Ordering of interstitial oxygen atoms inLa2NiO4+δobserved by transmission electron microscopy

Abstract

Structural properties of ${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4+\mathrm{\delta }}$ 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 2a×ka×lc (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 √2 a×p √2 a×qc (most frequently p=2 and q=1), where a×a×c stands for the ${\mathrm{K}}_2$ ${\mathrm{NiF}}_4$-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 ${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4+\mathrm{\delta }}$ with δ=0,...,1/8, 1/6, 1/4 seem to exist.