Metastable States in Alumina‐Rich La ₂ O ₃ :Al ₂ O ₃ and Nd ₂ O ₃ :Al ₂ O ₃

Abstract

Alumina‐rich mixtures of La 2 O 3 and Al 2 O 3 have equilibrium phases that are perovskite (LaAlO 3 ) and β‐alumina (LaAl 11 O 18 ). Annealing initially glassy samples with compositions 3La 2 O 3 –5Al 2 O 3 or 11La 2 O 3 –14Al 2 O 3 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 Nd 2 O 3 :Al 2 O 3 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.