HREM Study of Fluorinated Nd_2CuO_4

摘要

Nd2CuO4 was fluorinated using XeF2 as a soft fluorinating agent at temperatures ranging from 2000C to 5000C. The struc-ture of the fluorinated samples was studied using a combination of X-ray powder diffraction, electron diffraction, and high-resolu-tion electron microscopy. Samples annealed in the temperature range of 3000C to 4000C reveal the formation of a new mon-oclinic phase (a 12.96 A, b 5.5 A, c 5.8 A, and /? 920; space group C2/c). The structural model was deduced from electron microscopy investigations. It involves an ordered occu-pation of anion sites between two adjacent Nd layers by anions and anion vacancies and the partial migration of anions from these tetrahedrally coordinated fluorite-type positions to octa-hedral interstices forming apical vertexes of Cu (0, F)6 octa-hedra. No superconductivity was observed for this new monoclinic phase. The relationships between the structures of fluorinated T, T’, and T* phases are discussed.