Ordering of oxygen vacancies and magnetic properties in La0.5Ca0.5MnO3-delta (0 <=delta <= 0.5)

The study by electron diffraction and high resolution electron microscopy o f several samples of the system La0.5Ca0.5MnO3-delta, apart from strikingly confirming the correctness of the orthorhombic perovskite-related unit cel l of a approximate to a(c)root 2, b approximate to a(c)root 2, c approximat e to 2a(c) for 0 less than or equal to delta less than or equal to 0.25, po ints to the existence of three dimensional multitwinning which gives rise t o an apparently double cubic cell. For delta=0.5, the ordering of the anion ic vacancies leads to a new material isostructural to Sr2FeO5 and unit cell a = 5.361(3), b = 16.463(9), c = 5.340(3) Angstrom, space group Ibm2. The magnetic characterization of the three compositions has been carried out an d related to the structure. (C) 1999 Academic Press.