ORDERING OF INTERSTITIAL OXYGEN-ATOMS IN LA2NIO4+DELTA

Two types of homologous series of superstructures have been found for La2NiO4 + delta containing excess oxygen, i.e. delta > 0. They are 2axkax1c (k, 1 = n or 2n depending upon n) and square-root-2axp-square-root-2axqc (most frequently p = 2 and q = 1), where axaxc stands for the K2NiF4-type cell. The origin of these superstructures has been assigned to the ordering of excess oxygen atoms located interstitially between two adjacent LaO layers. The most probable ordering schemes are proposed. Phases with discrete oxygen contents of delta = 1/2n seem to exist.