Synthesis and magnetic studies of nanocrystalline nickel nitride material

Article abstract of DOI:10.1002/1521-396X(200202)189:3<691::AID-PSSA691>3.0.CO;2-3

Single domain Ni₃N nitride particles are synthesized by simultaneous decomposition and nitridation in ammonia atmosphere of [Ni(NH₃)₆](NO₃)₂ complex crystals at 650 K. The Ni₃N phase crystallizes in the hexagonal structure with unit cell parameters a = 4.624(4) A and c = 4.316(4) A, and has a crystallite size of 16 nm. In TEM study, these nanosize particles show spherical shape, and form particle aggregates of 18 nm size. Ni₃N particles exhibit a Curie temperature of T_C = 634 K. In the field dependence magnetic measurements, the presence of hysteresis loop indicates ferromagnetic behavior with σ_s = 1.678 emu/g, σ_R = 0.50 emu/g and μ₀H_c = 0.022 T. In this phase, the density of states (DOS) of Ni is dominated by 3d states and is mixed with 2p DOS of N. The contribution of d-electrons in the intra-band polarization affects the magnetic moment of Ni and thus the magnetic moment of Ni₃N.

Full text of DOI:10.1002/1521-396X(200202)189:3<691::AID-PSSA691>3.0.CO;2-3

phys. stat. sol. (a) 189, No. 3, 691–695 (2002)
Synthesis and Magnetic Studies
of Nanocrystalline Nickel Nitride Material
1
*
N. S. Gajbhiye ) Þ (a), R. S. Ningthoujam (a), and J. Weissmu¨ller (b)
(a) Department of Chemistry, Indian Institute of Technology, Kanpur-208016, India
(b) Institute of Nanotechnology, Forschungszentrum Karlsruhe, P.O. Box 3640,
D-76021 Karlsruhe, Germany
(Received May 1, 2001; accepted September 30, 2001)
Subject classification:75.50.Tt; 75.60.Ej; S1.62
Single domain Ni3N nitride particles are synthesized by simultaneous decomposition and nitrida-
tion in ammonia atmosphere of [Ni(NH3)6](NO3)2 complex crystals at 650 K. The Ni3N phase crys-
ꢀ
ꢀ
tallizes in the hexagonal structure with unit cell parameters a ¼ 4.624(4) A and c ¼ 4.316(4) A,
and has a crystallite size of 16 nm. In TEM study, these nanosize particles show spherical shape,
and form particle aggregates of 18 nm size. Ni3N particles exhibit a Curie temperature of
TC ¼ 634 K. In the field dependence magnetic measurements, the presence of hysteresis loop indi-
cates ferromagnetic behavior with ss ¼ 1.678 emu/g, sR ¼ 0.50 emu/g and m0Hc ¼ 0.022 T. In this
phase, the density of states (DOS) of Ni is dominated by 3d states and is mixed with 2p DOS of
N. The contribution of d-electrons in the intra-band polarization affects the magnetic moment of
Ni and thus the magnetic moment of Ni3N.
Introduction The transition metal nitrides are very important for technological appli-
cations as well as fundamental studies because of their unusual combination of physical
properties such as very high melting point, extreme hardness and metallic conductivity
[1]. Interestingly, transition metal nitrides in the nanostructured form are most attrac-
tive for magnetic storage devices, catalytic studies, sensors, fuel cells, superconducting
applications [2, 3]. However the paucity of literature on metal nitrides illustrates that
the much greater challenges are both in synthesizing and characterizing them with re-
spect to their composition, structure and physical properties. The metastable Ni3N
phase is reported as nanocomposite films of Ni3N/AlN along with the presence of ele-
mental nickel [4]. In this paper, we present the synthesis of nanocrystalline nickel ni-
tride (Ni3N) particles. These particles are obtained in the temperature range 613 to
673 K in NH3 atmosphere from the nickel hexaamine nitrate complex where M–N link-
age is present. Nanocrystalline Ni3N particles are characterized and studied for their
interesting magnetic properties.
Experimental Ni3N was prepared by simultaneous thermal decomposition and nitrida-
tion of [Ni(NH3)6](NO3)2 in ammonia atmosphere in the temperature range of 613–
673 K. The complex was prepared by the addition of 10 ml ammonia solution slowly
into 1.0 g of Ni(NO3)2 Á 6H2O (Merck, India, 97%) with rapid stirring to get a crystal-
line complex. The final product obtained was nanocrystalline Ni3N which was con-
1) Corresponding author; Tel.:+91 521 597080; e-mail:nsg@iitk.ac.in
*
) Presently at Institute of Nanotechnology, Forschungszentrum Karlsruhe, P.O. Box 3640,
D-76021 Karlsruhe, Germany; Tel.:+49 7247 82 6377; Fax:+49 7247 82 6368;
e-mail:gajbhiye@int.fzk.de
#
WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2002 0031-8965/02/18902-0691 $ 17.50þ.50/0

692
N. S. Gajbhiye et al.:Synthesis and Magnetic Studies of Nanocrystalline Ni N
3
firmed for its stoichiometry by chemical analyses for nickel and nitrogen amount. The
metal to nitrogen content ratio was found to be Ni:N ¼ 3:0.9984.
The powder X-ray diffraction (XRD) patterns of Ni3N were recorded by using a
Rich-Seifert X-ray diffractometer, model Isodebyeflex 2002 with Cu Ka radiation in the
2q range of 20ꢀ –100ꢀ. The crystallite size was determined from line broadening using
the Scherrer formula, D ¼ Kl/B cos q, where D is the particle size, K is a constant
(shape factor), B is the half-maximum linewidth, and l is the wavelength of the X-rays.
TEM (Philips model E-301) was used to study the morphology and particle size distri-
bution of powder samples. Magnetization measurements were carried out using a vi-
brating sample magnetometer, in conjunction with a Varian electromagnet assembly
that provides a magnetic field up to 2.1 T.
Results and Discussions Nanocrystalline Ni3N particles in the size range 15.0 to
20.0 nm were prepared by simultaneous thermal decomposition and nitridation of
[Ni(NH3)6](NO3)2 in the ammonia atmosphere for 4 to 12 h duration in the tempera-
ture range of 613–673 K. The reactivity and particle size are significantly influenced
by the calcination temperature and thus the formation of pure Ni3N materials. Figure
1a shows the XRD pattern of Ni3N. The decomposition of Ni3N starts above 683 K
and leads to a mixture of Ni3N and Ni metal with the escape of interstitial nitrogen.
The metastability of Ni3N is attributed to the low maximum solubility of nitrogen in
Ni lattice. The stepwise decomposition is unique to nickel nitride and takes place
because of the formation of an intermediate Ni4N phase in the conversion of nitride
to metallic nickel [5]. At high temperature, the removal of nitrogen is enhanced with
simultaneous reduction of Ni3N by the hydrogen molecules from the ammonia de-
composition. The main gas evolved is nitrogen and the final product at 873 K is pure
Ni metal (Fig. 1b). The XRD pattern of Ni3N was indexed to a crystal structure with
the symmetry found in the disordered Ni3C [6]. Ni3N belongs to the hexagonal sys-
ꢀ
ꢀ
tem with lattice parameters a ¼ 4.624(4) A and c ¼ 4.316(4) A. These parameters
ꢀ
ꢀ
also match with the reported values of a ¼ 4.621 A, c ¼ 4.304 A [7]. The lattice param-
eters of Ni metal having cubic (fcc) sys-
tem are 3.523 A [8]. It means that fcc Ni
ꢀ
metal changes to hexagonal Ni3N by
considering the cubic diagonal as the
hexagonal c-axis. There is a regular pla-
cement of nitrogen atoms in one third
of the distorted octahedral interstices.
Nitrogen–nitrogen distance is large
compared to the nickel–nitrogen which
leads to strong hybridization. The Ni3N
peak positions are uniformly shifted to
Fig. 1. XRD Patterns of a) Ni3N and b) Ni
metal

phys. stat. sol. (a) 189, No. 3 (2002)
Fig. 2. TEM micrograph of Ni3N particles
693
lower angles of 2q value by 0.4ꢀ, compared to pure Ni
and suggests an increase of lattice parameters. The crys-
tallite size of 16 nm was calculated by using Scherrer’s
formula from the line broadening. Figure 2 shows the
TEM micrograph of Ni3N particles obtained at 633 K.
The average particle size observed was about 18 nm
which is close to the XRD crystallite size. These parti-
cles are spherical in size and showed uniform distribu-
tion. The formation of particle aggregates or magnetic clusters is observed because of
strong interaction among the magnetic dipoles of particles. However, we did not find
evidence for the crystalline nickel metal in Ni3N matrix either by XRD or electron
diffraction.
Figure 3 shows the temperature dependence of magnetization of Ni3N particles of
16 nm size, when the applied field was 0.005 T and the inset figure shows the magneti-
zation as a function of field. The Ni3N sample exhibits ferromagnetic behavior with a
coercivity of 0.022 T. The loop is saturated when the maximum field applied is raised
up to 1.1 T. The s-value rises rapidly with a sizable hysteresis (up to the field of 1.1 T),
expected for the domain rotation in a ferromagnetic/ferrimagnetic substances [9]. The
specific saturation magnetization (ss) is found to be 1.6 emu/g and the retentivity (sR)
0.5 emu/g at room temperature. The reported value is 1.2 emu/g where the sample is a
nanocomposite film of Ni3N on alumina substrate having an excess amount (3.0%) of
elemental nickel [4]. The magnetic moments reported for Ni and Ni3N are 0.615mB at
273 K and 0.057mB at 292.5 K respectively [9]. Oyama [10] has also reported that Ni3N
shows ferromagnetic behavior. In our study, the ss values observed for Ni3N at a satura-
tion field of 1.1 T are 1.678 emu/g and 1.793 emu/g in the +ve and ––ve directions,
respectively. This difference is due to the forced magnetization as the single domain
rotation in the field direction is predominant at higher fields [9]. The coercivity mea-
sured from the hysteresis is 0.022 T at room temperature and is expected from the
Fig. 3. Specific magnetization as a
function of temperature. Inset
shows the field dependence of
magnetization

694
N. S. Gajbhiye et al.:Synthesis and Magnetic Studies of Nanocrystalline Ni N
3
non-interacting Stoner-Wohlfarth type particles. However, the bulk ss of Ni metal is
54.39 emu/g at room temperature [9]. This value is different from the values of Ni3N
nitride material. The Curie temperature (TC) for Ni3N is 634 K (Fig. 3) which is higher
than for the nickel (fcc) metal at 627.4 K [10].
The contribution to the magnetic moment in Ni3N system could be due to an unba-
lanced charge distribution between spin-up and spin-down states. From the magnetic
valence model discussed for Fe3N [10, 11], both N and Ni contribute to the magnetic
moment in Ni3N. The density of states of Ni (DOS) is dominated by 3d states and
mixed with 2p (DOS) of N. This results in the electron exchange processes having some
sort of covalent bonding between Ni and N atoms. However, in Ni3N, N acts as the
electron donor, and ionic bonding or charge transfer is also expected. These aspects are
confirmed from the electrical resistivity measurements and our Ni3N showed a higher
electrical resistivity of 151.7 ꢁ 10––6 W cm. compared to 7.0 ꢁ 10––6 W cm. of Ni metal at
300 K [13], where metal–metal bonding (Ni–Ni) only is present. It means that there is
a covalent bonding between Ni and N in Ni3N in addition to Ni–Ni bonding resulting
into high resistivity. Therefore, the number of unpaired d-electrons available for intra-
band polarization in Ni is reduced resulting in lower magnetization value for Ni3N
phase compared to Ni metal.
Yue et al. [6] have reported Ni3C phase exhibiting ferromagnetic nature with a room-
temperature coercivity of 70 Oe and a magnetization of 0.8 emu/g. But the observed
ferromagnetic behavior does not agree with the theoretical predictions. Such kind of
effects cannot be ruled out in case of Ni3N nitride material because of metastable nat-
ure and the observation of ferromagnetic nature of Ni3N can be explained by the dis-
order in the structure. The magnetic and structural measurements are consistent with
the microscopic observations of nanostructured materials with locally produced Ni-rich
Ni3N regions of aggregates/clusters which may permeate the Ni3N particles. These as-
pects are under investigation even though we have not found the presence of Ni in
Ni3N by XRD or electron diffraction.
Conclusions In the present study, we have discussed the synthesis and characterization
of nanocrystalline Ni3N particles that are stable up to 683 K in ammonia atmosphere.
TEM data showed spherical morphology, uniform distribution and strong dipolar inter-
actions among particles. Ni3N exhibited ferromagnetic behavior having a coercivity of
0.022 T and Curie temperature 634 K. The contribution to the magnetic moment in
Ni3N system is due to an unbalanced charge distribution between spin-up and spin-
down states of Ni-3d and N-2p orbitals.
Acknowledgements The authors gratefully acknowledge the financial support from
CSIR, New Delhi. One of us (N.S.G.) wishes to thank Professor H. Gleiter, Institute of
Nanotechnology, Forschungszentrum Karlsruhe for the encouragement.
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