Chemical bonding study in Ca substituted Y 2 BaNiO 5 by analysis of charge density distribution Marcello Merli a , Doretta Capsoni b , Marcella Bini b , Vincenzo Massarotti b, * a Dipartimento di Scienze della Terra e CNR-Centro Studio per la Cristallochimica e la Cristallogra®a, Universita Á di Pavia, via Ferrata 1, 27100 Pavia, Italy b Dipartimento di Chimica Fisica `M. Rolla' , Universita Á di Pavia e CSTE-CNR, viale Taramelli 16, 27100 Pavia, Italy Received 25 September 2001; accepted 17 October 2001 by P. Wachter Abstract Y 2 BaNiO 5 , along with its Ca-substituted analog Y 22x 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 in¯uences the electronic charge distribution and the bond character of these octahedra. The electron density map r 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 Pro®le Fitting WPPF). We compute the Laplacian 7 2 r 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 con®rms the existence of a singularity of the x  0:24 composition, which is a threshold for the re-arrangement of the crystal structure. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 61.10.My; 82.20.Wt Keywords: C. X-ray powder diffraction; Maximum Entropy Method; D. Electron density distribution; Bond characterization 1. Introduction The structure and basic properties of Y 2 BaNiO 5 and its isostructural cation-substituted compounds have been recently investigated [1,2]. These materials contain distorted NiO 6 octahedra arranged in one-dimensional 1D) chains, where the apical Ni±O bonds are shorter than the planar bonds. Y 2 BaNiO 5 is a 1D antiferromagnetic system whose magnetic properties depend upon doping. Several studies have characterized the structural and magnetic properties of Y 2 BaNiO 5 and its Ca-substituted samples [1,3±5]. This work focuses upon the chemical bonds in these compounds. In particular, we retrieve accu- rate electronic density maps from powder X-ray diffraction data. The key computation tool is a Maximum Entropy Method MEM) [6], a technique which has been applied to single crystal X-ray diffraction data to determine the charge density distribution and the bond type in crystalline solids. So far, encouraging results have been achieved with simple structures using both synchrotron and conventional X-ray powder diffraction data [7,8]; the adopted procedure [7] involves the extraction of the uFu 2 set by ®tting the whole powder pattern WPPF), followed by application of MEM to compute a compatible electron density. On the other hand, Izumi [9] retrieves an initial set of uFu from the Rietveld method and a structural model); then, he performs a cyclic optimization involving both MEM and WPPF until a stable model-independent set of uFu is achieved. The application of this method to powder diffraction data in Ca-substituted Y 2 BaNiO 5 compounds appears to be particularly appropri- ate; we begin with a well-known orthorhombic structure where peak overlapping is never too large [4]. We present Solid State Communications 121 2002) 193±198 0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0038-109801)00452-5 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Fax: 139-382-507575. E-mail address: vimas@chi®s.unipv.it V. Massarotti).