Experimental Investigation and Thermodynamic Modeling of the Li\(_2\)O–Al\(_2\)O\(_3\) System

In the present work, phase equilibria in the Li\(_2\)O–Al\(_2\)O\(_3\) system were experimentally studied and calorimetric measurements were performed. Based on obtained results and data from literature, thermodynamic parameters of the system were assessed. The solid solution phases were modeled using Compound Energy Formalism (CEF) and liquid phase was described by two-sublattice partially ionic liquid model. The experimental investigations for selected compositions of isothermally heat-treated samples were performed using x-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) were used to measure the temperature of the reactions as well as the heat capacities, respectively. After DTA, the microstructure was analyzed using SEM. Temperature of peritectic melting of h-LiAl\(_5\)O\(_8\) was determined to be 2222 K and temperature of eutectic reaction Liq \(\leftrightarrow\) \(\gamma\)-LiAlO\(_2\) + h-LiAl\(_5\)O\(_8\) to be 1965 K. Heat capacity of LiAlO\(_2\) and LiAl\(_5\)O\(_8\) was measured in the temperature range of 100-1300 K. The degree of inversion of spinel phase (Al\(^{+3}\), Li\(^{+1}\))\(_1^T\):(Al\(^{+3}\), Li\(^{+1}\), Va)\(_2^O\):O\(_4\) was modelled assuming Al\(^{+3}\) and Li\(^{+1}\) can occupy tetrahedral (T) and octahedral (O) cationic sublattices while its composition extension in Al\(_2\)O\(_3\) enriched region was described by introducing vacancies in octahedral sites. Thermodynamic description derived in the present study reproduces the degree of inversion close to that of the high-temperature spinel phase, which means that Al\(^{+3}\) ions preferentially occupy the tetrahedral sites. The calculated phase diagram satisfactorily agrees with the experimental results. Available experimental thermodynamic data are also reproduced within uncertainty limits.