The Measurements of High Temperature Thermochemical Properties of Rare Earth Manganites with Electrical Conductivity

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The aim of this project is to obtain the high temperature thermochemical properties of rare earth manganites. We could obtain these quantities from the measurements of isothermal electrical conductivity and thermogravimetric analysis as a function of oxygen partial pressure. When these manganite phases is annealed under low oxygen partial pressure at high temperature, they decompose into rare earth sesqui oxides and manganese monoxide. Both results from thermogravimetric analysis and electrical conductivity measurements were in good agreement for the temperature and oxygen partial pressure at the decomposition. Therefore the chemical potential diagram for the equilibria between rare earth manganites and the decomposed oxides could be obtained in satisfactory precision. By using these results at several temperatures (900 - 1200 C of temperature and 1 - 10^<-22> atm of oxygen partial pressures), the free energies, enthalpy and entropy changes for LnMnO_3 phases could be obtained from the second law treatment of thermodynamics.Also heat capacities of several manganites at high temperatures and enthalpy changes of phase transitions of some of these manganites and another type of rare earth manganites with higher structure, e.g. BaLn_2Mn_2O_7, were measured by DSC method. After these measurements are accomplished with a satisfactory precision, the thermochemical properties of these rare earth manganites are to be obtained with more precise from by using the third law treatment of thermodynamics.