The characterization and influence of stoichiometry on the properties of lanthanum nickel oxide

Density measurement were performed on single crystals of lanthanum nickel oxide. The results clearly show that deviations from the ideal stoichiometric ratios are accommodated by an anion interstitial mechanism. Next, thermogravimetric experiments were performed. The results of the density and thermogravimetric investigations were combined and the point defect equilibria determined. Magnetic susceptibility measurements were made on single crystals of La$\sb2$NiO$\sb{4+\delta}$ as a function of composition. Samples with nearly ideal stoichiometric ratios ($\delta$ $\approx$ 0) display a previously unreported transition to long range antiferromagnetic order which occurs in the same temperature range as the semiconductor-metal transition. An increase in $\delta$ leads to a suppression of the magnetic ordering temperature while the transition in resistivity remains unaltered. This result was interpreted in terms of an oxygen interstitial defect mechanism and the quasi-two-dimensional structure of the material.