Ordering of interstitial oxygen atoms in La2NiO4+y observed by transmission electron microscopy

Structural properties of La2NiO4+δ have been investigated by electron diffraction and transmission electron microscopy. Two types of homologous series of superstructures have been found for samples containing excess oxygen, i.e., δ>0. They are 2aÃkaÃlc (k,l=n or 2n depending upon n; the most frequently observed sets are k/2=l/2=n=2 and k=l/2=n=3) and â2 aÃp â2 aÃqc (most frequently p=2 and q=1), where aÃaÃc 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 La2NiO4+δ with δ=0,...,1/8, 1/6, 1/4 seem to exist.