Assessment of electronic states and local structure of Mn-atoms in nanometric-sized La_0.7Ca_0.3Mn_1−xNi_xO₃ manganites by means of X-ray-absorption fine structure measurements

Single-phase, nanosized La_0.7Ca_0.3Mn_1−xNi_xO₃ (x = 0, 0.02, 0.07, 0.1) manganites were synthesized via the autocombustion route. Information on the local geometric structure and the charge state of the Mn ions in the nanosized samples was gleaned from extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data analysis, respectively. The experimental absorption spectra were recorded at room temperature at the K-edge of the Mn-ions. Analysis of the normalized XANES spectra showed that the Mn formal valence remained practically unchanged upon Ni²⁺ doping. Nevertheless, the observed broadening of the ruling absorption edge suggested that the repulsive nearest-neighbor potential, stemming from the shortening of the distances of Mn to the nearest-neighbor oxygen atoms (Mn–O bonds) in the coordination shell, was slightly modified. The values of the Mn–O distances were obtained from the Fourier transformed EXAFS spectra. A slight but sizeable decrease in the value of the Mn–O bond distances was verified, which pointed to a slight variation in the Mn³⁺/Mn⁴⁺ ratio sparked by the Ni²⁺ doping. Here, a generation of more Mn⁴⁺ ions with smaller radius (0.53 Å) was expected. The obtained Mn–O distances were compared with those resulting from the Rietveld refinement of the X-ray powder diffraction data. The variation of the 〈Mn–O–Mn〉 bond angle with Ni²⁺ doping was also determined from the analysis of the X-ray diffraction patterns, which allowed visualizing the small distortion of the MnO₆ octahedra on substitution of Mn with Ni²⁺.