Geometric local structure at the Mn site in charge-ordered mixed valence manganites

An extensive study of Mn K-edge absorption spectroscopy has been carried out in La_1-xCa_xMnO₃ and Tb_1-xCa_xMnO₃ (x=0, 0.33, 0.5, 0.67, 1) series as a function of temperature. The EXAFS analysis for the charge ordering compounds (x ≥ 0.5) shows the presence of a static distortion of the MnO6 octahedron in the whole temperature range. However, this local distortion is lower than the distortion found for Jahn-Teller compounds. Magnetoresistive compounds (x=0.33) show a regular octahedron in the low temperature metallic phase. Instead, no changes are found for the first coordination shell (Mn-O) across the charge ordering transition while Mn-Mn distribution shows minor variations with the temperature. The structural analysis of the second coordination shell suggests that the Mn-O-Mn angle is the most relevant parameter to describe the electric and magnetic behaviour of these compounds.