Oxygen induced phase changes in YBa₂Cu₃O_6+δ. transport, structural and spectroscopic evidence

YBa₂Cu₃O_6+δwas prepared for various values of δ by equilibrating pieces of fully oxidized orthorhombic and fully reduced tetragonal material at 600°C.T _cwas found to decrease rapidly with decreasing δ and reached zero comptely wihin the orthorhombic phase. Strong changes in frequency and intensity of Raman and IR spectra were observed and could be correlated to the structural changes. In particular two IR-active modes at 580cm^-1and at 630cm^-1were observed which allowed to observe directly the doping process by the O(1)-oxygen. Also, the ratio of two special Raman lines was found to be a critical function of the oxygen concentration. From the experiments an assignmnt of vibrational modes was obtained in very good agreement with a recent normal coordinate calculation.