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Abstract  

The results of first principles calculations of band structure, density of states and electron density topology of CdC2O4 and Ag2C2O4 crystals are presented. The calculations have been performed with WIEN2k ab initio program, using highly precise full potential linearized augmented plane wave (FP LAPW) method within Density Functional Theory formalism. The obtained SCF electron density has been used in calculations of Bader’s AIM (atoms in molecules) topological properties of the electron density in crystal. The obtained results show important similarities in electronic structure and electron density topology of both compounds and allow supposing, that during the thermal decomposition process these compounds should behave similarly, which is in agreement with the experiment.

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Theoretical studies of thermal decomposition of anhydrous cadmium and silver oxalates

Part II. Correlations between the electronic structure and the ways of thermal decomposition

Journal of Thermal Analysis and Calorimetry
Authors:
A. Koleżyński
and
A. Małecki

Abstract  

Detailed analysis of the results of full potential linearized augmented plane wave (FP LAPW) ab initio calculations for anhydrous silver and cadmium oxalates, reported in first part of this paper [1] has been presented. Additional calculations of Bader’s AIM (Atoms in Molecules) topological properties of the electron density, bond orders (Pauling, Bader, Cioslowski and Mixon) and bond valences according to bond valence model have been done. The obtained results show the similarities in electronic structure of both compounds and support the conclusion, that during the thermal decomposition process, these compounds should most probably decompose to metal and carbon dioxide, in agreement with the experiment.

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Abstract  

The results of first principles calculations of band structure, density of states and electron density topology of ZnC2O4 crystal are presented. The calculations have been performed with WIEN2k FP LAPW ab initio package. The obtained SCF electron density has been used in calculations of Bader’s QTAIM (quantum theory of atoms in molecules) topological properties of the electron density in crystal. Additional calculations of bond orders (Pauling, Bader, Cioslowski and Mixon) and bond valences according to bond valence model have been done. The obtained results are analyzed from the point of view of the thermal decomposition process, and this analysis indicates, that most probably this compound should decompose to metal oxide, carbon oxide and carbon dioxide, in agreement with the experiment.

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Abstract  

The results of theoretical analysis of the properties of crystal structure and bonding in relation to thermal decomposition process in chosen anhydrous metal oxalates (Cd, Co, Zn) are presented. The methods used in this analysis — the Bader’s quantum theory of atoms in molecules and bond order model (as defined by Cioslowski and Mixon), applied to topological properties of the electron density, obtained from DFT calculations performed by Wien2k package (full potential linearized augmented plane wave method), as well as Brown’s bond valence model (bonds valences and strengths, and bond and crystal strains, calculated from crystal structure and bonds lengths data) are described. Presented results allow us to state, that these methods, when used simultaneously, make possible the description and analysis of the crystal structure and bonding properties and give us the additional insight into its behavior during thermal decomposition process. The proposed theoretical approach can be considered as promising and reliable tool for theoretical analysis, allowing explanation and prediction of the properties of the structure and bonding and hence the most probable way of thermal decomposition process to take place in such structure.

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Abstract  

The results of theoretical analysis of the electronic and crystal structural properties and bonding in relation to thermal decomposition process in anhydrous calcium oxalate are presented. The methods used in this analysis—topological analysis of electron density (Bader’s Quantum Theory of Atoms in Molecules approach) obtained from DFT calculations performed by Wien2k package (Full Potential Linearized Augmented Plane Wave Method); bond order model (Cioslowski&Mixon), applied to topological properties of the electron density; as well as Brown’s Bond Valence Model (bonds valences and strength’, and bond and crystal strains, calculated from crystal structure and bonds lengths data) are described. The analysis of the obtained results shows that these methods allow us to explain the way of thermal decomposition process of anhydrous calcium oxalate to calcium carbonate as a decomposition product, and to describe the structural transition taking place during such process (from monoclinic anhydrous CaC2O4 to rhombohedral calcite structure). In the light of the results of our similar calculations performed previously for other anhydrous oxalates (zinc, cadmium silver, cobalt, and mercury) the proposed theoretical approach can be considered as promising and reliable tool, which allow analyzing the properties of the structure and bonding and hence predicting the most probable way of thermal decomposition process for given crystal structure.

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Abstract  

The results of theoretical analysis of the crystal structure and bonding in relation to thermal decomposition process in anhydrous mercury oxalate are presented. The methods used Bader’s Quantum Theory of Atoms in Molecules formalism with bond order model (by Cioslowski and Mixon), applied to electron density obtained from ab initio calculations carried out with FP-LAPW Wien2k package (Full Potential Linearized Augmented Plane Wave Method) and Brown’s Bond Valence Model are described. The analysis of the obtained results shows that most probably the thermal decomposition process of mercury oxalate should lead to metal and CO2 as products (as it is experimentally observed). Presented results (as well as the results of our similar calculations carried out previously for zinc, cadmium silver, cobalt and calcium oxalates) allow us to state that such methods (topological and structural), used simultaneously in analysis of the crystal structure and bonding properties, provide us with the additional insight into given compound’s behavior during thermal decomposition process. As a result, these methods can be considered as valuable supporting tool in the analysis of thermal decomposition process in given compound.

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Abstract  

DTA in conjuction with X-ray diffraction analysis with a high-temperature camera and infrared spectroscopy was employed to determine the mechanism of oxidation of Ni-P alloys. Amorphous Ni-P powders were obtained from a nickel(II) sulphate bath as a nickel source and sodium dihydrophosphate(I) as a reducing agent. The crystallization product is composed of two phases: (f.c.c.) Ni and (b.c.t.) Ni3P. The amorphous to crystalline transformation takes place in the temperature range 280–330C. Ni3P samples were heated from room temperature to 1050C in air atmosphere at 5C min−1. It was found that the first stage of oxidation of Ni3P goes through the intermediate phase of Ni12P5 formation to Ni2P. Some exothermic reactions were observed. Heating runs were interrupted after each reaction for crystal structure determination by IR spectrometry. Infrared spectra are reported and it is shown that the structure units present in the amorphous products at about 700C were the oxoanions PO3 and P2O7 . The final products of the oxidation process are NiO and Ni3(PO4)2.

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Abstract  

The oxidation of Ni100–xPx(7.3 at%<x<25.0 at%) powders in air in the temperature range 350–450C was determined by kinetics and X-ray diffraction. The isothermal kinetics was modeled using theGinstling–Brounstein equations. The oxidation process was found to be thermally activated with activation energy 127.8 kJ mol–1 for x=7.3 at% to 157.7 kJ mol–1 for x=25.0 at%. It was found that the rate constants for x=7.3 at% were approximately 100 times lower than those for x=25.0 at%.

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Abstract  

A simple method of determination of kinetic parameters by analysis of DTA(t) function was developed for the case of systems undergoing transitions without mass change and when kinetic equation describing transition rate is known. The presented method also permits the determination of transition rate dα/dt (or α(t )) when the kinetic equation of transition is unknown. The developed method was tested using DTA data of crystallization of 2CaOAl2 O3 1.95SiO2 glass pure and doped with Cr3+ and Nd3+ .

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