Metastable States in Alumina-Rich La2O3:Al2O3 and Nd2O3:Al2O3

Alumina‐rich mixtures of La2O3 and Al2O3 have equilibrium phases that are perovskite (LaAlO3) and β‐alumina (LaAl11O18). Annealing initially glassy samples with compositions 3La2O3–5Al2O3 or 11La2O3–14Al2O3 between 860° and 1075°C creates a metastable state that is clearly detected by X‐ray diffraction (XRD). While structures have not been determined, the new state appears to have two crystalline phases. This metastable state is favored by low annealing temperatures and by compositions with more rare‐earth oxide. Similar behavior is seen with Nd2O3:Al2O3 compositions, although the smaller Nd ion is less effective in promoting the metastable structure. The detection of equilibrium β‐alumina by XRD can be difficult, leading to possible misinterpretation of phases that are present.