The results of crystal structure determination and theoretical analysis of electronic structure and bonding properties in relation to thermal decomposition process in anhydrous nickel oxalate are presented. The details of the methods used in this analysis i.e., the Bader's quantum theory of atoms in molecules and bond order models (as defined by Pauling, Bader, Cioslowski and Mixon-modified by Howard and Lamarche), applied to topological properties of the electron density, obtained from ab initio calculations carried out by Wien2k FP-LAPW package (full potential linearized augmented plane wave method), as well as Brown's bond valence model (bond valences and strengths, and bond and crystal strains, calculated from experimental crystal structure data) are described. Nickel oxalate dihydrate was prepared by precipitation from water solutions of nickel nitrate (V) with oxalic acid at about 60°C. The crystalline powder was filtered, washed, and dried at 80°C on air. Anhydrous nickel oxalate sample was measured by XRD method applying Philips X'Pert Pro MD diffractometer equipped with MRI high temperature cell. Structural as well as qualitative and quantitative phase analyses were made by Phillips X'Pert HighScore Plus version 2.1 software with implemented full-pattern fit by means of Rietveld method. The detailed analysis of the obtained results shows that anhydrous nickel oxalate has monoclinic crystal structure (P2 1 /c, sg 14), the carboncarbon bond is the weakest one, and the process of thermal decomposition of this structure should begin with the breaking of this particular bond followed by nickel-oxygen bonds, which will lead to metallic nickel and carbon dioxide as final products, in agreement with the experiment. These results, supported by our earlier ones show clearly that such methods (topological and structural), when used simultaneously in analysis of the crystal structure and bonding properties, provide us with the additional insight into the behavior of given compound during thermal decomposition process and thus allow predicting and explaining of its most probable pathway. Keywords Crystal structure bond order Á Bond strength Á Bond valence Á Electron density topology Á FP-LAPW DFT calculations Á Thermal decomposition of anhydrous nickel oxalates
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