Shennan Hu¹ , Baihong Sun^1,2 , Wenting Lu^1,2 , Shiyu Feng^1,2 , Azkar Saeed Ahmad^1,2,3 , Hiroki Habazaki⁴ , Bihan Wang⁵ and Elissaios Stavrou^1,2,3
1Department of Materials Science and Engineering, Guangdong Technion-Israel Institute of Technology, Shantou 515063, China
² Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel
³Guangdong Provincial Key Laboratory of Materials and Technologies for Energy Conversion, Guangdong Technion – Israel Institute of Technology, Shantou, Guangdong 515063, China
⁴SPring-8/JASRI, 1-1-1 Kouto, Sayo-gun, Sayo-cho, Hyogo 679-5198, Japan
⁵Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, Hamburg 22607, Germany
Elissaios.stavrou@gtiit.edu.cn
Yttria-Stabilized Zirconia (YSZ) is an important material for a wide range of industrial applications. Variation of Yttria quantity (Y2O3) within YSZ results in different properties. For instance, 3mol%YSZ (3YSZ) is a major contributor in cutting tools ^[1] and dental implants because of its superb hardness as well as excellent resistance to corrosion. Likewise, 8mol% (8YSZ) is a preferred electrolyte for Solid Oxide Fuel Cells (SOFC) ^[2]. In this study, our main aim is the investigation of the structural evolution of YSZ compounds under high pressure. For this reason, a concomitant high-pressure in-situ Raman spectroscopy and synchrotron angle dispersive X-ray diffraction (XRD) study using a diamond anvil cell (DAC) at room temperature was performed. Two specimens with 3 mol% (3YSZ) and 8 mol% (8YSZ) Y2O3 have been studied. Pressure was determined by ruby fluorescence and/or gold equation of state (EOS), and neon was utilized as the pressure-transmitting medium that remains fairly hydrostatic up to at least 40 GPa.
Smith et al. ^[3] suggested that doping of ZrO2 with Y2O3 stabilizes highsymmetry crystal structure (tetragonal or cubic) as opposed to monoclinic structure for pure ZrO2. This was confirmed in our study; XRD measurements at ambient conditions for both 3YSZ and 8YSZ indicate the coexistence of a, predominately, tetragonal (space group P42/nmc (137), t phase) phase and a monoclinic (space group P21/b (13)) phase. Upon increasing pressure, the monoclinic phase completely and irreversibly transformed into the tetragonal phase above 4GPa for both compounds. Upon further pressure increase, 8YSZ remains in the tetragonal phase up to 23 GPa, consistent with the results reported by Alzyab et al. ^[4] and Smith et al. ^[3] . On the other hand, in the case of 3YSZ, a pressure-induced phase transition towards the cubic fluorite (Fm3m (225)) or the pseudocubic tetragonal (space group P42/nmc (137), t’’ phase) structure was concluded above 10 GPa, following the analysis of Lamas et al. ^[5] .
Owing to the second-order nature of the tetragonal to cubic phase transition and the group-subgroup relation between the two structures, the transition can be only determined by observing the merging of the relevant Bragg peaks, making it challenging to distinguish individual reflection within one observed diffraction peak. Thus, making the exact determination of the critical pressures and high-pressure phases (pseudo-cubic t’’ or cubic) ambiguous. For this reason, Raman spectroscopy, which is highly sensitive to local order modification, was further employed. In the case of 8YSZ, Raman spectroscopy results confirm XRD findings up to 23GPa, revealing two prevailing Raman active modes (Eg and A1g), starting from 4GPa, which belong to the tetragonal phase. Furthermore, a phase transition to the cubic phase was also observed above 27GPa, based on the disappearance of Raman active modes above this pressure. Finlay, from XRD diffraction results, the relevant structural parameters (lattice parameters and cell volume) were obtained as a function of pressure, and a 3 rd order Birch-Mumaghan EOS was fitted to the experimental results in order to determine the bulk moduli. This study documents the close synergy between XRD and Raman spectroscopy techniques needed for the full and accurate characterization of secondorder phase transitions under pressure.

Reference
[1]. Yanagida, H., Koumoto, K., & Miyayama, M. (1996). The chemistry of ceramics. Wiley.
[2]. Mahato, N., Banerjee, A., Gupta, A., Omar, S., & Balani, K. (2015). Progress in material selection for solid oxide fuel cell technology: A review. Progress in Materials Science, 72, 141-337.
[3]. Smith, Q. B. (2016). Impedance Spectroscopy Studies of Yttria Stabilized Zirconia Under Extreme Conditions. UNLV.
[4]. Alzyab, B., Perry, C. H., & Ingel, R. P. (1987). High‐Pressure phase transitions in Zirconia and Yttria‐Doped Zirconia. Journal of the American Ceramic Society,
70(10), 760–765.
[5]. Lamas, D. G., & Walsöe De Reca, N. E. (2000). X-ray diffraction study of compositionally homogeneous, nanocrystalline yttria-doped zirconia powders. Journal of Materials Science, 35, 5563-5567.

Raman spectroscopy,high pressure,YSZ,synchrotron x-ray diffraction,in-situ XRD