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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 162 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 163 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 164 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 165 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. 166 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 167 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. 168 Fig.4.3. XRD patterns of BaCoO3 169 Fig. 4.4. FTIR spectra of carbonate precursor and BaCoO3 170 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. 171 Fig.4.5. FT-Raman spectra of precursor and BaCoO3 Fig. 4.6 EDX elemental composition of BaCoO3 172 Fig. 4.7 SEM images of carbonate precursor and BaCoO3 173 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)]. 174 Fig. 4.8 UV-absorbance spectra of carbonate precursor and BaCoO 3 Fig. 4.9 UV-DRS band gap of carbonate precursor and BaCoO3 175 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. 176 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 177 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 178 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. 179 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 180 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 181 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 182 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. 183 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) 184 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. 185