Study of the local distortions of the perovskite systemLa1−xSrxCoO3(0≤x≤0.35)using the extended x-ray absorption fine structure technique

Abstract

We present a temperature-dependent extended x-ray absorption fine structure (EXAFS)/x-ray absorption near edge structure (XANES) investigation of ${\text{La}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{CoO}}_{3}$ (LSCO) over a wide doping concentration range $(0\ensuremath{\le}x\ensuremath{\le}0.35)$. Five of the samples are nanoparticles ($x=0.15$, 0.20, 0.25, 0.30, and 0.35) and four are bulk powders ($x=0$, 0.15, 0.20, and 0.30). From the EXAFS analysis, we find that the Co-O bonds are well ordered for both bulk and nanoparticle materials and there is no clear evidence for a Jahn-Teller (JT) distortion in the LSCO system (either static or dynamic). The distortion of the Co-O bond with increasing $T$, parameterized by the width of the pair distribution function (PDF), $\ensuremath{\sigma}(T)$, can easily be modeled using a correlated Debye model with a high correlated Debye temperature $\ensuremath{\sim}800\text{ }\text{K}$. There is also no evidence for a step in plots of ${\ensuremath{\sigma}}^{2}$ vs $T$. In addition, the very small nonthermal contribution to ${\ensuremath{\sigma}}^{2}$ for the Co-O (PDF), ${\ensuremath{\sigma}}_{\text{static}}^{2}$, sets an upper limit on the extent of any Jahn-Teller distortion at low $T$. These experiments are inconsistent with the existence of a significant fraction of Co sites with an intermediate spin (IS) state, for which there is a JT active ${e}_{g}$ electron on the Co atoms. We cannot, however, exclude the possibility of a tiny fraction of sites having a JT distortion or some other (non-JT active) means of producing an IS state. The bulk samples are well ordered out to at least the third neighbors (Co-Co) while the nanoparticles show increased disorder and a reduction in coordination for Co-Co. XANES data are also presented, and, for both bulk and nanoparticle samples, there is essentially no edge shift with increasing Sr concentration. Bond-valence sums also indicate no change in effective Co valence. These results indicate that when holes are introduced via Sr doping, they have little $\text{Co}(3d)$ character and the Co configuration remains close to $3{d}^{6}$; we argue that the holes have mostly $\text{O}(2p)$ character and are localized more on the O sites.