Assessment of electronic states and local structure of Mn-atoms in nanometric-sized La${}_{0.7}$Ca${}_{0.3}$Mn${}_{1-x}$Ni${}_x$O${}_3$ manganites by means of X-ray-absorption fine structure measurements

Single-phase, nanosized La${}_{0.7}$Ca${}_{0.3}$Mn${}_{1-x}$Ni${}_x$O${}_3$ (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 Ni${}^{2+}$ 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 Mn${}^{3+}$/Mn${}^{4+}$ ratio sparked by the Ni${}^{2+}$ doping. Here, a generation of more Mn${}^{4+}$ 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 Ni${}^{2+}$ doping was also determined from the analysis of the X-ray diffraction patterns, which allowed visualizing the small distortion of the MnO${}_6$ octahedra on substitution of Mn with Ni${}^{2+}$.