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Chapter-4
Synthesis, characterization, microstructure, electrical, optical and
magnetic properties of barium cobalt carbonate precursor and BaCoO3
oxide material
4.1 Introduction of BaCoO3
In the past few years, the main interest in cobalt oxides has been their applications
in solid state fuel cells. Cobalt oxides have drawn considerable attention in the last few
years due to their interesting electronic structure and magnetic properties. In particular,
the behavior of BaCoO3 has been thoroughly analyzed recently. The first studies on the
material showed that its ground state was a long-range antiferromagnet. However, more
recent works indicate that its magnetic properties are more complicated and a competition
between antiferromagnetic (AF) and ferromagnetic (FM) interactions was observed. On
the other hand, electronic structure calculations yield that the ground state of the material
is a long-range ferromagnet with an alternating orbital order along the CoO6 chains (the
material has a quasi-one-dimensional structure). BaCoO3 is a transition metal oxide with
interesting properties, which have been studied in recent years. In its highly anisotropic
structure, face-sharing CoO6 octahedra form chains that are likely to produce onedimensional effects in its magnetic properties.
In perovskites, oxygen defect structure have much interest due to their versatile
properties as electrical, magnetic, superconductivity and also plays an important role as a
catalyst for degradation of pollutants. The important aspect of semiconducting perovskite
oxide BaCoO3 material. BaCoO3 layer structure has reduced cations Co3+ (d6) present in
the octahedral coordination and Co4+ (d5) present in the both tetrahedral and octahedral
sites. The different phases are possible with BaCoO3 depend on the partial oxygen
pressures such as 2H –perovskite structure has BaCoO2.94, 5H and 12H layer structure
[Olivier Mentre et al., (2008); A.J. Jacbson et al., (1980); Ghislaine Ehora et al., (2007)].
[Beatrice et al., (1957)] first reported the single crystal of BaCoO3, but this flux method it
takes long hours to prepare the compound. Although in solid state process has more
disadvantage as high reaction temperatures, low chemical homogeneity and long time
process. Botta et al reported citrate sol-gel method to prepare BaCoO3 as high
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temperature and less homogeneity [P.M. Botta et al., (2007); Beatrice E. Gusheel et al.,
(1957)]. By wet-chemical process, offers advantages of good mixing of the starting
materials leads to excellent chemical homogeneity of the final product.
4.2 Structure of BaCoO3
BaCoO3 is a transition metal oxide that recently has been a matter of interest of
both experimental and theoretical work. Its structure as shown in Fig.4.1. described as a
2H-hexagonal pseudo perovskite formed by chains of distorted, tilted, and face-sharing
CoO6 octahedral. The plane perpendicular to the chains consists of a hexagonal array of
Co atoms, which would lead to frustration for any in-plane collinear antiferromagnetism.
The only possible long-range antiferromagnetic state (for collinear moments) could
couple ferromagnetic planes antiferromagnetically (so-called A-type structure). The
magnetic properties of BaCoO3 have still not been resolved in literature. It is certain that
the Co4+ ions are in a low-spin state S=1/2, t2g5 eg 0, since an ion with such a high valency
produces a large crystal field. BaCoO3 is a material with an orbital degree of freedom. In
the distorted octahedral environment, the Co4p is in a low-spin configuration. The d
levels of the cation split (we take the z-axis along the Co chains and the x- and ydirections perpendicular to it, i.e., they fall in the hexagonal plane) into the eg levels (dxz
and dyz), which are at higher energy and fully unoccupied, and the t 2g levels at lower
energy. Due to the distortions of the octahedra, the latter split further into the dz 2 and the
degenerate dx2 y2 and dxy orbitals. Co4þ has a d5 configuration and hence one hole in the
spin-down t2g manifold. While the dz2 orbital is always fully occupied, the hole is
actually in the doublet state, so that the system has an orbital degeneracy.
The hexagonal structure, however, may lead to magnetic frustration when the Co
atoms perpendicular to the chains (on a triangular arrangement) should have an
antiferromagnetic coupling. The experimental studies on BaCoO3 have established that
the Co ions are in a low-spin state S=1/2 as to be expected for a Co 4+ ion, with the
configuration t2g5 eg0 in the case of octahedral symmetry. In the temperature range from
70 to 300 K, the material is found to be semiconducting, with conduction occuring
through n-type carriers. The origin of the gap is not yet resolved.8 Possible reasons are a
Mott-Hubbard-type transition, an Anderson localization due to some structural disorder
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or a “Peierls” dimerization of the Co chains, but none have been confirmed
experimentally. First measurements reported a Néel temperature TN=8 K, but recent
experiments reveal a more complex magnetic structure7 beyond the simple
antiferromagnetic behavior. Paramagnetism is found above 250 K, where a coupling
constant of J/kB=10 K is estimated by fitting to a onedimensional (1D) Heisenberg
model. For temperatures between 70 and 250 K, ferromagnetic coupling is predominant
whereas for T,70 K antiferromagnetic couplings become more important.
Fig. 4.1 Structure of BaCoO3
4.3 Physical Properties of BaCoO3
The magnetic properties of BaCoO3 were analyzed considering the formation of
nanometric magnetic clusters, ferromagnetic regions embedded in a nonferromagnetic
matrix, in the material, starting from experimental data and interpreting them by means of
ab initio calculations. This is usually related to having a sort of self-generated assembly
of magnetic clusters in which magnetic interactions introduce glassiness among them or a
competition between ferromagnetic and antiferromagnetic couplings that may lead to
frustration of any long-range magnetic order, as it is the case in BaCoO3. The 2H-BaCoO3
is formed of isolated columns of face-sharing CoIVO6 octahedra and so represents the
archetype of the so-called 1D structural type with LS CoIV, S ¼ 1/2. The intra-chain
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interactions are preferentially ferromagnetic along c and a competition between F and AF
inter-chain interactions produces a weak ferromagnet at low temperature, i.e. M5T ¼
0.15 mB/Co. However, the possibility of super-paramagnetism between non-interacting
ferromagnetic clusters embedded in a non-ferromagnetic matrix has been recently argued.
(ii) The introduction of ordered oxygen vacancies leads to two 5H- and 12HBaCoO3
polymorphs that display linear [Co 3O12] and [Co4O15] face-sharing octahedra oligomers
with terminal tetrahedra [A.J. Jacobson et al., (1980); M. Parras et al., (1995)]. The
magnetic structure described for the ferromagnetic 5H-BaCoO2.8 [K. Boulahya et al.,
(2005)] shows magnetic moments of 3.1-3.5 μB (octahedral CoIII/IV) and 4.2 μB
(tetrahedral CoIV).
In this present report discuss the solid solution carbonate precursor method for
preparation of BaCoO3. The incorporation of divalent Co2+ cations during the
precipitation of barium carbonate (witherite mineral). The researchers report a
distribution coefficient for Ba2+ from an aqueous solution into Cobalt carbonate
(spherocobaltite) in calcite structure. The crystallization mainly depends on the
physiochemical conditions of precipitate formation [C.M. Holl et al., (2000); K.
Vidyasagar et al., (1984)]. Whitherite form of BaCo(CO3)2 has orthorhombic crystal
system formed during precipitation synthesis. This carbonate precursor further
decomposed to BaCoO3. By this precursor method, BaCoO3 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 sphere shape oxide product. For optical band gap, it reveals the
precursor as insulating material and the BaCoO3 as semiconducting nature. The room
temperature magnetic study indicates the carbonate precursor as paramagnetic but its
oxide BaCoO3 as super paramagnetic behavior.
4.4 Materials and Methods
4.4.1. Experimental part
In this work stoichiometry ratios of cobalt nitrate and barium nitrate dissolved in
deionized water under constant stirring at 80°C. Add 2M sodium carbonate as a
precipitating agent was dripped into the mixed salt solution under mild stirring until the
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pH value of the salt solution reached 8.5. The precipitated carbonate stirred for 0.5h to
attain homogenization then washed with distilled water and dried at 100°C for 4h. The
colorless filtrate tested for complete precipitation. Then the obtained carbonate precursor
can be further decomposed to 900°C for 3hours the presence of air, it forms the single
phase BaCoO3 as a product.
4.4.2. Characterization Technique
Thermogravimetry differential thermal analysis of precursor carried out with a
SDT Q600 V20.9 model. The powders characterized by X-ray diffraction technique
(XRD, Bruker D8 Advance, by CuKα radiation, kα = 1.5406Å). FTIR spectrums
examined using JASCO 400 Infrared spectrometer. The surface morphology and the
microstructure studied by a scanning electron microscope (HRSEM FEI Inspect F50).
The chemical composition determined with an EDX analyzer attached with a (HRSEM
FEI Inspect F50) instrument. Optical band gap obtained by using Jasco V-670-UVVisible diffused reflectance spectrometer. The magnetic properties analyzed by VSM
(Lakeshore VSM 7410). The dielectric studies carried out by LCR meter (HIOKI 353250 LCR meter HITESTER) in the frequency range from 50Hz to 5MHz for variation of
temperature.
4.5 Results and Discussion
4.5.1. Thermal analysis
Fig.4.2. shows the thermal analysis of barium cobalt carbonate precursor. The
decomposition mechanism of carbonate precursor and product formations investigated by
TGA-DTA. Thermal analysis carried out from room temperature to 950°C in an oxygen
atmosphere at a heating rate of 4°C/min. The endothermic peak at 183°C small weight
loss due to the evaporation of residual water. The weight loss at 212°C it loses one
molecule of carbon dioxide then it forms oxycarbonates. Further increase the temperature
to 730°C small endothermic peak reveals the decomposition oxycarbonate to oxide. The
stable BaCoO3 product formed at a temperature of 900°C. The overall weight loss
23.49% and phase identification by XRD it proves the product formation.
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Plausible stepwise decompositions
O2
Step-1
BaCo(CO3)2
250-650°C
BaCoO(CO3) + CO2
O2
Step-2
BaCoO(CO3)
BaCoO3 + CO2
800-900 °C
Overall decomposition step
BaCo(CO3)2
(purple colour)
900 °C
BaCoO3 + 2CO2
(black colour)
Fig. 4.2. Thermal analysis of barium cobalt carbonate precursor
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4.5.2 XRD analysis
Fig.4.3. Shows the XRD pattern of the product calcined at 250-900°C for 2 h.
After calcinations at 250-650°C for 2 hours, BaCO3 phase detected. Increasing the
temperature to 800°C for 2 h, barium carbonate decomposed it form a mixed phase of
BaCO3 and Co3O4. Further increase the temperature to 900°C the XRD pattern proves the
BaCoO3 single phase formed as hexagonal system and its lattice parameters a
=b=5.6831Å, c =4.7183Å are consistent with those reported in JCPDS (card No 01-0700363).
Using the XRD diffraction data, the crystallite sizes of samples were able to be estimated
using the Scherer equation.
(4.1)
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. The most intense (101)
plane of BaCoO3 selected to calculate the average crystallite size as 400nm.
4.5.3 FTIR Studies
Fig.4.4. shows the FTIR spectra recorded for the carbonate precursor and its
decomposed products at different temperature. The carbonate precursor obtained at
100°C it shows the broad peak at 3480 and 1630cm-1 can be assigned to absorbed
water,1435cm-1 it reveals the witherite carbonate mineral. When the precursor calcined at
250°C, two bands appear at 1435 and 860cm-1 assigned to the stretching vibration of
carbonate peak. From the spectra at 800°C that the carbonate starts to decomposed the
bands at 1440 and 850cm-1 peak intensity decrease. After calcination at 900°C, the bands
appear at 730cm-1 and 570 cm-1, 469 cm-1 due to the asymmetric stretching vibrations of
Ba-O and Co-O bonds.
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Fig.4.3. XRD patterns of BaCoO3
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Fig. 4.4. FTIR spectra of carbonate precursor and BaCoO3
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4.5.4 FT Raman analysis
Fig.4.5. FT-Raman spectra recorded in KBr matrix for the barium cobalt
carbonate precursor shows broad peak at 3085cm-1 and 1065cm-1 it corresponds to
witherite carbonate mineral [S. M. Antao et al (2000)]. The oxide product BaCoO3
appeared at (A1g) 682cm-1 and (Eg) 475cm-1 because of the formation of Ba-O and Co-O
groups.
4.5.5 SEM Micrograph analysis
Fig.4.7. Shows the SEM image of barium carbonate precursor and BaCoO3
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.
4.5.6 EDX analysis
Fig.4.6. shows the energy dispersive X-ray spectroscopy (EDX) used to quantify
the elements exists 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
composition of BaCoO3.
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Fig.4.5. FT-Raman spectra of precursor and BaCoO3
Fig. 4.6 EDX elemental composition of BaCoO3
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Fig. 4.7 SEM images of carbonate precursor and BaCoO3
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4.5.7 Diffuse Reflectance spectroscopy studies
Fig.4.8. Shows the UV-Visible absorbance spectra for barium cobalt carbonate
precursor contains major peaks at wavelengths of 210, 245, 334 and 376nm corresponds
to carbonate mineral. The oxide BaCoO3 product contains major peaks at wavelengths of
509nm due to the presence of Ba-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 (4.2)
(αhυ)n = A(hυ−Eg)
(4.2)
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.4.9. Optical band gap properties of carbonate precursor and BaCoO3 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 barium 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 [S. K. Jaiswal et al., (2011)].
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Fig. 4.8 UV-absorbance spectra of carbonate precursor and BaCoO 3
Fig. 4.9 UV-DRS band gap of carbonate precursor and BaCoO3
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4.5.8. Electrical Resistivity Measurements
BaCoO3 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 electrical resistivity measurement by
four probe method, applied current (I) is 8mA and the resistance is measured in the
temperature range of 300-473K. The method can eliminate the effects of contact
resistance between the sample and electrical contacts and therefore is most suitable for
low and accurate resistance measurements [Yadunath Singh (2013)]. The resistivity of
the material is obtained from the equation (4.6).
(4.6)
where S is the distance between probe S= 0.1875, V is the obtained voltage across the
two inner contacts, I is the current passing through the sample. Fig.4.10. shows the
relation between resistivity of material with increasing temperature. The temperature
dependence electrical resistivity proves a semiconducting behavior. BaCoO3 n-type
semiconductor must have a wide band gaps so that lesser number of charge carriers are in
the valence and conduction bands at room temperature. The maximum electrical
resistivity (0.80Ωcm) was exhibited by BaCoO3 that was decreased to 0.76Ω cm
temperature were increased from 300K to 473K. If we increase the temperature, however
thermal agitation increases and some valence electron gain energy greater than E a and
then jump to the conduction band. The activation energy of the material is calculated
from the relation in equation.(4.7), where k is Boltzmann constant.
(4.7)
Fig.4.11. shows the graph is plotted between log10ρ versus T -1 shows higher
electrical resistivity at room temperature as the temperature increases the resistivity
decreases due to hopping conduction mechanism. The slope value is calculated from the
graph and it is substituted in equation 4, to calculate the activation energy of BaCoO3 is
0.6eV.
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Fig.4.10 Electrical resistivity (ρ) as a function of temperature for BaCoO3
Fig.4.11. log10ρ versus 1000T-1 to calculate the activation energy
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4.5.9 Dielectric Studies
The carbonate precursor and oxide powder pelletized using a hydraulic press
technique employing a pressure of 7tons, and it forms pellet of 12mm in diameter and
1mm in thickness. The pellets finely polished by electronic grade silver paints. The
dielectric constant (ε’) calculated from the formula (4.8)
ε
(4.8)
ε
σac = ε0ε'tan
(4.9)
=2 f
(4.10)
(4.11)
where C is the capacitance obtained from the analysis, t is the thickness of the pellet, ε0is
the permittivity of the free space, A is the area of the pellet, f is the applied ac.
Frequency, k is Boltzmann constant, and T is the temperature [Banwari Lal et al.,
(2004)]. The ac conductivity calculated from the formula (4.9, 4.10 & 4.11). The
activation energy obtained from the graph σac versus inverse of temperature in kelvin.
Fig.4.12. Shows the dielectric constant (ε’) of BaCoO3 vary with frequency at room
temperature. Strong relaxation of ε’ observed in the investigated frequency increases the
dielectric constant decreases suggesting a relaxor – like behavior. This behavior
attributed to weak interactions between the dipoles in the layered structure [A.L.Kholkin
et al., (2001); N. Rezlescu et al., (1974); Banwari Lal et al., (2004)]. The conduction
process can be attributed to the presence of two types of charge carriers in carbonate and
oxide, that is, n type, as a hole exchange between Co3+ and Co2+ and transfer of O2−
between filled side with vacant oxygen side. The following equation (4.12 & 4.13) can
explain the mechanism as follows.
Co3+
1/2O2 + V2*
Co2+ + h*
O2-
(4.12)
(4.13)
Fig.4.13. shows the frequency dependence of dielectric loss (or) tan
also
decreases with increasing frequency. The dielectric loss gets decreases with increasing
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frequency and in a certain period it remains constant up to 5MHz. Then loss gets
increases due to stray of capacitance. Fig.4.14. shows the temperature dependence of σac
for carbonate precursor and BaCoO3 at a frequency of 5 kHz. Hence the study of
electrical conductivity is very important. The conductivity gets decreases with increased
temperature. The activation energies calculated by curve fitting equation (6) the
activation energy of the crystal is less than 1eV. The low activation energy suggests an
intrinsic conduction due to the contribution of space charge. The space charge created
between the sample and electrode [H. M. Zaki et al., (2005); D. Szwagierczak et al.,
(2005); Chien-Chih Huang (2007)]. Fig.4.15 shows the frequency dependence of
electrical conductivity decreases for both precursor and oxide due to the ordered motion
of weakly bound charged particles. The charge carriers dominate the external field due to
space charge polarization conduction process. The activation energies calculated by curve
fitting the activation energy is Ea = 0.6eV. The low activation energy suggests an intrinsic
conduction due to the contribution of space charge carriers and holes in the carbonate
precursor and oxide material. This proves the conduction behavior of carbonate and its
oxide material.
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Fig.4.12. Dielectric constant versus logf for carbonate precursor and BaCoO3
Fig. 4.13. Dielectric loss versus log F for carbonate precursor and BaCoO3
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Fig.4.14. Variation of electrical conductivity (σ) as a function of temperature
for carbonate precursor and BaCoO3
Fig. 4.15 Conductivity (σ ac) versus logf for carbonate precursor and BaCoO3
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4.5.10 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.
BaCoO3 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 (4.14 & 4.15)
(4.14)
(4.15)
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 (4.16)
(4.16)
where B is the applied magnetic field, R is the change in resistance; R 0 is the resistance
of the material.
Resistivity (ρ) = R0T
ρ
Hall coefficient (RH) = μHρ
(4.17)
(4.18)
(4.19)
(4.20)
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 8x10-1 Ωm, hall mobility (μH) is 2x10-3 m2/Volt Sec, hall
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coefficient (RH) is 1.6x10-3m2/columb and carrier concentration (n) is -4.0x1023 m-3 was
obtained by using the above equations (4.17 - 4.20).The charge carrier concentration
values lies near to the reported value and it proves as n-type semiconductor. This
observation indicate that the electronic structure of BaCoO3 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.
4.5.11 Room temperature Magnetic studies
Fig.4.16. shows the room temperature magnetic properties of barium cobalt
carbonate precursor and 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-1.
It reveals that both carbonate precursor and oxide material are paramagnetic. 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 barium 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.
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Fig 4.16. Magnetic moment Vs H for carbonate precursor and BaCoO 3
Table-4.1. Comparison of room temperature magnetic behavior of carbonate precursor and
BaCoO3 oxide
Barium cobalt carbonate precursor
-3
BaCoO3 oxide product
Magnetic saturation (Ms=9.79x10 emu)
Magnetic saturation (Ms=9.92x10-3 emu)
Magnetic retentivity (Mr =14.63x10-6emu)
Magnetic retentivity (Mr =12.77x10-6emu)
Magnetic coercivity (Hc=13.98G)
Magnetic coercivity (Hc=22.74G)
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4.6. Conclusion
Co-precipitation carbonates precursor method as a useful technique to prepare
BaCoO3. The phase formation temperature by thermal analysis technique indicates that
single phase formation of oxide at 900°C. The scanning electron microscopic studies
indicate that agglomerated spheres and have average grain size ∼140 to 250 nm. The
elemental analysis by EDX and thermal analysis was in good agreement with the
stoichiometry of the product BaCoO3. The optical band gap for carbonate precursor at
4eV behaves as an insulating material but its oxide BaCoO3 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 precursor method represents
a useful technique for the preparation of cobalt based perovskite.
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