ORDERING OF CHAIN OXYGEN IN YBA2CU3OX - OPTICAL INVESTIGATIONS ON SINGLE-DOMAIN CRYSTALS

We have determined the polarized reflectance spectra of (001) surfaces of YBa2Cu3O(x) single-domain crystals with intermediate oxygen conten t. Using the in-plane anisotropy of the optical properties, we could s eparate the contribution of the Cu-O layers and the Cu-O chains to the electronic transport properties. Taking the spectra in situ while the samples were heated up from room temperature to 150-degrees-C, thermo dynamical equilibrium states were established. We observe a strong dec rease of the free carrier and the Cu1+ concentration and the chain len gth with increasing temperature. Comparing the total number of free ca rriers with the number of Cu1+ ions, we find a direct correlation in a ccordance with a simple chemical model.