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For x = 1 (SrFeO3) the periodically cubic model was used with the symmetry group Pm3m. The Fe-O distance and lattice parameters are 1.928 ? and 3.85 ?, respectively (Fig. 1c).
Another important feature is the bonding length of Fe and O ions. This is valuable information that helps indicating valence states of iron. For iron oxides, the Fe-O distance varies in the range of 1.91 ?-1.95 ? for Fe4+, about 1.990-2.006 ? for Fe3+, and to that of Fe2+ is about 2.14 ? [7].
Table 2 illustrates the Fe-O distance in these compositions. For SrFeO3 the Fe-O distance is 1.92 ?, the Fe ions will largely take the form of Fe4+. In fact, it is hardly found any iron with valence other than 4. For three remaining materials, there exist distances correspond to Fe3+ and Fe4+. This is in accordance with the fact that there happens a charge disproportion as the schedule 2Fe4+ ? Fe3+ + Fe5+. Especially in compound La0.33Sr0.67FeO3, the existence of small Fe-O distance of 1.87 ? along y-axis direction strongly suggests the presence of Fe5+. In reality, they have already found the charge disproportion in this specimen as stated above.
Fig. 3 gives a comparison of Fe-O bonding distance got from EXAFS data [7] and the calculation. There are good coincidences with experiment for the doped perovskites while notable differences happen only to the pure ones.
Another approach to the Fe valence states is to use the Mulliken population analysis. Owing to the Mulliken charges in Table 3, we will have a closer look on a partial charge of each atom. The low values of Fe partial charge suggest that it is strongly bonded to other oxygen ions in the octahedral. The electrons tend to move away from Fe ion leads to low partial charges, or in other words, high valence states are observed with Fe ions.
3.3 Dipole Moment by Hirshfeld Method
Hirshfeld method was used to examine the polarization of La0.5Sr0.5FeO3 with cluster model for two and four units cell. The results showed a polarization vector of high magnitude 0.123 au (Px =-0.088 au, Py = 0.064 au, Pz = 0.057 au) or 8410 μC/m2 compared with 26 μC/m2 to that of BaTiO3 [8]. Another notice is that, in cubic structure, although the distortions appear but they are very symmetric. This leads to the elimination of ion polarization. Our investigation shows no electron polarization in this specimen too. It could be explained that the polarization does not exhibit in periodical cubic system.
LaFeO3 behaves as a semiconductor with energy gap of 0.816 eV and crystal field splitting of 2.72 eV comparable to that of 1.5 ± 0.2 eV obtained from experiments. SrFeO3 is a conductor containing a small ferromagnetic phase. The Fe-O bond distances were calculated and discussed the relation of this quantity with the Fe valence states to find the existence of high valence states of Fe ions. The spontaneous electric polarizations is found out 8,410 μC/m2 in accordance with the high value obtained from the experiments.
Acknowledgments
The Authors would like to thank the Projects QGTD 09-05 (VNUH) and NAFOSTED 103.02.111.09 for support.
Another important feature is the bonding length of Fe and O ions. This is valuable information that helps indicating valence states of iron. For iron oxides, the Fe-O distance varies in the range of 1.91 ?-1.95 ? for Fe4+, about 1.990-2.006 ? for Fe3+, and to that of Fe2+ is about 2.14 ? [7].
Table 2 illustrates the Fe-O distance in these compositions. For SrFeO3 the Fe-O distance is 1.92 ?, the Fe ions will largely take the form of Fe4+. In fact, it is hardly found any iron with valence other than 4. For three remaining materials, there exist distances correspond to Fe3+ and Fe4+. This is in accordance with the fact that there happens a charge disproportion as the schedule 2Fe4+ ? Fe3+ + Fe5+. Especially in compound La0.33Sr0.67FeO3, the existence of small Fe-O distance of 1.87 ? along y-axis direction strongly suggests the presence of Fe5+. In reality, they have already found the charge disproportion in this specimen as stated above.
Fig. 3 gives a comparison of Fe-O bonding distance got from EXAFS data [7] and the calculation. There are good coincidences with experiment for the doped perovskites while notable differences happen only to the pure ones.
Another approach to the Fe valence states is to use the Mulliken population analysis. Owing to the Mulliken charges in Table 3, we will have a closer look on a partial charge of each atom. The low values of Fe partial charge suggest that it is strongly bonded to other oxygen ions in the octahedral. The electrons tend to move away from Fe ion leads to low partial charges, or in other words, high valence states are observed with Fe ions.
3.3 Dipole Moment by Hirshfeld Method
Hirshfeld method was used to examine the polarization of La0.5Sr0.5FeO3 with cluster model for two and four units cell. The results showed a polarization vector of high magnitude 0.123 au (Px =-0.088 au, Py = 0.064 au, Pz = 0.057 au) or 8410 μC/m2 compared with 26 μC/m2 to that of BaTiO3 [8]. Another notice is that, in cubic structure, although the distortions appear but they are very symmetric. This leads to the elimination of ion polarization. Our investigation shows no electron polarization in this specimen too. It could be explained that the polarization does not exhibit in periodical cubic system.
LaFeO3 behaves as a semiconductor with energy gap of 0.816 eV and crystal field splitting of 2.72 eV comparable to that of 1.5 ± 0.2 eV obtained from experiments. SrFeO3 is a conductor containing a small ferromagnetic phase. The Fe-O bond distances were calculated and discussed the relation of this quantity with the Fe valence states to find the existence of high valence states of Fe ions. The spontaneous electric polarizations is found out 8,410 μC/m2 in accordance with the high value obtained from the experiments.
Acknowledgments
The Authors would like to thank the Projects QGTD 09-05 (VNUH) and NAFOSTED 103.02.111.09 for support.