Study of the local distortions of the perovskite system La1-xSrxCoO3 (0≤x≤0.35) using the extended x-ray absorption fine structure technique

We present a temperature-dependent extended x-ray absorption fine structure (EXAFS)/x-ray absorption near edge structure (XANES) investigation of La1-xSrxCoO3 (LSCO) over a wide doping concentration range (0 <= x <= 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), sigma(T), can easily be modeled using a correlated Debye model with a high correlated Debye temperature similar to 800 K. There is also no evidence for a step in plots of sigma(2) vs T. In addition, the very small nonthermal contribution to sigma(2) for the Co-O (PDF), sigma(2)(static), 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 Co(3d) character and the Co configuration remains close to 3d(6); we argue that the holes have mostly O(2p) character and are localized more on the O sites.