Correlation between the structure and the spin state in R1-xSrxCoO3 (R=La, Pr, and Nd)

The crystal structures of the perovskites R1-xSrxCoO3 (R=La, Pr, and Nd) with 0 <= x <= 0.3 were refined from x-ray powder diffraction spectra with the Rietveld method. The result of the Rietveld analyses indicates the presence of three unequal Co-O bonds with different lengths for all samples, and we found that the difference between the two Co-O(2) bond lengths increases with temperature and Sr content. The intermediate-spin (IS) state of the Co3+ ion is Jahn-Teller (JT) active, and the structural data show that the concentration of Co3+ ions increases with temperature and Sr content. For the non-Sr-doped R=La sample, we observed only a small distortion at 90 K while the JT distortion was large at 300 K. With substitution of Sr for La, the distortion at 90 K was found to increase. This observation can be accounted for by the increased stability of the IS state with Sr doping. On the other hand, the distortion was small even at 300 K for nondoped PrCoO3 and NdCoO3, indicating that the low-spin state is stable up to higher temperatures than in LaCoO3. The distortion at 90 K was larger for the Sr-doped samples as in the R=La system, but the Sr content at which the JT distortion started to increase significantly was larger than that in the R=La system.