Chemical bonding study in Ca substituted Y 2BaNiO 5 by analysis of charge density distribution

Abstract

Y 2BaNiO 5, along with its Ca-substituted analog Y 2− x Ca x BaNiO 5 (0< x≤0.33), displays peculiar physical properties related to the presence of linear chains of NiO 6 octahedra. These compounds can be considered prototypical Haldane systems and one-dimensional Heisenberg antiferromagnets. In this work, we investigate how the presence of calcium influences the electronic charge distribution and the bond character of these octahedra. The electron density map ρ( r) is derived from powder X-ray diffraction patterns with a novel procedure based upon a correlative use of two methods: Maximum Entropy Method (MEM) and Whole Powder Profile Fitting (WPPF). We compute the Laplacian ∇ 2 ρ( r) at the Bader critical point of the M–O bond; this quantity is shown to be directly related to structural parameters and their dependence upon composition. In particular, the substantial contraction of NiO 6 octahedra with increasing Ca content is related to an Ni–O bond which becomes more ionic as x increases up to x=0.24. On the other hand, covalency becomes stronger above x=0.24, particularly along some Ba–O bonds. Computation of the Shannon entropies confirms the existence of a singularity of the x=0.24 composition, which is a threshold for the re-arrangement of the crystal structure.