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FOKIN et al.
Experimental procedure. The chemical interaction mation of Mg2Ni-based hydride phases is accompanied by
between Mg2Ni and ammonia (hydriding/nitriding pro-
cess) was studied in a 60-ml high-pressure laboratory
apparatus at an initial ammonia pressure of 0.6–0.8 MPa,
using ammonium chloride (10 wt % relative to the inter-
metallic compound) as a reaction activator. A weighed
amount (1.0–1.5 g) of the starting mixture was loaded into
a stainless-steel container, which was then mounted in an
autoclave. After pumping to a residual pressure of ꢀ1 Pa
at ꢀ20°C over a period of 30 min, the autoclave was
charged with NH3 to a pressure of 0.6–0.8 MPa and left
standing at this temperature for 30 min. Next, the reactor
was heated to a preset temperature, held there for 3 h,
cooled to ꢀ20°C, and heated again. Since the pressure in
the system increased during the reaction owing to hydro-
gen and nitrogen release (no more than 1.5 MPa), the pro-
cess was considered to reach completion when the pres-
sure stopped varying.After an appropriate number of heat-
ing–cooling cycles, the ammonia was discharged to a
buffer tank, and the reaction products were drawn from the
reactor in an inert atmosphere and analyzed.
the incorporation of a certain amount of nitrogen into the
metal matrix (we failed to find any experimental evidence
for this assumption). In addition, it is well known that
hydrogen exposure reduces the particle size of metallic
systems. Therefore, given the above two circumstances,
the formation of the magnesium hydride MgH2 (a =
0.454 nm, c = 0.309 nm) and magnesium amide
Mg(NH2)2 (a = 1.038 nm, c = 2.006 nm) at 150°C is attrib-
utable to the decomposition of the Mg2NiH4 tetrahydride,
with the resultant magnesium metal entering into hydrid-
ing and amidation reactions. Note that all of the synthe-
sized phases were stable under the experimental condi-
tions of this study: according to X-ray diffraction data, all
of the above compounds were among the reaction prod-
ucts after pressure release. The hydriding/nitriding process
at 150°C further increases the specific surface area of the
material (25.5 m2/g). According to thermal analysis data,
the decomposition temperatures of Mg2NiH4 and
Mg(NH2)2 are 260 and 365°C, respectively, in agreement
with earlier results [6, 12].
At a hydriding/nitriding temperature of 200°C, the
reaction products did not contain the parent intermetallic
compound, which was probably among the factors
responsible for the drastic increase in the specific surface
area of the material (to 113.3 m2/g). Moreover, its diffrac-
tion pattern showed weak reflections characteristic of
nickel nitride, Ni3N.
As reported previously, massive nickel is nonreactive
with ammonia, whereas fine-particle nickel reacts with it
at 300–450°C to form Ni3N. Therefore, the observed
reflections from this nitride indicate that the decomposi-
tion of the Mg2NiH4 tetrahydride is accompanied by the
formation of both magnesium and nickel, the latter enter-
ing into a nitriding reaction.
Note also that the decomposition of the Mg2NiH4 tet-
rahydride competes with disproportionation of this com-
pound, as evidenced by the formation of another interme-
tallic compound, MgNi2 (a = 0.482 nm, c = 1.597 nm), in
the reaction at 200°C without NH4Cl (sample 4). In addi-
tion, the reaction products contained metallic nickel (a =
0.353 nm) and the magnesium nitride Mg3N2 (a =
0.997 nm), which were the major products of the reaction
between the intermetallic compound Mg2Ni and ammonia
at temperatures from 300 to 450°C (samples 6–10).
The presence of ammonium chloride in the reaction
system leads to its reaction at relatively low temperatures
(150–200°C) with the magnesium resulting from
Mg2NiH4 decomposition and the formation of trace
amounts of MgCl2. After ammonia exposure of the parent
intermetallic compound at 300°C and higher temperatures
(samples 6–10), no magnesium chloride was detected
among the reaction products.
The NH4Cl was removed from the hydriding/nitriding
products by washing with ethanol or evacuation [11].
Characterization and chemical analysis. X-ray dif-
fraction patterns were collected on an ADP-1 computer-
controlled diffractometer (CrKα radiation).
The specific surface area of the samples was evaluated
using the BET equation from low-temperature krypton
adsorption measurements after degassing the solid phase
in a vacuum of 1.3 × 10–3 Pa at 300°C for 15 h. The error
of determination was within 10%.
Thermal analysis was carried out with a Netzsch STA
409 Luxx simultaneous thermal analyzer (TG–DTA/DSC)
in flowing argon at a heating rate of 10°C/min.
The hydrogen content of the reaction products was
determined by a standard procedure, by burning samples
in flowing oxygen. Nitrogen was determined by the
Kjeldahl method, and chlorine was determined turbidi-
metrically.
RESULTS AND DISCUSSION
The Mg2Ni–ammonia reaction conditions and the
results obtained are summarized in the table.
At 100°C, the intermetallic compound shows little or
no visible reaction with ammonia, but ammonia exposure
increases the specific surface area of Mg2Ni by several
orders of magnitude, from 0.04 to 14.7 m2/g, and, accord-
ing to X-ray diffraction results, slightly changes its lattice
parameters (sample 1). Raising the temperature to 150°C
(sample 2) leads to hydrogen uptake and the formation of
an Mg2NiH0.3 solid solution, which gradually absorbs
hydrogen, converting to the Mg2NiH4 tetrahydride (a =
1.33 nm, b = 0.65 nm, c = 0.66 nm).
Note that the hydriding/nitriding process yields mag-
tallic compounds and alloys lead us to assume that the for- nesium amide even at 300°C (sample 7). At higher tem-
Our previous results on hydriding/nitriding of interme-
INORGANIC MATERIALS Vol. 45 No. 8 2009
Fokin
