Yttria-stabilized zirconia (YSZ) is known for its high ionic conductivity at elevated temperatures, which makes it ideal for usage as an oxygen ion conducting electrolyte. Thus, it is often utilized in chemical sensors or solid oxide fuel cells. In SOFCs, it is also employed as a component of the anode (a Ni/YSZ cermet). There, besides facilitating the transport of oxygen anions from the electrolyte to the three-phase boundary, it plays a role in the catalytic process involving the activation of various fuels fed to the SOFC.

In both cases, the relevant properties – be it the ionic/electronic conductivity or the catalytic activity – are defined by the existence of oxygen vacancies and the electronic structure of the complex oxide. In order to investigate this systematically, thin film model systems with varying compositions, ranging from 3 mol% Y₂O₃ to 40 mol% Y₂O₃, were deposited by means of a home-built direct current ion beam sputter source.[1] This source allows for the preparation of thin films (approximately 25 nm) at a controlled growth rate, facilitating the epitaxial growth. As substrates, freshly-cleaved NaCl(001) single crystals were employed, which allowed the thin films to be used as unsupported transmission electron microscopy (TEM) specimens.

Using selected area electron diffraction, the crystallography was investigated. However, since the tetragonal and cubic polymorphs contain lattice planes with the same spacings, a distinction is not trivial. Nevertheless, this can be circumvented by calculating the unit cell height, i.e. the lattice parameter c, which is equivalent for the tetragonal and cubic structures, and plotting it as a function of the yttria-content. There, a retention of the unit cell height between 8 and 20 mol% Y₂O₃ can be observed, which indicates that a phase transition from tetragonal to cubic is present between 8 and 20 mol% yttria as, due to the higher yttria concentration (with Y³⁺ being the larger ion than Zr⁴⁺), the unit cell volume must increase. Hence, the retention of the unit cell height can only be explained by an expansion in the lateral dimensions, i.e. the tetragonal cell becomes cubic.[1] In fact, using magnetron sputtering, the location of the phase transition could be narrowed down to be between 8 and 9.3(4) mol% Y₂O₃,[2] which is crucial knowledge if, for instance, the cubic structure for the electrolyte is desired.

Very similar trends can be found for the direct (by means of valence EELS) and indirect band gaps (UPS), which show a decreasing trend as more Zr is substituted by Y – and, thus, as more oxygen vacancies are generated. However, at the phase transition, there is a discontinuity as the band gaps increase significantly. Furthermore, the electron affinity, i.e. the energy difference between the vacuum level and the conduction band minimum, is shown to follow the same function as well. This parameter exhibits another rare phenomenon: it is negative. Because of this, the vacuum level is within the band gap, which causes electrons that have been excited into the conduction band by, for instance, optical absorption, to be emitted with almost no barrier. This makes these materials also suitable as photocathodes, for example in spin-polarized electron microscopes.

With Čerenkov emission spectroscopy, where the light emitted when an electron traverses the sample at a speed larger than the speed of light inside the medium is recorded, another electronic transition at slightly above 3 eV (also displaying the same trend as the lattice parameter as a function of the Y₂O₃content) was found. By comparison with densities of states from DFT calculations, this can be determined as being the excitation of electrons from the valence band maximum into the unoccupied oxygen vacancy-related gap states, i.e. these values depict the gap state level.

With help of these investigations, not only can a crystallographic thin film phase diagram be constructed, but also an electronic structure phase diagram, which displays the relation of various electronic niveaus as a function of the Y₂O₃content in the YSZ thin films relevant for applications in SOFCs.

[1] Götsch, T.; Wallisch, W.; Stöger-Pollach, M.; Klötzer, B.; Penner, S. AIP Adv. 2016, 6 (2),025119

[2] Götsch, T.; Schachinger, T.; Stöger-Pollach, M.; Kaindl, R.; Penner, S. Appl. Surf. Sci. 2017, 402,1–11

Figure 1: The unit cell height (top panel) as well as the band gaps (lower panel) show the same behavior as a function of the yttria content.