MODELING THE STRUCTURES OF LA2NIO4

Starting with only the chemical formula, La2NiO4, the phase diagram and the various structures it contains have been modelled using bond valences. The bonding graph is generated from the formula and is mapped into 3-dimensional space to give the archetypal K2NiF4 structure. The inconsistencies between the requirement that the layers in this structure be commensurate and the requirement that the two bond valence network equations (the Valence Sum Rule and the Equal Valence Rule) be satisfied, leads to the prediction that, under different conditions, one of four different structures should be found. The stoichiometric material is predicted to transform with reducing temperature from a structure in space group F4/mmm through one in Bmab to (possibly) one in P4(2)/ncm. A superstructure (e.g. 2 x 3 x 1 in space group Ibam) with a composition La2NiO4.17 is predicted for samples prepared in the presence of oxygen, corresponding to the observed orthorhombic (or tetragonal) interstitial phase. The model accounts for the miscibility gap observed between the stoichiometric and oxygen rich phases and indicates that all the structures are only marginally stable with the Ni-O bonds usually in compressive strain. The predicted structures (latice parameters and atomic positions) agree with those observed (where known) to within a few picometres.