Characterization of metastable tetragonal hafnia

Conditions for retaining metastable tetragonal HfO₂ at room temperature were determined. Mechanical properties were measured as a function both of the fraction of retained metastable HfO₂ and of the porosity. The metastable phase was retained by 1) thermal decomposition of pure Hf(OH)₄ or HfOCl₂ and 2) hot pressing of HfO₂ + 1% Er₂O₃. The critical size for retaining pure tetragonal HfO₂ at room temperature in a stress free condition is 100 Å, whereas the critical size for the 1% Er₂O₃ hot pressed composition is significantly larger. The elastic moduli were found to vary linearly with porosity and showed a small effect due to the metastable phase presence.