Topochemical reduction of the oxygen-deficient Ruddlesden−Popper phase (n=1) La1.85Ca0.15CuO4−δ and electrical properties of the La1.85Ca0.15CuO3.5

AbstractThe present paper reports on the synthesis, structure and electrical properties of the La1.85Ca0.15CuO3.5 compound. The topotactic reduction of the tetragonal oxide T-La1.85Ca0.15CuO4−δ with CaH2 results in the formation of La1.85Ca0.15CuO3.5. The powder X-ray diffraction data have shown that La1.85Ca0.15CuO3.5 adopts a face-centered monoclinic crystal structure (A 2/m, a=8.6224(6), b=3.8446(2) Å, c=13.0179(10) and β=109.690(5)°) with anion vacancies located within the CuO2 (CuO1.5□0.5, where □ represents an anion vacancy) layer of the phase. The topotactic reduction of the complex transition metal La1.85Ca0.15CuO4−δ may lead to the preparation of novel anion-deficient phases with unique transition metal-oxygen sublattices which cannot be prepared via the reduction of all-oxide substrates. The structural and chemical selectivity of the reduction process by anion de-intercalation and oxidation by anion insertion are reviewed. The oxidative monoclinic (La1.85Ca0.15CuO3.5) to tetragonal (T′-La1.85Ca0.15CuO4−δ) transition occurs at 380°C. The structural determination combined with accurate compositional and electrical characterization indicates a change in the charge-compensating mechanism with phase transition