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Chapter-5
Synthesis, characterization, microstructure, optical and magnetic
properties of strontium cobalt carbonate precursor and Sr 2Co2O5 oxide
material
5.1 Introduction of Sr2Co2O5
Brownmillerite materials have frequently attracted enormous attention for
applications as sensors, substrates, and catalytic electrodes, and are also promising
candidates for advanced microelectronic, optoelectronic, and spintronic devices.
Compared to ferrite based materials, pervoskite related cobaltites are characterized by
high electrochemical activity, faster oxygen –ionin and p-type electronic conductivities
and greater oxygen permeability [Eirin Sullivan et al., (2012); V.V. Vashook et al.,
(1997); F. Lindberg et al., (2004)]. Cobaltite cathode exhibits a superior electrochemical
performance with respect to common electrode materials. Sr2Co2O5 has two
crystallographic forms (Brownmillerite) and (2H- related hexagonal pervoskite) the
existence of which depends on the spin configuration of cobalt. The alternating CoO6
octahedra and CoO4 tetrahedra sub-layers stacked along the c-axis, forming
1Dimensional oxygen vacancy like channels [A.M. Abakumov et al., (2004); JeanClaude Grenier et al., (1986)]. The overall Cobalt valence state is 3+ with 3d6 electrons,
producing an electronically insulating ground state. The optical properties and electronic
structures of Sr2Co2O5 were found to be highly sensitive to change in the oxygen
stoichiometry, exhibiting a metal-insulator transition (MIT) [T. Nagaia et al., (2006); C.
de la Calle et al., (2008)]. The crystalline and magnetic structure of Sr 2Co2O5 was
studied by Takeda etal. The octahedral and tetrahedral Co 3+ ions are in high spin state
leading to g-type antiferromagnetic structure. Sr2Co2O5 is stabilized by high spin(HS)
Co3+ cations in octahedral and tetrahedral coordination both sites have four unpaired
electrons with configurations t 42ge2gand e3t23,this represented by a typical sequence of
polyhedral “OTOTOTOT”. The brownmillerite-type polymorph of Sr2Co2O5 is
metastable and only formed by quenching from temperatures above to 910°C [Wu HaiPing et al., (2009); Lassi Karvonen et al., (2011)]. The solid state process has more
disadvantage as high reaction temperatures, low chemical homogeneity and long time
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process. The disadvantage of sol-gel method as high temperature and less homogeneity.
By wet-chemical process, offers advantages of good mixing of the starting materials
leads to excellent chemical homogeneity of the final product [Ondrej Jankovsky et al.,
(2014); Zhang Gaoke et al., (2006); K .Vidyasagar et al., (1984)].
5.2 Structure of Sr2Co2O5
The brownmillerite type structure A2B2O5 represents one of the most common
types of ordering of oxygen vacancies in oxygen-deficient perovskites. The structure can
be described as consisting of alternating layers of BO6 octahedra and BO4 tetrahedra,see
Fig. 5.1. It is found among complex first-row transition metal oxides, Sr2Co2O5 [T.
Takeda et al., (1972)] and copper LaSrCuGaO5 [J.T. Vaughey et al., (1991)]. Depending
on the nature of the B cation and the synthesis conditions used,compo unds with
brownmillerite structure generally crystallize with one of three space group symmetries:
Ibm2, Pcmn and Icmm. The first two space groups represent ordered variants of the
structure,which differ from each other by the relative orientation of the chains of
tetrahedra in different layers.
Fig.5.1. The Brownmillerite Structure
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The structure in space group Icmm comprises incomplete ordering of the chains
within the tetrahedral layers. One example of an intermediate between the ordered
structure in space groups Ibm2, Pcmn and the disordered in space group Icmm is the
recently reported compound Ca2(Co,Al)O5 [ S. Lambert et al., (2002)],with a modulated
brownmillerite type structure due to long range ordering of the chains of tetrahedral
[F.Lindberg et al., (2004)]. Among brownmillerites of transition metals there are
examples of compounds with Ca or Sr cations in the A sublattice demonstrating
distinctions in behavior upon the variation of composition or formal valence of the
transition metal. orthorhombically distorted structure with a Co 3+ cations tend to adopt a
Jahn Teller distorted octahedral environment which displays reasonable oxygen nonstoichiometry. Attempt has therefore been made here to synthesize strontium cobaltite via
oxalate based and investigate its formation optical absorption and magnetic behavior.
5.3 Physical Properties of Sr2Co2O5
The strontium cobaltite is reported to be ferromagnetic in nature with Curie
temperature of 222 or 290K. On the other hand, SrCoO 3− ( = 0.5 or Sr2Co2O5) phase
depicts antiferromagnetism with Neel temperature of 545K or 570K [J.C. Granier et al.,
(1986)]. The magnetic nature in fact depends on oxygen stoichiometry and relative
amounts of Co3+ and Co4+ ions with their spin states high, intermediate or low. SrCoO2.5
with brownmillerite-type orthorhombic structure exhibits (i) antiferromagnetism with
Neel temperature of 570 K, (ii) Co 3+ ions in high spin state, and (iii) strong super
exchange interaction via O2− ;exchange integral (Jex) being 28.5kB with kB as the
Boltzmann constant. The domain wall in the brownmillerite-type structure with Z –
contrast imaging and electron energy loss spectroscopy (EELS) in the scanning
transmission electron microscope (STEM) and showed cobalt atoms on octahedral and
tetrahedral sites to be in 4+ and 2+ states, respectively. Moreover, Co–O columns in
tetrahedral sites are oxygen deficient with ordering of the anion vacancies. Structural
relaxation of the CoO6 octahedra occurs by reduction of the Co 4+ to Co3+ ions with
creation of extra oxygen vacancies. According to (H. Taguchi et al., 1976), SrCoO2.97
possesses spontaneous magnetization of 34 emu/g (or 1.18μ B per cobalt atom) at 77K due
to one unpaired electron of Co 4+ ion in low spin eg0t2g5 state. SrCoO3 to be a ferromagnet
with Co4+ ions in low spin state and Curie temperature of 222 K. Also, oxygen content
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(i.e., -value) in SrCoO3− strongly influences the magnetic parameters. For example,
increase in ı from 0.05 to 0.26 leads to (i) emergence of Co 3+ (low spin) at the cost of
Co4+ (low spin) ions and (ii) linear decrease of Curie temperature (Tc) from ∼212 to∼178
K, (iii) magnetic moment from ∼1.5 to 0.6 when extrapolated to 0K, and (iv)
paramagnetic Curie temperature ( ) from ∼270K to 216 K. It is weakly ferromagnetic
with two Curie temperatures (TC) 750 and 790 K; the coercivity and remanent
magnetization at room temperature being 70Oe and ∼9×10−4 emu/g, respectively. The
optical absorption occurring at 4.12, 1.90, 1.58, 1.23 and 0.87 eV can be attributed to
ligand metal charge transfer (LMCT) from O2− to Co3+, Co3+ octahedral d–d metal metal
charge transfer (MMCT) [S.K. Jaiswal (2011)].
In this present report discuss the solid solution carbonate precursor method for
preparation of Sr2Co2O5. The incorporation of divalent Co2+ cations during the
precipitation of barium carbonate (strontionite mineral name). The researchers report a
distribution coefficient for Sr2+ from an aqueous solution into Cobalt carbonate
(spherocobaltite) in calcite structure. The crystallization mainly depends on the
physiochemical conditions of precipitate formation. strontionite form of Sr2Co2(CO3)4
has orthorhombic crystal system formed during precipitation synthesis. This carbonate
precursor further decomposed to Sr2Co2O5. By this precursor method, Sr2Co2O5 has not
yet been reported. The advantage of precursor method is maintaining the metals ratio in a
homogenous form compared to other ceramic methods. The SEM micrographs reveal the
rod shape carbonate precursor and diffused sphere shape oxide product. For optical band
gap, it reveals the precursor as insulating material and the Sr2Co2O5 as semiconducting
nature. The room temperature magnetic study indicates the carbonate precursor as
paramagnetic but its oxide Sr2Co2O5 as superparamagnetic behavior.
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5.4. Materials and Methods
5.4.1. Experimental part
Sr2Co2O5 synthesized by using a co-precipitation carbonate precursor method.
The starting materials were of analytical-grade Sr(NO3)2.4H2O, Co(NO3)2.6H2O and
anhydrous sodium carbonate. The stoichiometry ratios of metal nitrates dissolved in
deionized water under constant stirring at 80°C. Then 2M sodium carbonate as a
precipitating agent was dripped slowly into the mixed salt solution under mild stirring
until the pH value of the salt solution reached 8.5. The suspension stirred for 30 minutes
to attain homogenization. The purple color precipitated carbonate washed repeatedly with
hot deionized water and dried at room temperature. The colorless filtrate tested for
residual metal ions to determine the complete precipitation. This strontionite single phase
carbonate precursor then heated at 800°C for 6h to obtain the single phase Sr2Co2O5
product by removing carbonaceous material.
5.4.2. Characterization Technique
Thermogravimetry differential thermal analysis of precursor was carried out with
a SDT Q600 V20.9 model. The powders were characterized by X-ray diffraction
technique (XRD, Bruker D8 Advance, by CuKα radiation, kα = 1.5406Å). FTIR
spectrums were examined using JASCO 400 Infrared spectrometer. The surface
morphology and the microstructure were studied by a scanning electron microscope
(HRSEM FEI Inspect F50). The chemical composition was determined with an EDX
analyzer attached with a (HRSEM FEI Inspect F50) instrument. Optical band gap
obtained by using Jasco V-670-UV-Visible diffused reflectance spectrometer. Hall
measurement by using a Four probe measurement setup with a field of 8T and a current
of 4A in room temperature. The magnetic properties were analyzed by VSM (Lakeshore
VSM 7410).
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5.5 Results and Discussion
5.5.1. Thermal analysis
Thermogravimetry and differential thermal analysis carried out from room
temperature to 800°C in an oxygen atmosphere at a heating rate of 4°C/min. The
decomposition mechanisms of strontium cobalt carbonate precursor and product
formation investigated by TGA-DTA. Fig.5.2. shows the thermal analysis of strontium
cobalt carbonate precursor. The endothermic peak at 84°C small weight loss due to the
evaporation of surface adsorbed water. The two endothermic peaks at 213°C and 310°C,
it loses two molecules of carbon dioxide then it forms oxycarbonates. Further increase the
temperature to 751°C small endothermic peak reveals the decomposition of oxycarbonate
to oxide. The stable product Sr2Co2O5 formed at a temperature of 800°C. The overall
weight loss 24.94% and phase identification by XRD it proves the product formation.
O2
Step-1
Sr2Co2(CO3)4
Sr2Co2O2(CO3)2 +2 CO2
250-350°C
Step-2
Sr2Co2O2(CO3)2
O2
Sr2Co2O5 + 2CO2
750-800°C
Overall decomposition step
Sr2Co2 (CO3)4
(Purple color)
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Sr2Co2O5 + 4CO2
800°C
(Black color)
Fig.5.2. Thermal analysis of strontium cobalt carbonate precursor to Sr2Co2O5oxide product
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5.5.2 XRD analysis
Fig.5.3. Shows the XRD pattern of the product calcined at various temperatures.
The carbonate precursor calcined at 100-250°C shows the presence of corresponding
strontium carbonate mineral named as strontianite. After calcination at 650°C for 2 hours,
SrCO3 phases and Co3O4 detected. Further increase the temperature to 800°C the XRD
pattern proves the Sr2Co2O5 single phase formed as orthorhombic crystal system and its
lattice parameters a =b =5.5883Å, c =4.8183Å are consistent with those reported in
JCPDS (card No 040-0874).
Using the XRD diffraction data, the crystallite sizes of samples were able to be estimated
using the Scherer equation (5.1).
(5.1)
In this equation, D is the crystallite size (nm); K is the so-called shape factor, which
usually takes a value of about 0.9; λ is the X-ray wavelength; β is the full width at half
maximum of the diffraction peak at
is the diffraction angle. Since the peak from the
most intense among all of the planes of Sr2Co2O5. The plane (300) diffraction selected to
calculate the average crystallite size as 150-300nm.
5.5.3 FTIR Studies
Fig.5.4. Shows the FTIR spectra recorded for the carbonate precursor and product
calcined at different temperature. As illustrated in two bands at 3480 and 1630cm-1 can
be assigned to absorbed water which decreases with calcination, when precursor calcined
at 250°C, two bands appear at 1440 and 850cm-1 assigned to the stretching vibration of
carbonate peak it reveals the strontianite carbonate mineral. From the spectra illustrated
at 800°C that the carbonate structure decomposed and the band at 1440 and 850cm-1 peak
intensity decrease. After calcination at 800°C, the bands appear at 690cm-1 and 520cm-1,
440cm-1 due to the asymmetric stretching vibrations of Sr-O and Co-O groups.
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Fig.5.3.XRD patterns of Sr2Co2O5
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Fig.5.4. FTIR spectra of carbonate precursor and Sr2Co2O5
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5.5.4 SEM Micrograph analysis
Fig.5.5. (a) & (b) Shows the SEM image of Strontium carbonate precursor and
Sr2Co2O5 particles prepared by the carbonate precursor method. The figures represent
several magnifications. In order to investigate the morphology evolution of the precursor
samples, the material shows a micro porous combined rod like morphology and the
product oxide samples shows a diffused micro porous sphere like morphology composed
of agglomerated spheres of different grains of good connectivity between the grains.
5.5.5 EDX analysis
Fig.5.6. Shows the energy dispersive X-ray spectroscopy (EDX) used to quantify
the elements exist in the prepared sample by taking a selective portion of SEM image in
the form peaks. The EDX pattern confirms the findings of XRD pattern and shows the
combination of Sr2Co2O5. The elemental composition listed in the Table-5.1.
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Fig.5.5. Higher and lower magnifications SEM image of (a) strontium carbonate precursor and
(b) Sr2Co2O5
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Table-5.1.EDX composition of Sr2Co2O5
Element
Wt%
At%
OK
17.04
48.97
CoK
29.44
22.96
SrK
53.52
28.08
Matrix
Correction
ZAF
Fig.5.6. EDX spectra of Sr2Co2O5
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5.5.6 Diffuse Reflectance spectroscopy studies
Fig.5.7. Shows the UV-Visible absorbance spectra for Strontium cobalt carbonate
precursor contains major peaks at wavelengths of 210, 245, 334 and 376nm corresponds
to carbonate mineral. The oxide Sr2Co2O5 product contains major peaks at wavelengths of
509nm due to the presence of Sr-O, Co-O metal oxygen bonds. The relationship between
the absorption coefficients α is an incident photon energy hυ. In order to calculate the
optical band gap of sample by using Tauc’s relation in equation (5.2)
(αhυ)n = A(hυ−Eg)
(5.2)
where α denotes the absorption coefficient, A is constant, Eg is band gap and
exponent n depends on the type of transition. The optical band gap calculated by using
Tauc relation by plotting (αhν)2 against h where α and hν denote the absorption
coefficient and photon energy respectively and by extrapolating the curve to photon
energy axis. The band-gap energy (Eg) values evaluated using Tauc’s plot as shown
Fig.5.8. Optical band gap properties of carbonate precursor and Sr2Co2O5 oxide product
studied by UV-Visible DRS absorption spectra. The optical absorption plot of carbonate
precursor shows wide energy band gap as 4eV due to the ligand to metal charge transfer
(LMCT) takes place from O2- to octahedral Co2+, it reveals the insulating property.
The optical absorption band gap of oxide product at 2eV due to ligand to metal
charge transfer (LMCT) takes place from O2- to mixed oxidation state of cobalt as
(Co2+&Co3+). Hence d-d charge transfer takes place in the strontium cobalt oxide system
it shows a semiconducting behavior. For the sample under investigation, the semi
conducting process can be attributed to the presence of two types of charge carriers, that
is, p type, as a hole exchange between Co 4+, Co3+ and Co2+ and transfer of O2− between
filled side with vacant oxygen side. The following Eq. (5.3), (5.4) and (5.5) can explain
the mechanism as follows.
Co4+
Co3+ + h*
(5.3)
Co3+
Co2+ + h*
(5.4)
1/2O2 + V2*
O2-
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(5.5)
Fig.5.7.UV-absorbance spectra of carbonate precursor and Sr2Co2O5
Fig.5.8. UV-DRS band gap of carbonate precursor and Sr2Co2O5
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5.5.7 Four probe Hall effect measurements
Hall effect measurements performed with vander Pauw geometry using a Four
probe measurement setup with a field of 8T and a current of 4A in room temperature.
Sr2Co2O5 sample pressed into a pellet with a diameter of 12mm and sintered at 800°C for
12 hours. The pellet used as a sample for four probe hall measurement. The contacts
between the sample pellet and Au lead wires made by modified analytical grade silver
paste. The contact resistance between the sample surface and each probe was less
than100 Ω.
The resistance of the material (R0) measured by using the equation (5.6&5.7)
(5.6)
(5.7)
where R1 is the resistance when voltage is measured across the terminal 3 and 4, then
current is passed between terminal 1 and 2. R2 is the resistance when voltage is measured
across the terminal 1 and 2, then current is passed between terminal 3 and 4.
The hall mobility was measured by using equation (5.8)
(5.8)
where B is the applied magnetic field, R is the change in resistance; R 0 is the resistance
of the material.
Resistivity (ρ) = R0T
(5.9)
ρ
Hall coefficient (RH) = μHρ
(5.10)
(5.11)
(5.12)
Where T is the thickness of the pellet and e=1.6x10 -19 is the charge of an electron.
The resistivity of material (ρ) is 0.2x10-2 Ωm, hall mobility (μH) is 4x10-3 m2/Volt Sec,
hall coefficient (RH) is 0.8x10-4m2/columb and carrier concentration (n) is 7.0x10 26 m-3
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was obtained by using the above equations (5.9-5.12).The charge carrier concentration
values lies near to the reported value and it proves as p-type semiconductor. This
observation indicate that the electronic structure of Sr2Co2O5 material contains donors as
intrinsic oxygen vacancy and acceptors of cobalt ion in these oxidation states Co 3+ and
Co2+ double layer interactions in the ceramics.
5.5.8 Room temperature Magnetic studies
Fig.5.9. Shows the room temperature magnetic properties of strontium cobalt
carbonate precursor and Sr2Co2O5 oxide product obtained by using Vibrating Sample
Magnetometer (VSM). The graph plotted between the magnetic moment versus applied
magnetic field (M-H curves) at room temperature. Comparing the magnetic behavior of
carbonate precursor and oxide material in Table-5.2. It reveals that both carbonate
precursor and oxide material are paramagnetic.
Table-5.2.Comparison of room temperature magnetic behavior of carbonate precursor and
Sr2Co2O5oxide
Strontium cobalt carbonate precursor
Sr2Co2O5 oxide product
Magnetic saturation (Ms=9.87x10-3 emu)
Magnetic saturation (Ms=7.9x10-3 emu)
Magnetic retentivity (Mr =15.16x10-6emu)
Magnetic retentivity (Mr =15.36x10-6emu)
Magnetic coercivity (Hc=14.72G)
Magnetic coercivity (Hc=28.50G)
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Fig.5.9. Magnetic moment vs. Field for carbonate precursor and Sr2Co2O5
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The carbonate precursor magnetic retentivity does not change but increasing magnetic
saturation moment due to change in domain size. The decreasing external field coercivity
of strontium cobalt carbonate precursor twice to that of oxide due to the presence of
cobalt in single Co2+ oxidation state. The carbonate to oxide materials align its moment in
one direction of spontaneous magnetization and it shows superparamagnetic behavior in
anisotropic region. This magnetic behavior is due to the zener double exchange
mechanism is possible in the oxide due to the presence of different valence (Co2+, Co3+&
Co4+) of cobalt ion.
5.6. Conclusion
Co-precipitation strontium cobalt carbonate precursor method as a useful
technique to prepare Sr2Co2O5. The phase formation temperature by thermal analysis
technique indicates that single phase formation of oxide at 800°C. The scanning electron
microscopic studies indicate that agglomerated spheres and have average grain size ∼150
to 300 nm. The elemental analysis by EDX and thermal analysis was in good agreement
with the stoichiometry of the product Sr2Co2O5. The optical band gap for carbonate
precursor at 4eV behaves as an insulating material but its oxide at 2eV as a
semiconducting material due to ligand to metal charge transfer. The carbonate to oxide
materials align its moment in one direction of spontaneous magnetization and it shows
super paramagnetic behavior in anisotropic region. The results demonstrate that
carbonate precursor method represents better preparation method of layered cobalt based
perovskite.
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