The atomistic structure of yttria stabilised zirconia at 6.7 mol%: an ab initio study

Abstract

Keywords: solid oxide fuel cell, yttria stabilised zirconia, ab initio, density functional theory, empirical potential, Born-Mayer-Huggins, ReaxFF, point charge model. AbstractYttria stabilized zirconia (YSZ) is an important oxide ion conductor used in solid oxide fuel cells, oxygen sensing devices, and for oxygen separation. Doping pure zirconia (ZrO 2 ) with yttria (Y 2 O 3 ) stabilizes the cubic structure against phonon induced distortions and this facilitates high oxide ion conductivity. The local atomic structure of the dopant is, however, not fully understood. X-ray and neutron diffraction experiments have established that, for dopant concentrations below 40mol% Y 2 O 3 , no long range order is established. A variety of local structures have been suggested on the basis of theoretical and computational models of dopant energetics. These studies have been restricted by the difficulty of establishing force field models with predictive accuracy or exploring the large space of dopant configurations with first principles theory. In the current study a comprehensive search for all symmetry independent configurations (2857 candidates) is performed for 6.7mol% YSZ modelled in a 2x2x2 periodic supercell using gradient corrected density functional theory. The lowest energy dopant structures are found to have oxygen vacancy pairs preferentially aligned along the ⟨2 1 0⟩ crystallographic direction in contrast to previous results which have suggested that orientation along the ⟨1 1 1⟩ orientation is favourable. Analysis of the defect structures suggests that the Y 3+ -O vac interatomic separation is an important parameter for determining the relative configurational energies.Current force field models are found to be poor predictors of the lowest energy structures. It is suggested that the energies from a simple point charge model evaluated at unrelaxed geometries 3 is actually a better descriptor of the energy ordering of dopant structures. Using these observations a pragmatic procedure for identifying low energy structures in more complicated material models is suggested. Calculation of the oxygen vacancy migration activation energies within the lowest energy ⟨2 1 0⟩ oriented structures gives results consistent with experimental observations.

Revisiting the Temperature Dependent Ionic Conductivity of Yttria Stabilized Zirconia (YSZ)

“…with mass action constant K = K 0 · e − Has / kT [21] ( H as = standard association enthalpy, E a = migration barrier height, D 0 = high temperature limit of the diffusion coefficient D). For given doping concentration, four parameters (E a , H as , K 0 , D 0 ) can be determined by fitting Eq.…”

Unexpectedly large energy variations from dopant interactions in ferroelectric HfO2 from high-throughput ab initio calculations

“…29 Another example is yttrium-doped ZrO 2 in the c-phase (YSZ), an important material for solid oxide fuel cells and gas sensors. 30 Finally, HfO 2 and ZrO 2 can exhibit ferroelectricity in thin films for certain doping concentrations. 4,18 The ferroelectricity arises from the polar, orthorhombic Pbc2 1 phase (p-o-phase).…”

“…41,42 As pointed out in the last paragraph, each of the three processing techniques reveals different dopant distributions, which only in one case represents the thermodynamic equilibrium configuration with the lowest energy. Doping of HfO 2 [25][26][27][43][44][45] and ZrO 2 46 was extensively researched utilizing the density functional theory (DFT) and using cluster expansion 30,33,47 or high-throughput calculations 48 under the premise that the lowestenergy structure is realized in thin films. Although all these studies attempt to answer the question of phase stabilization with doping, they do not consider the energy variation from the doping distribution enforced and biased by the processing technique.…”

Third-order nonlinear optical susceptibility of crystalline oxide yttria-stabilized zirconia

“…First, we focused on the static non-linear optical properties of pure c-ZrO 2 aiming at a reliable estimation of the order of magnitude of its third order NLO responses in idealized conditions to be used as a guide for our qualitative conclusions. Since little is known about the true local atomistic structure of YSZ, to circumvent the challenging task [25,26] of determining the most stable local crystal structure of each system considered, the second step comprised computations addressing the importance of the vacancy/dopant distribution on the third order susceptibilities. For this task, we chose two doping concentrations of 3.2 and 33 %mol.…”