O. Dolotko et al. / Journal of Alloys and Compounds 506 (2010) 224–230
225
studied to gain basic insights on the phase transformation mech-
anisms. For instance, Luo [4] and Xiong e al. [5] demonstrated
that binary systems such as Mg(NH ) –2LiH and LiNH –CaH
exhibit high hydrogen storage capacity with improved thermo-
dynamics. Xiong [5] and Wu et al. [13] reported that hydrogen
absorption–desorption characteristics and final products of inter-
action in Li–Mg–N–H and Li–Ca–N–H systems are similar. For
gas outlet port. During the ball milling of ∼3 g of hydride mixture,
179.6 g steel balls were used (8 balls weighing 16 g each, 4 balls
weighing 8 g each, 5 balls weighing 3.5 g each and 15 balls weigh-
ing 0.14 g each). Pressure changes were measured by connecting
the vial to the PCTPro 2000 system after the mechanochemical
treatment. Volume calibration was performed at room tempera-
ture before the ball milling. The RGA-100 residual gas analyzer was
used for the qualitative analysis of the released gas.
2
2
2
2
example, Li Ca(NH)2 ternary imide was successfully synthesized
2
by dehydrogenating a mixture of LiNH2 and CaH2 taken in a 2:1
molar ratio using a synthetic route similar to (2) [13]. Hydrogen
storage properties of the Li–Ca–N–H system were recently exam-
ined by Tokoyoda et al. [14]. They prepared two kinds of mixtures:
Ca(NH ) –2LiH and CaH –2LiNH that were ball milled for 2 h,
and their hydrogen desorption and absorption characteristics were
examined by thermal desorption and mass spectroscopy combined
with thermogravimetry and differential thermal analysis. Both
Solid reaction products were characterized by X-ray powder
diffraction at room temperature on a Scintag powder diffractome-
ter using Cu K␣ radiation, in the range of Bragg angles 2ꢀ from
◦
◦
10 to 80 . The measurements were performed using a sample
holder, covered by two kinds of thin plastic films, used to protect
the sample from air. One of the films produces an amorphous-like
2
2
2
2
◦
◦
background between 13 and 18 of 2ꢀ and another film produces
◦
◦
broad halos in the region between 15 and 23 of 2ꢀ. Due to air sen-
sitivity of the starting materials and the products, all manipulations
have been carried out in argon atmosphere in a glove box.
composites form Li NH and CaNH compounds after dehydrogena-
2
◦
tion at 200 C; both systems were transformed into an “unknown
imide phase” after heating to 400 C. Authors of Ref. [14] concluded
◦
that the overall reversible reaction in the Li–Ca–N–H system can be
expressed as (4), and that the hydrogen capacity is 4.5 wt.%:
3. Thermodynamic analyses
3
.1. Computation procedure
CaH + 2LiNH → CaNH + Li NH + 2H ↔ Ca(NH ) + 2LiH (4)
2
2
2
2
2 2
To gain an insight on the stability of the intermediate phases that
◦
The “unknown imide phase”, which forms after 400 C, is
Li Ca(NH) ternary imide isolated and characterized by Wu et al.
could form during mechanochemical transformations of lithium
and sodium amides with calcium hydride, taken in 1:1 and 2:1
molar ratios, we have calculated the crystal parameters, elec-
tronic structure, total energies, and the enthalpy of formation of a
wide range of hydrides [CaH , CaNH, Ca(NH ) , LiNH , Li NH, LiH,
Li Ca(NH) , NaNH , Na NH, NaH and Na Ca(NH) ]. Total energy
2
2
[
[
13]. Recently, Ca–Na–N–H system was evaluated by Xiong et al.
15]. Similar to other amide–hydride systems, interaction between
Ca(NH2)2 and NaH (1:1) was observed in the temperature range of
2
2
2
2
2
◦
1
20–270 C with 1.1 wt.% of hydrogen desorbed, of which 0.96 wt.%
2
2
2
2
2
2
of hydrogen can be recharged.
and geometry optimization calculations were carried out using
density functional theory (DFT) [29] and generalized gradient
approximation (GGA) [30] for exchange and correlation potentials.
The calculations were performed using Vienna ab initio simulation
package (VASP) [31] and the projector augmented wave (PAW) [32]
method. High precision calculations with a cutoff energy of 500 eV
for the plane-wave basis were performed. The equilibrium crystal
structures and lattice parameters were obtained by starting with
initial structures given by existing experiments and optimizing the
geometries without any symmetry constraints. In all calculations,
self-consistency was achieved with a tolerance in the total energy
of at least 0.1 meV.
Building upon our previous research, in which we showed that
ball milling may be an excellent tool to rapidly destabilize a com-
plex hydride system and may lead to an array of solid state reactions
[
16], here we investigate room temperature mechanochemical
reactions between MNH2 (M = Li, Na) and CaH2 taken in differ-
ent molar ratios. Even though ball milling is a common technique
employed to mix solid hydride phases for further investigations of
their thermochemical behaviors, it is also a very convenient and
effective, yet an underappreciated tool to conduct solid state reac-
tions. As we will show below the mechanisms of mechanochemical
transformations of the same system may be different from those
induced by temperature.
Furthermore, existence of several calcium nitrides, amides and
imides, such as Ca N2 [17,18], Ca N [19–22], CaNH [23], Ca NH
3.2. Crystal structure
3
2
2
[
24,25], and Ca(NH ) [26–28] prompted us to also investigate mix-
2 2
Among the above-mentioned compounds, LiNH2 has been well
tures in molar ratios from which the corresponding compounds
may be produced from CaH2 and MNH . Thus, to obtain Ca N
3
1
studied. It has a tetragonal structure and belongs to space group
a
2
3
2
¯
symmetry I4. The structure was determined experimentally [33]
:2 molar ratio of the components is required, for CaNH it becomes
:1, Ca NH and Ca N 2:1, and for Ca(NH ) a 1:2 mixture of CaH2
and detailed calculations can be found in Refs. [34–36]. For Li NH,
2
2
2
2 2
we have used the Li ND (orthorhombic Ima2) as the starting struc-
2
and MNH2 is needed.
. Experimental details
The starting materials LiNH2 (95 wt.% purity), NaNH2 (>90 wt.%
ture [35]. Our calculated parameters given in Table 1 agree well
with those of Herbst and Hector [35]. We considered two structures
2
for NaNH ; the first one has lithium amide tetragonal structure
2
¯
(
space group I4), while the other one is orthorhombic (space group
Fddd) [37,38]. Our calculations show that the orthorhombic struc-
ture is the ground state for NaNH . For Na NH, which is not found
purity) and CaH powder (90–95 wt.% purity) were purchased from
2
2
2
Sigma–Aldrich. Mixtures of the alkali metal amides with calcium
hydride taken in different molar ratios (∼1 g of mixture total) were
loaded in a 50 ml hardened steel vial and ball milled using 20 g
of steel balls (two large balls weighing 8 g each and four small
balls weighing 1 g each) in an 8000M SPEX mill. The ball milling
was stopped at different time intervals and phase analysis of the
mixture was carried out.
For the gas volumetric measurements after the ball milling, a
magnetic ball mill “Uni-Ball-Mill 5” was used. Mixtures of hydrides
were ball milled in a 90 ml hardened steel vial, equipped with a
experimentally, the Li NH structure as the input geometry was
used.
2
There are several possible products or intermediate phases
that can form during the dehydrogentaion of lithium (sodium)
amide and calcium hydride mixture. These are: CaNH, Ca(NH ) ,
2
2
Li Ca(NH)2 and Na Ca(NH ) . To obtain the ground state geom-
2
2
2 2
etry of the CaNH crystal, we used two initial structures. One is
reported by Sichla and Jacobs [23], which is a face centered cubic
¯
(fcc) structure with space group Fm3m, Z = 4, a = 5.143 Å. The other,
taken from the work of Wegner et al. [39], is a BaND structure hav-