Role of compensating Li/Fe incorporation in Cu0.945Fe0.055- xLixO: Structural, vibrational and magnetic properties

Article abstract of DOI:10.1039/c7ra03960c

Doped transition metal oxides, like CuO, are spintronic materials. An increase of magnetic moment has been reported in Fe-doped CuO.1 Additional secondary doping elements such as Li may further modify magnetism in transition metal oxides2,3 due to changes

Full text of DOI:10.1039/c7ra03960c

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Role of compensating Li/Fe incorporation in
Cu0.945Fe0.055ꢀxLixO: structural, vibrational and
magnetic properties
Cite this: RSC Adv., 2017, 7, 31970
Mohd. Nasir,a N. Patra,b Md. A. Ahmed,c D. K. Shukla,d Sunil Kumar,e D. Bhattacharya,b
f
d
b
g
a
*
*
C. L. Prajapat, D. M. Phase, S. N. Jha, Sajal Biring and Somaditya Sen
Doped transition metal oxides, like CuO, are spintronic materials. An increase of magnetic moment has
been reported in Fe-doped CuO.1 Additional secondary doping elements such as Li may further modify
magnetism in transition metal oxides2,3 due to changes in valence state and size. Such complex doping
may generate secondary impurity phases, restricting the success of such applications. Enhancement of
magnetic properties strengthens applicability of the materials in devices. Single phase monoclinic
Cu0.945Fe0.055ꢀxLixO crystalline powders without any impurity are synthesized. Structural studies,
electronic valence state, fine structure, and local geometry of constituent elements confirm the purity of
the materials and provide clues to probable reasons leading to enhanced magnetism. Ferromagnetic
coupling between neighboring spins is most likely dependent on the spin and separation between the
spin species.
Received 7th April 2017
Accepted 26th May 2017
DOI: 10.1039/c7ra03960c
rsc.li/rsc-advances
than the cubic salt structure.14 It consists of irregular Cu–O
chains along the [10ꢀ1] and [101] directions.
1. Introduction
Following the theoretical predictions of Dietl et al.,4 several
efforts have been made to synthesise nanostructured dilute
magnetic semiconductor oxides such as CuO, TiO2, In2O3,
ZnO, SnO2, Fe2O3 etc. by doping with 3d transition metals
because of their novel optical, magnetic and electric proper-
ties.5 Copper oxide has several exotic properties and potential
applications in areas like spintronic devices,6 optoelec-
tronics,7 photocatalysis,8 gas sensing,9 solar energy conver-
sion,10 catalysis,11 electrode materials in Li-ion batteries12 and
supercapacitors.13 As a building block of high temperature
superconductors, a superexchange mechanism between Cu2+
ions coordinated with O in CuO (tenorite) has been widely
discussed. CuO is an antiferromagnet in its ground state
above the three-dimensional phase transition temperature at
TN1 ¼ 229 K and TN2 ¼ 213 K.14 Monoclinic CuO (space group
C2/c) has unique properties among all 3d monoxides since it
has a square planar coordination of copper–oxygen, rather
CuO nanostructures have been synthesized by hydrothermal
process, spray pyrolysis, etc.15 The above techniques usually
require longer reaction time and a larger amount of energy. Sol–
gel technique involves less processing time but produces good
yield. It has better control on stoichiometry, purity and homo-
geneity. Doping makes profound changes in optical, magnetic
and magneto-transport properties of CuO by modifying its
electronic structure. Electrical, optical, excitonic, spectroscopic
and neutron diffraction studies on pristine and doped CuO with
Mn, Ni, Zn, Cd, and Ti16–18 have been studied. A few reports are
also available on ‘Fe’ doped CuO.1,19,20 However, only limited
reports exist on codoped CuO.21,22 Ferromagnetic properties are
less understood in CuO and depend on synthesis method,
annealing and processing conditions. As a result different
researchers arrived at diverse results for CuO nanoparticles. The
changes were attributed to presence of various defects such as
cation or anion vacancies, quantum size effects and uncom-
pensated spins.23,24 Defects are generated due to the differences
in atomic size, valence state and the crystal radius. The differ-
ence of crystal radius generates lattice strain which sometimes
limits the substitution level. Crystal radius compensation may
be done by compensating the shortage by a larger ion giving
support to the structure. By proper choice the changes due
different valence state replacement may also be avoided.
aDepartment of Physics, Indian Institute of Technology Indore, Indore, 453552, India.
E-mail: sens@iiti.ac.in
bAtomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai,
400085, India
cDepartment of Physics, University of Calcutta, Kolkata, 700009, India
dUGC-DAE, Consortium for Scientic Research, Indore, 452017, India
eMetallurgical Engineering and Material Science, Indian Institute of Technology
In our previous study,25 we have shown solid solution of Fe
substituted CuO. Lattice parameters reduced due to Fe3+
substitution. Being a more positive ion than Cu2+, Fe3+ will
increase O-affinity in the lattice reducing O-vacancies and
Indore, 453552, India
fTechnical Physics Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
gElectronic Engg., Ming Chi University of Technology, New Taipei City, 8802, Taiwan
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providing scope of O-interstitial points. For every two Fe3+ morphology was determined using eld emission scanning
substitutions, an oxygen atom interstitial is expected, electron microscope (SUPRA 55 FESEM). Raman spectroscopy
increasing the chances of structural breakdown. Such a struc- was done at UGC-DAE, Indore, using a micro-Raman system from
tural breakdown was observed for Fe content more than 9.7% in Jobin Yvon Horiba LabRAM HR visible (200–800 nm) spectrom-
Fe-doped CuO. Reection peaks such as (110), (201), (020) and eter and an argon 488 nm laser. Magnetic eld and temperature
(113) splitted up indicating that certain planes are preferential dependent magnetization of the samples were examined using
to strain in the lattice and were responsible for modication of Quantum Design SQUID VSM (model SVSM-050).
the structure from a simple CuO to some other structure. These
For XPS measurements, samples were xed on a specimen
splitting vanished by prolonged heating at higher temperatures holder using silver paste. To avoid the surface charging effect,
but several additional peaks appeared belonging to CuFe2O4 the conducting path was provided from bottom to the top
structure. The lattice was at an intermediate condition of surface of the sample. The chamber was then evacuated to
a phase transition or a solubility limit. We want to verify the role a vacuum ꢁ1 ꢃ 10ꢀ9 Torr. Samples were investigated using XPS
of such structural strain and how it can be controlled to increase system of SPECS with monochromatic Al-Ka X-ray (hn ¼ 1486.74
the solubility limit. Hence we have substituted Li1+[IV] ion eV) radiation as the primary radiation (energy resolution of 0.5
(crystal radius ꢁ 0.73 A) in place of Fe3+[IV] (ꢁ0.63 A) in eV). Initially, survey spectra were recorded over a range of 0 to
Cu0.945Fe0.055O system.
1000 eV for each composition. The high-resolution spectra were
It has been reported that magnetic properties of various taken for 2p3/2 level of Fe & Cu and for 1s level of oxygen, lithium
metal oxides can be enhanced with codoping.2,3,22,26,27 The hole and carbon. The energy calibration has been done with respect
and electron doping increases ferromagnetic coupling in Co to the reference (C 1s peak at a binding energy value of 284.6 eV)
and Mn doped ZnO.28 Li incorporation in Fe-doped CuO is for all edges. The so X-ray absorption spectroscopy (SXAS) of
studied as Li does not have d electrons to interact with magnetic the samples at Cu and Fe L-edges and O K-edge have been
ordering.29 Li codoping enhances the ferromagnetic behaviour performed at the SXAS beamline (BL-01) of Indus-2, RRCAT,
in CuO:Fe as it brings the d electron closer to the optimal values Indore in total electron yield (TEY) mode.
for double exchange for various dopants such as Fe, V, Co, etc.
The XANES and EXAFS measurements on the samples at Cu
We discuss the detailed structural and electronic properties and Fe K-edges have been carried out at room temperature at
of the substituted pure phase materials in this report. In the EXAFS beamline (BL-9) at INDUS-2 synchrotron source (2.5 GeV,
present experiment, synchrotron based so X-ray absorption 300 mA) at RRCAT, Indore, India,31,32 operating in the energy
spectroscopy (SXAS) measurements at O K-edges and Cu and Fe range of 4–25 keV with a resolution of 1 eV at 10 keV. To select the
L3,2 edges and X-ray Absorption Near Edge Structure (XANES) X-ray energies a Si (111) double-crystal monochromator is used
and Extended X-ray Absorption Fine Structure (EXAFS) studies with a pair of vertically focusing Rh/Pt coated mirrors for focusing
at Cu, Fe K-edges have been carried out, of the Fe doped CuO and higher harmonics rejection. XANES and EXAFS signals of the
samples. SXAS and XANES measurements have been employed samples were collected in transmission mode at the Cu K-edge
to examine the oxidation state while EXAFS measurements have (8979 eV) and uorescence mode at the Fe K-edge (7112 eV).
been carried out to know the local neighborhood around each Reference standard metal foils of iron and copper were used for
species present in the sample.
each energy calibration. To avoid the Bragg condition, each
sample is oriented in a specic way. It should be noted that
measurements at Li K-absorption edge (55 eV) could not be done
since the above beamline works in the range of 5–25 keV.
For data collection in the transmission mode, the sample is
2. Experimental methods
Nanocrystalline Cu0.945Fe0.055ꢀxLixO (0 # x # 0.055) powders
were synthesized by sol–gel method as previously reported.30 placed between two ionization chambers. The rst ionization
Copper(II) oxide (99.7%, Alfa Aesar, metal basis) iron(III) nitrate chamber measures the intensity of incident ux (I0) and the
nonahydrate (98%, Alfa Aesar), lithium nitrate, anhydrous second ionization chamber is used for measuring transmitted
(99.999% Alfa Aesar, metal basis), citric acid anhydrous (assay ux (It). From these intensities the absorbance [m ¼ It/I0] of the
99% Merck) and nitric acid (assay 69% Merck) were used as sample is found as a function of energy. To improve the signal
precursors. Required amount of precursors were dissolved in DI to noise ratio, an appropriate mixture of gases with optimum
water separately and mixed together. A solution of citric acid pressure were chosen to achieve 10–20% absorption in rst
and glycerol was prepared in a separate beaker to serve as ionization chamber and 70–90% absorption in second ioniza-
chelating agent. The chelating agent was added to the mixed tion chamber. The higher harmonics in the X-ray spectra were
aqueous solution of precursors. The resulting solution was kept rejected by detuning of the second crystal (30%) using the piezo
under constant stirring at 75 ꢂC for 6 h to form a gel which was motor. For measurements in the uorescence mode, the sample
then dried to produce uffy black powders. This black powder is is placed at 45ꢂ to the incident X-ray beam and the uorescence
again ground into a ne powder. Finally, the powder was signal (If) is detected using a Si dri detector placed at 90ꢂ to the
annealed at 450 C for 6 h in air.
incident X-ray beam. The 1st ionization chamber measures the
ꢂ
The crystallinity of the prepared samples was examined using incident X-ray ux (I0) and the absorbance of the sample
If
X-ray diffraction technique (Bruker D2 Phaser X-ray diffractom-
(m ¼ ) is obtained as a function of energy by scanning the
monochromator over the specied energy range.
˚
I0
eter) with Cu Ka radiation having wavelength of 1.54 A. The
scanning was done in the region of 2q from 10ꢂ to 90ꢂ. The
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The adequate amount of samples is mixed thoroughly with competing mechanisms involved. Note that Li1+(IV) has slightly
2+
˚
˚
cellulose powder. The samples were pelletilised into disc of 13 higher crystal radii ꢁ0.73 A (Shannon radii) than Cu (IV) ꢁ 0.71 A
3+
˚
mm diameter to get reasonable edge jumps. The experimental and Fe (IV) ꢁ 0.63 A. The higher charge and lower crystal radius
X-ray absorption data were processed using IFEFFIT soware of Fe3+ facilitates O-content to increase thereby removing oxygen
packages.33
deciency and even introducing oxygen interstitials. On the other
hand the increasing substitution by the larger size but with lesser
charge Li1+ ion tends to reduce the oxygen content and restore the
actual size of the ion at the Cu-site. No doubt this substitution not
only controls oxygen content, but also introduces a lot of combi-
nations of exchange interactions between cations, oxygen and
oxygen vacancies. Such modications and introduction of
different exchange integrals will provide rich magnetic properties
of these materials.
3. Results and discussions
The structural and vibrational studies of Cu0.945Fe0.055ꢀxLixO (0
# x # 0.055) nanoparticles have been examined with XRD and
Raman spectroscopy. Note that the Cu content has been kept
xed while the ratio of Fe to Li content has been varied, i.e. Fe
has been substituted by Li.
Raman spectroscopy was done in the range of 200–800 cmꢀ1
.
X-ray diffraction (XRD) patterns shown in Fig. 1(a) reveal
monoclinic crystalline phase of Cu0.945Fe0.055ꢀxLixO (0 # x #
0.055) nanoparticles. All the samples are single phase. No peak
belonging to impurity phases is detected. Le Bail prole tting of
XRD data was performed. Lattice parameters a, b and c are smaller
for x ¼ 0, i.e. entirely Fe-substitution, than pure CuO. With
nominal amount of Li1+ introduction, for x ¼ 0.012, a, b, and c
increase to values even more than that of pure CuO and thereaer
decrease gradually with increased substitution to values close to
pure CuO [Fig. 1(b)]. The crystallite size decreases with increasing
x and is much smaller than pure CuO and purely Fe-substituted
CuO [Fig. 1(c)]. This emphasizes that Fe and Li are incorporated
in monoclinic structure of CuO. The average crystallite size, D,
(ꢁ40–50 nm) and lattice strain (ꢁ0.48%) is estimated from the
In CuO, 4Au + 5Bu are IR active and Ag + 2Bg are Raman active
modes.34 Raman active modes (Ag, Bg, and Bg) appear at ꢁ293,
342 and 628 cmꢀ1 respectively35,36 for all samples [Fig. 3]. No
extra vibrational modes are observed ensuring the phase purity
and good crystallinity of the samples. We observe a nominal
blue shi in all samples with non-zero values of x. This may be
due to the reduced effective oscillator mass with Li incorpora-
tion. Only for x ¼ 0 we observe a red shi from the pure CuO
which may be related to reduction of oxygen vacancies.
The valence state of the Cu, O, Li and Fe ions of the
Cu0.945Fe0.055ꢀxLixO (0 # x # 0.055) has been examined by high
resolution XPS [Fig. 4]. Adventitious carbon was observed from
the presence of C 1s feature. Nominal changes are observed in
the binding energies of the Cu 2p3/2 (933.75 eV) and Cu 2p1/2
(953.23 eV) peaks [Fig. 4(b)] in all the substituted samples,
consistent with previous reports.35,36 Here, we have shown XPS
spectra for x ¼ 0.027. A satellite peak is observed at ꢁ8.88 eV
above the major peak (Cu 2p3/2). This peak is characteristic of
materials like copper halides, having open shell 3d9 L (L for
ligand) conguration,37 i.e. Cu exists as Cu2+ state with an
incomplete d-shell. The incomplete d-shell leads to the satellite
peak. The full lled d-shell as in Cu2O containing Cu1+ ions is
not associated with a satellite feature [Fig. 4(b)]. Thus presence
of any Cu1+ phase in our sample is ruled out.
ꢀ
FWHM of prominent diffraction peak (111), using Scherrer
equation aer subtracting the instrumental broadening, D ¼ 0.9l/
˚
˚
b cos q [where, l(Cua1) ¼ 1.5406 A, l(Cua2) ¼ 0.15418 A, b ¼ line
broadening at half the maximum intensity (FWHM) and, q ¼
Bragg diffraction angle]. FESEM images of Cu0.945Fe0.055ꢀxLixO (0
# x # 0.055) powders [Fig. 2(a)–(d)] reveal particles of sizes ꢁ100–
200 nm and is likely to be composed of several crystallites. With
increasing x, the morphology of samples change from elongated
to spherical ones. The strain reduced from pure CuO to x ¼ 0.027
sample and increased somewhat thereaer. The strain and
disorder created due to the substitution of Fe by Li has two
Fig. 1 (a) XRD spectra of Cu0.945Fe0.055ꢀxLixO (0 # x # 0.055) ensuring pure phase for all substitutions, (b) displays the variation in lattice parameter
“a”, “b” and “c” (in unit A˚) estimated from Le Bail profile fitting, and (c) variation in crystallite size and strain with increasing substitution, x.
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Fig. 2 Represents the FESEM images of Cu0.945Fe0.055ꢀxLixO (a) x ¼ 0, (b) x ¼ 0.027, (c) x ¼ 0.043, and (d) x ¼ 0.055.
O2ꢀ ion in CuO.38,39 The peak appearing at 532.4 generally
corresponds to the adsorbed H2O, or O2.38,39 However, a peak at
531.4 eV has also been observed which is related to oxygen
decient CuO materials.39 Hence, we may say that O2 is less
likely to be adhered to the surface and is also decient in the
lattice. Chances of H2O adsorption cannot be ruled out from
these data. A peak ꢁ56 eV [Fig. 4(e)] is in proximity to Li 1s edges
reported in other Li containing materials.40,41 Interstitial Li ions
in CuO are reported as a signicant peak at 52 eV.40,41 We do not
see any signature of such a feature in our samples. This
emphasizes that Li has substituted Cu in the lattice.
So X-ray absorption spectroscopy (SXAS) of Cu0.945
-
Fe0.055ꢀxLixO (0 # x # 0.055) samples at the Cu 2p, Fe 2p and O
1s, was performed to investigate the valence state, density of
empty/partially lled electronic states and local geometrical
structure of the samples.42
We observe two double peaks matching considerably with
the peak prole and position of Fe L3 (2p3/2 / 3d) and L2 (2p1/2
/ 3d) edges of Fe2O3 [Fig. 5(a)] separated by 20 eV.43 The
double peaks of both the L2 and L3 features, are each composed
of a low intense peak, A1 (Fe 2p – t2g) and a main peak, A2 (Fe
2p3/2 – eg).44 To be noted that the XAS data of FeO contains only
one peak. Thus the double peak nature is a certain signature
that Fe is in Fe3+ state. However, small changes in energy
positions of the double peaks hint at probable minor deviations
in the oxidation state.
A double peak pre-edge feature due to O 1s / hybridized
O(2p)-metal (3deg/t2g) states is reported in Fe2O3;45 S1 (O 1s /
Fe 3d – t2g) at ꢁ531 eV and, S2 (O 1s / Fe3d – teg) at
ꢁ533.6 eV.45,46 The pre-edge is followed by a combination of
broad peaks in the range R ꢁ 535–555 eV (O 1s / hybridized O
(2p)-metal (4s/4p) states.42,45 For the pure CuO sample the XAS
data consist of one pre-edge peak, S1 (ꢁ532.6 eV), and R,
Fig. 3 Room temperature Raman spectra of Cu0.945Fe0.055ꢀxLixO. The
Raman shift is observed in all Raman modes with increase in x.
A Fe 2p doublet (Fe 2p1/2 and Fe 2p3/2) is observed at ꢁ723 eV
and ꢁ710 eV, respectively in the Fe 2p spectrum [Fig. 4(c)] of all
the Cu0.945Fe0.055ꢀxLixO samples. Due to spin–orbit coupling,
splitting of 12.7 eV is observed in Fe 2p spectra. A shake-up
satellite feature is observed at about 717.59 eV which is in
close agreement with available literature, indicating that Fe is in
+3 valence state.35 There is no peak in the region 706–707 eV,
indicating the absence of metallic Fe.
A broad peak corresponding to O 1s [Fig. 4(d)] was tted with
three components, at 532.7, 531.4, and 530.13 eV. The lower
binding energy peak positioned at 530.13 can be attributed to
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Fig. 4 (a) The full survey spectrum, fitted XPS spectra at the (b) Cu 2p (c) Fe 2p (d) O 1s and (e) Li 1s edges of Cu0.945Fe0.027Li0.027O.
a region of six other peaks [Fig. 5(b)]. For the substituted Fe substitution, containing no Li) the S1 feature moves to
Cu0.945Fe0.055ꢀxLixO samples we see similar spectra for most of a lower energy value at par with the S1 value of Fe2O3. Addi-
the samples and the second pre-edge peak, S2, is missing. Note tionally, a negligibly small hump is observed in close proximity
that with the incorporation of Li (x ¼ 0.055) the S1 peak position to the position of S2, but not exactly at the S2 position of Fe2O3
moves towards slightly higher values suggesting changes in the (ꢁ533.6 eV). Due to the extra charge of Fe3+, oxygen vacancies
t2g level of Cu 3d state. Fe is introduced in x ¼ 0.027, 0.043 are reduced. These extra electrons will be available in the eg
substituted samples, but the S1 value remains unchanged. The levels, thereby a small hump is visible at S2.
S1 level is highest for x ¼ 0.012. Thereaer, for x ¼ 0, (i.e. 5.5%
Fig. 5 XAS spectra of Cu0.945Fe0.055ꢀxLixO at (a) Fe L2,3-edge with reference to Fe2O3, shows predominantly Fe3+ state, and (b) the O K-edges
ensure the phase purity of all samples.
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The R region shows [Fig. 5(b)] multiple peaks ꢁ6, corre- report, but with minimal Li incorporation of x ¼ 0.012 the
sponding to O 1s / hybridized O 2p-M (4s, 4p) states. There is changes in the intensity was observed. The intensity increased
remarkable resemblance with the CuO features,47 rather than further in x ¼ 0.027 and thereaer started to reduce gradually
any Fe oxide feature. However, the peak positions are not exactly but was higher than the pure CuO intensity. Hence, from this
same as pure CuO. The small changes in the binding energy result it seems that 1s / 4p transition probability increases in
further conrm substitution and hint at larger separation of Cu ions as Li is introduced and reaches maximum at x ¼ 0.027.
energy levels with Li incorporation. The lesser crystal radius of This trend is even better observed for the 1s / (4s4p) transi-
Fe3+ helps in reducing strain in the lattice. But it also introduces tion. The shoulder of the absorption edge of the x ¼ 0.027
more oxygen. With the gradual incorporation of larger size Li1+ sample is entirely different from the other samples. To under-
ion the trend tries to reverse and strain increased. These types stand better we derived the rst derivative of the edges. The
of changes in local strain result in changes in energy levels. derivative shows one minima in between two maxima, the
However, one must also recollect that the charge of Li1+ is lesser minima corresponding to the energy of transition. It seems that
than Fe3+ and Cu2+. Thereby the oxygen retentive properties will the x ¼ 0.027 sample has the most transition probability for the
be less and the structure will start generating strain due to 1s / 4s4p transition.
lattice disorder. Note that for the x ¼ 0 sample, the binding
energy has decreased but have increased with even nominal Fe0.055ꢀxLixO samples has been compared with FeO, Fe2O3 and
substitution of Li1+.
Fe metal foil [Fig. 6(b)]. The spectra of all the samples are
The normalized Fe K-edge XANES spectra of Cu0.945-
The normalized Cu K-edge XANES spectra [Fig. 6(a)] of identical to that of Fe2O3. However, the edges shied nominally
Cu0.945Fe0.055ꢀxLixO (0 # x # 0.055) samples are almost iden- to relatively lower values. This suggests the possibility of Fe
tical with that of pure CuO. A small change in the edge shape being predominantly in Fe3+ state with nominal Fe2+ presence
and position is observed with Li and Fe incorporation in host in all samples. A quantitative analysis of the normalized XANES
lattice. In the CuO K-edges XANES spectra mainly two regions spectra revealed almost 80–85% Fe3+ oxidation state. A very
appear.48,49 A shoulder in the energy range of 8982–8986 eV weak pre-edge feature occurs around the energy of 7112 eV
appears and is attributed to a forbidden 1s / 4s transition but characteristic of Fe3+ (3d5) ion. This corresponds to a transition
allowed due to mixing of 4s and 4p orbitals. This feature is involving (a) electric quadruple transition, (b) hybridization of
characteristic of Cu2+
.
The main feature appears around the vacant d orbital and (c) the presence of distorted octahedral
50
9000 eV is due to the allowed 1s / 4p transition which merges structure or the little presence of non-centrosymmetric tetra-
to the continuum.51 No pre-edge peak appears in the XANES hedral sides.56 In an ideal octahedron where we expect electric
spectra in agreement with available literature.52–55 This suggests dipole moment contributions only such transitions are not
that Cu ions in Cu0.945Fe0.055ꢀxLixO samples remain in octahe- seen. Thus the change in the intensity and width in the pre-edge
dral symmetry.55 To be noted that the energy of the maxima of region is the signature of change in the centrosymmetric nature
the white line peaks [Fig. 6(a)] initially decreased and thereaer around the absorbing atom. Note that the pre-edge becomes
increased with increase of Li content. These peaks, in the co- stronger with substitution indicating a stronger distortion in
doped samples become more intense with substitution. the tetrahedral which indicates possibility of a stronger polari-
Unfortunately we could not provide the x ¼ 0 sample in this zation of the materials [Fig. 6(b)]. Beyond this pre-edge, the
Fig. 6 Normalized XANES spectra at (a) Cu K-edges, and (b) Fe K-edges for Cu0.945Fe0.055ꢀxLixO samples showing predominantly Fe3+
substitution at Cu2+ site.
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main absorption peak around 7127 eV appears due to the 1s / tting of the Fe K-edge spectra in the 1st co-ordination shell (Fe–
4p electric dipole transition. There is not much change in the O) same assumption has been taken and it also shows the dis-
white line intensity between the doped and co-doped samples torted octahedral structure which agrees with the XANES
unlike the case of Cu K-edge XANES.
spectra at the pre-edge region. The second intense peak
˚
The local atomic structure of Cu0.945Fe0.055ꢀxLixO was between 2 and 2.5 A in the c(R) versus R plots appears due to
investigated with X-ray absorption ne structure (EXAFS) at the scattering from the 8 Cu/Fe atoms at the 3rd shell at a nominal
Cu and Fe K-edges. The k2-weighted Cu/Fe K-edge EXAFS distance of 2.90 A from the absorbing core atom as also seen in
˚
oscillations and the corresponding Fourier transforms are dis- case of Cu K-edge data.
played in Fig. 7(a). The similarities of the c(R) data of the Cu and
Hence bond distances for both Cu–O and Cu–Cu shells
Fe K-edges data [Fig. 7(b)], reveal the similarity in local envi- increase with increasing Fe content or decreasing x i.e. Li
ronment in between a Cu and Fe site. The fact that this data content [Fig. 8(a)]. The reason for increment in bond distances
matches along with that of pristine CuO, manifests that the is the smaller crystal radii of Fe3+(IV) ꢁ 0.63 A than Cu2+(IV) ꢁ
˚
˚
structure of CuO is retained in the substituted samples. This 0.71 A. Similar variation in bond lengths were reported in Ni
indicates that Fe3+ ions are substituting Cu2+ ions without doped ZnO due to difference in crystal radii of Ni2+ and Zn2+
.
57
signicant distortions in the neighboring shells. This also rules So from variations in bond distances, it can be assumed that Fe
out presence of interstitial Li ions consistent with the XRD, goes to host CuO lattice as Fe3+. Also, we notice a decrease in
SXAS, XPS and XANES results.
atomic distances in Fe–O and Fe–Cu shells due to the reduction
Two dominant peaks, one at ꢁ1.5 A (Cu–O scattering path) in lattice parameter because of the smaller crystal radius of Fe3+
˚
˚
˚
and the other ꢁ2.4 A (Cu–Cu/Fe coordination shell) occur at ions (0.63 A). However, the atomic distances of Fe–O and Fe–Cu,
both Cu and Fe K edge. Theoretical simulation has been done show an expansion compared with that of Cu–O and Cu–Cu in
using structural parameters (lattice constants, Wyckoff posi- the pure CuO. This is probably due to change in the charge and
tions) of monoclinic CuO (space group – C2/c) structure. The crystal radii of two species due to Li incorporation.
theoretical c(R) vs. R plot along with corresponding scattering
paths were generated as previously described model for CuO.25 [Fig. 8(b)] around the central Cu atom with an increase in Fe
There is an increment of C. No. of the 1st oxygen shell
The set of EXAFS data analysis soware available within IFEF- doping concentration and decreasing Li content to maintain
FIT package have been used for EXAFS data analysis.33
the charge balance with reduction in oxygen vacancies. It
The 1st co-ordination shell having 4 oxygen atoms at 1.95 A should be noted that even at the lowest doping concentration
˚
distance contribute to this scattering path and 8 Cu/Fe atoms at Fe, the oxygen C. No. in the rst shell is less than that of
rd
˚
2.90 A distance in the 3 co-ordination shell contribute to the theoretical values for monoclinic CuO (which are 4 for the Cu–
next scattering path while the 2 oxygen atoms between these co- O1 shell). This is due to the reason that oxygen vacancies exist in
ordination shells at 2.78 A distance contribute very little to this pure CuO36 as native defects. However, no change in C. No. of
˚
and has higher s2 value which is a clear evidence of having the Cu/Fe–O2 and Cu/Fe–Cu1 shells is observed. The gradual
distorted (Jahn–Teller distortion) octahedral structure. For increase of Cu coordination with increasing Fe concentration is
Fig. 7 (a) k2 c(k) spectra at Cu and Fe K-edges of Cu0.945Fe0.055ꢀxLixO for (0 # x # 0.055) alternatively, and (b) experimental c(R) versus R plots
along with the theoretical fitted plot of Cu0.945Fe0.055ꢀxLixO at both Cu and Fe K-edges respectively. c(R) plots emphasise on similar environment
around both Cu and Fe edges. The c(k) and c(R) data have been vertically shifted for clarity.
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Fig. 8 The variation of (a) bond distances, (b) coordination numbers (C. No.) and (c) Debye–Waller factor (s2) with substitution, x. The best fit
parameters hint on the proper substitution of Fe/Li in CuO.
attributed to the replacement of Cu2+ ions by Fe3+ ions. The magnetic moment than Cu2+–Cu2+. Our investigations (to be
Debye–Waller (s2) factor of the Cu/Fe–O1 bond is found to be published elsewhere) in only Fe-doped samples reveal increasing
quite low as compared to Cu/Fe–Cu shells which shows lesser magnetization with increasing iron content. However, in
disorder around the central atom due to the incorporation of x ¼ 0.012 magnetic moment is increased drastically to 0.075 emu
a foreign cation in the lattice. This emphasize on the fact that gꢀ1. This is due to expansion of lattice parameters with nominal
the 3rd coordination shell (metal–metal) are signicantly Li1+ introduction. Hence lattice expansion leads to stronger
affected by the doping.
ferromagnetic coupling. This is evident from the fact that with
Field dependent magnetism (M–H studies) studies for further Li1+ substitution, the lattice parameters decrease, thereby
Cu0.945Fe0.055ꢀxLixO (0 # x # 0.055) samples at room temperature decreasing the magnetization. To be noted that Li is replacing Fe,
[Fig. 9(a)] seems weak ferromagnetic in nature. The hysteresis hence for higher x values Fe contribution is less. Finally, for x ¼
loops at room temperature change with substitution. Compared 0.055, i.e. only Li1+ substitution, the lattice parameters are even
to pure CuO (remnant magnetization ¼ 0.0028 emu gꢀ1), the smaller than pure CuO, thereby the magnetic moment is lower
magnetic moment increases in x ¼ 0 (0.0097 emu gꢀ1) [Fig. 9(b)]. than pure CuO. Since Li1+ ion does not have its own d-electrons to
With ve d electrons Fe3+ ion has total spin, S ¼ 5/2, whereas Cu2+ interfere with magnetic ordering thereby Li1+ by itself does not
ion has 1/2. Fe3+–Cu2+ exchange interaction results in larger contribute to enhance magnetization.
Fig. 9 (a) M–H plot at room temperature showing the hysteresis loop for Cu0.945Fe0.055ꢀxLixO nanoparticles, and (b) the variation of remnant
magnetization (Mr), and (e) coercivity (Hc) with increase in additional Li doping.
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In a similar study on Li co-doping in ZnO 2, it has been
observed that substitutional Li induces a non-spin polarized s-
like state. Moreover, electron deciency at oxygen in the
neighborhood of Li induces a charge transfer from electrons at
the Fermi level linked with minority spin of late transition
metals, thereby increasing the local magnetic moments of late
transition metals. Thus, Li co-doping enhances the long range
ferromagnetic ordering as it brings the d electron or TM closer
for double exchange to be dominated for TM dopants.2
Initially for nominal doping of Li we do see an increment in
the magnetic properties which may agree with this explana-
tion. But, in higher Li-doped samples we should expect
a higher magnetization due to more Li-assisted exchange
mechanisms. However, we see magnetization to decrease. This
emphasizes that most probably magnetization is not a Li-
mediated but is more dependent on the ionic spin and
separation.
Acknowledgements
The authors gratefully acknowledge IIT Indore for supporting
the project and providing the XRD facility. We gratefully
acknowledge Dr Vasant Sathe for providing access to Raman
spectroscopy. Md. Nasir is also thankful to UGC, New Delhi, for
providing Maulana Azad fellowship. One of authors (S. Biring)
acknowledges nancial grant received from MOST, Taiwan
(105-2218-E-131-003).
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