Charge order transitions in manganese oxides

The reflectance of a La0.85Sr0.15MnO3 crystal and the transmittance of LiMn2O4 powders have been measured for different temperatures ranging from 330 K down to 10 K. Synchrotron radiation was used in the far infrared range, while laboratory sources were used above 600 cm(-1). These oxides exhibit major changes of the phonon spectrum as T becomes lower than T* (T* = 180 K for La0.85Sr0.15MnO3 and T* = 290 K, with a hysteresis of about 15 K for LiMn2O4). For both of these manganese oxides, the increasing number of phonon structures observed for T <T* is ascribed to a rupture of the cubic crystal symmetry. The deformation of the high temperature cubic crystal follows the development into a charge ordered phase at T*, as recently observed by neutron scattering studies.