Influence of Mn–O bond length on carrier localization in La_1−x (Ca,Sr)_x MnO₃, where x = 0.28 to 0.375

A series of rhombohedral alkaline-earth-doped manganites, La_1−x (Ca,Sr)_x MnO₃, were studied, with x = 0.28 to 0.375, a constant tolerance factor of 0.927 ± 0.002, a very small variation of the Mn–O bond angle (<0.3°), and constant ion size variance (ς2) of 0.0020 ± 0.0010. Compositional uniformity and crystallinity were excellent, and porosity low. The temperature of the maximum in the resistivity, T_m, for x 4 0.28 was higher (by more than 20 K) than previously reported. With increasing x (and at the same time increasing Ca for Sr substitution), the Mn–O bond length shortened, T_m decreased, and the polaron hopping activation energy increased, indicative of increased carrier localization. The behavior is opposite to the La_1−xSr_xMnO₃ series and can be explained by a strong dependence of the electron–phonon interaction on Mn–O bond length. In order to maximize T_m, Mn–O bond length must be controlled to a precise degree.