Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

Abstract

Inelastic neutron scattering measurements on monoclinic zirconia (ZrO2) and 8 mol% yttriumstabilized zirconia were performed at temperatures from 300 to 1373 K. Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves, and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS, with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic, and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.

Raman spectra analysis of ZrO2 thermally grown on Zircaloy substrates irradiated with heavy ion: Effects of oxygen isotopic substitution

“…Second, in the case of a strong disorder, the reduced Raman spectra ultimately represent the frequency distribution of the phonon density of states (VDOS). In the present case, the observed broad signal has an apparent cut‐off wavenumber at about 830 cm −1 , which matches the cut‐off frequencies of the m‐ZrO 2 , t‐ZrO 2 , and even c‐ZrO 2 polymorphs calculated VDOS . Thus, it is tempting to interpret these spectra in terms of disorder.…”

“…Again, a precise result interpretation requires a complete normal coordinate analysis to determine the effective masses for each mode. However, for our purpose, it is sufficient to make rough estimates based on limiting cases referring to the partial VDOS available from the literature . In these partial VDOS and for all ZrO 2 phases, the heavier metal‐dominated modes lie in the low energy part of the VDOS, whereas the lighter oxygen atoms tend to dominate in the high energy part of the frequency spectrum, more or less schematically above ~ 400–450 cm −1 , slightly depending on the specific polymorph.…”

Effects of strain on the stability of tetragonalZrO2

“…Our quasi-harmonic calculations suggest that the approximation becomes invalid for tetragonal ZrO 2 above 1500K, when the predicted equilibrium volume becomes larger than the volume at which dynamical instabilities emerge. Recent work by Fultz and co-workers has also suggested the inadequacy of the quasi-harmonic approximation in capturing anharmonicity in rutile TiO 2 and monoclinic ZrO 2 at high temperatures 70,71 . Similar phonon-mode softening with increasing volume was predicted for rutile TiO 2 , for example, which is not observed experimentally 70 .…”

Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion and its stabilization of rutileTiO2

“…In the QHA, all shifts of phonon frequencies from their low temperature values are the result of thermal expansion alone. 20,21 For a lattice expansion of 0.5%, the energies of transverse phonons decrease by 10% to 50% in the QHA, giving Grüneisen parameters as large as 100 for rutile TiO 2 . 16,17 In the QHA, many transverse acoustic modes decrease to zero frequency at a lattice expansion of 0.75%, so the QHA predicts a collapse of the rutile structure at temperatures below 1000 K. The QHA does not account for all non-harmonic effects, however.…”