ELECTRON-MICROSCOPY STUDY OF ELECTROCHEMICALLY PREPARED LA2NIO4+DELTA(0.17-LESS-THAN-OR-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-0.26)

A new material with the composition of La2NiO4.26 exhibiting a large interstitial oxygen amount with regard to the basic K2NiF4-type structure has been prepared using an electrochemical process. This material has been examined by TEM and various superstructures either commensurate or incommensurate have been found. On the basis of the obtained ED patterns a structural model is proposed to account for the commensurate superstructures; they correspond to different oxygen stoichiometry, delta = 0.17 and delta = 0.25. In this structural model the interstitial oxygen atoms are located along the [111] directions of the K2NiF4-type unit cell. Two triclinic unit cells have been proposed for these two phases. Moreover an idealized structure involving an ordering of the nickel sites is depicted for La2NiO4.25. The ED patterns of the incommensurate structures have been described using a q* modulation vector in addition to the three fundamental vectors a(t)*, b(t)* and c(t)* of the 14/mmm structure. It was then pointed out that the modulation vector of the incommensurate structures lies between the two q1* and q2* modulation vectors that can be defined for the commensurate phases (delta = 0.17 and delta = 0.25 respectively) in the reciprocal plane corresponding to the [311BAR] zone axis. We assume that a direct relation exists between q* and delta.