Ferroelastic switching of doped zirconia: Modeling and understanding from first principles

Abstract

The properties of materials at high temperatures are often determined by complex thermodynamic mechanisms. One of the most prominent examples is the stabilization of tetragonal and cubic zirconia, which we investigate using density functional theory. The results show that the minimum energy path for the tetragonal-to-cubic phase transformation differs significantly from the paths discussed in the literature so far. This provides insight into the properties of compositions codoped with yttria and titania, an approach that has recently been proposed for the design of thermal barrier coatings. Zirconia-based materials offer many appealing properties, including a low thermal conductivity and a high ionic conductivity, as well as the potential for remarkable toughness. Hence, they are employed in a wide variety of applications, e.g., as catalyst supports, ionic conductors in sensors and solid oxide fuel cells, and thermal barrier coatings (TBC) for gas turbines in propulsion and power generation. In the latter case, a 150-1000 μm thick zirconia-based coating is applied to the turbine's superalloy components to protect them from the extreme temperatures generated during combustion. This improves the durability of the components and also enables an increase of the operation temperature and fuel efficiency [1].Pure ZrO 2 is not suitable for most applications because of the monoclinic-tetragonal phase transition at approximately 1500 K, which involves a disruptive volume change (∼4%) that degrades its mechanical integrity [1]. Thus, most ZrO 2 materials are doped to control or suppress the transformation, as it is the case for state-of-the-art TBCs based on singlephase tetragonal zirconia stabilized with 8 ± 1 mol % YO 1.5 (8YSZ). The remarkable durability of 8YSZ is ascribed to its superior toughness [2] based on a ferroelastic domain switching mechanism [1,3]. However, 8YSZ is metastable as a single tetragonal phase and eventually decomposes into Y-rich cubic and Y-lean tetragonal domains, the latter susceptible to the disruptive monoclinic transformation [4]. Accordingly, important goals in the development of advanced TBCs are phase stability at the higher temperatures (>1400 K) and improved toughness to mitigate a host of potential damage mechanisms [1]. For this purpose, it is essential to understand the fundamental origins of the mechanisms underlying the stabilization and toughening of the tetragonal phase.A common approach to stabilize the higher temperature forms of ZrO 2 involves doping with trivalent cations, most commonly Y 3+ , with the concomitant introduction of charged oxygen vacancies (F 2+ ) [5,6]. For TBCs, the useful composi-* carbogno@fhi-berlin.mpg.de Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.tional range is very narrow (8 ± 1 mol % YO 1.5 ): Underdoping renders the structure transformable to monocl...

Low-energy paths for octahedral tilting in inorganic halide perovskites

“…These are symmetrically distributed around the tiltless cubic perovskite. Similar transition mechanisms have recently been proposed also in several non-perovskite materials [18][19][20][21][22].…”

Nature of the octahedral tilting phase transitions in perovskites: A case study of CaMnO3

“…This allows for the emergence of the cubic perovskite structure as a dynamical average at lower temperatures than what would be expected in a picture where the phase transition happens by a displacive mechanism. A similar situation was found for the tetragonal-to-cubic phase transition in ZrO 2 [51].…”

Effects of strain on the stability of tetragonalZrO2

“…Their analysis predicted a second-order tetragonal-to-cubic transition of a displacive nature, whereby the average value of the shuffle amplitude gradually decreases to zero upon approaching the transition temperature from below. Carbogno et al 25 also investigated the tetragonal and cubic forms of ZrO 2 with molecular dynamics simulations using PBEsol. They found that thermal excitations induce spontaneous switching between tetragonal shuffle orientations and concluded that the experimental cubic form of ZrO 2 exists as a thermodynamic average of all three tetragonal displacement orientations.…”

“…Previously calculated lattice parameters for the cubic phase have ranges between 5.034 -5.151Å 22,23,25,36 . Calculated tetragonal lattice parameters vary between a = 5.029 − 5.161Å and c = 5.100 − 5.383Å 22,23,25,36 . The variation in calculated lattice parameters can likely be attributed to a variety of pseudopotentials being used.…”

“…It is well known that cubic ZrO 2 is dynamically unstable at zero Kelvin with respect to a cooperative oxygen shuffle that takes it to the tetragonal form [19][20][21][22][23][24] . The emergence of cubic ZrO 2 at high temperatures has been attributed to large anharmonic vibrational fluctuations that on average impart the crystal with cubic symmetry 21,25 . In constrast, various approximations to density functional theory (DFT) predict lowertemperature tetragonal and monoclinic variants of ZrO 2 to be dynamically stable at their zero Kelvin equilibrium volumes [22][23][24] .…”