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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 186 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 187 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 188 (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. 189 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). 190 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) 191 Sr2Co2O5 + 4CO2 800°C (Black color) Fig.5.2. Thermal analysis of strontium cobalt carbonate precursor to Sr2Co2O5oxide product 192 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. 193 Fig.5.3.XRD patterns of Sr2Co2O5 194 Fig.5.4. FTIR spectra of carbonate precursor and Sr2Co2O5 195 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. 196 Fig.5.5. Higher and lower magnifications SEM image of (a) strontium carbonate precursor and (b) Sr2Co2O5 197 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 198 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- 199 (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 200 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-3 201 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) 202 Fig.5.9. Magnetic moment vs. Field for carbonate precursor and Sr2Co2O5 203 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. 204