Cobalt spin states investigation of Ruddlesden-Popper La2-xSrxCoO4, using X-ray diffraction and infrared spectroscopy

Ruddlesden-Popper (RP) La2-xSrxCoO4 (x = 0.7, 0.9, 1.1, 1.3) were synthesized by a modified sol-gel route. The synthesized samples were characterized by XRD, FE-SEM technique, iodometric titration, FT-FIR spectroscopy and VSM analysis. The variation of Co-O bond length obtained from Rietveld refinements and the related peak shifts in the FT-FIR spectra showed that Co3+ has both LS and IS spin states. It is found that the population of Co3+ in the LS state decreases and IS state increases with increasing Sr concentration. Jahn-Teller effect was identified in the FT-FIR absorption bands, corresponding to the vibration modes of Co bonds with the apical and equatorial oxygen atoms. The ferromagnetic characteristic of La0.7Sr1.3CoO4, obtained by VSM, revealed the increase of Co3+ IS population by Sr doping, in agreement with XRD and FT-FIR results. The shift and split of the IR absorption bands with the change in the structural parameters are proposed to be an applicable tool to probe the spin states and Jahn-Teller effect in La2-xSrxCoO4 compounds.