  The c-axis polarized phonon spectra of single crystals of YBCO_{6+x} have
been measured for the doping range x=0.5 -> 0.95, between 10 K 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_{1u} 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-oxygen sticks. The 155 cm^{-1} low-frequency mode,
which involves the apical and chain-oxygens, splits into at least three
components with decreasing doping. Some phonon anomalies which 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 approx
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 approx 150 K.
