Phonon Modes in CMR Manganites at Elevated Temperatures

Phonon modes of the colossal magnetoresistance manganites La_0.7Ca_0.3MnO₃ and La_0.8Li_0.2MnO₃ 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 Grüneisen relationship. The La(Ca,Li)–(MnO₃) vibration (170 cm^–1) varies with cation mass as expected. The Mn–O stretching mode (580 cm^–1) has a higher frequency in the Li-doped compound than in the Ca-doped, whereas the O–Mn–O bending mode )340 cm^–1) occurs at the same frequency in each, demonstrating that the cation strongly influences the Mn–O– length but not the bond angle.