structures for MgH2, 50% Zn substituted
MgH2, and ZnH2 are shown in Fig. 8(a)e8(c), respectively. The
band structure shows that MgH2 is an indirect bandgap ma-
terial since the valence band maxima (VBM) and the conduc-
tion band minima (CBM) does not meet at the same k-point in
the Brillouin zone. The calculated bandgap of MgH2 is 3.69 eV
and this is consistent with previous studies [59,109]. However,
this value is deviated from experimental optical measure-
ments [110] for MgH2 (5.6 ± 0.1 eV) since the bandgap calcu-
lated from GGA/LDA is usually underestimate. To predict the
accurate bandgap one should use either hybrid functional or
GW calculations but this is out of the scope of the present
study.
For 50% Zn substituted MgH2 (see Fig. 8(b)), the calculated
bandgap value is 4.06 eV. In this system, the VBM and CBM are
located at the same G-point and hence, it is a direct bandgap
material. Moreover, Zn substitution brings additional states to
the valence band (VB). For ZnH2, the calculated band structure
(see Fig. 8(c)), demonstrates a direct bandgap (3.56 eV) feature
with both VBM and CBM located at G-point. Our calculations
suggest that the Mg1�xZnxH2 systems can be classified as an
insulator. Moreover, Zn substitution inMgH2 brings indirect to
direct transition and hence this may improve the optical ab-
sorption coefficient of MgH2 by Zn substitution.
The total DOS for Mg1�xZnxH2 systems in their ground state
crystal structures are shown in Fig. 9. The calculated bandgap
values of Mg1�xZnxH2 listed in Table 2. The non-linear varia-
tion of the bandgap of MgH2 as a function of Zn concentration
in P42/mnm phase is mainly due to the change in unit cell
volume aswell as competition between ionicity and covalency
by Zn substitution in MgH2. It is evident from Fig. 9 that the
bandgap of pure ZnH2 is smaller compared with that of pure
MgH2. The smaller bandgap value of ZnH2 is associated with
weaker bond strength (as evident from H site energy calcula-
tions). Moreover, there is a gradual decrease in the band gap
value as a function of Zn concentration until 37.5% Zn sub-
stitution. Consistent with our results, previous theoretical
studies reported that 6% Zn substituted MgH2 (Mg15ZnH32)
[67,111] and 20% Zn substituted MgH2 (Mg8Zn2H20) [63] have
the band gap value of around 3.1 eV and 2.5 eV, respectively.
Analysis of chemical bonding in MgH2 by Zn substitution
Partial density of states, charge density, and electronlocalization function analysesTo characterize the bonding interaction between the constit-
uents of Mg1�xZnxH2 systems, we have plotted partial DOS,
charge density, and electron localization function (ELF)
[112e114]. In the partial DOS ofMgH2 (refer Fig. 10(a)), it is to be
noted that the VB is mainly originated from H-s state with a
Fig. 8 e (Color online) The calculated band structures of (a) MgH2, (b) Mg0.5Zn0.5H2, and (c) ZnH2. The Fermi level is set to zero.
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 en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 6 13639
small contribution from Mg-s and Mg-p states since there is a
charge transfer from the Mg sites to H sites. The small Mg-s,p
electronic states present in the valence band are energetically
degenerate with the H-s states in the entire valence band
indicating that there is noticeable covalent interaction present
in-betweenMg and H. Hence, one can conclude that MgH2 has
a strong ionic bonding with small covalent character.
For Mg0.5Zn0.5H2 system, the partial DOS is plotted in
Fig. 10(b). From the VB, one can notice that the Mg-s states are
smaller than that of neutral Mg and at the same time H-s
states at the VB have higher value indicating that Mg donated
Fig. 9 e (Color online) The calculated total DOS of MgH2 as a f
its valence electrons to H. On the other hand, the Zn-s states in
the VB is higher than Mg-s states indicating that Mg donated
most of its s electrons to H. From the partial DOS of
Mg0.5Zn0.5H2 and ZnH2 shown in Fig. 10(b) and 10(c), respec-
tively, one can notice that the Zn(s)�H(s) bonding hybrids are
formed around �3.5 and �4.5 eV and also Zn-s and H-s states
are energetically degenerate in the entire valence band
reflecting the presence of substantial covalent type of inter-
action between Zn andH. The charge transfer fromMg/Zn-s to
H states along with charge sharing between Zn-s states and H
are the dominant bonding interaction in Zn substituted MgH2
unction of Zn substitution. The Fermi level is set to zero.
Fig. 10 e (Color online) The calculated partial DOS of (a)
MgH2, (b) Mg0.5Zn0.5H2, and (c) ZnH2. The Fermi level is set
to zero.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 613640
systems. The overall observation is that Mg1�xZnxH2 systems
are governed by mixed iono-covalent bond and the covalent
contribution increases with the increase of Zn concentration.
The charge density and ELF of MgH2 are plotted in Fig. 11(a)
and 11(d), respectively. Due to charge transfer from Mg to H
site in MgH2, the highest charge density resides in the im-
mediate vicinity of the nuclei at the H site. Almost spherical
charge distribution at the H site reconfirms that the interac-
tion between Mg and H is dominantly ionic. However, the
finite non-spherical charges present in-between Mg and H as
well as in-between H atoms indicate the presence of notice-
able covalent bonding between the constituents. It is known
that the region with an ELF value of 1.00, 0.50, and 0.00 rep-
resents fully localized electrons, fully delocalized electrons,
and very low charge density, respectively. The calculated ELF
plot for MgH2 (see Fig. 11(d)) shows that the electrons have a
paired character with predominant maximum ELF of about
0.9 at theH sites and non-spherical ELF distribution betweenH
atoms indicate that there is finite covalent bonding between
the H atoms. Moreover, there is a non-spherical ELF distrib-
uted between Mg and H indicating partial covalency. The
overall conclusions from charge density and ELF analyses for
MgH2 is that there is predominantly ionic bonding present
between Mg and H with small directional character present
between Mg and H as well as between H atoms.
In order to understand the role of Zn substitution on vari-
ation in the bonding behavior of MgH2, we have plotted charge
density and ELF plot for Mg0.5Zn0.5H2 in Fig. 11(b) and 11(e),
respectively. The nature of charge distribution seen in
Fig. 11(b) appears to be typical for compounds with iono-
covalent bonding. The charge density analysis shows that
the H closer to Mg has more spherically distributed charge
than that closer to Zn. Further, there is negligible charge
density present between Mg and H and in contrast, there is
substantial charge accumulated in between Zn and H. The
important observation is that the Zn substitution introduces
covalency which weakens the strong ionic bonding present
between Mg and H in the undoped MgH2. ELF of Mg0.5Zn0.5H2
system is shown in Fig. 11(e) also clearly indicate that there is
negligible amount of paired electron present in-between Mg
and H and there is a large amount of paired electrons present
in-between Zn and H. Further, the non-spherical distribution
of ELF in-between H atoms and in-between H and Zn along
with higher non-spherical character of ELF than that in MgH2
substantiate that Zn substitution enhances the covalency.
From the partial DOS, charge density, and ELF plot of
Mg0.5Zn0.5H2 system, it was indeed confirmed that the Zn
substitution reduced the ionic bonding interaction between
Mg and H and enhance the covalency in the system.
In contrast to MgH2, in the case of ZnH2, the calculated
charge density and ELF showed in Fig. 11(c) and 11(f) clearly
indicate that Zn is bondedwith H in a directionalmanner. The
non-spherical distribution of charge density and ELF along
with the large value of charge and ELF value in-between Zn
and H as well as in-between H indicates that there is a strong
covalent interaction between these atoms. So, when we
compared the bonding interaction of MgH2 with that of ZnH2,
we conclude that ZnH2 has more covalency.
Bader topological analysisTo quantify the amount of electrons present at the constitu-
ents of the compounds, we have made Bader topological
analysis. In the Bader charge analysis, each atom in a com-
pound is surrounded by a surface (called Bader regions) that
run throughminima of the charge density and the total charge
of an atom is determined by integration within the Bader re-
gion [115e117]. The calculated Bader effective charge (BC) for
the constituents in Mg1�xZnxH2 are listed in Table 3. The
positive value of BC represents electron depletion while the
negative value represents electron accumulation. The esti-
mated BC of the constituents in MgH2 shows that Mg donates
around 1.59 electrons to the H sites which reflects ionic
character. But the smaller value of the BC at the H sites (�0.79)
over the pure ionic value (�1) clearly indicates that MgH2 do
not reach a purely ionic bond with H in �1 state. Our calcu-
lated BC values for the constituents in MgH2 is in good
agreement with the corresponding value calculated by
Vajeeston et al. [118].
In the case of Mg0.5Zn0.5H2, Mg and Zn donate 1.56 and
0.69 electrons to the H sites respectively which is much
smaller than the value obtained from the pure ionic picture.
Moreover, H atom neighboring to Mg and Zn receives 0.54
electrons whereas that neighboring to Mg atoms alone re-
ceives 0.78 electrons (see Table 3). This is mainly due to
disproportionate bond induced by Zn substitution in MgH2.
Our analyses show that for 50% Zn substituted MgH2, the H
atoms receives an average of around 0.23 electrons less than
that in pure MgH2. These results reflect that the Zn substi-
tution weakens the ionic bond between the Mg and H. In
contrast to the MgH2 system, the calculated BC for Zn and H
Fig. 11 e (Color online) The charge density and ELF plot for (a) and (d) MgH2, (b) and (e) Mg0.5Zn0.5H2, (c) and (f) ZnH2. (Note:
Planes are chosen such a way that the MgeH, HeH, and ZneH bonds are clearly seen).
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 en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 6 13641
atoms in ZnH2 indicate that the ionicity is drastically
reduced in ZnH2 over MgH2. The conclusions arrived from
the BC analysis are consistent with the charge density, ELF,
and partial DOS analyses.
Born effective charge analysisBorn effective charge (BEC) analysis is another tool to quantify
charges in each atomic sites to evaluate the bonding charac-
teristics and were calculated using the Berry-phase approach
based on the modern theory of polarization which is imple-
mented in the VASP code. We have used the King-Smith and
Vanderbilt [119,120] method to calculate the polarizations of
perturbed cells. The Born effective charge tensors Z* of the
ions of Mg1�xZnxH2 systems have been calculated and listed in
Table 4. The diagonal components of the BEC tensors will
carry information about howmuch charge is transferred from
one site to other sites. For an ionic compound, the off-diagonal
components of the BEC tensorwill be zero or very small. In the
case of MgH2, the diagonal components are almost equal
Table 3 e Calculated Bader effective charges (BC) for theconstituents of Mg1¡xZnxH2 systems.
Compound atom BC (e)
MgH2 Mg 1.5869
H �0.7934
Mg0.5Zn0.5H2 Mg 1.5613
Zn 0.6955
H3 (neighboring to Mg) �0.7749
H8 (neighboring to Mg and Zn) �0.5382
ZnH2 Zn 0.7585
H �0.3792
(Zxx ¼ Zyy z Zzz) at the Mg as well as H sites. For both atoms,
most of the off-diagonal components are turn out to be zero
except Zyx components indicating strong ionic bonding. The
overall conclusion is that the MgeH bond has a large ionic
character with a small woof of covalency.
For Mg0.5Zn0.5H2, the diagonal components of the effective
charges at the Zn, Mg, and H sites are not equal (Zxx s Zyy s
Zzz) and the off-diagonal components have finite values. This
clearly reflects that the Zn atom brings covalency when it is
substituted in the MgH2 system. Further, the H neighbored by
Mg has the BEC value of �0.82 and that neighboring to Mg and
Zn has a value of �0.57 clearly showing that the introduction
of Zn brings a disproportionate bond. As like Mg0.5Zn0.5H2, the
pure ZnH2 has a covalent bond since the diagonal components
of the effective charges at Zn and H sites are not equal and the
off-diagonal components have noticeably large values. More-
over, the diagonal components of BC at H site is more than
nominal ionic value due to the fact that the dynamic charges
arising from covalency also added to the static charge. These
results are consistent with the other chemical bonding anal-
ysis done above.
Crystal orbital Hamiltonian population analysisIn order to get more insight into the chemical bonding, we
have also calculated the crystal orbital Hamiltonian popula-
tion (COHP). COHP [121e123] is the DOS weighted by the cor-
responding Hamiltonian matrix elements and this identifies
the location of bonding, antibonding, and nonbonding states
of the bonding pair and alsomeasures the strength of bonding
(magnitude of bonding) interaction between the constituents.
The negative COHP designates the bonding character whereas
the positive COHP denote the antibonding character. The
Table 4 e The calculated diagonal and off-diagonal components of Born-effective-charge-tensor elements (Z*) for theconstituents of Mg1¡xZnxH2 systems.
Compound atom Zxx Zyy Zzz Zxy Zyz Zzx Zxz Zzy Zyx
MgH2 Mg 1.851 1.851 1.942 0.120 0 0 0 0 0.120
H �0.902 �0.902 �0.948 �0.201 0 0 0 0 �0.201
Mg0.5Zn0.5H2 Zn 1.052 1.462 1.138 �0.216 0.106 �0.130 0.083 �0.050 0.213
Mg 1.705 1.692 1.835 0.112 0.066 0.138 �0.177 0.033 �0.069
H1 �0.577 �1.018 �0.415 �0.229 �0.114 �0.075 �0.122 �0.098 �0.239
H3 �0.854 �1.045 �0.658 �0.134 0.032 �0.005 0.107 0.027 �0.125
H5 �0.787 �0.556 �0.839 0.059 �0.039 0.251 0.302 0.004 0.025
H8 �0.563 �0.521 �1.061 �0.036 �0.040 �0.188 �0.189 �0.019 0.005
ZnH2 Zn 1.503 1.335 1.483 �0.281 0.304 �0.130 0.010 �0.263 0.166
H1 �0.466 �0.679 �1.009 ..0.030 �0.302 �0.090 �0.077 �0.270 �0.149
H2 �1.042 �0.649 �0.479 �0.301 �0.002 �0.013 �0.016 �0.000 �0.325
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 613642
calculated bond strength of the bonding pairs obtained from
integrated COHP (iCOHP) up to Fermi level for Mg1�xZnxH2
systems are listed in Table 5. Fig. 12(a) shows the calculated
COHP of bonding pairs in MgH2. If we analyze the COHP of
MgeH bonding pair, all the bonding states are filled and the
antibonding states are empty (see Fig. 12(a)) so one could
expect high stability. Apart from our enthalpy of formation
analysis, the high stability of MgH2 was already found through
experimental [21] as well as other theoretical studies [26]. Due
to the filling of maximum bonding states in MgeH pair, one
can expect strong bond between Mg and H. Consistent with
this viewpoint, the calculated iCOHP value for MgeH bonding
pair is relatively high. So if one wants to reduce the decom-
position temperature of MgH2, one should weaken the MgeH
bond. Moreover, the bonding interaction between H atoms are
considerably weak since both of the bonding and antibonding
states are present within the VB region of COHP and hence the
iCOHP value is very small.
From the analyses of the calculated COHP for Mg0.5Zn0.5H2
(see Fig. 12(b)), we found that the MgeH bonding pair has
bonding states located in the whole valence band range and
the calculated average iCOHP value for MgeH pair is �0.63 eV.
As the substitution of Zn introduces disproportionate bonds,
the detailed analysis of bond strength for MgeH bonds from
the calculated iCOHP values in Table 5 vary between �0.49 to
Table 5 e The calculated bond strength between thebonding pairs in Mg1¡xZnxH2 systems from integratedcrystal orbital Hamiltonian population.
Compound Interaction Bond strength (eV)
MgH2 MgeH �0.63
HeH �0.07
Mg0.5Zn0.5H2 MgeH �0.63 (average)
MgeH4 �0.69
MgeH5 �0.49
MgeH8 �0.71
ZneH �0.52 (average)
ZneH2 �0.36
ZneH5 �0.66
ZneH8 �0.56
HeH �0.09
ZnH2 ZneH �0.40
HeH �0.02
�0.71 eV. Because of this disproportionate bond formation,
the H site energy calculated for H neighboring to Mg atoms
alone has a value of 108.85 kJ/mol and that neighboring to Zn
and Mg has a value varying from 91.14 to 98.07 kJ/mol. The
hydrogen closer to Zn is weakly bonded with neighboring Mg
and that neighboring to Mg atoms alone is strongly bonded to
Mg as evident from calculated iCOHP values. So, the present
study suggests that the Zn substitution weaken some of the
MgeH bonds in Mg0.5Zn0.5H2 and hence one can expect that H
will be removed in relatively lower temperature than that in
Fig. 12 e (Color online) The calculated COHP between
constituents in (a) MgH2, (b) Mg0.5Zn0.5H2, and (c) ZnH2.
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 en e r g y 4 4 ( 2 0 1 9 ) 1 3 6 3 2e1 3 6 4 6 13643
pure MgH2 system. If we analyze the COHP of ZneH pair in
Mg0.5Zn0.5H2, we found that both bonding and antibonding
states are present within the valence band and hence the
calculated average iCOHP value for ZneHpair is only�0.56 eV.
It is well known that the filling of antibonding states lowers
the stability of the system. The observation of filling of anti-
bonding states in ZneHpair could explainwhy the enthalpy of
formation for Mg0.5Zn0.5H2 is lower than that of MgH2.
In the case of ZnH2, the COHP for the ZneH pair is shown
in Fig. 12(c). It may be noted that for the ZneH bonding pairs,
the antibonding states are dominated in the top of the
valence band. The band filling of the bonding analysis shows
that in order to achieve the maximum stability, all the
bonding states should be filled and the antibonding states
should be empty. However, the filling of antibonding states in
the COHP of ZneHpair could explainwhy ZnH2 ismeta-stable
in nature.
Conclusions
Though ZnH2 is synthesized experimentally long back, due
to the meta-stable nature of this compound its ground state
crystal structure is unknown till date. Using the state-of-
the art density functional calculations we have predicted
the ground state structure and the unit cell parameters of
ZnH2 by considering 37 potential trial structures. Also, in
order to reduce the strong ionic bonding between Mg and H,
we have substituted Zn at the Mg sites in MgH2 and
executed a systematic study of the phase stability, phase
transition, phase mixing, electronic structure, and chemical
bonding of Mg1�xZnxH2 systems and made the following
conclusions.
� Using the total energy minimization, we have predicted
that the ground state crystal structures of ZnH2 is ortho-
rhombic with space group Pna21 and the calculated equi-
librium structural parameters are listed. The calculated
enthalpy of formation for ZnH2 is a small positive value
and this could explain why this material is meta-stable.
� The calculated enthalpy of formation and H site energy of
Mg1�xZnxH2 systems affirm that MgH2 is highly stable, and
Zn substituted MgH2 systems are less stable than MgH2.
� From the calculated total energy as a function of Zn
substitution in MgH2, we have predicted a composition
induced structural phase transition from tetragonal P42/
mnm to orthorhombic Pna21 structure occurred at 40% Zn
substitution in MgH2.
� From the calculated enthalpy of mixing, we concluded that
Mg1�xZnxH2 systems can form single phase and they can be
experimentally synthesized using appropriate thermody-
namical conditions.
� The calculated H site energy of Mg1�xZnxH2 systems in-
dicates that the H neighboring to Zn can be removed more
easily than that neighboring to Mg atoms alone.
� The electronic structure studies reveal that Mg1�xZnxH2
systems are insulators. The non-linear variation in the
bandgap values as a function of Zn substitution is mainly
due to the competition between ionic and covalent bond in
these systems.
� The chemical bonding analyses from partial DOS, charge
density, ELF, BC, BEC, and COHP concludes that Mg1�xZnx-
H2 systems have iono-covalent interaction between the
constituents.
� From the detailed chemical bonding analysis, we have
found that Zn substitution induced disproportionate bonds
and this could reduce the decomposition temperature.
Overall, the present results suggest that one can reduce the
stability ofMgH2 by Zn substitution. From the calculatedH site
energy and the identification of the formation of dispropor-
tionate bonds by Zn substitution in MgH2, we have indicated
that H can be removed easily in Zn substituted MgH2. Hence,
the Mg1�xZnxH2 systems can be considered as a promising H
storage materials.
Acknowledgement
We gratefully acknowledge the Department of Science and
Technology, Ministry of Science and Technology, India for the
financial support via grant no. SR/NM/NS-1123/2013 to carry out
this research and Research council of Norway for computing
time on the Norwegian supercomputer facilities (Project No:
NN2875K). R.Varunaa wishes to thank Dr. A. Krishnamoorthy
andMr.M. R. AshwinKishore for their valuable discussions and
critical reading of the manuscript.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.ijhydene.2019.04.016.
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