Spectroscopic investigation of the electronic structure phase diagram of yttria-stabilized zirconia in an electron microscope

Abstract

Yttria-stabilized zirconia (YSZ) is one of the most frequently used ionic conductors since the substitution of Zr4+ by Y3+ ions results in the formation of oxygen vacancies. These vacancies lead to a significant O2- ion conductivity at elevated temperatures. Therefore, YSZ is employed as an electrolyte in devices like solid oxide fuel cells or chemical sensors. In this work, we investigate the influence of the Y2O3 concentration in the complex oxide on the crystallographic as well as the electronic properties. For this, we employed our self-built direct current ion beam sputter gun,(1) and deposited thin films with a nominal thickness of 25 nm for four different compositions, ranging from 3 mol% to 40 mol% Y2O3. These specimens were investigated using transmission electron microscopy as well as photoelectron spectroscopy to gain complementary information about their electronic structures. Usually, differentiating the tetragonal and cubic polymorphs of zirconium dioxide species is impossible with diffraction methods (both crystal structures yield the same diffraction patterns). The crystal class determination is nevertheless achievable by calculation of the unit cell height (i.e. the lattice parameter c), since this parameter is equivalent for both crystal structures (because the (002) spots in the diffraction patterns are identical). If this lattice parameter is subsequently plotted as a function of the yttria content, a distinct stagnation between 8 and 20 mol% Y2O3 can be observed (see Figure 1a). However, due to the significantly increased amount of Y3+, which is the larger ion when compared to Zr4+, the unit cell volume has to increase, which can only be explained by a phase transformation from tetragonal to cubic (i.e. a growth in the lateral dimensions). In later experiments, the phase transition could even be narrowed down to lie 158 between 8 mol% and 9.3 mol% Y2O3.(2) The same trend as a function of the yttrium oxide content is also observed for other parameters such as the direct band gaps, displayed in Figure 1b. These have been obtained by Valence EELS (VEELS),(3) for which the acceleration voltage is lowered to prevent Čerenkov losses from occluding the onset of the interband transitions, which corresponds to the band gap. Figure 1b reveals that the behavior of the band gaps is a mirrored version of that of the lattice parameter: when the yttria-concentration increases and the unit cell becomes larger, the band bap decreases, whereas it features a significant rise at the phase transition. A combination of core-loss EELS and UPS also gives rise to information about the O-vacancy related defect states within the band gap. Furthermore, it is shown that, using a cathodoluminescence spectrometer, the absorption of the emitted Čerenkov radiation inside the TEM can be used as a precise tool for the determination of the distance between the occupied and the unoccupied defect states (Čerenkov emission spectroscopy), as shown in Figure 2.