Effect of nitrogen substitution on the structural and magnetic ordering transitions of NiCr2O4

Article abstract of DOI:10.1039/c6ra22773b

The nitrogen (N) doped spinel NiCr₂O₄ has been synthesized at 773 K (N500) and 873 K (N600) by ammonolysis of NiCr₂O₄ powders to study the effect of anion doping on its structural and magnetic properties. The N contents are determined by thermogravimetric oxidation, yielding a composition that can be described as NiCr₂O_3.68N_0.21 (N500) and NiCr₂O_3.55N_0.30 (N600). X-ray photoelectron spectroscopic studies suggest that N^3? species partly substitute the oxygen in the lattice and oxygen vacancies exist in the N doped samples. There is evidence that in the nitrided sample, the Cr ion is most likely in a mixed oxidation state. As the N content increases, the structure at room temperature changes from tetragonal to the cubic phase; N500 is only partially tetragonal; N600 is completely cubic. Such structural change is the consequence of the depression of the cooperative Jahn-Teller effect of Ni²⁺ in the tetrahedral A site caused by the presence of N^3?. Combined heat capacity and temperature dependent magnetic susceptibility measurements give clear evidence of the magnetic and structural transitions in the N doped NiCr₂O₄. The Jahn-Teller transition temperatures decrease with increasing N content; this is likely due to increased covalency and hence enhanced contribution of the angular momentum and the spin-orbit coupling to local chemical bonding around Ni²⁺. Antiferromagnetic transitions are observed at T_S = 23 K and 22 K for N500 and N600, respectively. Hence there is indeed a lowering of transition when compared to pure NiCr₂O₄ (28 K). The magnetic loops at different temperatures confirm that the material behaves as a paramagnet over a wide range of temperatures T ～ 80-350 K. The material also exhibits a canted ferrimagnetic structural transition between 30 and 70 K. We also report evidence for increased frustration and lowered correlation length in N doped compounds compared to the parent NiCr₂O₄. The present study on N^? doping effects on the structure and magnetic properties of this NiCr₂O₄ is expected to be useful for tailoring the ferric phase transitions through anion substitutions.

Full text of DOI:10.1039/c6ra22773b

View Article Online
View Journal
RSC Advances
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: X. Liu, N. Yin, T.
Thomas, M. Yang, J. Wang and Q. Shi, RSC Adv., 2016, DOI: 10.1039/C6RA22773B.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
rsc.li/rsc-advances

Please do not adjust margins
RSC Advances
Page 1 of 9
View Article Online
DOI: 10.1039/C6RA22773B
Journal Name
ARTICLE
Effect of nitrogen substitution on the structural
and magnetic ordering transitions of NiCr2O4
Xin Liu,a,b* Nan Yin, a Tiju Thomas,c Minghui Yang, a* Junhu Wanga,b and Quan Shi a*
†
Received 00th January 20xx,
Accepted 00th January 20xx
The nitrogen(N) doped spinel NiCr2O4 has been synthesized at 773 K (N500) and 873 K (N600) by
ammonolysis of NiCr2O4 powders to study the effect of anion doping on its structural and magnetic
properties. The N contents are determined by thermogravimetic oxidation, yielding a composition
that can be described as NiCr2O3.68N0.21 (N500) and NiCr2O3.55N0.30 (N600). X-ray photoelectron
spectroscopic studies suggest that N3- species partly substitute the oxygen in the lattice and oxygen
vacancies exist in the N doped samples. There is evidence that in the nitrided sample, Cr ion is most
likely in a mixed oxidation state. As the N content increasess, the structure at room temperature
changes from tetragonal to the cubic phase; N500 is only partially tetragonal; N600 is completely
cubic. Such structural change is the consequence of the depression of the cooperative Jahn-Teller
effect of Ni2+ in tetrahedral A site caused by the presence of N3- . Combined heat capacity and the
temperature dependent magnetic susceptibility measurements give clear evidence of the magnetic
and structural transitions in the N doped NiCr2O4. The Jahn-Teller transition temperatures decrease
with increasing N content; this is likely due to increased covalency and hence enhanced contribution
of the angular momentum and the spin-orbit coupling to local chemical bonding around Ni2+.
Antiferromagnetic transitions are observed at Ts=23 K and 22K for N500 and N600, respectively.
Hence there is indeed a lowering of transition when compared to pure NiCr2O4(28 K). The magnetic
loops at different temperatures confirm that the material behaves as a paramagnet over a wide
range of temperatures T~80-350 K. The material also exhibits a canted ferrimagnetic structural
transition between 30 and 70 K. We also report evidence for increased frustration and lowered
correlation length in N doped compounds compared to the parent NiCr2O4. The present study on N-
doping effects on the structure and magnetic properties of this NiCr2O4 is expected to be useful for
DOI: 10.1039/x0xx00000x
www.rsc.org/
tailoring
the
ferric
phase
transitions
through
anion
substitutions.
1. Introduction
Ni2+ (with 3d8 configuration) are found in the tetrahedral
sites11. The tetrahedral crystal field around Ni2+ (3d8) in the
cubic phase results in fully occupied low energy e levels and
triply degenerate t2 levels, rendering this structure potentially
unstable. Consequently, a cooperative lattice distortion – from
cubic to tetragonal symmetry - lifts the orbital degeneracy in
NiCr2O4 at 310K12. This tetrahedral distortion results in the
elongation of NiO4 tetrahedra and c/a > 1. NiCr2O4 shows the
Nickel chromite (NiCr2O4) is a normal spinel and a common
mineral in Earth’s mantle and crust1, 2. It is commonly used for
catalytic applications3-6 and gas sensors7. It is also a well
known magnetodielectric material that was first reported by
Mufti et al8. The spinel NiCr2O4 exhibits interesting structural-
magnetic correlation9, 10. In the cubic structure, Cr3+ (with 3d3
configuration) preferentially occupies the octahedral sites
because of the strong crystal field stabilization of the half filled
non-degenerate t2g orbitals and empty eg orbitals, while the
occurrence of two magnetic transitions
corresponding to the onset of a ferrimagnetic (longitudinal)
component and an antiferromagnetic (transverse)
- TC and TS,
a.Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
China.
component13. By using neutron scattering, Ishibashi and
Yasumi have confirmed a further structural distortion from
tetragonal symmetry to orthorhombic symmetry at 65 K,
which is correlated with the onset of ferrimagnetic ordering14,
15. At lower temperatures, the NiCr2O4 exhibits a canted
magnetic structure in which the ferromagnetic component and
the antiferromagnetic (AF) component forms is consistent with
long-range order below TS=31 K. A distortion within the
b
Mössbauer Effect Data center, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, China.
Department of Metallurgical and Materials Engineering, Indian Institute of
c
Technology Madras, Chennai 600036, Tamil Nadu, India. E-mail:
shiquan@dicp.ac.cn (Q.S), myang@dicp.ac.cn (M.Y), liuxin01@dicp.ac.cn (X.L)
†Electronic Supplementary Informaꢀon (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 2 of 9
View Article Online
DOI: 10.1039/C6RA22773B
ARTICLE
Journal Name
orthorhombic structure takes place at the same temperature, NiCr2O4. We find this spinel exhibiting interesting physical
where anomalies in magnetic susceptibility and heat capacity properties: the Jahn-Teller (JT) distortion temperature
are found16, 17. The investigation of temperature and magnetic decreases with increasing N concentration. In addition, the
field dependence of dielectric properties reveals a new magnetic structure and magnetic interaction are also affected
anomaly at 20 K, corresponding to the completion of structural by the N doping.
and magnetic order10.
Considering the rich structural chemistry of NiCr2O4 a lot of
2. Experimental details
research efforts are ongoing to do with tailoring the phase
transitions. Most of the work has been focused on
Materials
modifications of cation composition, through doping of Cu18, 19
,
NiCr2O4 is prepared by a solid state reaction of a stoichiometric
mixture of powders of NiO (99.99%, Aldrich) and Cr2O3
(99.99%, Aldrich). The powders are ground together and
heated for 24 h in air at a temperature of 1473 K with
intermediate regrindings. 0.2 – 0.3 g of these samples are
placed in an alumina boat. The boat is then placed in a silica
tube which has air tight stainless steel end caps; these caps
have welded valves and connections to input and output gas
lines. All gases used are purified to remove trace amounts of
oxygen or water using pellet copper, nickel, palladium and
platinum with zeolites as support. The silica tube is then
placed in a split tube furnace and the appropriate connections
to gas sources are made. Argon gas is passed over the sample
for 15 min to expel air before establishing a flow of ammonia
gas (anhydrous, Air Gas). The NiCr2O4 is heated at the reaction
conditions as summarized in Table 1. After treatment for the
specified period, the furnace power is turned off and the
product is cooled to room temperature for ~ 4 h under
ammonia flow conditions. Before the silica tube is taken out of
the split tube furnace, argon gas is flowed through the silica
tube to expel the ammonia gas. The samples treated at 773 K
and 873 K are represented as N500 and N600, respectively.
Co20-22, Mg23, Fe24, etc. into the NiCr2O4 lattice. In effect these
studies explore the effect of the transition metal cations with
different numbers of d electrons on observed structural
distortions and the magnetic transition temperatures. A less
explored approach is to investigate impact of anion
composition on the structural transitions in NiCr2O4. Morris et
al. reported that NiCr2S4 and NiCr2Se4 both have the Cr3S4
structure25. It is relevant to note that the Cr3S4(I2/m) system
does not have
a spinel structure, and that is is not
antiferromagnetically ordered at low temperatures. In fact in
this system, the order moments are reduced from spin only
values, due to the appreciable covalency of the metal-sulfur
bonds26. Hence the impacts of anion doping in an oxide
material such as NiCr2O4 is certainly interesting.
Transition-metal oxides can be transformed into oxynitrides by
reacting with ammonia at elevated temperatures. From the
structural point of view, oxynitrides often show the same
structural type as the parent oxides for moderate
substitutional rates. Furthermore partial replacenent of
oxygen with nitrogen is readily obtained via amnonolysis27-29
.
The anionic substitution of divalent oxygen for trivalent
nitrogen additionally induces a change in the oxidation state of
the transition metals involved. This oftentimes yields insulating
materials as a consequence of strong perturbation of the
periodic potential at the anion substructure and due to the
Characterization
Finely ground powders are examined with a Rigaku Ultima VI
powder X-ray diffractometer (PXRD) with monochromatic
CuKα1 radiation (λ = 1.5406 Å). Crystal structures of the oxide
and the resulting nitride are confirmed by PXRD profiles using
the GSAS package. Scanning electron microscopy (SEM) is
performed with a LEO-1550 field emission SEM (FSEM). The dc-
magnetic susceptibility is measured with a Physical Property
Measurement System (PPMS) over the 4-350 K temperature
range. Magnetization isotherms are measured at selected
temperatures in the magnetic field range from -4T to +4T.
Thermal gravimetric analyses (TGA) are performed with a
SETSYS 16/18 (SETARAM) in a temperature range of 300 - 1273
K. The heat capacity measurement is performed using a
Physical Property Measurement System (PPMS) in the
temperature range from (1.9 to 400) K, and the measurement
uncertainties are verified to be ±3% below 20 K and ±1% from
(20 to 400) K.32 The detailed heat capacity measurement
procedure can be found in the related work reported by Shi et
al. 32, 33. The binding energy of the surface Ni, Cr, O, N species
are determined by X-ray photoelectron spectra (XPS) with an
ESCALAB250 X-ray photoelectron spectrometer using
contaminated C as internal standard (C1s=284.6 eV)
increased covalency in the materials since
electronegative than O30.
N is less
A number of theoretical and experimental studies have
focused on the electronic properties and electronic band gaps
of the oxynitrides with spinel structure. Several studies have
been carried out regarding ternary nitrides and oxynitrides31 in
an effort to tailor the desirable mechanical and electronic
properties associated with spinel oxynitrides. Research has
expanded further into spinel-type oxynitrides in an effort to
engineer the band gap value by varying the N:O ratio.
However, the highly reducing atmosphere during thermal
ammonolysis hardly permits middle-to-late transition metals
to form oxynitrides without being reduced to elemental metals
or metal nitrides. For this reason, the highest content of
nitrogen reported in materials with spinel crystal structures is
rather low.
In this study, we first doped N in the lattice of NiCr2O4 spinel
by ammonolysis of the precursor oxides under favourable
conditions. Subsequently we employed X-ray powder
diffraction (XRD), magnetic susceptibility, X-ray photoelectron
spectra (XPS) and heat capacity (Cv) measurements to
investigate the structural and magnetic transitions in N doped
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 3 of 9
View Article Online
DOI: 10.1039/C6RA22773B
Journal Name
ARTICLE
NiCr2O3.55N0.30 for products ammonolysed at 773 and 873 K,
respectively.
Results and discussion
Table 1: Refined lattice parameters and calculated domain size of
NiCr2O4 and its ammonolysis products.
Formula
NiCr2O4
NiCr2O3.68N0.21 NiCr2O3.55N0.30
Formula weight 226.68
224.50
223.69
Cubic
Crystal system Tetrahedral Cubic
Space group
a/Å
I41/amd
Fdꢀ3m
Fdꢀ3m
5.8319(1)
8.3181(2)
8.3184(1)
ꢀ
c/Å
V/ Å3
8.4433(1)
287.17(1)
ꢀ
575.53(1)
575.60(1)
calcd/kg mꢀ3
5.243
5.232
5.232
ρ
0.0462
0.0455
0.0410
WRp
0.0274
77
0.0263
78
0.0249
66
Fig. 1 PXRD patterns for NiCr2O4: N500 and N600. T stands for
tetrahedral phase and C stands for cubic phase of NiCr2O4.
* stands for CrN.
Rp
Domain sizeCalcd
Fig. 2 (a-d) presents the O 1s, N 1s, Cr 2p and Ni 2p core level
X-ray photoelectron spectra(XPS) of N500 respectively. Each
spectrum is fitted by a least squares method using Gaussian-
Lorentzian envelopes. The surface stoichiometry of the sample
calculated from the XPS data is N(3.73%), O(64.86%),
Ni(10.98%), Cr(20.43%). As seen, the surface stoichiometry of
the sample is close to the bulk of the sample; surface oxygen
concentration however is quite different. The O 1s spectra are
fitted with an O2- component at 529.9eV along with additional
peaks with binding energy (BE) at 530.9 eV and 531.9 eV. The
most prominent peak at 529.9 eV corresponds to the lattice
O2-, while the peak at 530.9 eV is assigned to the defect states
within the crystal.35-38 The additional peak at 531.9 eV is
associated with the O present in Cr-N-O or Ni-N-O bonds.39
The XRD patterns for the NiCr2O4 and the ammonolyzed
products can be seen in fig. 1. The X-ray diffraction profile is
fitted to confirm the phase purity of the NiCr2O4 products as
shown in Figure S1. The as prepared NiCr2O4 has a tetragonal
structure in space group I41/amdz with refined lattice
parameters a=5.8319(2) Å and c=8.4433(2) Å (ref: Table 1). The
ammonolysis product of NiCr2O4 at 873 K has a cubic structure
in the space group Fd-3mz with lattice parameters a =
8.3181(2) Å. The ammonolysis product of NiCr2O4 at 773 K is a
mixture of two phases, containing mainly cubic phase.
Presence of tetragonal NiCr2O4 in N500 is indicated by the
small shoulder peak at ~37 degrees in PXRD pattern. The
slightly larger lattice parameters and volume for the oxynitride
is due to the ionic radius of N3- (1.46Ǻ) being greater than the
ionic radius of O2-(1.38 Ǻ)34. Ammonolysis treatment of the
NiCr2O4 at higher temperature (>873 K) results in
decomposition to NiCrO3 and CrN. Both as-prepared and
ammonolysis products of NiCr2O4 have similar average
crystalline domain sizes of ~76 nm, calculated from PXRD
refinements. These results agreed well with results obtained
from electron micrography. The precise location of the
nitrogen atoms of the oxynitrides cannot be obtained by XRD
due to the similar X-ray scattering factors of N3- and O2-. Hence
measurement of neutron diffraction is needed; this will be
pursued in future.
SEM images are used to observe the surface morphology of
the products given in Figure S2. This figure confirms that the
powder sample is contains large particle of size ~70-400 nm.
The ammonolyzed samples are designated as NiCr2O4-xN2x/3
.
The nitrogen contents are analyzed by converting NiCr2O4-
xN2x/3 back to NiCr2O4 using TG in air flow as shown in Figure
S3. The calculated average compositions are NiCr2O3.68N0.21 and
Fig. 2 The O 1s (A), N 1s (B), Ni 2p (C) and Cr 2p(D) XPS spectra
for N500.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 4 of 9
View Article Online
DOI: 10.1039/C6RA22773B
ARTICLE
Journal Name
In Figure 2(a), the N 1s features show only one asymmetric
peak with binding energy (BE) of 395-402 eV. On
a
deconvolution, it can be seen that the broad peak consists of
three different peaks at ~396.2, 397.7 and 399.2 eV
respectively. The main peak with the binding energies of 397.7
eV is assigned to Cr-N or Ni-N species40, 41; this suggests that
nitrogen substitutes for oxygen in the spinel matrix. The peak
with binding energy at ~396.2 eV is indicative of a N3- species42,
43
and confirms the presence of oxynitride species with lower
binding energy44. Interestingly the BE observed here for N3- is
lower than that for NiN45 or CrN(396.3 - 397.1 eV)46, 47; this
suggests greater charge transfer to the nitrogen in NiCr2O4,
when compared to the pure nitrides. However there could be
a contribution from oxygen vacancy formation induced by N
doping as well48. The peak at 399.2eV is attributed to nitrogen
in the interstitial sites, where nitrogen has mixed Cr-O-N or O-
Cr-N (Ni-O-N, O-Ni-N) environment. In this state, N atoms are
bonded to one or more lattice oxygen ions47, 49 and therefore
are in a relatively positive oxidation state50. From literature, 18,
Fig. 3 Plot of heat capacities of NiCr2O4 (measured by
21
the values of BE for the Ni 2p level and of the satellite shift Klemme et al.
17 and in this work), N500 and N600 as a function
ΔE permit identification of the Ni ions; they are divalent in of temperature over the temperature ranges: (a) from (1.9 to
tetrahedral environments. The Cr 2p spectra of the sample has 400) K, (b) from (250 to 340) K, (c) from (60 to 75) and (d) from
a
multiplet shape that is typical for Cr3+ ions51. The (1.9 to 40).
incorporation of nitrogen into lattice results in Cr-N or Ni-N
bonds, which is supported by the lower BE of Ni 2p and Cr 2p
in N doped NiCr2O4. Marginal decrease in BE of Cr and Ni are heat capacities in the corresponding temperature windows to
reasonable since addition of N reduces the overall ionic a combination of Debye and Einstein functions, and the fitting
character of the compounds, increasing the electron density equation can be expressed by
around cations. Hence, in Fig. 2 (c) and (d), the negative shifts
Cp,m
=
m
·
D
(
ΘD
/
T
)+n1
·
E
(
ΘE,1
/
T
)+n2
·
E
(
ΘE,2
/
T
)
(1)
are observed in Cr 2p and Ni 2p binding energies. The peak of
Cr 2p is close to 576.0 eV which corresponds to Cr2+. Hence it is
difficult to prove the existence of Cr2+ ions because of the
small difference between the obtained value of binding energy
and that of Cr2+. Therefore it is very likely that Cr ions exist in
mixed oxidation states (i.e 3+ as well as 2+) in N doped NiCr2O4.
To investigate the magnetic and structural evolution of
the NiCr2O4 samples during the ammonolysis process in
thermodynamics, the heat capacities of the NiCr2O4 and N
doped samples are measured using the PPMS in the
temperature range from (1.9 to 400) K. The experimental heat
capacities are plotted against the temperature and compared
with the reported values from Klemme et al17 (Fig. 3). It can be
seen from the figure that the heat capacity of the as-prepared
NiCr2O4 are in agreement with published values revealing
three thermal anomalies at around 310 K, 66 K and 29 K,
respectively. Based on the previous magnetic and structure
studies13, the thermal anomaly appearing at 310 K (the first
transition) has been attributed to a cooperative Jahn-Teller
distortion lifting the orbital degeneracy in NiCr2O4, resulting in
a structural transition from cubic to tetragonal. The other two
anomalies at 66 K (the second transition) and 29 K (the third
transition) have been accounted for by a longitudinal and a
transverse antiferromagnetic ordering, respectively. As for the
nitride samples; with increase in N content, the first and third
transitions move to lower temperatures, while the second one
moves to higher temperatures. The lattice heat capacities of
these samples have been estimated by fitting the experimental
where
temperature Einstein functions, respectively.
and ΘE,2 are adjustable parameters; the sum of (
D(ΘD/T), E(ΘE,1/T) and E(ΘE,2/T) are Debye, low and high
m
,
n1
m
,
n2
,
ΘD
, ΘE,1
+
n1
+
n2) is
approximately equal to the number of atoms in the
molecule52-54. The fitting parameters from Eq. (1) are listed in
Table S1. The heat excess heat capacities due to the three
transitions have been calculated by subtracting the estimated
lattice heat capacities from the total, and the results are
shown in Figure S4. Consequently, the starting, ending and
peak temperatures and the entropy changes related to the
three transitions have been determined in Table S2 based on
the excess heat capacities.
As can be seen from Figure S4(a), the peak temperature
of the first transition moves to the lower temperatures with
increase in N concentration. Also, the transition entropy
decreases from 1.14 J·K-1mol-1 to 0.59 J·K-1mol-1 as the N
anions concentration increases from NiCr2O4 to N600. This
indicates that the Jahn-Teller distortion effect on the structural
transition has been suppressed in these samples. The second
transition due to the paramagnetic to ferrimagnetic ordering
only has a slight influence on transition temperatures and
entropies, suggesting that the doped N hardly affects the
magnetic ordering. However, as for the third transition related
to the antiferromagnetic ordering, the peak temperature
moves from 29 K to 24.3 K, and the transition entropy changes
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 5 of 9
View Article Online
DOI: 10.1039/C6RA22773B
Journal Name
ARTICLE
to almost half the amount, decreasing from 2.53 J·K-1mol-1 to corresponds to the ferrimagnetic transition. Further change in
1.30 J·K-1mol-1. This suggests that the doped N is likely to magnetic structure occurs at TS=28 K, where another anomaly
produce a significant effect on the thermodynamic behaviour
of the third transition.
The heat capacity of a substance can be generally
expressed as a sum of contributions from various modes
related to the lattice, electronic, magnetic and others. Each of
these contributions may be fitted to the corresponding
theoretical models which would help in elucidation of
important information about the physical properties of the
substance55. The lattice heat capacity far outweighs all other
contributions at high temperature regions (above 15-20 K).
However, at low temperatures (below 15-20 K) the lattice is
comparable to the other contributions, and therefore each of
these contributions can be extracted by fitting the total heat
capacity to theoretical models in carefully chosen temperature
regions. In order to further understand the observed specific
heat capacity trends measured for NiCr2O4 and N-doped
samples, the experimental heat capacities of these compounds
below 10 K have been fitted to a theoretical function of:
/T
Cp,m
=
γT
+
B3T3
+
B5T5
+
B7T7
+
BaswT
3e-∆
(2)
Here the linear term is usually a contribution from electrons as
well as vacancies or dislocations existing in the samples. The
odd-powers in temperature represent the lattice vibration
contribution. As for the magnetic contribution, it is
/
represented using the term Basw
T
3e-∆ T, where Basw is defined as
the antiferromagnetic constant proportional to molar volume
and the spin-wave stiffness constant. is the magnetic spin-
wave gap written in Kelvins52-54
The antiferromagnetic
Δ
.
contribution decreases in the N-doped samples, implying that
the magnetic ordering is likely reduced to some extent as the
nitrogen atoms get implanted in the lattice. Moreover the
linear term tends to increase as we go from NiCr2O4 to N600,
suggesting that the increasing nitrogen doping does result in
increasing vacancy concentrations. The values relevant to the
specific heat capacity measurements are listed in Table S3. It
can be seen from the results that the antiferromagnetic
contribution does exist in the heat capacities. This further
confirms the presence of antiferromagnetic ordering.
Fig. 4(a-c) shows the temperature dependence of
susceptibility in NiCr2O4 and N doped samples, as measured Fig. 4 The temperature dependence of susceptibility (Blue) and
after field cooling; a magnetic fields up to 1000 Oe is applied inverse susceptibility curves (Black) for NiCr2O4(a), N500(b) and
for getting the data. The observed magnetic data is analyzed N600(c). The arrows indicate the J-T transitions. The insets are
using the equation given below below:
the related temperature derivative of the susceptibilities, and
the peaks corresponding to the TC and TS respectively
C
(3)
χ
= χ0 +
T − Θcw
in this curve is observed. These structural and magnetic
Where C is the Curie constant, ΘCW is the Curie Weiss
temperature and χ0 is a temperature-independent Pauli-like
term. In Figure 4(a), as the temperature decreases, there is a
small kink at 315 K in the inverse susceptibility curve,
corresponding to the cooperative Jahn-Teller (J-T) transition
transition temperatures are in good agreement with other
14, 56
reports for polycrystalline NiCr2O4
.
For the N doped samples, the J-T distortions occur at 275 K
(N500) and 268 K (N600) respectively, which become lower
than the transition temperature of pure NiCr2O4. The
ferrimagnetic ordering (TC) occurs at 67 and 68 K, which is a
An abrupt rise in the susceptibility curve at TC=66
K
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 6 of 9
View Article Online
DOI: 10.1039/C6RA22773B
ARTICLE
Journal Name
minor increase after the N doping. In contrast to ferrimagnetic with N doping level. Furthermore, the frustration index
transition, the antiferromagnetic (transverse) ordering (ΘCW/TN) of N600 is 8.74, indicating that N doped samples are
transition (TS) is decreasing from 28 K to 22 K with N doping. the more frustrated compounds compared to the parent
These transition temperatures corroborate well with the heat NiCr2O4.
capacities results. When N atoms replace O atoms, the N ions It should also be mentioned that the deviation of the reverse
would provide empty 2p orbitals for the formation of covalent susceptibility against T curve from a straight line begins below
bonds with Ni2+ and Cr3+ 57. This likely causes a reduction of the a temperature of 176 K, 160K, and 152 K respectively. The
J-T potential energy; the driving force for the J-T distortion deviation observeed is a slow downward drop. This downward
would hence be reduced58, 59. The decreasing of distortion deviation from the Curie-Weiss law indicates the onset of a
would enhance the contribution of the angular momentum magnetic short-range interaction just above TC. The decrease
and the spin-orbit coupling, and as a result, J-T transition of deviation temperature means that N doping produces a
temperature would further fall off60. This in turn would decrease in correlation length.
stabilizes the cubic structure and leads to the gradual decrease
in the tetragonal distortion temperature.
The Curie Weiss (CW) equation (3) is applied to fit the
paramagnetic regimes of the susceptibility before the onset of
the J-T distortion for the NiCr2O4 and N doped samples. The
obtained effective moments (μeff) and CW temperatures are
listed in Table S4. The experimental value of μeff is 6.43 μB for
NiCr2O4, which is slightly larger than the expected 6.16 μB per
formula unit16. Hence we infer that although the angular
momentum is largely quenched due to the tetragonal
distortion, there is still a small contribution of the partially
quenched orbital moments of Ni ions to the overall magnetic
moment per formula unit. For the N doped NiCr2O4, the
temperature independent Pauli term χ0 increases due to the
covalent effect. The effective moments increase with
increasing N content from 7.64 μB to 8.04 μB. This further
corroborates the hypothesis that unquenched orbital
moments of Ni2+ in the tetrahedral sites of the cubic structure
make an important contribution to the magnetic moment61.
Magnetic contributions may also come from the
impurity/dopant atoms62, as well as the oxygen vacancies63.
The CW temperature of NiCr2O4 is -489 K，while N600 present
even larger CW temperatures of -594 K. The exchange coupling
between magnetic ions (Ni2+ and Cr3+) is directly proportional
to CW temperature and can be expressed as:
J = AΘCW
(4)
Where A=3kB/ZS(S+1), kB is the Boltzmann constant, Z is the
number of the interactions of nearest neighbours, and S is the
total spin. The negative value of ΘCW indicates the presence of
antiferromagnetic or ferrimagnetic correlation interaction,
while the magnitude of ΘCW for the oxynitrides suggests that
the antiferromagnetic interaction is enhanced. The orbits of
tetrahedral Ni2+ ions and octahedral Cr3+ can be characterized
by e4t2 and t2g3eg electronic configurations respectively. It is
known that the Cr3+ ion, in octahedral site with empty eg orbits
antiferromagnetically interact with Ni2+ in tetrahedral site, 64 In
fact, calculation suggests that the direct Cr-Cr exchange
constant (JCr-Cr) could be over three times larger than the
superexchange Cr-O-Ni interaction constant65. In the NiCr2O4
chromite, increase in CW suggests stronger B-B direct
exchange interaction among magnetic ions, which increases
Fig. 5 Isothermal field-dependent magnetization of N500 (a)
and N600 (b) measured at 4 K, 50 K, 100 K and 320 K
respectively
4
1
To gain further insight into the magnetic properties of the
synthesized N doped NiCr2O4 samples, magnetization (M) as a
function of applied magnetic field (H) is measured at different
temperatures (Figure 5). For N500 and N600, the
magnetization linearly increases with field while measured at
320 K and 100 K, and there is no magnetic hysteresis and
hence the remanence (Mr) and coercivity (HC) are practically
zero. This indicates a paramagnetic behavior. This is consistent
with the M-T plots in Figure 4. However, very clear magnetic
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 7 of 9
View Article Online
DOI: 10.1039/C6RA22773B
Journal Name
ARTICLE
Table II: The magnetic parameters of NiCr2O4, N500 and N600
Sample
C
θCW
(K)
Meff
(ꢁB)
χ0
TS
TN
MS
HC
(emu*K/mol)
(emu /mol*Oe)
(K)
(K)
(ꢁB)
(Oe)
NiCr2O4
N500
5.18
7.30
8.08
ꢀ486
ꢀ456
ꢀ594
6.43
7.64
8.04
0.00008
0.00084
0.00188
28
23
22
66
67
68
0.30[]
0.30
0.28
8300[]
12500
15000
N600
hysteresis loops appear at 50 K with HC, Mr and spontaneous The N doped NiCr2O4 has been synthesized successfully by
magnetization (Ms) values listed in Table II. The area under nitridation of the solid NiCr2O4 oxides at 773 K(N500) and 873
hysteresis increases with significant increase in coercivity (Hc). K (N600) respectively. The incorporation of N into the lattice
We observe higher HC of 14 kOe for N600 compared with 8.3 has been confirmed using XRD and XPS measurements. The
kOe for NiCr2O410. The larger coercive field indicates presence susceptibility and heat capacity based investigations indicate
of ferromagnetic long-range order and stronger magnetic that structural and magnetic properties undergo significant
anisotropy which may be caused by the spin-orbit interaction. changes upon N doping. Pure NiCr2O4 exists in tetragonal
At 50 K, the magnetization readily increases at low field and structure, however N500 and N600 contains cubic phase at
then linearly increases up to 40 kOe without showing any room temperature. The temperature of cooperative Jahn-
saturation for both N doped samples.
Teller distortion decreases with increase in N content. This
When temperature goes down to 4 K, the isotherm M(H) still observation is related to change in nature of bonding brought
did not exhibit saturation, and the spontaneous magnetization about by introduction on N into the lattice. The ferrimagnetic
of N doped NiCr2O4 is estimated to be 0.2 μB per formula unit. ordering temperature (TC) of the material exhibits a minor
Considering the fact that HC is found to be less than 15 kOe, increase indicating that the A-B superexchange interaction is
we suggest that this linear contribution signals a non-collinear not affected much by the N-doping due to the empty eg orbital
spins arrangement of the NiCr2O4 ferrimagnet. As is stated in of Cr ions (which is present in the octahedral site). The N
references66, 67, in non-collinear configuration the applied doped NiCr2O4 exhibits a canted ferrimagnetic structural
magnetic field exerts a torque that could change the angles transition between 30 and 70 K. We also report evidence for
between the canted magnetic moments. Hence the increased frustration and lowered correlation length with N
magnetization is expected to increase linearly with the doping in NiCr2O4. The hitherto unexplored details of
magnetic field in this temperature regime. Since the magnetic magnetostructural coupling are likely to stimulate further
moment of the tetrahedral sublattices in NiCr2O4 is greater in investigations on this interesting material.
magnitude than that of the octahedral sublattice (MA>MB)68,
and the magnetic structure consists of longitudinal and
transverse magnetic sublattices with slight canting of spins13.
This suggests that when N is incorporated in the lattice,
crystalline anisotropy and magnetostrictive effects associated
with spin-orbit coupling have increased the noncollinearity of
tetrahedral sbulattice spins. This decreases the noncollinearity
of octahedral sublattices68. Hence the magnetic ordering in
NiCr2O4 and the N doped samples is strongly coupled to
structure. Further investigations are required to obtain greater
clarity regarding correlations between magnetic and structural
changes in this interesting material.
Acknowledgements
This work is supported by National Natural Science Foundation
of China through grant 11205160, 21471147, 21473198, and
Liaoning Provincial Natural Science Foundation through grant
2014020087. Q. Shi would like to thank Hundred-Talent
Program founded by Chinese Academy of Sciences. M. Yang
would like to thank the National "Thousand Youth Talents"
program of China. Tiju Thomas thanks the Department of
Science and Technology, Government of India for Financial
Support through INSPIRE Faculty Award (DST 01117), and for
the Fast Track Young Scientist Award.
Conclusions
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 8 of 9
View Article Online
DOI: 10.1039/C6RA22773B
ARTICLE
Journal Name
28.
29.
M. T. Weller and S. J. Skinner, Int. J. Inorg. Mater., 2000, 2,
463.
M. Retuerto, C. de la Calle, M. J. Martinez-Lope, F.
Porcher, K. Krezhov, N. Menendez and J. A. Alonso, J. Solid
State Chem., 2012, 185, 18.
G. M. Veith, M. Greenblatt, M. Croft and J. B.
Goodenough, Mater. Res. Bull., 2001, 36, 1521.
T. D. Boyko, C. E. Zvoriste, I. Kinski, R. Riedel, S. Hering, H.
Huppertz and A. Moewes, Phys. Rev. B: Condens Matter
Mater. Phys., 2011, 84, 085203.
R. Dai, S. Zhang, N. Yin, Z.-C. Tan and Q. Shi, J. Chem.
Thermodyn., 2016, 92, 60.
Q. Shi, C. L. Snow, J. Boerio-Goates and B. F. Woodfield, J.
Chem. Thermodyn., 2010, 42, 1107.
R. D. Shannon, Acta Crystallogr., Sect. A: 1976, A32, 751.
H. A. E. Hagelin-Weaver, J. F. Weaver, G. B. Hoflund and G.
N. Salaita, J. Electron Spectrosc. Relat. Phenom., 2004, 134,
139.
P. R. Norton, R. L. Tapping and J. W. Goodale, Surf. Sci.,
1977, 65, 13.
B. P. Payne, M. C. Biesinger and N. S. McIntyre, J Electron
Spectrosc. Relat. Phenom. 2012, 185, 159.
M. C. Biesinger, B. P. Payne, A. P. Grosvenor, L. W. M. Lau,
A. R. Gerson and R. S. Smart, Appl. Surf. Sci., 2011, 257,
2717.
X. B. Chen and C. Burda, J. Phys. Chem. B, 2004, 108,
15446.
S. Agouram, F. Bodart and G. Terwagne, J. Electron
Spectrosc. Relat. Phenom., 2004, 134, 173.
I. Milosev, H. H. Strehblow and B. Navinsek, Surf. Coat.
Technol., 1995, 74-75, 897.
References
1.
A. E. Ringwood, McGraw-Hill International Series in the
Earth and Planetary Sciences: Composition and Petrology
of the Earth's Mantle, 1975.
30.
31.
2.
3.
F. C. Romeijn, Philips Research Reports, 1953, 8, 304-320.
N. J. Jebarathinam, M. Eswaramoorthy and V.
Krishnasamy, Bull. Chem. Soc. Jpn., 1994, 67, 3334.
G. Vivekanandan and V. Krishnasamy, Hung. J. Ind. Chem.,
1995, 23, 21.
J. Sloczynski, J. Ziolkowski, B. Grzybowska, R. Grabowski, D.
Jachewicz, K. Wcislo and L. Gengembre, J. Catal., 1999,
187, 410.
4.
5.
32.
33.
6.
B. L. Dubey, N. B. Singh, J. N. Srivastava and A. K. Ojha,
Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. &
Anal. Chem., 2001, 40A, 841.
34.
35.
7.
C. L. Honeybourne and R. K. Rasheed, J. Mater. Chem.,
1996, 6, 277.
36.
37.
38.
8.
N. Mufti, A. A. Nugroho, G. R. Blake and T. T. M. Palstra, J.
phys. Condens. matter., 2010, 22, 075902.
C. O. Augustion, D. Prabhakaran and L. K. Srinivasan, J.
Mater. Sci. Lett., 1993, 12, 383.
9.
10.
T. D. Sparks, M. C. Kemei, P. T. Barton, R. Seshadri, E.-D.
Mun and V. S. Zapf, Phys. Rev. B: Condens. Matter Mater
Phys., 2014, 89, 024405.
39.
40.
41.
42.
11.
12.
13.
J. D. Dunitz and L. E. Orgel, J. Phys. Chem. Solids, 1957, 3,
20.
G. Ueno, S. Sato and Y. Kino, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 1999, C55, 1963.
K. Tomiyasu and I. Kagomiya, J. Phys. Soc. Jpn., 2004, 73,
2539.
M. Batzill, E. H. Morales and U. Diebold, Phys. Rev. Lett.,
2006, 96.
14.
15.
E. Prince, J. Appl. Phys., 1961, 32, 68.
H. Ishibashi and T. Yasumi, J. Magn. Magn. Mater., 2007,
310, e610.
43.
44.
S. Q. Xiao and O. Takai, Thin Solid Films, 1998, 317, 137.
I. Milosev, J. M. Abels, H. H. Strehblow, B. Navinsek and M.
Metikos-Hukovic, J. Vac. Sci. Technol., A: 1996, 14, 2527.
G. Soto, W. de la Cruz and M. H. Farias, J. Electron
Spectros. Relat. Phenom., 2004, 135, 27.
C. Palacio, A. Arranz and D. Diaz, Thin Solid Films, 2006,
513, 175.
B. Subramanian and M. Jayachandran, Corros. Eng. Sci.
Technol., 2011, 46, 554.
S.-H. Lee, E. Yamasue, K. N. Ishihara and H. Okumura, Appl.
Catal. B-Environ., 2010, 93, 217.
Y. Cong, J. Zhang, F. Chen and M. Anpo, J. Phys. Chem. C,
2007, 111, 6976.
R. A. R. Monteiro, S. M. Miranda, V. J. P. Vilar, L. M.
Pastrana-Martinez, P. B. Tavares, R. A. R. Boaventura, J. L.
Faria, E. Pinto and A. M. T. Silva, Appl. Catal. B-Environ.,
2015, 162, 66.
M. C. Biesinger, C. Brown, J. R. Mycroft, R. D. Davidson
and N. S. McIntyre, Surf. Interface Anal., 2004, 36, 1550.
Q. Shi, L. Zhang, M. E. Schlesinger, J. Boerio-Goates and B.
F. Woodfield, J. Chem. Thermodyn., 2013, 62, 35.
Q. Shi, L. Y. Zhang, M. E. Schlesinger, J. Boerio-Goates and
B. F. Woodfield, J. Chem. Thermodyn., 2013, 62, 86.
Q. Shi, L. Y. Zhang, M. E. Schlesinger, J. Boerio-Goates and
B. F. Woodfield, J. Chem. Thermodyn., 2013, 61, 51.
E. S. R. Gopal, Specific heats at low temperatures, The
International Cryogenics Monograph Series, 1966.
O. Crottaz, F. Kubel and H. Schmid, J. Mater. Chem., 1997,
7, 143.
16.
M. R. Suchomel, D. P. Shoemaker, L. Ribaud, M. C. Kemei
and R. Seshadri, Phys. Rev. B: Condens. Matter Mater.
Phys., 2012, 86, 054406.
45.
46.
47.
48.
49.
50.
17.
18.
19.
20.
S. Klemme and J. C. van Miltenburg, Phys. Chem. Miner.,
2002, 29, 663.
M. Lenglet, A. D'Huysser, J. Arsene, J. P. Bonnelle and C. K.
Joergensen, J. Phys. C: Solid State Phys., 1986, 19, L363.
M. Tovar, R. Torabi, C. Welker and F. Fleischer, Phys. B,
2006, 385-386, 196.
A. S. Mikheykin, D. Y. Chernyshov, A. A. Bush, A. S.
Prokhorov, Y. I. Yuzyuk and V. P. Dmitriev, Phys. Solid
State, 2014, 56, 785.
21.
A. G. Kochur, A. T. Kozakov, K. A. Googlev, A. S. Mikheykin,
V. I. Torgashev, A. A. Bush and A. V. Nikolskii, J. Electron
Spectrosc. Rela. Phenom., 2014, 195, 208.
M. Ptak, M. Maczka, A. Pikul, P. E. Tomaszewski and J.
Hanuza, J. Solid State Chem., 2014, 212, 218.
B. D. Hosterman, J. W. Farley and A. L. Johnson, J. Phys.
Chem. Solids, 2013, 74, 985.
L. G. Antoshina, A. N. Goryaga and D. A. Chursin, Phys.
Solid State, 2002, 44, 747.
B. L. Morris, P. Russo and A. Wold, J. Phys. Chem. Solids,
1970, 31, 635.
22.
23.
24.
25.
26.
27.
51.
52.
53.
54.
55.
56.
A. V. Powell, D. C. Colgan and C. Ritter, J. Solid State
Chem., 1997, 134, 110.
M. J. Martinez-Lope, M. T. Casais and J. A. Alonso,
Z.Naturforsch.(B), 2006, 61, 164.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 9 of 9
View Article Online
DOI: 10.1039/C6RA22773B
Journal Name
ARTICLE
57.
M. H. Whangbo, H. J. Koo, A. Villesuzanne and M.
Pouchard, Inorg. Chem., 2002, 41, 1920.
58.
59.
J. B. Goodenough and A. L. Loeb, Phys. Rev., 1955, 98, 391.
L. Cianchi, M. Mancini and P. Moretti, Phys. Rev. B: Solid
State, 1973, 7, 5014.
60.
J. B. Goodenough, Magnetism and the chemical bond.
1963.
61.
62.
J. S. Smart, Phys Rev, 1954, 94, 847.
A. Stashans and S. Jacome, Comput. Mater. Sci., 2014, 81,
353.
63.
64.
65.
A. Sundaresan, R. Bhargavi, N. Rangarajan, U. Siddesh and
C. N. R. Rao, Phys. Rev. B, 2006, 74.
D. G. Wickham and J. B. Goodenough, Phys Rev, 1959, 115,
1156-1158.
S. Blanco-Canosa, F. Rivadulla, V. Pardo, D. Baldomir, J. S.
Zhou, M. Garcia-Hernandez, M. A. Lopez-Quintela, J. Rivas
and J. B. Goodenough, Phys. Rev. Lett., 2007, 99.
D. Tobia, J. Milano, M. T. Causa and E. L. Winkler, J. Phys.-
Condes. Matter, 2015, 27.
66.
67.
68.
I. S. Jacobs, J. Phys. Chem. Solids, 1960, 15, 54.
A. N. Goryaga, L. G. Antoshina, A. I. Kokorev and D. A.
Chursin, Phys. Solid State, 2002, 44, 759.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins