HREM study of fluorinated Nd2CuO4

Nd2CuO4 was fluorinated using XeF2 as a soft fluorinating agent at temperat ures ranging from 200 degreesC to 500 degreesC, The structure of the fluori nated samples was studied using a combination of X-ray powder diffraction, electron diffraction, and high-resolution electron microscopy. Samples anne aled in the temperature range of 300 degreesC to 400 degreesC reveal the fo rmation of a new monoclinic phase (a approximate to 12.96 Angstrom, b appro ximate to 5.5 Angstrom, c approximate to 5.8 Angstrom, and beta approximate to 92 degrees; space group C2/c), The structural model was deduced from el ectron microscopy investigations, It involves an ordered occupation of anio n sites between two adjacent Nd layers by anions and anion vacancies and th e partial migration of anions from these tetrahedrally coordinated fluorite -type positions to octahedral interstices forming apical vertexes of Cu(O, F)(6) octahedra, No superconductivity was observed for this new monoclinic phase, The relationships between the structures of fluorinated T, T ', and T* phases are discussed. (C) 2001 Academic Press.