Charge ordering and phonon anomalies in Pr0.5Ca0.5MnO3

We investigate the optical conductivity of a half-doped manganite Pr0.5Ca0.5MnO3 as a function of the temperature. Although optical phonon modes correspond to a slightly distorted cubic structure above the charge and orbital ordering TCO–OO, additionnal peaks are evidenced at low temperature. These new vibration modes are assigned to changes in MnOMn bond angle and length that we interpret as a signature of a cooperative Jahn–Teller distortion associated to charge/orbital ordering. Spectra exhibit a broad mid infrared band which transfers to high energy below TCO–OO resulting in an optical gap. We argue that this large spectral weight shift is due to freezing of Jahn–Teller polarons with the establishment of the charge ordered CE-type structure.