Monoclinic to tetragonal transformations in hafnia and zirconia: A combined calorimetric and density functional study

We use a combination of density functional theory and calorimetric measurements to investigate the monoclinic to tetragonal transition in hafnia and zirconia. We measure the transition enthalpies to be 8.4 +/- 0.7 kJ/mol in hafnia and, as previously reported, 5.272 +/- 0.544 kJ/mol in zirconia. Calculated values are 10.21 and 7.50 kJ/mol for hafnia and zirconia, respectively. We formulate a theoretical model of the phase transition consistent with the martnesitic character of the transformations. The transition barriers of 20.3 and 16.3 kJ/mol are estimated for hafnia and zirconia, respectively. We report the phonon spectra of monoclinic and tetragonal phases of both oxides and identify the Raman and IR active modes. The theoretical results compare well with available experiments. We present a comprehensive theoretical comparison of thermodynamic properties of zirconia and hafnia including the temperature dependence of specific heat.