Yttria-stabilized zirconia (YSZ) is a technologically important oxide ceramic whose properties are closely linked to phase stability and structural symmetry. It is widely used because of its practical importance. Its properties are strongly influenced by yttria content. For example, 3YSZ is used in cutting tools and dental implants because of its superb hardness and corrosion resistance [1], while 8YSZ is utilized in solid oxide fuel cells (SOFCs), oxygen sensors, and oxygen pumps because of its high ionic conductivity [2]. YSZ is also of considerable importance in thermal barrier coatings owing to its excellent shock resistance, low thermal conductivity, and relatively high coefficient of thermal expansion [3]. In addition, zirconia-based ceramics, notably YSZ, are considered promising candidates as host matrices for radioactive waste elements, where higher-symmetry structures improve the solubility of radioactive elements like Th under extreme conditions [4].
    Because of these practical and scientific interests, understanding the structural behavior of YSZ under extreme conditions is essential. Pure ZrO2 crystallizes in the monoclinic structure at ambient conditions, while higher-symmetry tetragonal and cubic structures are stable at elevated temperature. In YSZ, the introduction of yttria stabilizes these higher-symmetry phases at room temperature, depending on the doping level. These structural changes strongly affect the properties and industrial performance of the material. However, pressure, which is another important thermodynamic parameter, has not been equally exploited for understanding the structural behavior of YSZ under extreme conditions. Therefore, the structural evolution of two selected compositions of YSZ, with 3 mol% (3YSZ) and 8 mol% (8YSZ) of Y2O3, has been investigated under pressure using in-situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy in a diamond anvil cell up to 40 GPa at room temperature.
    The close crystallographic relation between the observed structures and the relatively large difference in the atomic numbers of Y/Zr and O imposes the simultaneous study using both techniques, aiming to fully elucidate the structural evolution under pressure. In this study, neither powder XRD nor Raman spectroscopy alone can uniquely resolve the pressure-induced phase transitions among t-, t”-, and c-phases in YSZ. Powder XRD is insensitive to oxygen displacements, while Raman spectroscopy cannot distinguish t- and t”-phases due to their identical space-group symmetry. Thus, only the combination of both techniques can accurately provide exact information about the critical pressures and the high-pressure phases. The 3YSZ composition lies at the stability limit of the tetragonal phase, where a coexistence with the monoclinic phase is observed, while 8YSZ is known to adopt the t”-phase under ambient conditions. We combined in-situ Raman spectroscopy together with high-resolution synchrotron XRD measurements using neon as the pressure-transmitting medium.
    The results, by combining both techniques, reveal that for both 3YSZ and 8YSZ, pressure promotes higher-symmetry structures. Under initial compression, the minority monoclinic phase at ambient conditions gradually transforms towards the tetragonal phase, and this transition is concluded for both 3YSZ and 8YSZ at approximately 10 GPa. For 3YSZ, after the disappearance of the monoclinic phase, the solely remaining t-phase transforms to the t”-phase, which in turn transforms to the c-phase above 28 GPa. The c-phase remains stable up to the highest pressure of this study. Likewise, for 8YSZ, the coexistence of t- and t”-phases continues up to 31 GPa, where both transform towards the c-phase, which also remains stable up to the highest pressure reached.
    Upon pressure release, all observed transitions are fully reversible with negligible hysteresis, with the exception of the practical disappearance of the monoclinic phase at ambient conditions. After full pressure release, both 3YSZ and 8YSZ recover their initial dominant tetragonal and t”-type character, respectively, as evidenced by both XRD and Raman measurements. However, the intensities of the characteristic Bragg peaks and Raman modes of the monoclinic phase become exceedingly weak, indicating that the amount of recovered monoclinic phase is minuscule. Taken together, these results indicate a suppression of the monoclinic phase upon decompression and a concomitant tendency to retain higher-symmetry structures, pointing to a route toward structural purification in YSZ.
    Our study underscores the significance of simultaneously performing and analyzing XRD and Raman spectroscopy in relevant crystallographic systems and delivers structural evolution insights in defect-stabilized oxide ceramics under high pressure. Moreover, it provides pressure as a composition-preserving route for suppressing monoclinic phase toward a structural purification in YSZ, as well as in other related stabilized ceramic systems.