ИСТИНА

This work continues a set of systematic investigation of thermochemical properties of copper-containing metal-oxides materials and is concerned with the determination of the enthalpy of formation of some phases in the La-M-Cu-O system (M = Ca, Sr, Ba). The solid solutions La¬2-xMxCuOy are of a particular interest as copper-containing high-Tc superconductors, cathode material for fuel cells and catalysts. These systems have high electronic and oxygen-ion conductivity. The only two works [1, 2] devoted to determinations of La¬2-xMxCuOy enthalpies of formation. The enthalpies of formation of La¬1.8Ca0.2CuO3.98, La¬1.8Sr0.2CuO3.99 were not determinated early. The enthalpies of formation of La¬1.8Ba0.2CuO3.976 were obtained by calorimetry at 298.15 K in [2] (oxHo = -35.0 1.6 kJ/mol), this sample was prepared from BaCO3. To prevent the sample from BaCO3 impurity we synthesized this sample by using other precursor. The samples of La¬2-xMxCuOy (M = Ca, Sr) were prepared by a method of solid state synthesis from lantanum and copper oxides and calcium (strontium) carbonate, the sample of La¬1.8Ba0.2CuO3.97 was synthesised by using BaO2. The synthesized samples were characterized by X-ray. The formal copper valence and extra oxygen contents in samples were determinated by iodometric titration. The composition of samples was confirmed by ICP-MS method. The enthalpy of formation of investigated solid solutions were determined by solution calorimetry method. The enthalpies of La¬2-xMxCuOy reaction with 1 M hydrochloric acid were measured at 298.15 K in sealed swinging calorimeter with an isothermal coat. The enthalpies of formation from the elements and oxides of studied solid solutions were calculated on basis of experimental data and the realiable literature data on thermochemical properties of simple oxides of copper, lanthanum and alkaline-earth elements.