Phonons and phase transitions in zirconia

The martensitic‐austenitic phase transition in zirconia was investigated as a function of temperature and pressure using Raman and IR spectroscopy. The frequencies and symmetries of the phonon modes corresponding to the monoclinic (M), C   2h 5(4 molecules per unit cell) structure were identified at 300 K and 1 atm. The temperatureinduced transition to the tetragonal (TI), D   4h 15structure (2 molecules per unit cell) showed thermal hysteresis (1225 ↔ 1425 K). The corresponding pressure‐induced transition at room temperature showed a pressure hysteresis (3.4 ↔ 3.95 GPa). However, the structural transition was to a different tetragonal modification (TII) with D 2h or C 4v symmetry with 4 molecules per unit cell.