Al and PEG Effect on Structural and Physicochemical Properties of CoFe2O4

In this work, pure and Alumina doped cobalt ferrite nanoparticles CoFe2−xAlxO4 (for x = 0.44) have been synthesized by the sol gel method. The influence of alumina doping on the morphological and mechanical properties of CoFe2O4 nano-particles were investigated by means of X-ray powder diffraction (XRD) and rietveld analysis. XRD analysis confirmed that the single phase formation of pure nano particles with the expected cubic inverse spinel structure with Fd3m space group and without any impurity phase. Alumina doping were led to a decrease in the crystallite size, lattice parameter, elastic constants and magnitude of moduli. It is explained on the basis of the replacement of Fe ions with half-filled d-shell (3d5) and larger radius by Al3+ ions with a completely filled shell (2p6) and smaller radius.


Introduction
Spinels of the type AB 2 X 4 are one of the most interesting and important families of crystalline compounds due to their broad applications in magnetic materials, ceramics, catalysis, etc. Spinels of the type A 2+ B 3+ 2 O 4 belong to a large group of composite oxides with a cubic symmetry (space group Fd3m), where A and B are cations with variable valence.][3] Among spinel composite oxides, spinel ferrite nanoparticles have been studied for many years due to their magnetic and electrical properties.][25][26][27][28] The present work is focused on the effects of Al 3+ doping on the structural, morphological and elastic properties of aluminium ion doped cobalt ferrite obtained via sol-gel method.The X-ray powder diffraction patterns, the microstructure and the elastic properties are discussed as a function of the Al 3+ doping.O as source of metal ions and polyethylene glycol (average molecular weight: 4000, Qualigen) as a surfactant were used.The pH was controlled by amonium hydroxide NH 4 OH.All reactants were purchased from Sigma-Aldrich.Deionized water served as reacting medium.

Synthesis of nanoparticles
Nanocrystalline powders of CoFe 2-x Al x O 4 (x = 0, 0.4) were prepared by the sol-gel method.In a typical reaction, cobalt nitrate (Co(NO 3 ) 2 .6H 2 O), iron nitrate (Fe(NO 3 ) 3 .9H 2 O), and aluminum nitrate (Al(NO 3 ) 3 .9H 2 O) were individually dissolved in 10 ml of deionized water in their respective stoichiometry.The solutions were then mixed and stirred for 30 minutes at room temperature.In addition, polyethylene glycol (average molecular weight: 4000, Qualigen) served as a surfactant for this reaction and poured into the above mixture.Then, the 25% ammonia solution was added drop by drop and stirred vigorous on a magnetic stirrer.The final pH of the mixture was about 8.The resulting solution was kept under stirring at 60 o C for 1 hour.The obtained gels were dried at 80ºC and calcined at 500 o C for 3 hours.
Crystal structure of nanoparticles was determined by a Bruker make diffractometer, Cu-K α X-rays of wavelength (λ=1.5406Å).The XRD patterns were recorded in the 2θ range of 10-90 o with a step width of 0.02 s -1 .

Simulation analysis
The accelrys materials studio 6.0 visualisation package was used to create the nanoparticles.The X-ray diffraction (XRD) pattern was analysed with the help of reflex module by employing rietveld refinement technique.The XRD pattern of the pure and aluminia doped CoFe 2 O 4 was refined using the Fd3m space group.The Castep module was employed to calculate the structural, electronic, and elastic properties of two samples.Generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof was used for all calculations.Peak details of synthesized nanoparticles sumerised in Table 1 and 2. The sharp and broad peaks in these XRD patterns, confirming the single phase formation of pure nano particles with the expected cubic inverse spinel structure with Fd3m space group and without any impurity phase.It can be observed that the intensities of the 220 planes increase and the intensity of 440 plane decrease with the addition of aluminium indicating the preference of the Co 2+, Fe 3+ and Al 3+ ions by the octahedral B and tetrahedral A sites.Indeed, this may be indicated the redistribution of the cations in the nanostructure aluminium cobalt ferrite.From this data it is evident that the ratio of Fe 3+ (oct.)/Fe3+ (tet.)changes with addition of Al 3+ ion.In the present case the cubic phase spinel ferrite structure for two composition is mainly due to deficit of Co 2+ ions in the octahedral sites(<85%) that leads to the absence of co-operative active Jahn-Teller distortion.The crystallite size of the nano-particles were calculated from the most intense peak (3 1 1) of XRD data using Debye-Scherer equation.The crystallite size of Al 3+ doped and pure CoFe 2 O 4 nano-particles were 18.9 and 24.3 nm respectively.The lattice constants of Al 3+ doped and pure CoFe 2 O 4 , calculated from F(θ) were 8.22 and 8.32 repectively.

X-Ray diffraction studies
As can be seen, the crystallite size and lattice constant were decreased with alumina doping.Similar results have been reported in the literature. 29,30This behavior of the lattice constant with alumina doping is due to the replacement of larger Fe 3+ ions (0.645 o A) by smaller Al 3+ ions (0.535 o A) in the system.
The x-ray density (ρx) of the samples was determined using relation given by Smith and Wijn: 31,32 ( ) where, M is the molecular weight of the composition, N A is the Avogadro's number and 'a' is the lattice constant.As there are 8 formula unit in the unit cell so 8 is included in the formula.The x-ray density (ρx) of alumina doped and pure CoFe 2 O 4 , were 3.605 and 3.746 g/cm 3 repectively.
It can be observed that the x-ray density (ρx) decreased with alumina doping, because the decrease in molecular weight overtakes the decrease in volume of the unit cell.

Rietveld analysis
Various structural parameters can be determined from rietveld analysis and are found useful to explain other physical properties. 33This method is based on a least-squares fit between step-scan data of a measured diffraction pattern and a simulated X-ray-diffraction pattern.
In this study the crystalline structure of the pure and aluminia doped CoFe 2 O 4 were analized in detailed by rietveld profile refinement method in reflex module.The XRD patterns for these samples have refined using the Fd3m space group (Figure 2).

/ /
Where Io and Ic are respectively observed and calculated integrated Bragg intensities (without background).R p and R wp values for pure and aluminia doped CoFe 2 O 4 were (6.01, 9.27) and (6.91, 9.30) respectively.The calculated R factors indicated that the models were correct.
Oxygen positional parameter (u) for each composite was calculated using the formulae available in the literature.Where u 3 ' gives u assuming centre of symmetry at (1/4, 1/4, 1/4) for which u idea = 0.250 (origin at B-site), while u 43 give 'u' assuming centre of symmetry at (3/8, 3/8, 3/8) for which u idea = 0.375 (origin at A-site).These two factors are related by following formula:

S X
The radius of the tetrahedral and octahedral sites in a spinel ferrite can be calculated using the following formulae: 37,38 Where R O is the radius of the oxygen ion (1.32 o A) and u represents the oxygen positional parameter (table 3).
Using the lattice constant as well as oxygen parameter (u) of each composite, interatomic distances have been calculated from following equations. 39,40The calculated values are presented in Table 3.
It is observed from Table 3 that the doping with alumina brought about a decrease in the values of R A , R B , edge lengths and bond lengths.This could be related to the smaller radius of Al 3+ ion as compared to Fe 3+ ion.In fact, the ionic radius of the Al 3+ ion is 0.050 nm, while the ionic radius of the Fe 3+ ions is 0.064 nm.
The interionic distances between the cations (b, c, d, e, and f) and between the cation and anion (p, q, r and s) were also calculated using the experimental values of lattice constant (a) and oxygen positional parameters (u 3m ) (Tables 4 and 5) by the following relations: 41,42 ( ) p a u b a 4 2 12 5 8 = S X Table 4 indicates that, both interatomic distances between the cation-anion (p, q, r and s) and between the cations (b, c, d, e and f) decrease with alumina doping.This result is accordance with decrease in unit cell volume.
The bond angles (θ 1 , θ 2 , θ 3 , θ 4 and θ 5 ) were calculated by simple trigonometric principle using the interionic distances with the help of following formula: 41,42 These values are tabulated in Table 5.
It is observed from Table 5 that angles θ 3 and θ 4 decrease while θ 1 , θ 2 and θ 5 increase with alumina doping.The observed decrease in θ 3 and θ 4 indicative of weakening of the B-B interaction while increase in θ 1 , θ 2 and θ5 suggest strengthening of the A-B and A-A interactions on Al 3+substitution in the system.

Elastic properties
The elastic constants of solids provide a link between the mechanical and dynamical behaviors of crystals.Cubic crystals have three independent elastic constants: C 11 , C 12 , and C 44 .In the present work, we studied effect of alumina doping on the elastic constants Cij, bulk moduli B, shear moduli G, , , ( ) Young's moduli Y and the Poisson ratio υ.Values of these factors are computed using the following relationships: [43][44][45]  Complate set of the calculated elastic constants for two samples are collected in Table 6.
It can be seen that magnitude of moduli were decreased on Al 3+ doping.It is explained on the basis of the replacement of Fe 3+ ions with half-filled d-shell (3d 5 ) by Al 3+ ions with a noble gas outer electron shell (2p 6 ) structure which do not contribute to the bond formation.
However it is well known that the cations with a completely filled outer electron shell structure are more stable and less compressible than the cations with a halffilled or a incomplete outer shell.In other word the material is compressed easier when the radius of the ions is longer.Therefore, strength of bonding and magnitude of moduli are expected to decrease. 46,47

Conclusions
Pure and Al 3+ doped cobalt ferrite nanoparticles were successfully synthesized by sol-gel method and thermally treated at 500ºC for 3 hours.XRD and the rietveld analysis showed that two composition were formed into single phase cubic spinel structure.Various structural parameters can be determined from x-ray powder diffraction pattern analysis and are found useful to explain other physical properties.The lattice parameters and the crystallite size were found decreasing with Al 3+ doping.The strength of the A-B interaction increase while B-B interaction decreases with Al 3+ substitution for Fe 3+ in the system.Also magnitude of moduli were decreased on Al 3+ doping.This is due essentially to the ion size difference among Al 3+ and Fe 3+ ions.

Acknowledgement
We are thankful to the Payam Noor University for their support and encouragements.
X-ray diffraction pattern of synthesized aluminia doped and pure CoFe 2 O 4 are presented in Figure.1.

Table 1 .
Peak analysis of XRD pattern of synthesized CoFe 2 O 4

Table 2 .
Peak analysis of XRD pattern of synthesized Al-dopped CoFe 2 O 4 Figure 2. Rietveld plot obtained by using the true instrumental function, a: CoFe 2 O 4 , b: Al 0.44 CoFe 1.56 O 4 .

Table 3 .
Edge lengths [A o ], bond lengths[A o ], radius of the tetrahedral and octahedral [A o ], and anion parametes [A o ] of Al-Co-Fe-O system

Table 4 .
Inter-ionic distances [A o ], of Al-Co-Fe-O system

Table 5 .
Bond angles (degree) of Al-Co-Fe-O system

Table 6 .
Elastic constants for Al-Co-Fe-O system