Pursuing Colossal Magnetoresistance in Doped Lanthanum Cobaltates

CO2 treatment of the compound showed very different effects in the transmission and emission Mossbauer spectra, and actually confirmed the nonrandom distribution of Co and Fe in the lattice. 

Interestingly, despite the significant CO2 absorption, the emission spectra of Sro.5Cao,5Coo.5Feo.s0.. 2.s showed hardly any change after the treatments at either 650 or 825 °C where the CO2 uptake was 3 and 17%, respectively (Fig. 19.14). 

The transmission Mossbauer spectra revealed a completely different behavior. The effect of the CO2 treatment was clear. The ratio of the original sextets remained almost the same, but a new sextet appeared (Fig. 19.15). 

It was explained by the following reasoning. While ready absorption of CO2 in these perovskites can be due to the ordered oxygen vacancy arrangement, which is facilitated by the presence of Co, CO2 has a strong preference to react in a way that an almost pure Fe compound forms, and Co is retained in the brownmillerite structure  

The authors pointed out that an unequal decrease in the brownmillerite-type sextets v s not found when the new sextet grew, and it was not seen in the emission experiment either. This apparent contradiction could be resolved by assuming that since the oxygen stoichiometry cannot decrease further down below 2.5 in the brownmillerite structure, if Fe leaves the lattice from the preferred octahedral sites, Co has to substitute it, and one observes octahedrally coordinated nucleogenic Fe. Thus, the relative intensity of the two sextets has to remain the same. 

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PURSUING COLOSSAL MAGNETORESISTANCE IN DOPED LANTHANUM COBALTATES... 

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