Anion Rearrangements in Fluorinated Nd2CuO3.5

Abstract

Nd 2 CuO 3.5 , a derivative of Nd 2 CuO 4 with anion-deficient (CuO 1.5 □ 0.5 ) layers, has been fluorinated with XeF 2 at temperatures between 100 and 400 °C. Samples fluorinated at 200-300 °C exhibit superconductivity with critical temperatures of 6-11 K. Electron microscopy investigations revealed a complicated microstructure in both the initial and fluorinated phases. Nd 2 CuO 3.5 is characterized by the presence of numerous twins which are caused by the lowering of the symmetry after a topotactic oxygen removal from the parent Nd 2 CuO 4 phase. Fluorination at 200°C leads to an increase of the formal copper valence and to a restoration of the T'-type structure. It is suggested that fluorine replaces oxygen in the Nd 2 O 2 slab and the released oxygen atoms migrate into the (CuO 1.5 □ 0.5 ) layers thus forming conducting (CuO 2 ) planes. This rearrangement shows a close resemblance to that previously found in fluorinated A 2 CuO 3 (A = Ca, Sr). Local areas of fluorinated phases with larger values of the c parameter were found in the matrix of T' phase. At 300 °C the monoclinic Nd 2 Cu(O,F) 5 phase is also formed (a ≃ 13.2 A, b ≃ 5.5 A, c ≃ 5.8 A, β ≃ 92°, space group C2/c) with a chainlike ordering of the empty and filled anion positions in the Nd 2 (O,F) 3 blocks and buckled (CuO 2 ) planes due to a tilt of the Cu(O,F) 6 octahedra.