If a higher ionic radius element Sr2+(1.18 Å) doped to a smaller ionic radius site Bi3+ (1.03 Å) of BiFeO3, then what happened to the crystallite size and particle size? If the particle size decreased then what is the cause behind this?
More details are required to get an accurate answer. These include:
1- The details of the preparation method (any change in the pH or T).
2- Confirmations regarding the formation of phase-pure solid solution.
3- Confirming that Sr is really occupied the Bi sites (NOT Fe sites or both).
4- Specifying whether there is positive or negative enthalpy of mixing.
5- The effect of Sr addition on the interfacial free energy.
However, as a general answer, the particle size is highly affected by the rate of nucleation; the particle size decreases when the rate of nucleation is high, at a given rate of growth, and vise versa.
The rate of nucleation, at a given T and C, is indirectly proportional to the change in the free energy, due to the formation of nuclei, which itself indirectly proportional to the change in the volume free energy.
Thus, if we suppose that Sr-doped material have higher volume free energy as compared with the un-doped material, due to the higher enthalpy caused by (i) the strain due to the size difference and (ii) the formation of the vacancies due to the charge difference, then the change in the free energy due to the nucleation in the case of the doped material will be less than that of the un-doped one. Hence, the rate of nucleation will be higher and the particle size will be smaller.
This explanation assumed that (i) Sr addition does NOT affect the interfacial free energy, (ii) the preparation method is achieved at low T, and (iii) the Sr-O bond is similar or weaker than that Bi/Fe-O bond.
The crystallite size is a measure of the structural coherence length, which may be influenced significantly by structural distortions of the lattice. The average ionic radius at A sites and B sites of ABO3 perovskite determines the tolerence factor: tG=(rA+rO)/sqrt(2)(rB+rO), which determines the type of crystal distortions in the perovskite (for example, orthorhombic or rhombohedral distortions). Thus, it is not a straight forward conclusion that increasing the ionic radius should increase the lattice parameters. In fact, in a previous study, the substitution of Mn3+ions by the smaller Co3+ ions was found to induce reduction of the lattice parameters and cell volume for x = 0.1, and then an increase at higher Co concentrations [1]. The average cell volume depends critically on the bond length and bond angles. Further, poor crystallization due to significant crystal distortions may result in a decrease of the particle size.
[1] F. Ben Jemaa et al. (2015), Structural, magnetic, and magnetocaloric studies of La0.67Ba0.22Sr0.11Mn1-xCoxO3 manganites. J. Mater. Sci. 50: 620-633
Hi,
In yiur case the particle size shd not reduce, because crystalite is nothing but grain size only. More crystalite joined together then it will be called as particle. You are just coating the higher radi atom ti lower one. As a result
It may imprive the particle size not crystalite. Anyway check once again both the values.
Best.
Arunprakash Vincent
Obviously it depends on several factors, but if you try to visualise it at a glance then I should suggest valency and ionic radius has great impact on doping depending on the system you have chosen for doping. If your substitute a element with isovalent but lower ionic radius, it serves as possitive chemical pressure and vice- Versa. That is reduced unit cell parameters can be observed for lower ionic radius of isovalent doping and vice-Versa. But in case of non isovalent doping, you need to know several factors to be specific.
When a larger size cation is replaced with a smaller size cation or vice-verse from the crystal lattice, the crystallite size will be decreased, due to lattice strain induced by the size mismatch.
Dear Researcher,
whenever doping is performed with mismatch valence ion there must be significant distortion in the unit cell. Moreover, the physical properties and hence the volume of the unit cell are also altered. Please go through my article where you may get the detailed answer about this particular system (Sr doped BiFeO3).
Article Effect of Sr doping on the magnetocapacitive effect in Bi0.6...
with regards,
R. Rajesh
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