Phonon modes in CMR manganites at elevated temperatures

Phonon modes of the colossal magnetoresistance manganites La0.7Ca0.3MnO3 and La0.8Li0.2MnO3 have been investigated by far-infrared (100-700 cm(-1)) reflectivity spectroscopy at elevated temperatures (300-800 K). The three principal optical phonons all move systematically to lower frequency as temperature increases, in accordance with a Gruneisen relationship. The La(Ca,Li)-(MnO3) vibration (similar to 170 cm(-1)) varies with cation mass as expected. The Mn-O stretching mode (similar to 580 cm(-1)) has a higher frequency in the Li-doped compound than in the Ca-doped, whereas the O-Mn-O bending mode (similar to 340 cm(-1)) occurs at the same frequency in each, demonstrating that the cation strongly influences the Mn-O bond length but not the bond angle.