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Rietveld Refinement and Spectroscopic Analysis of Co3-xMnxO4

(0.1≤x ≤ 1.0) Ceramic Compositions

P. L. Meena*, Ravi Kumar**, K. Sreenivas***

*Department of Physics, DeenDayalUpadhyaya College (University of Delhi),Shivaji Marg, Karampura, New Delhi – 110015, India

**Beant College of Engineering andTechnology, Gurdaspur, Punjab – 143521,India ***Department of Physics and Astrophysics, University of Delhi, North Campus, Delhi– 110007, India

*Corresponding Author:plmeena@gmail.com Abstract: Ceramic compositions of Co3-xMnxO4 (x = 0.1−1.0) prepared by conventional solid state sintering have been characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy. Unit cell dimensions, coordinates of the atoms, oxygen parameter ‘u’ and metal-oxygen bond lengths have been determined through Rietveld analysis using FullProf program. Single phase formation along-with a linear increase in the lattice parameter ‘a’ is observed without any change in structural symmetry with increasing Mn content. A linear increase in ‘a’ and a related decrease in the oxygen parameter ‘u’ with increasing Mn content imply a decrease in the tetrahedral site as Mn occupation increases, and correspondingly the octahedral site expands. FTIR absorption bands show a significant broadening and shifting to lower wave numbers with increasing Mn content, and ultimately tend to merge at higher Mn content (x > 0.8) signifying strong interactions due to overlapping octahedra of Co and Mn. Raman spectroscopic analysis supports the observed increase in the lattice parameter and the occupation of Mn at the octahedral sites. Analysis of XRD data and Raman spectra show that Mn content x ≤ 1.0 is accommodated at the octahedral site, while retaining the cubic spinel structure.

Keywords: Ceramic, Co3-xMnxO4, Powder X-ray diffraction, Rietveld refinement,Fourier transform infrared, Raman spectroscopy, Oxygen parameter, Metal-oxygen bond lengths.

Accepted On: 10.02.2014

1. Introduction Mixed oxides known as spinels with the general formula AB2O4 where A = Mg, Cr, Mn, Fe, Co, Ni, Zn, Cd, andB = Al, Cr, Mn, Fe, Co, Ga represent divalent and trivalent transition metal cations respectively. They form a special class of compounds with interesting electronic, magnetic, optical, and catalytic properties, and have been extensively investigated by introducing a variety of dopants [1,2,3,4]. In recent years Co3O4 has also been identified as a magnetic semiconductor[5]and in one of our earlier reported works[6] an interesting evidence through dielectric phase transition and magnetic properties has been noted in single phase Mn doped Co3O4 indicating its multiferroic nature. Earlier studies on the magnetic properties of Co3-xMnxO4 (x = 0.1 to 1.0) have revealed its ordered ferrimagnetic behaviour, and a phase transition from para- to ferrimagnetic below 191K [7]. From the substitution point of view, it appears a lot of scope exists to substitute various trivalent cations by replacing a fraction of Co3+ ions by Al, Mn, Ni, Cr, Ti etc[7, 8, 9]. The motivation for the substitution of Mn3+ ions having a larger ionic radii was to look into the possibility of developing a new class of single phase multiferroic materials with the expectation that Mn can induce non-centro-symmetric charge ordering and consequent polarization. It is well known that cobalt manganese, Co3-xMnxO4 (CMO) spinel oxides crystallize in cubic form (space group ��3�) for x < 1.2, and for x > 1.2 the tetragonal (I41/amd) type structure observed[10]. The preparation methods and conditions are known to strongly influence the redistribution/repartition of the cationic oxidation states amongst the crystallographic sites and lead to complex structural

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possibilities[11]. In the reported literature the emphasis is more towards a better understanding of the cationic distribution and single phase formation over the whole solid solution range. Cobalt manganese oxide powders, films, and ceramics prepared by a variety of techniques are being investigated for achieving optimum material parameters for different applications[12]. Depending upon the compositional stoichiometry and the occupancy of the cations at A- and B-sites, some of the cobalt manganese spinel phases seem to be promising for their multiferroic properties in which both ferroelectricity and ferromagnetism can coexist[13]. Recently, first principle calculations have shown that it is more realistic to have ferrimagnetism, instead of ferromagnetism, through B-site cation ordering in single phase multiferroic oxides[14]. Subsequently, different ferrimagnetic oxides such as MnCr2O4[8], TmFe2O4[15],MnWO4[16] have been explored for their multiferroic properties, and a similar geometrical frustration has been considered as the origin for multiferroic behaviour in cubic sulpho-spinels[17]. The crystal structure of Co3-xMnxO4(x = 0.0 to 1.2) is a cubic spinel and belongs to ��3�(site symmetry O�

� )space group. Each unit cell of CMOcontains eightCo� ���Co���� ����O��formula units and

therefore 32 O2-ions. This close packing contains 64 tetrahedral (A-site) interstices and 32 octahedral (B-site) interstices coordinated with O2- ions in which Wyckoff positions 8a denote the tetrahedral sites (oxygen coordination 4) and 16d the octahedral sites (oxygen coordination 6) surrounded by O2-ions at 32e sites. Empty interstitial space is comprised of 16 octahedral sites (16c) and 56 tetrahedral sites (8b and 48f)[18]. The structure of Co3O4 is characterized by an oxygen parameter ‘u’ having a value around 0.25. Liu & Prewitt reported [19] that the arrangement of O2- ions form a cubic close packing (FCC) structure for u = 0.25, and with u> 0.25 there is a deviation in the spinel lattice with deformation of oxygen tetrahedrons and octahedrons. In this paper we focus on the structural and spectroscopic analysis of Co3-xMnxO4 (0.1 ≤ x ≤ 1.0) compositions prepared by solid-state reaction to identify the single phase formation, and study the effects on the lattice and oxygen parameter, and distortion of the octahedral site in Co3-xMnxO4 ceramic compositions.

2. Experimental

Ceramic compositions of cobalt based manganite spinel Co3-xMnxO4 (CMO) with x = 0.1 to 1.0 were synthesized by conventional solid-state reaction method using high purity starting(>99.97%) materials consisting of cobalt oxide (Co3O4) and manganese oxide (MnO). The solid-state reaction used for the preparation of Co3-xMnxO4 is given as follows:

24xx343 O2

xOMnCo3xMnO3OCo)x3( −→+−

− ................................................. (1)

Appropriate quantities of the reactants (Co3O4 and MnO) required for developing the Co3-xMnxO4 compositions with (0.1 ≤ x ≤ 1.0) were calculated. The powders were weighed and then mixed in stoichiometric proportion thoroughly.This was followed by grinding in a mortar and pestle using acetone to improve the degree of mixing, and was continued for several hours to achieve good homogeneity. The powders were calcined at 800 ºC for 12 hours in air, and the calcination step was repeated 4 times, starting at 800 ºC and followed by intermediate grinding and re-calcination at a higher temperature, and at each step the temperature was increased by 50 ºC ultimately increasing the temperature up to 950 ºC. Calcined powders were pressed at a pressure of 5 tons to obtain 1-2 mm thick circular discs of 10 mm dia, and the pellets were finally sintered at 950 ºC for 24 hours. The sintering step was also repeated 3 times by crushing the sintered pellets and grinding them to a fine powder size. After the final sintering step the ceramic pellets were slowly cooled at a rate of 5 ºC per minute. The density of the sintered pellets was measured using Archimedes method with distilled water as immersion fluid. X-ray powder diffraction (XRD) patterns were recorded at room temperature using a Bruker AXS X-ray diffractometer with Cu Kαradiation in the step scanning mode with a 0.02° step size in the range 20° ≤ 2θ ≤ 70°. Initially the X-

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ray diffraction (XRD) data was analyzed using PowderX software. Fourier transform infrared (FTIR) spectra was recorded using a Perkin Elmer RX-1 FTIR spectrometer, and the Raman spectra were recorded at room temperature using a RenishsawInVia reflex micro Raman spectrometer operating at 514 nm.

3. Results and Discussions Figure 1 shows the X-ray diffraction (XRD) patterns at room temperature for the Co3-xMnxO4 (CMO) ceramics prepared with varying compositions x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0.The diffraction patterns show the formation of a single phase, and all the observable reflections could be indexed as 220, 311, 222, 400, 422, 333/511, and 440 planes, and are in agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) file 80-1544, assuming face centered cubic (FCC) structure, and also in agreement with earlier reported studies[20].

Fig.1. Powder X-ray diffraction patterns for CMOceramic compositions recorded at room temperature.

The XRD data obtained on CMO ceramic samples was analysed in detailed by Rietveld profile refinement method by the computer program FullProf suite software (1.00) version Feb. 2007 JGP-JRC using the pseudo-Voigt profile function of Thompson et al. [21].The lattice parameters(with error in 2θ less than 0.05º) required for Rietveld refinement were initially determined by the PowderX computer program [22].The refinement of the XRD data was performed considering a spinel structure for the Co3O4 type phase (����cubic space group). In this structure, Co ions are located in the tetrahedral 8a sites (1/8, 1/8, 1/8) and in the octahedral 16d sites (1/2, 1/2, 1/2), O is located in the 32e sites [(u, u, u) with u~0.25, where ‘u’ is the oxygen parameter)].According to Young [23] the criteria for the fitting quality on the experimental data were confirmed on the following basis: (i)The goodness of fit (GoF), χ2, to be 1.0, or less than 1.3. (ii)The Durbin-Watson statistic (D-W statistic), d, must be 2.00 or very close to 2.00, and usually this islow in the beginning of refinement, and gets progressively closer to 2.00 [24].

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(iii)The weighted-profile R value, Rwp, for laboratory XRD data should be ~ 10% [25].

Fig.2. Rietveld refinement results of XRD data for Co3-xMnxO4 (a) x = 0.1, and (b) x = 1.0,experimental (black dots), calculated patterns (continuous red line), and, the difference plot (continuous blue line). Vertical markers

indicate the positions of the calculated Bragg reflections. The Rietveld [26] refinement analysis is a well-established technique for crystal structure determination and refinement from powder X-ray diffraction data.The typical profiles for the two extreme CMO compositions (x = 0.1, and 1.0), showing the calculated Bragg peak positions, XRD patterns (observed, calculated, and difference) resulting from Rietveld refinement are shown in Fig. 2 (a) & (b). All the peak positions and the relative intensity of the diffraction lines are found to be in good agreement with the ASTM X-ray powder data files JCPDS-ICDD, thus confirming the formation of a single phase for each composition. Crystal symmetry, refined lattice parameters, D-W statistic,GoF (goodness of fit),and reliability R-factor (Rwp) are presented in Table 1. The conventional Rietveld reliability R - factors (Table 1) obtained from Rietveld refinement for all the compositions are according to McCusker, et al. [25].

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Fig.3.Variation of lattice parameter, a, and linear fit of ‘a’ for the CMO ceramics as a function of composition (x), and inset shows the shift in the peak position for 311 reflections.

Figure 3 shows the variation of lattice parameter ‘a’ in the Co3-xMnxO4 compositions with varying x, and the inset in the Fig.3 shows the measured shift in the (311) peak position towards lower angles with increasing Mn content. The estimated value of the lattice constant ‘a’ increases linearly from 8.097 ±0.001Åfor x = 0.1 to 8.278 ± 0.002 Åfor x = 1.0 (Table 1, and Fig. 3), and is in agreement with earlier reported results[6, 7].The observed variation in the lattice parameter (a) with composition (x)was found to follow the trend as “a = 8.07365 (±0.00098) + 0.1942(± 0.0018) x”.Thecations (Co2+, Co3+, and Mn3+) in CMO occupy two non-equivalent sites: tetrahedral 8a (A-site) and octahedral 16d(B-site). The substitution of Co3+by Mn3+ leads to unit cell expansion, and the increase in the lattice parameter ‘a’isdue to the large ionic radius of Mn3+ (0.785 Å) in comparison to the ionic radius of Co3+ (0.65Å) occupying the octahedral sites. This effect causes the lattice plane spacing to change and the diffraction peaks shift to a lower (2θ) angles as seen in inset of Fig. 3. It is important to point out that CMO retains the normal cubic spinel structure with increase in Mn substitution up to x = 1.0 (Table 1). A linear variation in the lattice constant ‘a’ with varying Mn content up to x = 1.0 follows Vegard’s [27] law (Fig. 3, and Table 1), and suggests that the system has not approached the maximum solubility limit up to Mn content of (x = 1.0), and the Co3-xMnxO4system is able to accommodate Mn3+ up to 33%. The Rietveld analysis confirms single phase formation corresponding to the expected compounds which crystallize in the face-centered cubic system spinel type in accordance with spatial group ����(No. 227). The AB2O4 spinel structure, with eight formula units per unit cell, is based on cubic close packing (ccp) of oxygen. The oxygen parameter ‘u’ provides a quantitative measure of the distortion from the perfect ccp arrangement and is equal to 0.25 for an ideal ccp, whereas an increase in the value of ‘u’ has been attributed to a smaller octahedral site and a larger tetrahedral site in comparison to the perfect ccp[19]. However, O’Neill [29] in their investigations on the binary solid solution Co3O4-CoCr2O4 suggests equal tetrahedral and octahedral bond lengths for u = 0.2625, and this relative value of ‘u’ is also reflected in the relative intensities of Bragg peaks. In the present study the oxygen positional parameter ‘u’ is found to decrease (from 0.2626 to 0.2572) with increasing Mn content (x = 0.1 to 1.0) in Co3-xMnxO4 [Fig. 4(a), and Table 2]. A decrease in the ‘u’parameter has been reported by Liu & Prewitt [19] in their temperature dependent study on Co3O4 powders, and attributed the decrease in ‘u’ to a decrease in the tetrahedral, and an increase in octahedral bond lengths. A variation in the ‘u’ parameter therefore reflects the adjustmentof the oxygen positions to accommodate differences in the actual radius of the tetrahedral site (MT –O) and the octahedral site (MO – O) cations, where M denotes Co or Mn ions. It may be noted from Table 2 that the bond length�����(the shortest distance between A-site cations and oxygen ion) is greater than �����(the shortest distance between B-site cations and oxygen ion) and is consistent with the crystal radius of the divalent high spin (MT – O) (bigger ionic radii) being greater than the crystal radius of the trivalent low spin (MO – O)(smaller ionic radii), thus implying different spin states for both the cations and could be calculated in terms of the lattice constant ‘a’ and the oxygen parameter ‘u’ given by [29,30]

)8/1(3 −=−

uarOM

T ……………………………………………………..(2)

8/323 2+−=

−uuar

OM O ………………………………………………….(3)

The combined information on‘a’ and ‘u’unambiguously determine both the cation-to-oxygen inter-atomic distances. It is clear from Fig. 4 (b & c) and Table 2 that the average bond length r !�"for the octahedral site increases, and r #�"for the tetrahedral site decreases as Mn occupation increases. Mn atoms are more inclined to occupy the octahedral sites in Co3-xMnxO4 and are favoured by the polarization effects of intermediate oxygen atoms between the octahedral and the tetrahedral sites. As such, the tetrahedral sites can be contracted due to the displacement of the four oxygen ions toward the body diagonal of the cube.

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At the same time the oxygen ions connected with the octahedral sites can move in such a way, so as to expand the size of the octahedral site and shrink the tetrahedral site consequently.The observed discrepancy in the increase of the octahedral, or the decrease in the tetrahedral bond length [Fig. 4 (b & c), and Table 2] may be attributed to the relative occupancy of Mn3+ at Co2+ or Co3+ with different ionic radii of 0.785Å, 0.83 Å, and 0.65 Å respectively in the octahedral site. Since Mn3+ ion is bigger than the existing Co3+ ion, the r !�"distances in the octahedral sites increase as the value of x increases. This is in contrast to the observations of Levine [31] where an inverse relationship between the covalent character of the spinel and bond lengths has been reported. The increasing bond lengthsr !�"with increasing Mn content (Table 2) clearly suggest there is a decrease of iono-covalent character of the spinel with increasing Mn content, and similar results have been reported in the case of Zn-substituted Li-Cu spinel [32].

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Fig.4.Variation in (a) the oxygen parameter u, (b) the tetrahedral bond length(r #�"), and (c) the octahedral bond length (r !�") in Co3-xMnxO4 as a function of composition (x).

Table 1. Crystal symmetry, lattice constant (a), D-W statistic (d), goodness of fit (χ2) and conventional Rietveld reliability R- factor (Rwp) for polycrystalline single phase ceramic Co3-xMnxO4 (0.1 ≤ x ≤ 1.0) compositions

Compositions Crystal Lattice Constant D-W stat. GoF Rwp

Co3-xMnxO4 Symmetry a (Å) D χ2 (%)

x = 0.1 Cubic 8.097±0.001 1.922 1.03 10.30 x = 0.2 Cubic 8.118±0.002 1.883 1.13 13.10 x = 0.3 Cubic 8.129±0.001 1.917 1.06 09.47 x = 0.4 Cubic 8.146±0.002 1.995 1.06 13.20 x = 0.5 Cubic 8.167±0.001 1.840 1.11 11.50 x = 0.6 Cubic 8.188±0.001 1.831 1.05 10.00 x = 0.7 Cubic 8.208±0.001 1.875 1.12 10.20 x = 0.8 Cubic 8.222±0.003 1.942 1.11 12.00 x = 0.9 Cubic 8.250±0.003 1.775 1.09 11.25 x = 1.0 Cubic 8.278±0.002 1.739 1.24 12.10

Table 2. Oxygen parameter (u), tetrahedral bond length, (r #�"), octahedral bond length, (r !�"), X-ray

density(ρXRD), bulk density (ρM) and porosity (P) for polycrystalline single phase ceramic Co3-xMnxO4 (0.1 ≤ x ≤ 1.0) compositions

Compositions Oxygen Tetrahedral Octahedral X-ray Measured Porosity parameter Bond Length Bond Length density density

Co3-xMnxO4 (u) (r #�") Å (r !�")Å ρXRD(g/cm3) ρM(g/cm3) P x = 0.1 0.2626(14) 1.929(0.010) 1.928(0.010) 6.015 5.984 0.0052 x = 0.2 0.2630(14) 1.940(0.010) 1.930(0.010) 5.958 5.549 0.0686 x = 0.3 0.2628(12) 1.939(0.009) 1.934(0.009) 5.925 5.920 0.0008 x = 0.4 0.2624(20) 1.938(0.015) 1.941(0.015) 5.877 5.499 0.0644 x = 0.5 0.2612(16) 1.927(0.012) 1.954(0.012) 5.822 5.123 0.1200 x = 0.6 0.2610(14) 1.928(0.010) 1.961(0.011) 5.769 5.275 0.0856 x = 0.7 0.2607(14) 1.929(0.011) 1.968(0.011) 5.716 5.486 0.0402 x = 0.8 0.2591(14) 1.910(0.011) 1.983(0.012) 5.679 5.249 0.0758 x = 0.9 0.2589(15) 1.913(0.012) 1.992(0.012) 5.611 5.223 0.0690 x = 1.0 0.2572(20) 1.896(0.015) 2.011(0.016) 5.545 5.371 0.0313

The X-ray density for the polycrystalline ceramic CMO series was calculated from the molecular weight and the volume of the unit cell using the following relation [33]

V

A

VN

A

a

XRD

∑=

∑=ρ 66020.1 …………………………………………………………….. (4)

where∑Arepresents sum of the atomic weights of the atoms in the unit cell, Na is the Avogadro’s number and V is volume of unit cell. Fig. 5 shows the variation in the unit cell volume V = a3, (cm3) and the X-ray density,ρXRD(g/cm3), calculated from Rietveld refined XRD data. It is seen from Fig. 5 that the unit cell volume increases and X-ray density decreases with increasing Mn content up to x = 1.0. The estimated values of X-ray density and the measured bulk density are compared in Table 2. The value of the X-ray density (ρXRD) being slightly higher than the measured bulk density (ρM) indicates the presence of some porosity in the system. The porosity (P) is calculated using the following equation:

XRD

M1Pρ

ρ−= ……………………………………………………………………………..(5)

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whereρMis the measured bulk density andρXRDis the X-ray density of the samples. The porosity is found to fluctuate and found to be maximum 12% for x = 0.5 (Table 2).

Fig. 5.Lattice volume a3and X-ray density patterns for CMO compositions.

Fig. 6.Room temperature FTIR spectra for Co3-xMnxO4 (0.1 ≤ x ≤ 1.0) ceramic compositions.

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Fourier transform infrared (FTIR) spectra of Co3-xMnxO4 (x = 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) ceramic compositions are shown in Fig. 6. Two distinct and sharp absorption bands at579 cm-1 (ν1) and 667 cm-

1(ν2) are observed in all the compositions. An increasing Mn content in the range (x = 0.1 to 1.0) results in a significant shift in the bands to lower wavenumbers with (∆ν1 = 37 cm-1, ∆ν2 = 38 cm-1), and in addition the bands are broadening, and begin to coalesce at the higher Mn content (x>0.8). These absorption bands originate from the stretching vibrations of the metal oxygen bond, and with x = 0.1 they match very closely with the reported FTIR observations on nano-patriculate Co3O4 spinel and the formation of single phase[34,35,36]. The 579 cm-1 (ν1) band is characteristic of Co3+ vibration in the octahedral hole, and the 667 cm-1(ν2) band is attributed to Co2+ vibration in the tetrahedral hole of the spinel lattice[37].Insignificant changes inthe relative intensity ratio of the ν1 and ν2 bands (Fig. 6) indicate minimal changes in the particle size as confirmed earlier through the XRD and Rietveld refinement calculations. These observations and correlation on insignificant changes in the particle size are well supported by an earlier study where substantial changes in the intensity ratio have been prominently seen for a changing particle size in Co3-xMnxO4 (0.0≤ x ≤ 1.4) complex oxide powders prepared by sol-gel method[37]. The continuous shift in theν1 and ν2 bands to lower wavenumbers is due to the progressive incorporation of Mn into the cubic structure and is in agreement with the earlier observations of Tian et al.[4]on Co3-xMnxO4 (0 ≤ x ≤ 0.34) prepared by chemical vapour deposition technique. In addition to the shift of the ν1 and ν2 bands to lower wavenumbers, the decreasing intensity and the broadening of the bands is noteworthy which ultimately coalesce at the higher Mn content (x = 0.9 & 1.0). This could be due to the strong interactions between the two types of octahedra CoO6 and MnO6 which result in the overlapping of the bands corresponding to the different octahedra at higher Mn concentration[38]. Co3O4 crystallizes in the normal cubic spinel structure Co2+[Co3+]2O

2-, and Co2+ and Co3+ are located at A-site and B-site respectively, and belong to the space group (����) with�$

% spectroscopic symmetry. Group theory predicts the following modes in spinels, Γ= A1g (R) + Eg (R) + F1g (in) + 3F2g (R) +2A2u (in) + 2Eu (in) + 4F1u (IR) + 2F2u (in), where (R), (IR) and (in) represent Raman active, infrared-active, and inactive modes, respectively. The polycrystalline Co3O4 possess five Raman active modes, as A1g+ Eg + 3F2g, with wave numbers 194.4 cm-1 (F1

2g), 482.4 cm-1 (Eg), 521.6 cm-1 (F22g), 618.4 cm-1 (F3

2g) and 691.0 cm-1 (A1g)[39, 40].

Fig. 7. Raman spectra collected at room temperature for Co3-xMnxO4 (0.1 ≤ x ≤ 1.0) ceramic compositions.

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In polycrystalline Co3O4 spinel, the observed Raman mode at ~ 692.6cm-1 (A1g)is attributed to the characteristics of octahedral, (B-site, Oh) sites (Co/MnO6) in the O�

�spectroscopic symmetry, and the mode at ~ 194.6 cm-1 (F1

2g) is attributed to the tetrahedral (A-site, td) sites (CoO4). Raman spectra of Co3-

xMnxO4 (x = 0.1, 0.2, 0.5, 0.9, and 1.0) show a shift in the peak position towards lower wave number (Fig. 7). The mode A1g shifts from 689.6 cm-1 for x = 0.1 to 6561.7 cm-1 for x = 1.0, due to the increased unit cell parameters with Mn substitution, and indicates that most of the substituted Mncations are occupying the octahedral sites (Co/Mn-O6). The Raman bands in spectra corresponding to compositions with lower Mn content are relatively sharp in comparison to those with higher Mn content, and with increasing Mn incorporation into the lattice the bands are shifted, broadened, and some of them coalesce. For example, a pair of bands at 521.4 and 483.6 cm-1 with sharp FWHM as seen in the spectra for x = 0.1 tend to coalesce into a single diffused band at 503 cm-1. The broadening and shifting of the Raman modes with Mn-substitution into the parent Co3O4 lattice observed in the present study are in good agreement with the earlier reported observations[40,41].

4. Conclusions In summary, single phase bulk ceramic compositions of Co3-xMnxO4 (0.1 ≤ x ≤1.0) have been successfully synthesized by the solid state reaction method. The effect of Mn substitution in Co3O4 has been characterized and the quantitative analysis of the XRD data evaluated on the basis of Rietveld structure refinement methods yields detailed information about the structure. The lattice parameter ‘a’ is found to increase with increasing Mn content in the prepared compositions. Rietveld refinement data suggests that the Mn ions preferentially occupy the octahedral B-site, and a decreasing oxygen parameter ‘u’ indicates a decrease in tetrahedral and an increase in octahedral bond lengths. The combined information on ‘a’ and ‘u’ ambiguously shows that the tetrahedral site decreases as Mn occupation increases and the octahedral sites move in such a way so as to expand the size of the octahedral site and shrink the tetrahedral site. FTIR spectroscopic analysis reveals strong interactions and overlapping of the absorption bands with increasing Mn content due to the two types of octahedra CoO6 and MnO6, and Raman spectroscopic analysis supports the observed increase in the lattice parameter and the occupation of Mn at the octahedral sites. Analysis of XRD data and Raman spectra show that Mn content x ≤ 1.0 is not difficult to accommodate at the octahedral site, while retaining the cubic spinel structure. Acknowledgements The author wishes to express his sincere thanks to Dr. R. J. Choudhary, UGC-DAE CSR, Indore, for his kind help in XRD measurements and University Science Instrumentation Centre (USIC), University of Delhi for FTIR and Raman measurements. References:

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