Evaluation of the thermodynamic stability of REMgAl11O19 (RE = La, Pr, Nd, Sm) hexaaluminates with a magnetoplumbite structure in the high temperature region

This study is important due to the lack of reliable data about the properties of high temperature materials for energy production and aerospace engineering. The purpose of this article was to evaluate the thermodynamic stability of RE magnesium hexaaluminates REMgAl11O19 (RE = La, Pr, Nd, Sm) with a magnetoplumbite structure, which are promising components for thermal barrier coatings. For this, we calculated the values of the Gibbs energy of the decomposition reactions of RE magnesium hexaaluminates into simple oxides and aluminum-magnesium spinel MgAl2O4 and REAlO3 phases in the temperature range of 298–1,800 K. For calculations, we used data on the thermodynamic properties of hexaaluminates calculated from the values of heat capacity measured by differential scanning calorimetry in the range of 300-1,800 K and from values of thermodynamic properties of simple oxides, MgAl2O4, and REAlO3 provided in previous research. There is hardly any information about the thermodynamic properties of RE magnesium hexaaluminates, which are promising thermal barrier materials. The purpose of the article is to provide a thermodynamic evaluation of the probability of decomposition reactions of hexaaluminates in the high temperature region. Previously published data on the high temperature heat capacity of compounds with the composition of REMgAl11O19 (RE = La, Pr, Nd, Sm) were used to calculate temperature dependences of entropy and changes in enthalpy, which were used to evaluate the Gibbs energy of the decomposition reactions of hexaaluminates into constituent oxides. The temperature dependences of the Gibbs energy of the four possible decomposition reactions of hexaaluminates allowed drawing conclusions about thermodynamic stability in the high temperature region