Optical phonons polarized along the c axis of YBa2Cu3O6+x, for x=0.5->0.95

The c-axis polarized phonon spectra of single crystals of YBa2CU3O6+x, were measured for the doping range x = 0.5 --> 0.95, between 10 and 300 K. The low background electronic conductivity, determined by Kramers-Kronig analysis of the reflectance, leads to a rich phonon structure. With decreased doping the five normally-active B-1 alpha modes broaden and the high-frequency apical-oxygen mode splits into two components. We associate the higher of these with the two-fold coordinated copper ''sticks''. The 155 cm(-1) low-frequency mode, which involves the apical and the chain oxygens, splits into at least three components with decreasing doping. Some phonon anomalies that occur near T-c in the highly doped material occur well above T-c in the oxygen-reduced systems. An unusual broad phonon band develops in the normal state at approximate to 400 cm(-1), which becomes more intense at low doping and low temperatures, borrowing oscillator strength from apical- and plane-oxygen modes resulting in a major transformation of the phonon spectrum below approximate to 150 K.