YBa₂Cu₃O_7-x STRUCTURAL SYSTEMATICS FOR EQUILIBRIUM COOLED SAMPLES B. C. Chakoumakos¹, T. B. Lindemer², M. Yethiraj¹, ¹Solid State Division, ²Chemical Technology Division, Oak Ridge National Laboratory*, Oak Ridge, TN 37831

Constant wavelength neutron powder diffraction data have been refined using FAT-RIETAN for a new suite of YBa₂Cu₃O_7-x samples, specially prepared as a function of oxygen content, by dynamically changing the oxygen partial pressure during cooling while monitoring the sample weight. The lattice parameters variation as a function of oxygen content and the orthorhombic-to-tetragonal transition follow that observed by Anderson et al. (Physica C 172, 31, 1990) for in situ measurements, except that our c values are systematically larger by 0.03 Å. The oxygen contents determined by site occupancy refinements (no O(5) site included) agree within 1.5 s.u. of the nominal equilibrium values. From a bond valence sum analysis, the apparent copper valences vary from 1.3 to 2.3 for the chain site and from 2.15 to 2.20 for the plane site as the oxygen content is varied from 6.05 to 6.97 per formula unit. The hole content per CuO₂ sheet exhibits a parabolic dependence on T_c. For each x value a 12 g sample was prepared in a vacuum thermogravimetric apparatus by heating to equilibrium weight (+/-0.1 mg) at T-pO₂ conditions that would give the desired x value as calculated from the thermodynamic models that were fitted to experimental data for YBa₂Cu₃O_7-x (Lindemer et al., J. Am. Ceram. Soc. 72, 1775, 1989). The samples were then cooled at ~200deg.C/h while decreasing the pO₂ to maintain the equilibrium specimen weight, i.e., the desired x value; about 75% of the specimens were equilibrated at lower T-pO₂ conditions to demonstrate that the desired x values were maintained. Reduction in flowing Ar-4%H₂ was used to verify the nominal x values of all samples after neutron diffraction. Neutron powder diffraction data were collected using the HB4 high resolution powder diffractometer (1.03 Å, d-spacing range = 0.55 - 5.38 Å) at the High Flux Isotope Reactor, ORNL.

*Supported by the Division of Materials Sciences, U.S. D.O.E. (contract DE-AC05-96OR22464 with Lockheed Martin Energy Research).