Influence of Mn-O bond length on carrier localization in La1-x(Ca,Sr)(x)MnO3, where x=0.28 to 0.375

A series of rhombohedral alkaline-earth-doped manganites, La1-x(Ca,Sr)(x)Mn O3, 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 degrees) , and constant ion size variance (sigma(2)) of 0.0020 +/- 0.0010, Compositi onal uniformity and crystallinity were excellent, and porosity low. The tem perature of the maximum in the resistivity, T-m, for x = 0.28 was higher (b y more than 20 K) than previously reported. With increasing x land at the s ame time increasing Ca for Sr substitution), the Mn-O bond length shortened , T-m decreased, and the polaron hopping activation energy increased, indic ative of increased carrier localization. The behavior is opposite to the La 1-xSrxMnO3 series and can be explained by a strong dependence of the electr on-phonon interaction on Mn-O bond length. In order to maximize T-m, Mn-O b ond length must be controlled to a precise degree.